Advances in Agronomy, Volume 25

ADVANCES IN AGRONOMY VOLUME 25 CONTRIBUTORS TO THIS VOLUME K. BAEUMER W. A. P. BAKERMANS C. BLOOMFIELD J. K. COULT...

Author: N. C. Brady (Editor)

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ADVANCES IN

AGRONOMY VOLUME 25

CONTRIBUTORS TO THIS VOLUME

K. BAEUMER W. A. P. BAKERMANS

C. BLOOMFIELD J. K. COULTER A. E. FOSTER

R. F. HARRIS T. K. HODGES

E. A. HOLLOWELL W. E. KNIGHT G. A. PETERSON

MOSHEJ. PINTHUS J. R. QUINBY J. C. RYDEN

J. K. SYERS

ADVANCES IN

AGRONOMY Prepared under the Auspices of the AMERICAN SOCIETY

AGRONOMY

OF

VOLUME 25

Edited by N. C. BRADY International Rice Research Institute Manila, Philippines ADVISORY BOARD

D. G . BAKER

H. M. LAUDE

G. R. DUTT G . W. KUNZE

M. A. MASSENGALE

D. E. WEIBEL

1973

ACADEMIC PRESS

New York

San Francisco London

A Subsidiary of Harcourt Brace Jovanovich, Publishers

COPYRIGHT 0 1973, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. N O PART O F THIS PUBLICATION MAY BE REPRODUCED OR TRANSMImED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

ACADEMIC PRESS, INC.

111 Fifth Avenue, New York, New York 10003

United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London NWI

LIBRARY OF CONGRESS CATALOG CARDNUMBER:50-5598

PRINTED IN THE UNITED STATES OF AMERICA

CONTENTS CONTRIBUTORS TO VOLUME25 . . . . . . . . . . . . . . . . . . . . . , . . . . . . , . . . . . . . . . . . .

ix

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xi

PHOSPHORUS IN RUNOFF AND STREAMS

J. C. RYDEN,J. K. SYERS,AND R. F. HARRIS

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

11. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Factors Affecting the Dynamics of Phosphorus in Runoff and Streams . .

2

.

IV. Phosphorus Loads in Runoff and Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Impact of Phosphorus Carried in Streams on Standing Waters . . . . . . . . . VI. Present Status and Outlook . . . , . , . . . . . . . . , . . , . . . . . , . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

20 37 38 41

CRIMSON CLOVER

W. E. KNIGHTAND E. A. HOLLOWELL

I. 11. 111.

...............................................

Morphology . . . . . . . . . . . . . . . . . . . ................. . . . . ... .. .. .. .. Physiology . , . . . . . . . . . . . . . , . . . Culture . . . . . . . ... . . .. . ............... .. Utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IV. V. VI. VII. VIII. Conclusions

..,............

48 50 52

57 65 68

....................................

. . . . .. . . . . . .. . .. . . .

........................ ...............................................

12 73

ZERO-TILLAGE

K. BAEUMERAND W. A. P. BAKERMANS 1. Introduction: The Concept of Zero-Tillage

. . . . . . . . . . . . . . . . . ... . . . . . .

11. Comparison of Environmental Conditions in Tilled and Untilled Soils . . . 111. Effects of Zero-Tillage on Plant Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IV. Crop Husbandry . . . . . , . . . . . , . . . , . . . . . . . . . , . . . . , . . . . . . . . . . . . . . . . . V. Evaluation of Zero-Tillage in Farming Systems . . . . . . . . . . . . . . . . . . . . . . VI. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

78 80 95 103 113 119 120

vi

CONTENTS

THE GENETIC CONTROL OF FLOWERING AND GROWTH IN SORGHUM

J. R. QUINBY

................

I. 11. 111. The Floral Stimulus

IV. V. VI. VII. VIII. IX. X. XI. XII.

..

Implications to Plant Breeding References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

160

ION ABSORPTION BY PLANT ROOTS

T. K. HOLXES I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview of Nutrient Absorption by Roots . . . . . . . . . . . . . . . . . . . . . . . . . Energy-Dependent and Active Ion Transport . . . . . . . . . . . . . . . . . . . . . . . Kinetics and Selectivity of Ion Absorption . . . . . . . . . . . . ......... Energetics of Ion Transport . . . . . . . . . . . . . . . . . . . . . . . . Proposed Model for Ion Absorption by Roots . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . ................................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. 111. IV. V. VI. VII.

163 164 167 180

198 201 202

LODGING IN WHEAT, AND OATS: THE PHENOMENON, ITS CAUSES, AND PREVENTIVE MEASURES

MOSHEJ. PINTHUS I. 11. 111. IV. V. VI. VII. VIII.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Description and Causes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of Lodging on Crop Development and Yield . . . . . . . . . . . . . . . . Plant Characters Associated with Lodging . . . . . . . . . . . . . . . . . . . . . . . . . Environmental and Agronomic Factors Affecting Lodging . . . . . . . . . . . Prevention of Lodging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Breeding for Lodging Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Increased Exploitation of Yield-Promoting Factors Due to the Prevention of Lodging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References ...................................................

210 21 1 217 223 23 1 23 8 246 254 256

vii

CONTENTS

GENESIS AND MANAGEMENT

OF ACID SULFATE SOILS

C. BLOOMFIELDAND J. K. COULTER

I. Introduction . . . . . . . . . . . . . . . . . . . . ..................... 11. The Formation of Sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Oxidation of Sulfides . . . . . . . . . . ........... IV. Mining and Corrosion Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Classification and Mapping . . . . . . .............. VI. Conditions for Plant Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII. Management for Agriculture ......... VIII. Analysis of Pyritic Soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IX. Conclusions . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

266 267 278 290 292 296 3 08 3 15 318 319

MALTING BARLEY I N THE UNITED STATES G. A. PETERSON AND A. E. FOSTER

I. 11. 111.

IV. V. VI. VII. VIII. IX. X. XI. XII.

Classification of Cultivated Barleys of the United States

Quality Testing P Barley Varieties XI11. Malting Barley Pr References . .

cceptable Malting

.............................. .........................

364 375

AUTHORINDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SUBJECTINDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

379 398

This Page Intentionally Left Blank

CONTRIBUTORS TO VOLUME 25 Numbers in parentheses indicate the pages on which the authors' contributions begin.

K. BAEUMER(77), Faculty of Agriculture, University of Goettingen, Goettingen, Federal Republic of Germany W. A. P. BAKERMANS (77), Institute for Biological and Chemical Research of Field Crops and Herbage, Wageningen, The Netherlands C. BLOOMFIELD (265 ) , Rothamsted Experimental Station, Harpenden, Herts, England J. K. COULTER( 2 6 5 ) , Rothamsted Experimental Station, Harpenden, Herts, England A. E. FOSTER(327), Department of Agronomy, North Dakota State University, Fargo, North Dakota R. F. HARRIS( 1 ) , Department of Soil Science, University of Wisconsin, Madison, Wisconsin T. K . HODGES(163), Department of Botany and Plant Pathology, Purdue University, Lafayette, Indiana E. A. HOLLOWELL (47), W.S. Department of Agriculture, Beltsville, Maryland W. E. KNIGHT (47), U.S. Department of Agriculture, Mississippi State, Mississippi 0.A. PETERSON (327), Department of Agronomy, North Dakota State University, Fargo, North Dakota MOSHEJ. PINTHUS(209), The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot, Israel J. R. QUINBY ( 125 ) , Pioneer Hi-Bred Company, Plainview, Texas J. C. RYDEN(1 ), Department of Soil Science, University of Wisconsin, Madison, Wisconsin" J. K. SYERS( I ) , Department of Soil Science, University of Wisconsin, Madison, Wisconsin"

* Present address: Department of Soil Science, Massey University, Palmerston North, New Zealand. ix


PREFACE

Dramatic reductions during the past two years in the world food supply have jolted a complacent world into the realization that the food-population race remains unquestionably the most critical problem facing mankind. Population growth continues at alarming rates in those countries where food supplies are already inadequate. Food shortages are plaguing not only the poor countries where hunger, malnutrition, and starvation are a way of life, but have now reached the more affluent nations. Even the United States which for a generation has sought through public programs to limit crop production is now concentrating on programs to increase food supply. Once again tillers of the soil, and the crops and animals which supply our food have high national priorities. In this time of international concern over food supply, reviews of scientific advancement such as those contained in this volume are most reassuring. Papers contained in this volume are concrete evidence of the contribution of crop and soil scientists to mankind’s efforts to feed himself. Four of the papers deal with crops. One is concerned with research on crimson clover, a legume grown in the southern part of the United States and a plant which is most important to a growing animal industry in this area. Remarkable progress is reported on knowledge gained from the breeding of sorghum, a plant which is rapidly becoming a major crop in the semi-arid regions throughout the world. Factors affecting the lodging of small grains is the subject of one review. Recent advances in research on malting barley, a crop of expanded acreage and of increasing quality expectations is the subject of the fourth crops article. The reviews of advances in soil science are certainly not unrelated to crop production. The mechanisms of ion absorption by plant roots are the subject of one review. Plant root growth is one of the phenomena considered in the critical analysis of the practice of zero-tillage made by scientists who have devoted much of their research efforts to this cultural practice. Phosphorus accumulation in streams and lakes fed by runoff from agricultural lands is the subject of another review. The need to prevent environmental contamination from agricultural chemicals is considered. The genesis and management of acid sulfate soils, which occupy millions of acres of coastal areas in warm and hot humid climates are discussed. These soils are important especially to the rice growing areas of the world. The international focus of this journal is maintained not only by the nature of the subjects covered but by the selection of authors to write the reviews. Food production is truly an international problem to which crop and soil scientists throughout the world are addressing their attention.

N. C . BRADY xi


PHOSPHORUS IN RUNOFF AND STREAMS J. C. Ryden,' J. K. Syers,' and R. F. Harris Department of Soil Science, University of Wisconsin, Madison, Wisconsin

I. Introduction 11. Terminology

..................................................... . ...................... .............................

......................

B. Forms of 111. Factors Affecting

in Runoff and Streams

......................

B. Chemical Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Phosphorus Loads in Runoff and Streams . A. Influence of Point Sources on Phosphorus in Streams . . . B. Runoff from Forest Watersheds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Runoff from Agricultural Watersheds D. Runoff from Land Associated with Ani E. Urban Runoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Impact of Phosphorus Carried in Streams on Standing Waters . . . . . . VI. Prcsent Status and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I.

1 2 2 4 4 4

7 20. 21 22 25 32 33 37 38 41

Introduction

Increasing evidence suggests that phosphorus ( P ) in surface waters is a primary factor controlling the eutrophication of water supplies (Ohle, 1953; Mackenthun, 1965; Stewart and Rohlich, 1967; Vollenweider, 1968; Lee, 1970). Assessment of the relative contribution of the different sources of P to surface waters (Fig. 1 ) is of critical importance for implementation of control measures to prevent or reverse P-induced eutrophication. Although the importance of runoff and streams as major sources of P to standing waters is well recognized, little attempt has been made to differentiate between and quantify the P forms in runoff and streams which are of potential importance with respect to their impact on the biological productivity of standing waters. Furthermore, little emphasis has been placed on the reactions that may occur between dissolved inorganic P and Present address: Department of Soil Science, Massey University, Palmerston

North, New Zealand. 1

2

J. C. RYDEN, J. K. SYERS, AND R. F. HARRIS

the solid phases with which it is in contact in runoff and streams, as pointed out by Taylor ( 1967) and Biggar and Corey (1969). Critical concentration limits have been suggested for P in surface waters which, if exceeded, will lead to excessive biological productivity (Sawyer, 1947; Mackenthun, 1968). In this review, however, rather than emphasizing critical concentrations, P in runoff and streams will be discussed mainly from the standpoint that any P load constitutes a potential increase in the P fertility of surface waters.

II.

Terminology

A. HYDROLOGY AND PHOSPHORUS SOURCES This review will use essentially the definitions proposed by Langbein and Iseri ( 1960). Watershed (drainage basin; catchment area). A part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream, or a body of standing (impounded) surface water, together with all tributary surface streams and bodies of standing surface water. Stream. A general term for a body of flowing water. In hydrology the term is usually applied to the water flowing in a natural channel. Stream flow. The discharge (of water) that occurs in a natural channel. Runoff.That part of precipitation that falls on land and ultimately appears in surface streams and lakes. Runoff may be classified further according to its source. Surface runoff (overland flow). That part of rainwater or snowmelt which flows over the land surface to stream channels. Surface runoff may also enter standing waters directly or be consolidated into artificial channels, e.g., storm sewers in urban areas (urban runoff), before entering a stream or body of standing water. Subsurface runoff (storm seepage). That part of precipitation which infiltrates the surface soil and moves toward streams as ephemeral, shallow, perched groundwater above the main groundwater level. In many agricultural areas subsurface runoff may be intercepted by artificial drainage systems, e.g., tile drains, accelerating its movement to streams. Groundwater run08 (base runoff). That part of precipitation that has passed into the ground, has become ground water, and is subsequently discharged into a stream channel or lake as spring or seepage water. In addition to runoff, the other potential contributors to streams and standing waters are precipitation incident on the water surface and industrial and sewage effluents (Fig. 1 ) .


3

McCarty (1967) and Vollenweider ( 1968) have made a useful division of sources of P to surface waters based on the ease of quantification. Point sources enter at discrete and identifiable locations and are therefore amenable to direct quantification and measurement of their impact on the receiving water. Major point sources include effluents from indus-

FIG. 1. Schematic representation of the relationships between phosphorus sources and runoff, streams, and standing waters.

trial and sewage-treatment plants (Fig. 1) . Diffuse .wurces may be defined as those which at present can be only partially estimated on a quantitative basis and which are probably amenable only to attenuation rather than to elimination. Diffuse sources require the most investigative attention. Vollenweider ( 1968) further divided diffuse sources into: 1. Natural sources such as eolian loading, and eroded material from virgin lands, mountains and forests. 2. Artificial or semiartificial sources which are directly related to human activities, such as fertilizers, eroded soil materials from agricultural and urban areas, and wastes from intensive animal rearing operations. The loads of P imparted to runoff and streams from natural diffuse sources provide a datum line against which the magnitude of P loads from artificial sources may be compared.

4

J. C. RYDEN, J. K.

SYERS, AND

R. F. HARRIS

B. FORMS OF PHOSPHORUS

In natural systems, P occurs as the orthophosphate anion (Pod3-)which may exist in a purely inorganic form (H2P0,- and HP0,2-) or be incorporated into an organic species (organic P ) . Under certain circumstances inorganic orthophosphate may exist as a poly- or condensed phosphate. A secondary distinction is made between particulate and dissolved forms of P, the split conventionally being made at 0.45 pm. Other terminology used is as follows: Total P . All forms of P in a runoff or stream sample (dissolved and particulates in suspension) as measured by an acid-oxidation treatment (e.g., acid ammonium persulfate). Dissolved inorganic P . P in the filtrate after 0.45 pm separation determined by an analytical procedure for inorganic orthophosphate. Organic P . P that may be determined within the dissolved and particulate fractions by the difference between total P and inorganic P. Ill.

Factors Affecting the Dynamics of Phosphorus in Runoff and Streams

Before evaluating the magnitude of various P sources in terms of the loads of P in runoff and streams, and the extent to which previous studies of P loadings enable an adequate definition of P sources, it is important to understand the physical and chemical factors affecting the dynamics of P in runoff and streams. These factors determine not only the movement of P into runoff and streams, but also its distribution between the aqueous and particulate phases. A.

PHYSICALFACTORS

All terrestrially derived diffuse sources of P are associated with the movement of water in contact with a solid phase. The solid phase may be stationary with respect to water flow, or may move in the flow at some speed equal to or less than the flow. Precipitation disposed of as subsurface or groundwater runoff is primarily in contact with a stationary solid phase, namely the soil profile and, in the case of groundwater runoff, possibly the bedrock. Consequently, the amounts and concentrations of P carried in subsurface and groundwater runoff will be influenced by the time of contact with any component in the soil profile capable of interacting with dissolved P in the percolating water and by the concentration of dissolved P that the soil components maintain in the soil solution. Time of contact between the percolating solution and any soil component will in turn depend on the rates of infiltration and percolation into and through the soil.


5

Some of the theories developed to describe water movcment in soils can be applied to evaluate the potential loss of P from various soil types as a result of subsurface runoff. Gardner (1965) developed equations to describe the movement of nitrate in the soil profile due to leaching. The chemical interactions that occur between dissolved inorganic P and soil components (discussed later), when water percolates through the soil, must also be taken into consideration. Inclusion of a term in the equations developed by Gardner (1965) to describe the relationship between P in particulate and aqueous phases is therefore necessary. This could take the form of a linear adsorption isotherm relevant to the concentrations of dissolved inorganic P maintained in the solution of a particular soil. Biggar and Corey (1969) have also reviewed the literature on infiltration and percolation of water in agricultural soils as it pertains to nutrient movement. The movement of solid phase material in contact with natural waters occurs during surface runoff and in streams. The amounts of solid material capable of entering surface runoff will depend on the intensity of rainfall, physical and chemical attachment between various solid components, and the amounts and energy of runoff waters (Guy, 1970). It is the energy of surface runoff or stream water, however, that governs the amounts of a specific size fraction of particulate materials which will remain in suspension during water flow. The primary source of particulate material to surface runoff and streams is eroding soil (Guy and Ferguson, 1970), although in urban areas with little ongoing development, particulates may be dominated by specifically urban detrital material (e.g., street litter and dust) and organics derived from urban vegetation. The total on-site losses of soil due to sheet and rill erosion are not necessarily delivered to streams. The amount of sediment that travels from a point of erosion to another point in the watershed is termed the sediment yield (Johnson and Moldenhauer, 1970). Consequently the Universal Soil Loss Equation used to predict field soil losses on an average annual basis (Wischmeier and Smith, 1965) must be corrected when used to predict sediment loads in streams because deposition of particulates may occur on the land surface as a result of slope variations before surface runoff reaches a stream. It is for this reason that estimates of soil loss in surface runoff from sites within a particular watershed cannot be translated into total P losses through a knowledge of the total P content of the soil, if the P loss is to be related to P enrichment of surface waters. An associated complication arises from the fact that soil P is primarily associated with the solid phase. As soil erosion is a selective process with respect to particle size, selectivity has been observed for P loss in surface

6


runoff. The extent of the selectivity depends on the particle sizes with which most of the soil P is associated. This observation has led to the concept of enrichment ratios (ER) , which for P are calculated as the ratio of the concentration of P in the particulate phase of surface runoff to the concentration of P in the source of the particulate phase. This effect was first considered by Rogers (1941), who observed ER values of 1.3 for total P and 3.3 for “0.002 N H,SO, extractable” P for a silt loam situated on a 20-25% slope. Other values range from 1.5 to 3.1 for total P (Knoblauch et al., 1942; Neal, 1944; Stoltenberg and White, 1953), whereas Massey and Jackson (1952) observed values between 1.9 and 2.2 for “water-soluble plus pH 3 extractable” P for silt loams in Wisconsin. The selective nature of surface runoff with respect to P is due to selective removal of fine soil particulates as a result of the energy limitations of runoff and the fact that a large percentage of total soil P is frequently associated with clay-sized material (Scarseth and Chandler, 1938; Williams and Saunders, 1956; Syers et al., 1969). Greater selectivity of fines and consequently particulate P will occur as the energy of surface runoff decreases. Stoltenberg and White (1953) observed that as precipitation disposed of through surface runoff decreased from 70 mm to 0.25 mm per hour, the clay content of eroded material from a soil with a clay content of 16-18% increased from 25% to 60%. This has obvious implications in relation to the nature of the sediment load carried by a stream and the interactions of P between the solid and aqueous phases, particularly during periods of surface runoff. It should be pointed out, however, that although the P content of the sediment load may increase as surface runoff diminishes, as may be predicted from the work of Stoltenberg and White (1953), the total P load may not change, or may even decrease, owing to lower sediment loads. The particulate material carried in streams may be divided into bed load and wash or suspended load. The bed load, which may also have a contribution from existing stream sediment, is that which moves along or close to the stream bed, whereas the wash load is maintained in the flow by turbulence (Johnson and Moldenhauer, 1970). By inference from the selectivity of surface runoff for fine soil particulates, the wash load will be high during surface runoff events. Furthermore, Johnson and Moldenhauer (1970) suggested that the wash load travels at about the same velocity as the water with which it is in contact. Consequently, P associated with the clay- and silt-sized particulates constituting the wash load will move between any two points in the stream profile at the same speed as the ambient dissolved forms of P. Increased turbulence in streams during high flow, or arising from an increasing gradient, will tend to maintain in suspension particle sizes more


7

characteristic of the bed load, and may even resuspend existing stream bed sediment. In a study of total P loads in the Pigeon River, North Carolina, Keup (1968) noted that an increase in gradient from 2.81 to 4.35 m/km, over which no tributaries entered the main stream, resulted in a 90.8 kg/day increase in the total P load carried. It appears that in the majority of cases a large proportion of particulate P in streams arises from soil erosion. Phosphorus may be stored in stream bed sediments, but unless the stream is actively aggrading, the amount of P stored will be less than the inflow (Keup, 1968). That which is stored is liable to resuspension and transport owing to turbulence during periods of high flow.

B. CHEMICAL FACTORS 1 . Nature of Soil P Soil P may be divided into two broad categories: inorganic P, namely, that associated with soil mineral particles; and organic P, which forms an integral part of the soil organic matter fraction. a. Inorganic P . O n the basis of solubility product criteria, it has been postulated that discrete phase crystalline Fe and A1 phosphates exist in noncalcareous soils (Kittrick and Jackson, 1956; Hemwall, 1957; Chakravart and Talibudeen, 1962). The general occurrence of discrete Fe and A1 phosphates seems doubtful on the basis of the ion product data presented by Bache (1964) and the experimental observations of Hsu (1964). It is now generally accepted that secondary inorganic P in many soils exists primarily in association with oxides and hydrous oxides of Fe and Al, as surface-bound forms or within the matrices of such components. However, that discrete Fe and A1 phosphates are formed as temporary phases in the vicinity of phosphate fertilizer particles due to conditions of localized high acidity and P concentration is well established (Lindsay and Stephenson, 1959; Huffman, 1969). Such compounds will not be stable as the dissolved inorganic P concentration in the soil solution or aqueous portion of other soil-water ecosystems decreases. The calcium phosphate mineral, apatite (Shipp and Matelski, 1960) and calcic fertilizer-soil reaction products (Huffman, 1969) have been identified in soils. The amounts of apatite are appreciable only in weakly weathered soils (Williams et al., 1969), as predicted by the weathering indices of Jackson ( 1969). Calcic fertilizer-sail reaction products may be present in neutral and calcareous surface soil horizons, and their importance in maintaining high concentrations of dissolved inorganic P in soil-water ecosystems should not be overlooked.

8


Consequently three basic forms of inorganic P may exist in unfertilized soils (Syers and Walker, 1969; Williams and Walker, 1969): apatite, which is a discrete phase P compound; P sorbed on the surfaces of Fe, Al, and Ca soil components (nonoccluded); and P present within the matrices of Fe and A1 components (occluded). In fertilized soils, a variety of P fertilizer-soil reaction products may exist as transient phases. As the solubility product of pure apatite in water is low (0.03 pg per milliliter at pH 7, Stumm, 1964) and the P held within the matrices of Fe and A1 components is virtually chemically immobile, except under reducing conditions in the case of Fe, major emphasis should be directed toward the reactions involving P in solution and that sorbed on the surfaces of Fe, Al, and Ca components as well as the release of P due to dissolution of fertilizer-soil reaction products. b. Organic P. Elucidation of the composition of soil organic P is restricted by lack of extractants capable of removing organic P from soils in a relatively unaltered form and by the inadequacy of current methods for mildly degrading extracted organic P-organic matter complexes. Existing data indicate that most of the organic P in soils is associated, in an ill-defined manner, with the humic and fulvic acid complex of soil organic matter (Anderson, 1967). Of the specific forms of organic P that have been identified in soils, inositol phosphates are present in largest relative amounts, comprising up to 60% of the total organic P (Anderson, 1967; Cosgrove, 1967; McKercher, 1969). Other specific organic P compounds are present in soil in much lower quantities: nucleic acids account for 5-lo%, and other phosphate esters, such as phospholipids, sugar phosphates, and phosphoproteins, for less than 1-2% (McKercher, 1969). 2. Sorption of Dissolved P by Soils Whenever water containing a particular concentration of dissolved P comes into contact with soil material, there is a possibility for sorption, desorption, or dissolution reactions to take place. The types of reactions are the same regardless of whether they occur under conditions existing in the soil profile, surface runoff, or streams. Although in some cases biological assimilation may initially affect the distribution of P between dissolved and particulate phases of soil-water systems, the distribution of P between these phases will be determined by the nature of the inorganic particulates and the concentrations of dissolved P in solution (Keup, 1968; McKee et al., 1970; Ryden et al., 1972b). a. Inorganic P. It has been demonstrated that the uptake or sorption of P from solution by soils is significantly related to the presence of shortrange order (amorphous) oxides and hydrous oxides of Fe and A1 (Williams et al., 1958; Gorbunov et al., 1961; Bromfield, 1965; Hsu, 1964; Saunders, 1965; Syers et al., 1971). Furthermore, “pure” oxides and hy-


9

drous oxides of Fe and Al, and short-range order aluminosilicates have also been shown to be particularly effective in the sorption of inorganic P from solution (Gastuche et al., 1963; Muljadi et al., 1966; Hingston et al., 1969). The sorption of inorganic P by Fe and A1 oxides and hydrous oxides is known to be rapid, as is the sorption of P by soils. Furthermore, short-range order Fe and A1 oxides and hydrous oxides are ubiquitous in soils (Hsu, 1964), their relative amounts depending on parent material, climatic and drainage conditions, and occur mainly as coatings on other soil components. Shen and Rich (1962) and Jackson (1963) have noted the occurrence of A1 hydroxypolymers and Dion (1944), and Roth et al. ( 1969) have reported the presence of F e oxide and hydrous oxide coatings on clay mineral surfaces. Such coatings, in conjunction with the greater surface area of the clay fraction compared to that of the other particle-size fractions in a soil, explain the observation of Scarseth and Chandler (1938) that up to 50% of the total P in soils may be associated with the the clay fraction, as well as the enrichment ratio effect for P as a result of soil erosion. Attempts have been made to correlate P sorption with the clay content of soils (Williams et al., 1958). Correlations between P sorption and clay content after removal of Fe and A1 oxides and hydrous oxides often have been poor. Better correlations may be expected if P sorption is related to the content of water-dispersed clay. The sorption of P by water-dispersed clay and silt of soils has obvious implications to reactions occurring between dissolved and particulate P in surface runoff and streams. Sorption of inorganic P by CaC03 has also been demonstrated (Cole et al., 1953). The nature of the surfaces of calcite in calcareous soils may be very different from those of pure calcite (Buehrer and Williams, 1936; Lahav and Bolt, 1963; Syers et al., 1972). The sorption of dissolved inorganic P by soils may be described by sorption isotherms similar to that shown in Fig. 2. Numerous workers have also shown that sorption may be described by some of the adsorption isotherms developed to describe gas adsorption by solids (Russell and Prescott, 1916; Olsen and Watanabe, 1957; Rennie and McKercher, 1959; Syers et al., 1973). Similar observations have been made for the sorption of inorganic P by soil components such as kaolinite and short-range order Fe and A1 oxides and hydrous oxides (Gastuche et al., 1963; Muljadi et al., 1966; Kafkafi et al., 1967). Although these studies have been useful in describing relationships between various soils and soil components with respect to their P sorption capacities, they have provided little information regarding P sorption behavior from solutions containing the low dissolved inorganic P concentrations characteristic of most soil-water ecosystems, largely because of the high levels of added P used (Ryden et al., 1972b). Furthermore, Syers et al. (1973) obtained two linear Langmuir relation-

10


sorbed ( f )

APon sol I

released (3

FIG.2. Typical isotherm for the sorption of added inorganic phosphorus by a soil. E = equilibrium P concentration. (From White and Beckett, 1964.)

ships which intersected at equilibrium P concentrations varying from 1.5 to 3.2 pg P/ml, for three contrasting soils-an observation that probably invalidates interpretations of P sorption made from many previous studies where high levels of added P were used. The study of White and Beckett (1964), conducted at initial dissolved inorganic P concentrations, comparable to those existing in soil-water ecosystems, provides a useful basis for understanding the interactions between aqueous and particulate phases of P in runoff and streams. Figure 2 illustrates the principle of the approach used. White and Beckett (1964) defined the intersection of the P sorption isotherm and the abscissa, the “equilibrium phosphate potential” ( 5 p C a pH,PO,) , abbreviated to “equilibrium P concentration” by Taylor and Kunishi ( 1971) . The intersection is equivalent to the inorganic P concentration in the ambient aqueous phase when there is no net sorption or release of P, i.e., AP = 0. This is a point of reference which provides a predictive estimation of sorption or release of P should the P concentration in solution change. Furthermore, the average slope of the sorption curve over a given final P concentration range provides information on the ability of the soil to maintain the P concentration at the equilibrium P concentration. The steeper the slope, the closer will the final P concentration be to the equilibrium P concentration. The slope of the curve, although not related to total P sorbed, is related to the extent to which that soil may sorb P over the concentration range considered. The potential of this approach in predicting the chemical mobility of P in soil-water systems is clearly evident and has been used with regard to streams by Taylor and Kunishi (1971) and Ryden et al. (1972a,b) for rural and urban soils, respectively. The desorption of sorbed P from soils is not as simple as may be inferred from the sorption-release relationships obtained by White and

+

PHOSPHORUS I N RUNOFF AND STREAMS

11

Beckett (1964). In fact very few studies have been reported regarding the desorption of sorbed P, and those reported by Syers et al. (1970) and Ryden et al. (1972a), involved desorption following sorption of P from solutions containing P concentrations in excess of those commonly found in soil-water ecosystems. In studies involving the sorption of P by kaolinite from solutions containing realistic inorganic P concentrations, Kafkafi et al. (1967) observed that initially all the sorbed P was isotopically exchangeable. During a subsequent washing or desorption step, however, a portion of the sorbed P became nonexchangeable, or “fixed,” this portion being dependent upon the amount of P sorbed, the number of washings, and the nature of the previous P sorption cycle. Sorption of P was represented by either onestep sorption from a range of solutions of different initial P concentration or by successive additions of small amounts of dissolved inorganic P. Both these types of P sorption, as well as an effect analogous to washing, could occur in soil-water ecosystems. 6. Organic P . Although the mechanisms involved in the retention of organic P by soils have not been established fully, there is evidence that inositol hexaphosphate, and possibly other organic P compounds, are retained by a precipitation rather than a sorption reaction. Nevertheless, removal of dissolved organic P from solution appears to be a rapid process. Pinck et al. (1941 ) reported that many commonly occurring water-soluble organic phosphates, e.g., salts of glycerophosphate, hexose diphosphate, and nucleic acids, become nonextractable with water at almost the same rate and as completely as dissolved inorganic P. The retention of water-soluble organic P by sorption reactions may occur by at least two basically different mechanisms (Sommers et al., 1972). Goring and Bartholomew (1950) observed that removal of “free iron oxides” considerably reduced the amount of fructose 1,6-diphosphate sorbed by subsoil material, suggesting that the sorption of organic P may occur through orthophosphate groups by a similar mechanism to that for inorganic P. It is also possible that organic P can be retained by interaction of the organic moiety of the phosphate ester with inorganic soil components. For example, nucleic acids and nucleotides are protonated at pH 5 (Jordan, 1955) and could consequently be retained on clay surfaces by displacement of exchangeable cations. Furthermore, physical adsorption, also through the organic portion of the molecule, is possible, particularly if the molecular weight of the compound is high, as suggested by Greenland (1965). In such cases retention is weak and is accomplished by van der Waals and ion-dipole forces. Greaves and Wilson (1969) have implicated physical adsorption in the retention of nucleic acids by montmorillonite. It is also possible that retention occurs indirectly through other

12

J. C . RYDEN, J. K. SYERS, AND R. F. HARRIS

soil organic compounds such as fulvic and humic acids after interaction of organic phosphates with these species (Martin, 1964). The desorption of sorbed organic P has not been extensively studied. The hypothesis that inorganic P added to soils displaces sorbed organic P to solution (Latterell et al., 1971) was not supported by the data presented by Wier and Black (1968). Although organic P may be leached from soils, it appears that a large proportion of that removed may not be in a dissolved form. After incubating sucrose with ammonium nitrate in the upper portion of a calcareous soil, Hanapel et al. (1964) found that most of the organic P removed by leaching was present in a particulate rather than a dissolved form. 3. Chemical Aspects of P in Subsurface and Groundwater Runoig Losses of P in subsurface and groundwater runoff have been considered minimal in the past, but, as will be discussed later, such losses can amount to a significant proportion of losses from agricultural land, and possibly a major proportion from forest lands. The supposition that P losses in subsurface and groundwater runoff are low probably stems from the concept of P immobility based on the P sorption properties of soils using added inorganic P concentrations far in excess of those normally present in the soil solution. It is of interest to note that many of the reported mean concentrations of dissolved inorganic P in subsurface runoff are within the range of values expected to be maintained in the soil solution. Pierre and Parker (1927) reported values ranging from 0.020 to 0.350 pg P/ml, with an average of 0.090 pg/ml, for several surface soils from the southern and midwestern states of the United States. These workers also noted that dissolved inorganic P concentrations could be maintained at a fairly constant level. Barber et al. (1963) reported similar values for the upper 15 cm of 87 soils from the midwestern United States, with an average of 0.180 pg of P per milliliter; the frequency distribution of the values obtained, however, suggested a mode of between 0.040 and 0.060 pg of P per milliliter. As water percolates through the soil profile, there tends to be a “chemical sieving” of dissolved inorganic P (Black, 1970). This arises as a result of the sorption of inorganic P by soil components. The low concentrations of P found in groundwater runoff, which has experienced the maximum effects of deep percolation with concomitant increase of contact with P-deficient particulates of the subsoil, are undoubtedly a direct result of the chemical sieving effect. The principle of this effect is illustrated by other data presented by Barber et al. (1963). For the same 87 soils mentioned previously, the average dissolved inorganic P concentration at a depth of 46-61 cm was 0.089 pg/ml, less than half that for the upper


13

0-15 cm. Another illustration is observed in results presented by Ryden et al. (1972a) for the P sorption properties of successive soil horizons of a Miami silt loam profile. The concentrations of dissolved inorganic P maintained in solution after shaking with solutions of different initial added inorganic P concentrations at a solution: soil ratio of 40: 1 are given in Table I. TABLE I Dissolved Inorganic Phosphorus (P) Concentrations Maintained by Soil IIorizoiis of Miami Silt Loam after Equilibration with Solutions of Different Initial Added Inorganic P Concentrationsn

Horizon

Depth (cm)

Initial P conc. (ccg/ml)

Final P coiic. (/*g/mU

A1

0-15 15-38 56-66

0.0 0.471 0.030

0.471 0.030 0.007

B1 3C1

~

Data extrapolated from Ryden et

(11.

(197‘2a).

The concentration of dissolved inorganic P in subsurface and groundwater runoff will depend on the nature and amounts of P-retaining components in the profile, the surface area exposed to percolating waters, and the ease of percolation which affects the contact time of dissolved inorganic P with the retaining components. In studies of P leaching through columns of organic soils in the laboratory, Larsen et al. (1958) observed that P retention, measured by srP autoradiographs, was closely correlated with the total hydrous Fe and A1 oxide (“sesquioxide”) content. Similarly, losses of P due to leaching through a deep siliceous sandy soil were demonstrated in W. Australia by Ozanne (1963). When 225 kg/ha of 32P-labeled superphosphate was broadcast during winter on a fallow sandy soil, over 50% of the P had penetrated to more than 1 m below the surface within 38 days, during which 230 mm rain had fallen. Ozanne (1963) also demonstrated that the potentially large losses of P to subsurface and groundwater runoff from sandy soils compared to that from loamy soils were due to quantitative rather than qua1itativ.e differences in P-retaining components. Although major emphasis has been placed on P losses in surface runoff, it appears that losses of P to subsurface and groundwater runoff, although of little significance from an agricultural standpoint, may under certain conditions constitute a significant loss of P from agricultural watersheds in terms of the P enrichment of surface waters, as will be discussed

14

J. C. RYDEN, J . K . SYERS, AND R. F. HARRIS

later. Losses of P to subsurface and groundwater runoff are even more difficult to evaluate than those in surface runoff and demand further investigative attention.

4 . Chemical Aspects of P in Streams As discussed previously, surface runoff from agricultural land constitutes a heterogeneous and relatively short-lived system. Any attempt to consider the distribution and chemical mobility of P between solid and aqueous phases before entry into the receiving stream would be pointless as a new and more homogeneous system is rapidly established. Surface runoff in urban areas is somewhat different because in most cases it is channelized shortly after origin by alteration of surface drainage patterns; under such circumstances it is analogous to a stream in an artificial channel. Consequently, the chemical mobility of P will be discussed from the standpoint of the stream environment. The potential of suspended particulates derived from eroding soil to modify the dissolved inorganic P concentration of streams has been suggested by Taylor ( 1967) and Biggar and Corey (1969). Wang and Brabec (1969) also implied that inorganic P was sorbed by suspended particulate material from observations of dissolved inorganic P concentrations in the Illinois River at Peoria Lake. An evaluation of the possible effects of eroded soil materials on the dissolved inorganic P concentrations of streams may be obtained from P sorption studies (Taylor and Kunishi, 1971; Ryden et al., 1972a,b). It is essential, however, that conditions realistic of those existing in streams are used if meaningful results are to be obtained (Ryden et al., 1972a). Widely differing interpretations can be made as solution: soil ratios and initial dissolved inorganic P concentrations are changed from those conventionally used in P sorption studies to those realistic in terms of the stream environment (Fig. 3 a-c). The data in Fig. 3a suggest that inorganic P released from the A1 horizon, which contained a P fertilizer-soil reaction product, would be largely resorbed by the noncalcareous B1 horizon and to some extent by the calcareous 3C1 horizon, should the horizons erode together. Sorption studies employing low initial added inorganic P concentrations and a wide (400: 1 ) so1ution:soil ratio (Fig. 3c) indicate that the B1 horizon has a much lower ability to remove dissolved inorganic P from solution than expected, this being equal to or only slightly greater than that of the 3C1 horizon. In fact for mixtures of varying ratios of A1 and B l , and A1 and 3C1 horizons, it was found (Ryden et al., 1972b) that the latter mixtures were able to maintain lower dissolved inorganic P concentrations than the former. The conditions used by Ryden et al. (1972a,b) to predict the potential of eroding soils to modify the dissolved


15

+loo

+I00

’:; 0 1000

1000

3000

-SO

5-

+ Ir

+4

._ -

s

o

c O

n

Q

-1

.30

10

10

0

-

10

Final dissolved inorganic P Concentration Wgll)

FIG. 3. Sorption of added inorganic phosphorus by horizons of a Miami silt loam profile from solutions of varying initial dissolved inorganic P concentrations and at varying so1ution:soil ratios. ( a ) High added P (0-6 pg/ml) and narrow so1ution:soil ratio (50: 1 ) . ( b ) Low added P (0-0.2 pg/ml) and narrow solution:soil ratio ( 4 0 : l ) . (c) Low added P (0-0.2 pg/ml) and wide so1ution:soil ratio ( 4 0 0 : l ) . [From Ryden et al. (1972a), reproduced with permission of the American Society of Agronomy.]

inorganic P concentrations of streams, gave results comparable to those obtained in simulated stream systems using a solution: soil ratio of 1000:1 This is equivalent to a sediment concentration of 1000 mg/liter, which lies well within the range of values cited by Guy and Ferguson (1970) and Johnson and Moldenhauer ( 1970). The P sorption studies reported by Taylor and Kunishi (197 1) and Ryden et al. (1972a,b) involved closed systems, i.e., soil in contact with the same aqueous phase. This may be justified on the grounds that the wash load of a stream travels at the same velocity as the water in which it is suspended (Johnson and Moldenhauer, 1970), as discussed previously.

16


Sorption studies may be used to provide reasonable estimates of dissolved inorganic P concentrations in streams, under various flow conditions, draining rural watersheds. Taylor and Kunishi (1971 ) observed that dissolved inorganic P concentrations during base flow of a stream draining a small agricultural watershed in Pennsylvania, were in the range of 0.040 to 0.060 pg P/ml, values which were close to those predicted from P sorption studies using stream bank sediment and subsoil material. During periods of surface runoff, predicted dissolved inorganic P concentrations would be in excess of 0.200 pg of P per milliliter for the surface soil used by Taylor and Kunishi (1971) and 0.100 pg of P per milliliter for that used by Ryden et al. (1972a) due to release of P from eroded surface soil; however, predictions from the work of Taylor and Kunishi (1971) are based on the use of a narrow (10: 1 ) so1ution:soil ratio. The ability of eroding stream bank material or resuspended stream bed sediment to resorb inorganic P released to solution should not be ignored (Taylor and Kunishi, 1971 ) . In a more recent study of the same watershed in Pennsylvania, Kunishi et al. (1972) observed that during a heavy summer rainstorm only 31% of the total “available” P (total dissolved plus resin-extractable P on the suspended sediment) was in the resin-extractable form in a stream draining an agricultural subwatershed. At the outflow of the main watershed, however, over 50% of the total “available” P was in the resin-extractable form. Kunishi et al. (1972) suggested that for this watershed, as suspended material moves downstream and mixes with material from other parts of the watershed as well as that eroded from the stream banks, dissolved P is actively sorbed. During a second less intense storm, however, when stream bank erosion was less severe, the proportion of total “available” P associated with the sediment was virtually the same at both monitoring stations. A similar hypothesis might also explain the observation of White ( 1 972) at Taita, New Zealand, that the concentration of dissolved inorganic P at the outflow of small watershcds during base flow was lower than that recorded for groundwater seepage giving rise to the stream flow. It is important to distinguish between the quantities of various types of soil materials expected to enter streams in urban as opposed to agricultural surface runoff. In agricultural areas, surface runoff will carry primarily surface soil material to receiving streams. Surface soils may contain P fertilizer-soil reaction products capable of producing significant increases in dissolved inorganic P concentrations, due to their dissolution (Ryden et al., 1972a). In urban areas, however, land under development, which is prone to severe erosion, is frequently graded, exposing some or all horizons of the area profile to potential erosion. Dissolved inorganic P concentrations of receiving streams in urban areas may be sufficiently high that


17

the addition of eroded soil material may cause a reduction in the dissolved inorganic P concentration. An approach similar to that used by Taylor and Kunishi (1971) and Ryden et al. (1972a,b) could be used to identify other diffuse sources of potential P enrichment within a watershed. The approach would be particularly useful for estimating the potential of various forms of urban detrital material to influence the dissolved inorganic P concentrations of surf ace runoff. One diffuse source of considerable importance is the leachate from leaves, particularly during the autumn. An appreciable percentage of the total P in leaf tissue may be in a water-soluble form. Ash leaves may contain 62% of total P as water-soluble inorganic P (Nykvist, 1959). Cowen and Lee (1972) observed that 44 and 120 pg soluble inorganic P per gram air-dry weight of fallen oak and poplar leaves, respectively, could be leached by 1 liter of distilled water percolating at a rate of 8.4 ml per minute. Greater amounts of P were released from oak leaves during consecutive leaching cycles and after fragmentation of whole leaves. Similar experiments were conducted by Timmons et al. (1970) using agricultural crop residues. These were leached in a fresh condition and after drying, and freezing and thawing cycles. The data suggest that the leaching of crop residues is most likely to contribute to the dissolved inorganic P concentration of streams during spring thaw in certain areas when, after numerous freezing and thawing cycles, the residues will be carried over frozen ground in surface runoff. When greater infiltration can occur, a portion of the leached P may be retained in the soil due to sorption.

5 . P Chemistry of Stream-Bed Sediment Little is known of the chemistry of stream-bed sediment although it is conceivable that it is similar to that of the subsoil of the surrounding area (Taylor and Kunishi, 1971 ) . Consequently, P sorption studies using subsoil material may provide some information on the role of stream-bed sediment in regulating the dissolved inorganic P concentration due to its suspension during turbulence. This would be particularly true in watersheds with little contribution to stream-bed sediment as a result of surface runoff. In watersheds where surface runoff is a regular occurrence, however, stream-bed sediment is expected to have a significant contribution from surface horizon soil material, and the latter could contribute to base flow concentrations of inorganic P. Care should be taken, however, in the extension of the P sorption properties of field soils to stream-bed sediment. Hsu (1964) observed that the amount of inorganic P sorbed by soil after storage for 1 year in a continuously wet condition, increased from 69 to 99 pg of P per gram of soil.

18


The increased sorption was attributed to release of Fe to solution from crystalline phases due to the development of localized reducing conditions during storage, and reprecipitation of “ferric hydroxide” on contact with more aerobic conditions. The redox status of stream-bed sediments does not appear to have been studied, but it is reasonable to suggest that reduction occurs at depth in the sediment with the possibility of crystalline ferric components being transformed to short-range order ferrous forms. The importance of short-range order oxides and hydrous oxides of Fe in the sorption of inorganic P has already been discussed. The possible transformation of Fe from crystalline to short-range order forms represents the first stage of the more aggressive transformations which occur in lake sediments under anaerobic conditions (Shukla et al., 1971). The observation of Kafkafi et al. (1967) that the washing of kaolinite, on which P had been sorbed, produces a “pool” of nonexchangeable P is also of direct relevance to the P chemistry of stream-bed sediments, assuming a similar effect occurs. Stream-bed sediment with associated sorbed P could undergo a series of steps equivalent to sequential washing due to resuspension and settling as a result of minor turbulence. The observations of Kafkafi et al. (1967) suggest that sorbed P could become progressively less exchangeable and may constitute an essentially permanent removal of dissolved inorganic P from streams. When stream-bed sediment contains eroded fertilized soil materials, however, a different situation may prevail. Ryden et al. (1972a) showed that release of P from a surface soil horizon by repeated washing with P-free 0.1 M NaCl initially followed first-order kinetics, suggesting that release was due to the dissolution of solid phase P, probably a fertilizer-soil reaction product. 6 . Forms of P in Runoffand Streams

In many studies concerned with various aspects of P in runoff and streams there has been a tendency to measure total P. The measurement of total P discharged by streams does not provide any indication of the amounts of P available for aquatic plant growth. Consequently, the forms of P measured in streams that enter a lake or reservoir are of direct importance in assessing the impact of runoff- and stream-derived P on a body of standing water. Dissolved inorganic P is one of the obvious choices because this form of P is directly available for biological utilization. Objections to the measurement of dissolved inorganic P, as it is conventionally determined, have been raised by Frink (1971 ) on the basis that distinction between dissolved and particulate forms is based on filtration through a 0.45 pm filter. Although it is possible that filtration does not strictly differentiate between dissolved and particulate P, it provides a more


19

realistic measure in terms of the effects of runoff- and stream-derived P on the biological productivity of standing waters than the measurement of total P. Vollenweider ( 1968) has also pointed out the necessity to distinguish between total P and dissolved forms of P because it is possible that P exports from some watersheds occur mainly in biologically unavailable forms, such as apatite. This work showed that P exports from the Alpine portion of the Rhine Basin amount to 1.45 kg/ha per year. As this is mainly in the form of apatite, however, the contribution of biologically available P to Lake Constance is relatively small. In other regions it appears that a high proportion of particulate inorganic P in streams draining urban and rural watersheds may in fact be apatite. Eroding urban soils in the Lake Mendota watershed, Wisconsin, contain between 6 and 80% of the total inorganic P as apatite, with amounts exceeding 50% in the lower B and C horizons (J. K. Syers, J. C. Ryden, and J. G. Thresher, unpublished data). For the same soil materials, Sagher and Harris (1972) observed that algal cultures suffered P starvation when the sole P source in the growth medium was C horizon material, indicating the very low biological availability of the P present in apatite. Chemical fractionation schemes have been used to determine the forms of inorganic P in soils. These schemes evolved from the observations of Chang and Jackson (1957) that certain chemical reagents were able to solubilize inorganic P contained in various synthetic phosphates and phosphate minerals. Recent workers (Bromfield, 1967; Williams et al., 1967, 1971a,b; Syers et al., 1972) have questioned the validity of the separation of inorganic P into Al-, Fe-, and Ca-bound forms, as proposed by Chang and Jackson ( 1957). Providing that the problems inherent in inorganic P fractionation schemes are recognized, useful interpretations may be drawn from the data obtained. The form of particulate inorganic P which is expected to have the greatest potential impact on the biological productivity of standing waters is that which is nonoccluded. Part of the nonoccluded and even some of the occluded inorganic P associated with ferric components is released into solution when anaerobic conditions develop subsequent to sedimentation. Appropriate inorganic P fractionation schemes applied to suspended stream sediments may provide a more meaningful measure of the forms and amounts of particulate inorganic P carried in streams. As pointed out by Taylor et al. (1971), suspended sediment concentrations are frequently not high enough to provide adequate amounts of xaterial in a manageable volume of water. Evaluation of the forms of P in soil materials which are known to be transported to streams in surface runoff may overcome this problem to some extent. In the case of eroding soil materials, the inorganic P fractionation schemes should not

20

J. C. RYDEN, J. K. SYERS, A N D R. F. HARRIS

be applied to the whole soil, due to the ER effect resulting from erosion. Water-dispersed particle-size separates should be used. In spite of the possible errors involved in a dissolved-particulate P split based on filter pore size, it seems that in the majority of cases the most meaningful and useful measurements of P in runoff are dissolved forms, particularly dissolved inorganic P. Frequently dissolved forms of P account for a major percentage of total P (Sylvester, 1961; Sullivan and Hullinger, 1969), whereas dissolved inorganic P in many cases constitutes a major proportion of the total dissolved P. It should be noted that dramatic changes can occur in the concentration of dissolved inorganic P and other P fractions after sample collection, even after only a short period of time . some cases when samples are not analyzed im(Ryden et al., 1 9 7 2 ~ )In mediately after collection, the only valid P parameter that can be measured is total P.

IV.

Phosphorus loads in Runoff a n d Streams

The P content of precipitation reflects the amount of P subject to washout from the atmosphere at the time of the precipitation event. The amounts of P carried in precipitation rarely exceed 1 kg/ha per year as total P (Miller, 1961; Weibel et al., 1966; Allen et al., 1968; Gore, 1968). Weibel et al. (1966) reported that the average concentration of total acidhydrolyzable P in precipitation falling on Cincinnati, Ohio, was 0.080 pg/ml, whereas Taylor et al. ( 197 1 ) reported an average concentration of 0.020 pg/ml for total dissolved P in precipitation collected at rural Coshocton, Ohio. Data for the P content of precipitation should be viewed with some skepticism unless adequate precautions have been taken to guard against contamination of the collection vessel (Gore, 1968; White, 1972). White (1972) found that although rainwater collected over an extended period indicated a mean dissolved inorganic P concentration of 0.020 pg/ml, a mean concentration of 0.003 pg/ml, based on specific showers, might be a more accurate estimate. It is difficult to evaluate the effect of P carried in precipitation on P loads in runoff and streams. Phosphorus contained in precipitation which becomes a part of any soil-water ecosystem may undergo considerable change in form, depending primarily on the chemical factors discussed previously, and will become an integral part of the P forms in runoff and streams. Surface runoff water is the carrier of not only the P initially present in precipitation but also any P which enters surface runoff water because


21

of chemical interactions or the energy of the water itself. Several factors affect the amount and energy of surface runoff water at any particular location and, therefore, the amount of additional P entering and carried by it. These include nature of land use, extent of vegetative cover, slope, intensity of rainfall, and permeability of the land surface. The quantity of precipitation entering subsurface and groundwater runoff is inversely related to that disposed of in surface runoff and evapotranspiration. It is consequently affected by the factors listed above for surface runoff. The major portion of P in subsurface and groundwater runoff is expected to be in dissolved forms. If subsurface runoff is accelerated by artificial drainage systems, however, soil colloids, with associated P, may appear in the water as it enters streams. The P load carried by a stream under given flow conditions will represent the relative contribution of P loads in each of the runoff components, as well as the influence of any point source of P. A.

INFLUENCE OF POINT SOURCESON PHOSPHORUS IN STREAMS

Estimates of the contribution of P to surface waters from domestic wastes in the United States range from 91 x loo to 227 x l o G kg per year with total P concentrations ranging from 3.5 to 9.0 pg/ml (McCarty, 1967; Ferguson, 1968). Weibel et al. ( 1964) estimated that P discharged as raw sewage from combined storm sewers in Cincinnati, Ohio, amounted to 3.4 kg/ha per year as total dissolved P. In the area of Madison, Wisconsin, the per capita contribution of P to surface waters from treated domestic waste was estimated to be 0.544 kg/capita per year (Sawyer, 1947), whereas an estimate of 1 kg/capita per year was given by Metzler et al. (1958) for Chanute, Kansas. The difference between the estimates of Sawyer (1947) and Metzler et al. (1958) may reflect the increased use of P in domestic products, particularly detergents. The impact of sewage outfall on the dissolved inorganic P concentration of streams and rivers was studied by Brink and Gustafsson (1970). Their results are summarized in Table 11. Obviously the impact of the outfall is dependent on factors which include flow rate of the receiving stream and the P content of the effluent. Under certain agricultural management conditions animal excrement may constitute a point source of P to streams. Excrement may enter streams during surface runoff from feedlot operations or by the cleaning of milking sheds into open drains. The magnitude of these sources of P will be discussed later. McCarty (1967) was unable to estimate the magnitude of contributions of P made to streams from industrial wastes. The amounts of P discharged

22

J. C. RYDEN, J. K. SYERS, AND R. F . HARRIS

TABLE I1 Effect of Sewage Outfall on tllc Dissolved Inorganic Phosphorus Concentration of the Receiving Water" IXssolved inorganic P concentration

(pg

P/ml)

Receiving water

Before outfall

After outfall

River Stream Stream Ditch

0.09 0.05 0.11 0.01

0 4% 0.18 4.30 0.75

Data from Brink and Gustafsson (1970).

to streams will depend on the industrial process concerned and local legislation covering the discharge of industrial effluent. Mackenthun et al. (1968), for example, estimated that a potato canning factory and a woollen mill contributed 3446 and 835 kg of P per year, respectively, to the East Branch of the Sebasticook River, Maine. Domestic and many industrial wastes not only supply large amounts of total P to streams but also have a pronounced effect on the concentrations of dissolved forms of P in the receiving stream. Because domestic and industrial wastes are point sources, they are easily recognized within a watershed and are amenable to direct manipulation.

B. RUNOFFFROM FORESTWATERSHEDS A compilation of data from several studies of the quantities of P lost in streanis from stable forest and woodland watersheds is presented in Table 111. Exports of P in streams from long-established and stable forest watersheds provide a useful datum line against which losses of P from other land-use areas may be compared. The data in Table I11 show a considerable degree of uniformity. Total P losses range from 0.68 to 0.02 kg/ha per year with three out of the four values being less than or equal to 0.1 kg/ha per year. Only a few measurements have been made of the dissolved inorganic P concentration of stream water in forested watersheds. The values reported by Brink and Gustafson (1970) in Sweden show a mean of 0.015 pg/ml, with this fraction amounting to 33% of the total annual loss of P. The data suggest that total P and dissolved inorganic P concentrations rarely exceed 0.115 and 0.025 pg/ml, respectively. Two interesting points arise from the data in Table 111. From the study of a stream draining a

z

TABLE I11 Losses of P in Streams Draining Forest Watersheds

P concentration in streamwater (pg P/ml) Study

Location

Form measured

P loss (kg/ha/yr)

Range

Mean

El z 8 2 2 0

Bormann et al. (1968) Brink and Gustafsson (1970)

New Hampshire Sweden

Cooper (1969) Jaworslii and Hetting (1970) Sylvester (1961)

N. Minnesota Potomac River Basin Washington

Taylor et al. (1971)

Coshocton, Ohio

Total P Total P Dissolved inorganic P Not specified Total P Total P Dissolved inorganic P Total soluble P

0.02 0.06 0.02 0.1s 0.1 0.68 0.07 0.05

-

-

0.008-0.053 0.002-0.026 0.043-0.060

0.048 0.015 0.041

-

-

0.024-0.115 0.004-0.009

0.069 0.007 0.015

0.011-0.OPO

c.4 q

+

3 v)

+I

b

24

J . C. RYDEN, J . K. SYERS, A N D R. F. HARRIS

woodland area at Coshocton, Ohio, to which no fertilizer P had been applied for over 30 years (Taylor et al., 1971), it would appear that the woodland is conservative of P. The average total soluble P content of rainfall was 0.020 pg/ml, whereas that in the stream draining the watershed was 0.015 pg/ml. The extent of addition of total dissolved P to the woodland can be calculated from precipitation data given by Taylor et al. (1971 ) ; a value of 0.17 kg/ha per year is obtained. This value is more than three times greater than the annual P loss in the stream. The conservative nature of forests for P is further borne out by the fact that the annual contributions of P to the land surface in precipitation, quoted previously, are in most cases considerably greater than annual exports of P in streams from forest watersheds. In many cases there is an order of magnitude difference. This hypothesis assumes that data covering the P content of precipitation are correct. The second point of interest relates to the “background” P concentration in forest streams. The data suggest only minor seasonal fluctuations in P concentrations, particularly that of dissolved inorganic P. As a major portion of streamflow is considered to have a groundwater origin (Biggar and Corey, 1969; Johnson and Moldenhauer, 1970), it is conceivable that the dissolved inorganic P load in streams of forested areas is primarily due to that in groundwater runoff. If the reported mean P concentrations of forest streams are compared to those for groundwaters, a marked similarity is observed. Juday and Birge ( 193 1) found that the total dissolved P concentrations of 19 wells in northern Wisconsin, an extensively forested area, ranged from 0.002 to 0.197 pg/ml, with an average of 0.018 pg/ml when the highest value is omitted. This mean value is, if anything, slightly higher than the mean concentrations for dissolved fractions of P reported in Table 111. The higher mean concentrations of total P probably arise from suspended inorganic and organic solids that enter streamflow due to turbulence, especially during high flow. The minor fluctuations in P concentrations reported for forest streams suggest that P export is minimally affected by P input from surface runoff. Amounts of surface runoff in forest watersheds will be low owing to the protection afforded by canopy cover and/or forest floor vegetation. The “background” P export in forest streams is a direct reflection of the chemical and physical factors that affect P concentrations in groundwater and subsurface runoff. Because larger amounts of stream flow from forest watersheds will arise from groundwater and subsurface runoff, the “chemical sieving” action of the soil plays a major role in maintaining the consistently low dissolved inorganic P concentrations in forest streams and may also account in part for the apparent conservative nature of forest watersheds for P.


25

C. RUNOFFFROM AGRICULTURAL WATERSHEDS The loss of P in streams draining agricultural (in most cases arable) watersheds is far less well defined than that for forest streams. This is probably due to the fact that in studies designed to estimate this loss, little differentiation has been made with respect to the forms of runoff. Consequently, there are major problems in estimating P loss from agricultural watersheds using many of the data presented in the literature. Losses of P from agricultural land have not only been based on analyses of streams draining a specific watershed (Campbell and Webber, 1969; Taylor et al., 1971 ), but have also been estimated from data obtained in surface runoff studies (Timmons et al., 1968). Many previous reviews of this subject have relied on such data (Taylor, 1967). Losses of P in streams draining various agricultural watersheds are summarized in Table IV. The lowest loss of total P is from rangeland in Ontario, Canada (Campbell and Webber, 1969) which had received no P fertilizer in living memory. This loss is very similar to losses of total P from forest watersheds, suggesting a minimal contribution if P from surface runoff. Similarly, the total P carried in the base flow, primarily attributable to groundwater runoff, of several streams draining arable agricultural watersheds in S.W. Wisconsin (Minshall et al., 1969) is also little different from total P loads in streams draining forest watersheds. Minshall et al. (1969) reported the total P loss in base flow to be less than 0.12 kg/ha per year. If stream flow during periods of surface and subsurface runoff is included, however, the estimated annual loss of total P increases by one order of magnitude, as indicated by the data of Witzel et al. (1 969) for the same area of S.W. Wisconsin (Table I V ) . These studies suggest that the groundwater runoff or base-flow component of streams draining agricultural watersheds is little different from the total P load of forest streams. It is therefore necessary to estimate the extent to which the P load of streams draining agricultural watersheds may be augmented by P loads of surface and subsurface runoff. The major factors affecting the loads of P in surface runoff from agricultural land include time, amount, and intensity of rainfall, rates of infiltration and percolation, slope, soil texture, nature and distribution of native soil P, P fertilization history, cropping practice, crop type, and crop cover density. A selection of reported losses of P in surface runoff from arable land of various slopes and cropping practices is summarized in Table V. Losses range from the extremely high values of 67 kg/ha per year to almost zero. Losses of P in all studies listed in Table V have been based on the collection of surface runoff (water and particulates) from small experimental

TABLE I V Losses of P in Streams Draining Agricultural Watersheds

Study Campbell and Webber (1969) Fippin (1945) Taylor d al. (1971)

Witzel d al.

Location

S. Ontario, Canada Tennessee Valley Coshoeton, Ohio

S.W. Wisconsin

(1969)

a

January through September 1967.

Soil texture

Form of P measured

Slope (%)

I-r

P Crop

P applied (kg/ha/yr)

P lost (kg/ha/yr)

-

Total P

-

90% Rangeland

0

0.08

-

Total P

-

Row crops, open farmland 50% Permanent pasture; 50% winter wheatmeadow 100% Pasture cultivation-haypasture

-

6.26

3.5

0.07

Silt loam

Total dis-. solved P

Silt loam

Total P

12-18

6-8

1.88

4.08

1.000

9.64 4.27

1.51 1.20

I-r ?c


27

plots frequently no larger than 30 x 6 m, with subsequent analysis for one or more forms of P. Although this approach was originally developed to investigate soil fertility losses due to soil erosion, it is still used to estimate P loads in surface runoff as it relates to the fertility of surface waters (Timmons et al.,1968; Nelson and Romkens, 1969). It is difficult to make any generalizations regarding the P loads carried in surface runoff or to draw conclusions from them in terms of how agricultural practices and natural variables affect P loads in streams draining agricultural watersheds. This is due to the differences in forms of P measured and the lack of comparative studies with respect to slope, soil texture, cropping, and climatic variables. One of the few studies from which meaningful interpretations of P loss in surface runoff can be made in relation to degree of slope and cropping practice is that by Massey et al. (1953) in Wisconsin (Table V ) . As expected, greater “available” (water-soluble plus pH 3 extractable) P losses to surface runoff occurred on the steeper slopes when cropping practice was kept constant. The introduction of two years hay into the rotation reduced the P loss by a factor of approximately four. The value of “improved” or “conservative” agricultural practices in reducing the magnitude of P losses is illustrated in the studies at Coshocton, Ohio (Weidner et al., 1969) and at Lafayette, Indiana (Stoltenberg and White, 1953). It should be noted, however, that although the “improved” practice reduced the total amounts of acid-hydrolyzable P lost in surface runoff at Coshocton, the concentration of this fraction during surface runoff increased from 0.43 to 0.59 pg/ml. Attempts have been made to measure the relative contributions of the aqueous and particulate fractions of surface runoff to the total loss of a measured form of P. In a plot study using simulated rainfall, Nelson and Romkens ( 1969) obtained dissolved inorganic P concentrations of 0.05, 0.30, and 0.50 pg of P per milliliter in the aqueous phase of surface runoff from fallow plots 12 days after 0, 56, and 1 1 2 kg of P per hectare, respectively, had been disked into the soil, with only slight decreases in concentrations up to 75 days after fertilizer application. Although very high artificial rainfall rates were employed (up to 73.5 mm/hr), indications are that high concentrations of dissolved inorganic P may be maintained in surface runoff water. Timmons et al. (1968) determined the distribution of total P loss in surface runoff between the aqueous and particulate phases from plots under natural precipitation. Although these workers did not report P concentrations, losses of total P in the aqueous phase of surface runoff arising from snowmelt far o.utweighed those in the particulate phase. In contrast, total P loss in the aqueous phase varied in most cases between 5 and 40% of the loss in particulates in surface runoff arising from rainfall.

TABLE V Losses of Phosphorus in Surface Runoff from Field Plots Study

Location

Knoblauch et al.

New Jersey

(1942)

Massey et al.

Wisconsin

(1953)

Soil texture

Form of P measured

(%I

Sandy loam

Total P

Silt loam

Soluble pH 3 extractable P (available)

3.5

3 3

11

20

Lafayette, Indiana

Silt loam

0.5 M N H 4 F 0.1 N HCI

+

00 0.5

extractable P (“available”)

Thomas et al. (1968)

Tifton, Georgia

Sandy loam

Crop

+

0.05 N HCI 0.025 N H ~ S O I

extractable P

3

P lost (kg/ha/yr)

+

Vegetables (i) No manure (ii) Manure (iii) Cover crop (iv) Cover crop manure Corn-oats Corn-oats2 yr hay Corn-oats Corn-oats-? yr hay Corn-oats-4 yr hay Oats-5 yr hay Coma Cornh Soybeansa Soybeansb Wheat” Wheat* Meadow“ Meadow* corn Rye-peanuts-rye Rye-corn-oats Oats-rye

+

+

11

Stoltenberg and White (1953)

P applied (kg/ha/yr)

Slopc

40 06 67 07 59 66 49 65 0 91 0 24 2 91

0 73 0 75 0 13 2 86 0 86

3 82 1 93 0 0 0 0 0 0 0 0

84 48 99 74 02 07 05 02

Timmons et al. (1968)

W. Central Minnesota

Loam

Weinder et al. (1969)

Coshocton, Ohio

Silt loam

a

Total P

Total acidhydrolyzable P

6

-

Fallow Corn-continuous Corn-rotation Oats-rotation Hay-rotation Cornc Cornd Wheat" Wheatd

29.1 99.1 30.2

4.82 17.34 4.83 17.34

0.2-0.6 0.1-0. 2 0.1 0.0-0.1 0.1-0.3 10.24 3.11 1.33 0.41

Prevailing practice: moderate fertilizer levels; liming t o p H 6.0; straight row planting and cultivation.

* Conservation practice: higher fertilizer levels; liming t o p H 6.5; manure before corn; contour planting and cultivation. c

Prevailing practice: straight row tillage across slope; low P fertilizer level; alsike-red clover-timothy meadow mixture; liming t o p H 5.4. Improved practice: contour tillage; high P fertilizer level; clover-alfalfa-timothy meadow mixture; liming t o p H 6.8.

0

2 ? w

C

30


These observations are not unexpected because rainfall tends to loosen soil particles by drop impact, facilitating their entry into surface runoff waters. It is apparent that an appreciable dilution of dissolved P may occur when surface runoff augments base flow in streams. Taylor et al. (1971) reported a mean total dissolved P concentration of 0.022 pg/ml in a stream draining an agricultural watershed at Coshocton, Ohio; concentrations never exceeded 0.100 ,g/ml even under conditions of high stream flow when surface runoff was occurring. It is generally considered that P is retained sufficiently strongly by soil particulates that movement out of the soil profile in percolating waters is minimal (Way, 1850; Black, 1970). Subsurface runoff from agricultural land, however, may contain significant concentrations of dissolved inorganic P in relation to those present in surface waters (Table VI) . It should be noted, however, that the data in Table VI represent losses of dissolved inorganic P in tile and irrigation return flow drains. Artificial drainage systems increase the rates of infiltration and percolation, reducing contact times between the soil solution and soil components capable of sorbing inorganic P from solution. Furthermore, tile drains will remove water from surface horizons of the soil profile, diminishing the possibility for contact between percolating waters and more P-deficient subsoil material. Not all the data in Table VI, however, indicate a net loss of P from the soil profile to subsurface runoff. In the Snake River Valley, Idaho, Carter et al. (1971) found that only 30% of the dissolved inorganic P in irrigation water left an irrigation tract by return flow. When the dissolved inorganic P concentration in irrigation water exceeded 0.010-0.020 pg/ml, irrigation decreased the downstream P load, a useful field example of the chemical sieving action of soils. Johnston et al. (1965), however, reported a net loss of 3% at an applied P fertilizer rate of 51.9 kg/ha on irrigated land in the San Joaquin Valley, California. The data in Table VI indicate that a reasonable proportion of P loss to streams draining arable watersheds can be due to subsurface runoff. Although no data are available to compare P loads due to surface and subsurface runoff, Sylvester (1961) reported that total P loss by irrigation return flow in the Yakima Valley, Washington, ranged from 3.8 to 14.3 kg/ha per year, values higher than many reported for surface runoff losses. Under a nonirrigated farming system, Bolton et a!. (1970) observed losses of dissolved inorganic P in tile drain effluent ranging from 0.13 to 0.29 kg/ha per year at a fertilization rate of 28.9 kg of P per hectare per year. It would appear, therefore, that losses of P in subsurface runoff can be similar or even greater than those in surface runoff. Furthermore, subsurface runoff will occur not only during periods of surface runoff, but also when evapotranspiration is less than infiltration.

TABLE VI Losses of Dissolved Inorganic Phosphorus in Subsurface Runoff Dissolved inorganic P concentration (pglml) Study

Location

Bolton et al. (1970)

Ontario, Canada

Brink and Gustafsson (1970) Carter et al. (1971)

Sweden

Cooke and Wlliams (1970)

Soil texture

Drainage system Tile drains

Clay

-

Snake Valley, Idaho

Calcareous silt loam

Irrigation return flow

Woburn, England

Sandy

Tile drains

Voelecker (1874)

a

S. Central Michigan San Joaquin Valley, California Yakima Valley, Washington

Rothamsted, England

No P fertilizer applied. 28.9 kg P applied per hectare per year.

Clay to sandy loams Heavy silty clay Sandy loam

Clay loam

Range

Mean

Corn, oats Alfalfa, bluegrass

0.200-0.170 0,190-0.270 0.045-0.140

0.180" 0 . 210b 0.079

Alfalfa, corn, root crops, pasture Arable and grassland Arable grassland Root crops

0.007-0.23

0.012

0-0.300

0 . 0uo

0-0.700 0-0.750 0.010-0.300

0.440 0.080

0.053-0.230

0.079

-

Tile drains

Sandy drift Erickson and Ellis (1971 Johnston et al. (1965) Sylvester (1961)

Crop

Tile drains and ditches Irrigation return flow drains Surface return flow drains Subsurface return flow drains Tile drains

Cotton, rice, alfalfa

0.072-0.300

Wheat

-

0.161

0.029-0.460

0.182

0.054-0.802

-

32

J. C. RYDEN, J. K. SYERS, AND R. F . HARRIS

D. RUNOFFFROM LANDASSOCIATED WITH ANIMAL REARING Animal excrement is a source of P to surface waters (McCarty, 1967). Little direct information is available on the P load this source imparts to streams. Although some loss of P to subsurface and groundwater runoff can be expected, the two major ways in which animal excrement may enter streams are by surface runoff from land upon which manure has been spread and by surface runoff from feedlot operations. Manure spread on land in certain areas during the winter months is subject to transport in surface runoff waters. The amounts of surface runoff during spring thaws are particularly great owing to the combined effects of snowmelt and rainfall on frozen ground. A study of P loss from land manured during winter was conducted by Midgley and Dunklee (1 945). Manure was applied to study plots in Vermont during winter for a period of 6 years at a rate of 22.5 tonnes/ha. Losses of total dissolved P amounted to 2.1 and 2.5 kg/ha for 20 and 10% slopes, respectively. It was concluded that P losses were little affected by slope but more by the amount of snow. Using the data of Midgley and Dunklee (1945), Lee et al. (1969) estimated that 6810 kg of P is lost in surface runoff to streams in the Lake Mendota watershed, Wisconsin, from agricultural lands on which manure is spread during 5 months of the year when the ground is frozen. This amounted to approximately 60% of the total P losses from rural land in the watershed. Concern has also been shown in countries where large areas of land are used for pasture and high intensity grazing, such as in New Zealand, that dung pats may be carried in surface runoff and contribute significantly to the P loading of streams (Elliott, 1971). The magnitude of this problem is obviously very difficult to estimate, and its control virtually impossible, unless restrictions are placed on the proximity to streams that stock are allowed to graze. Again, there are few data from which the magnitude of P loss to streams via surface runoff from feedlot operations can be estimated. Surface runoff from feedlots could almost be regarded as a point source of P because such operations are highly concentrated and the area occupied is generally insignificant in relation to the area of the region in which they are located. In Nebraska the total area of concentrated feedlots amounts to no more than 5670 ha (Swanson et al., 1970). When it is considered, however, that cattle of 454 kg average weight excrete 7.7 kg of P per year (Millar and Turk, 1955), of which 60-80% may be in an inorganic form (Peperzak et al., 1959) and that 1.5 million cattle may be on feed at any one time, each occupying an area of 37 m? (Swanson et al., 1970), it is probable that the local impact of surface runoff from these operations on the

33


P status of streams in the area is considerable. The magnitude of this source of P to streams may result in a spread of effects far beyond the immediate vicinity. Data presented by Gilbertson et al. (1970) for the effect of slope and cattle density on the total P losses from unpaved feedlots in Nebraska are presented in Table VII. The greatest total P losses occurred during snowTABLE VII Effect of Slope and Cattle Density on Total P Loss from ITnpaved Fcedlotsn

Slope 9 6

3

Cattle density (in2/animal) 18.6 9.3 18.6 9.3 18.6 9.5

I,

P loss in winter runoff I, (kg/lla)

P loss i n rain-

80.5 469.3 146.4

58.5 34.2 36.6 29.3 34.2 29.3

256.9

78.1 514.5

storm runoff c (kg/W

Data from Gilbertson et al. (1970). January through April, 1969. April through July, 1969.

melt with a large effect of cattle density but only minor effects due to slope. The latter finding is in agreement with that of Midgley and Dunklee (1945), despite the different purpose of the two studies. The concentrations of total P in surface runoff ranged from 6.8 to 753.2 pg/ml during winter and from 13.9 to 46.6 pg/ml in rainstorm surface runoff. These concentrations are extremely high and would be expected to produce a significant change in the total P concentration of receiving streams.

E. URBANRUNOFF The load of P in streams draining urban watersheds which have a negligible contribution of P from point sources will generally be dominated by that carried in surface runoff. Drainage patterns in urban areas are, in most cases, altered so drastically by paving, the installation of storm sewers, and the channelization of water courses, that contributions from subsurface and groundwater runoff are probably small. It is probable that a large proportion of precipitation in an urban area, which in a rural area would con-

34

J . C. RYDEN, J. K. SYERS, AND R. F. HARRIS

tribute to subsurface and groundwater runoff, is intercepted by drains and becomes indistinguishable from urban surface runoff. Because many of the studies of surface runoff from urban areas have been conducted in areas served by combined sewer systems it is difficult to estimate the contribution of surface runoff to the P load carried by streams. It is virtually impossible to separate the component appearing in stormwater outlets due to overloading of sanitary sewers during wet weather flow from that due to urban surface runoff (Weibel, 1969). Several studies, however, have been conducted recently with the sole objective of determining the quality of urban surface runoff. A summary of these data is given in Table VIII. One of the first studies conducted was that by Weibel et al. (1964) in an 1 1 ha residential and light commercial section of Cincinnati, Ohio. The maximum mean total dissolved P concentrations were observed during the summer (0.36-0.39 pg/ml) whereas minimum values were observed in winter (0.16 pg/ml). By far the most extensive study of the quality of urban surface runoff took place in Tulsa, Oklahoma (Avco Corporation, 1970). The proportion of unused land, arterial streets, and industrial land were all found to be important in relation to the mean dissolved inorganic P concentrations observed in monitored storm Sewers. The highest annual load of 8.8 kg/ha was for urban surface runoff from a light industrial area, a large proportion of which was still under development. Other test areas, all except one including residential property, gave rise to dissolved inorganic P losses of 1.1 to 3.3 kg/ha per year. The largest loads of P per impervious area were from districts where tree cover was dense. This is probably due to the leaching of dissolved inorganic P from leaves, discussed previously. As reported by Kluesener (1972) and Harris et al. (1972), for urban watersheds in Madison, Wisconsin, leaching of leaves and seeds, coupled with the considerably reduced infiltration characteristics of urban areas, can be expected to result in high concentrations of dissolved inorganic P in urban surface runoff in the spring and autumn. Storm sewers draining runoff from residential areas into Lake Wingra, Madison, were monitored intensively during snowmelt, and spring, summer, and autumn storm runoff events; samples were taken every 2-5 minutes during peak flow and at longer intervals over the entire length of a storm to enable determination of the frequency of sampling needed to obtain, in conjunction with flow data, a reliable estimate of P input loads (Harris et al., 1972). Dissolved inorganic P generally constituted more than 80% of the total dissolved P in runoff at all times of the year. Although dissolved inorganic P was highest in the autumn (up to 2.4 pg/ml) and spring (up to 2.1 pg/ml), immediately following leaf and seed fall, respectively, the relative input

wz

TABLE VIII Losses of Phosphorus in Surface Runoff from Urban Areas

meable area Study Avco Corporation

Sylvester (1961)

Weihel et al. (1964)

X 0

runoff waters (pg/ml)

P loss

?? 1 vl

(%)

Form measured

(kg P/ha/yr)

Range

Mean

2

Tulsa, Oklahoma

37

2 80

0 54-3 49

1 15

Ez

Durham, North Carolina Seattle, Washington

29

Dissolved inorganic P Total P

3 4

0 15-52 50

0 55

;

Dissolved inorganic P Total P Total dissolved

-

Trace-0 78

0 08

-

0 01-1 40 0 052-1 452

Location

(1970)

Bryan (1971)

V

P concentration in

Imper-

Cincinnati, Ohio

Z

-

37

P

4

0.92

o

21 0 '26

P m

>

5

36

J . C. RYDEN, J . K. SYERS, AND R. F . HARRIS

loading of dissolved inorganic P was greater during the snowmelt period (levels of 0.4 to 0.9 pg/ml) because of the large volumes of water discharged in this period. Particulate inorganic P varied from 10 to 80% of the total particulate P and showed no consistent relationship to time of year. Levels of total particulate P tended to increase with increasing runoff flow. A substantial proportion of this particulate P was of sufficiently high density to settle rapidly out of the biologically active lake surface waters and probably have minimal effect on lake P fertility status. On the other hand, low density runoff particulate P may provide an important reservoir of biologically available P in lake waters, especially in late spring and summer when P-deficient algae and aquatic plants will tend to accelerate release of dissolved P from such suspended runoff particulates. Although total P levels during a specific runoff event tended to be highest during the initial flush, total P load was dictated essentially by flow rather than by fluctuations in P composition (Harris et a!., 1972). If these trends recur for runoff from different land-use areas, limited sampling of runoff from representative flow-gauged storm sewers during periods of high Row, and analysis of these samples for dissolved inorganic P and low-density particulate P should provide valid estimations of the loads of biologically important P components in urban runoff. Another potentially important source of P to urban surface runoff is that associated with eroding soil. During urban development, particularly on the fringes of urban areas, large tracts of land are frequently stripped of vegetation and graded, maximizing the possibility of erosion should surface runoff occur. There are no reported studies of P losses from such development projects, but Guy and Ferguson (1970) cite soil loss from highway construction in a watershed in the Potomac River basin. This averaged 1710 tonnes/km' per year over a three-year period. The amount of a specified form of P lost to flowing waters by such severe erosion will depend to a large extent on previous land use. As extensive construction programs frequently utilize land previously under agricultural management, high P losses can be expected, the distribution of inorganic P between the solid and aqueous phases being primarily determined by the nature of the inorganic particulates and the concentrations of dissolved inorganic P in solution (Ryden et al., 1972a,b). There is reasonable agreement between the estimates of the P loads carried in urban surface runoff. In many situations, however, urban surface runoff probably only amounts to a small percentage of that contributed by municipal and industrial wastes. As the amounts of P discharged to streams from the latter sources are reduced, urban runoff will become a much more significant source of P to receiving streams (Weibel et al., 1964).


V.

37

Impact of Phosphorus Carried in Streams on Standing Waters

The overall, short-term impact of surface runoff-derived P on standing waters is expected to be high because a large proportion of the average annual discharge of P occurs over only short periods of time during the annual cycle. Bryan (1971) pointed out that in Durham, North Carolina, there was an annual average of 40 day-long surface runoff cycles. Consequently, the major portion of the annual stream loading of P is concentrated into only 40 days, which potentially amplifies its effects by a factor of approximately nine. Furthermore, the impact of the P load carried in streams is expected to be greater during late spring and early summer when aquatic microorganisms are in the potentially active growth phase. The extent of any evaluation of the impact of P carried in streams on standing waters with respect to their biological productivity will also depend on the forms of P measured. If the only form measured is total P, then evaluation may be no more than one segment in a nutrient budget for the body of standing water. This is the easiest approach but one which allows no interpretation of the effects on biological productivity. Even when forms of P relevant to biological productivity are measured, a major unknown factor centers around the effects of mixing as streams enter standing waters. It is reasonable to suggest that temperature differences between stream water and standing water will have some effect on the degree of mixing. Stream water of a lower temperature than surface lake water would be expected to sink below the surface (Twenhofel, 1950), possibly minimizing effects on the photic zone. Such a situation may occur in summer and autumn when tcmperature differences will be the greatest. Furthermore, the effect of overall stream water density arising from sediment concentration, particularly during periods of surface runoff, causes entering water to sink to a lower level (Twenhofel, 1950). Temperature and density effects would tend to contribute P to deep waters and sediments, removing the P load from an immediately usable location. When minimal temperature and density differences exist between streams and standing waters, then a direct dilution effect will probably operate, providing entering waters produce enough turbulence to facilitate mixing. Under such circumstances the biological ‘availability of particulate forms of P will also depend on settling times which ultimately remove them from the photic zone. If the standing waters are in a stratified condition, however, entering stream waters could override the thermocline. In this case dilution would be limited by the amounts of epilimnetic waters. In spring-fed lakes, the primary source of water is groundwater runoff. The amount of water entering a lake from this source is difficult to eval-

38

5. C. RYDEN, J. K. SYERS, AND R. F. HARRIS

uate. The P load of groundwater runoff has traditionally been regarded as minimal (Taylor, 1967; Keup, 1968), and there is considerable evidence to support this contention. Rarely do P concentrations (in most cases dissolved inorganic P ) exceed 0.020-0.030 pg of P per milliliter (Juday and Birge, 1931; White et al., 1963; Mackenthun et al., 1968; Cooke and Williams, 1970). The importance of groundwater runoff as a P source to bodies of standing water has been based on flow rates and P concentrations of land springs and wells. Although it seems unlikely that groundwater runoff itself will contribute significant quantities of P to standing waters, the upward movement of ground water through the sediment may cause redistribution of P within the sediment and even release of dissolved P to the overlying water. The magnitude of this effect will depend on the P status and redox status of the sediment, the nature of the P-retaining components, the nature of groundwater entry (point or diffuse) into the lake, and its amount. The relocation of sediment P to groundwater runoff may be why Millar and Tash (1967) estimated that groundwater runoff or springs contributed 24.9% of the P inflow to Upper Klamath Lake, Oregon. At present it is difficult to estimate the impact of runoff- and streamderived P on standing waters, and such considerations can only be made if the forms of P relevant to biological productivity are measured. Furthermore, the mixing effects that occur as flowing waters enter lakes and reservoirs, as well as the potential of bottom sediments for the P enrichment of groundwater runoff, require further investigation.

VI.

Present Status and Outlook

The preceding discussion of the factors affecting the dynamics and loads of P in runoff and streams reveals various gaps in our knowledge and illustrates the problems in interpreting the data thus far obtained. The first and major difficulty in data interpretation and comparison is the lack of uniformity in the forms of P measured. In many cases this makes comparison between different studies virtually impossible, thereby prohibiting estimations of the relative importance of any particular source. In many studies, particularly those relating to surface runoff from agricultural land, the measurement of total P has been favored. This has led to the concept of nutrient budgets for P, whereby nutrient input and output for an ecosystem are used to determine whether P is lost. This approach is favored by Frink (1967, 1971). If the estimates of P input and output are based on measurements of total P, little information is gained because such deter-


39

minations override any knowledge of the distribution of P between various forms in runoff and streams, some of which will have a greater or lesser effect on the biological productivity of surface waters. Although relatively few studies have been conducted on the P loads of streams and surface runoff from forest and urban watersheds respectively, there is considerable agreement in the results so far obtained. The situation is quite different for P loads in runoff and streams from agricultural watersheds. Frink (1971) stated that an “average” agricultural watershed with respect to P loss is a “useless fabrication.” It would appear, however, that the major problem arises from the lack of relevant information upon which reliable estimates can be made, a situation which has arisen largely because of an apparent lack of definition of the system being investigated. The use of surface runoff plots to determine losses of P from agricultural watersheds presents several problems. Surface runoff is a spasmodic rather that a continuous phenomenon, its composition at any location being highly heterogeneous and likely to change over short distances because the energy of the aqueous component, and therefore its ability to carry particulate material, varies with slope. The studies cited previously (Timmons et al., 1968; Nelson and Romkens, 1969), in which attempts were made to measure the distribution of the P load between the solid and aqueous phases of surface runoff appear to have limited value. When surface runoff enters streams, a much greater degree of homogeneity will be assumed, resulting in a new and probably more stable distribution of P between the aqueous and sediment phases, as discussed previously. Measurement of dissolved P fractions in surface runoff itself may lead to erroneous conclusions regarding its impact on the dissolved P status of streams due to the transitory nature of surface runoff. In order to obtain more meaningful estimates of P loss from agricultural watersheds, detailed studies of the P load of streams draining the watersheds are required. Some such studies have been conducted (Minshall et al., 1969; Witzel et al., 1969; Campbell and Webber, 1969; Taylor et al., 1971); these will be referred to as watershed analyses herein. None of the watershed analyses cited, however, covered more than a 2-year period of monitoring; the duration of the study could lead to considerable variation in P loss estimates, due to yearly differences in weather patterns as noted by Timmons et al. (1968) for surface runoff studies. Future studies must be based on the watershed analysis approach in order to avoid bias in estimates of the P loss obtained in plot studies due to differences in the energy of surface runoff imparted by slope variations within the watershed. Furthermore, it is essential that studies be long-term to minimize yearly variation in weather patterns and that the forms of P measured be standardized. Although watershed analyses combine the P

40

J . C. RYDEN, J. K. SYERS, A N D R. F. HARRIS

loads of surface, subsurface, and groundwater runoff, these may be separated by determining P loads under various flow conditions in a way similar to that used by Minshall et al. (1969) and to some extent Taylor et al. ( 1971 ) . With careful selection of small watersheds in the same geographic and climatic area, accurate records of fertilizer practice, and cognizance of less diffuse or even point sources of P (e.g., effluent from animal-rearing or industrial operations) within the watershed, it should be possible to obtain meaningful estimates of the effects of various land use and fertilizer practices as well as physical variables on the loss of P from agricultural watersheds. This approach is similar to that which has been used to evaluate P loads in streams draining forest watersheds. It is also important that this be coupled with investigation to define diffuse sources of P more adequately in terms of the components which constitute such sources. Attempts have been made in this direction, as illustrated in the studies conducted by Taylor and Kunishi (1971), Cowen and Lee (1972), and Ryden et al. (1972a,b). Studies similar to these are necessary if any remedial steps are to be taken to reduce the magnitude of man-induced diffuse P sources and will be particularly valuable if carried out in conjunction with watershed analyses. Only by adopting such an approach will it be possible to provide adequate estimates of the potential of soil and fertilizer P for the P enrichment of streams; a topic which is currently surrounded by considerable controversy. Comparative tables of the relative magnitude of various P sources have been drawn up for individual watersheds (Miller and Tash, 1967; Lee et al., 1969; Jaworski and Hetting, 1970). Although such tables are useful for identification of problems within a specific watershed, extrapolation of this concept to a national basis is dangerous. Local and regional variations in land use can seriously distort the relative impact of any source of P on water quality. The way in which P source data are presented can also lead to different conclusions as to the impact of one source as opposed to another. This is particularly true for comparative tables of P sources compiled on a nationwide basis. McCarty (1967) estimates that in the United States, 4.9 X loGto 77.2 X loGkg of P per year is lost to surface waters through urban surface runoff, whereas 54.5 x lo6 to 544.8 x loG kg of P per year originates from agricultural runoff. If losses are expressed on a per area basis, relative contribution estimates are very similar if not reversed, losses being 0.23 to 3.59 and 0.12 to 1.23 kg/ha per year, respectively. These figures show the need for careful evaluation of problems within any given watershed or group of watersheds. Watershed analyses will provide more useful data than estimations of the magnitude of various P sources from a national standpoint.


41

ACKNOWLEDGMENTS Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, by the Office of Water Resources Research Project No. WRC 71-10 (OWRR A- 038- WIS), and by the Eastern Deciduous Forest Biome Project, International Biological Program, National Science Foundation subcontract 3351, under Interagency Agreement AG-199, 40-193-69, with the Atomic Energy Commission, Oak Ridge National Laboratory.

REFERENCES Allen, S. E., Carlise, A., White, E. J., and Evans, C. E. 1968. J . Ecol. 56, 497-504. Anderson, G. 1967. 111 ‘‘Soil Biochemistry” (A. D. McLaren and G. H. Peterson, eds.), pp. 67-90. Dekker, New York. Avco Corporation. 1970. “Storm Water Pollution from Urban Land Activity.” U.S. Dept. of the Interior, Fed. Water Qual. Admin., U S . Govt. Printing Office, Washington, D.C. Bache, B. W. 1964. J. Soil Sci. 15, 110-116. Barber, S. A., Walker, J. M., and Vasey, E. H. 1963. Trans. J . Meet. C o m m . IV and V . Int. SOC.Soil Sci., 1962, p p . 121-124. Biggar, J. W., and Corey, R. B. 1969. I n “Eutrophication: Causes, Consequences, Correctives,” pp. 404-445. Nat. Acad. Sci., Washington, D.C. Black, C. A. 1970. In “Agricultural Practices and Water Quality” (T. L. Willrich and G. E. Smith, eds.), pp. 72-93. Iowa State Univ. Press, Ames. Bolton, E. F., Aylesworth, J. W., and Hore, F. R. 1970. Can. J . Soil Sci. 50, 275-279. Bormann, F. H., Likens, G. E., Fisher, D. W., and Pierce, R. S. 1968. Science 159, 882-884. Brink, N., and Gustafsson, A. 1970. Lantbriikslroegsk. Inst. Markvetenskap-Vattenvard No. I. Bromfield, S. M. 1965. A u s f . J . Soil Rcs. 3, 31-44. Bromfield, S. M., 1967. Arcst. J. Soil Res. 5, 93-102. Bryan, E. H . 1971. S. Water Res. Pollilt. Contr. Conf. 20tlr, 1971. Buehrer, T. F., and Williams, J. A. 1936. Ariz., A y r . Exp. Sta., Bull. 64, 1-40. Campbell, F. R., and Webber, L. R. 1969. J . Soil Water Conserv. 24, 139-141. Carter, D. L., Bondurant, J. A,, and Robbins, C. W. 1971. Soil Sci. SOC. Amer., Proc. 35, 331-335. Chakravart, S. N., and Talibudeen, 0. 1962. J . Soil Sci. 13, 231-240. Chang, S. C., and Jackson, M. L. 1957. Soil Sci. 84, 133-144. Cole, C. V., Olsen, S. R., and Scott, C. 0. 1953. Soil Sci. SOC. Amer., Proc. 17, 352-356. Cooke, G. W., and Williams, R. J. B. 1970. J . SOC. Water Treat. Exani. 19, 253-276. Cooper, C. F. 1969. I n “Eutrophication: Causes, Consequences, Correctives,” pp. 446-463. Nat. Acad. Sci., Washington, D.C. Cosgrove, D. J . 1967. I n “Soil Biochemistry” (A. D. McLaren and G. H. Peterson, eds.), pp. 216-228. Dekker, New York. Cowen, W., and Lee, G. F. 1972. Leaves as a source of phosphorus. Mimeo. Water Chem. Program, University of Wisconsin, Madison.

42


Dion, H. G. 1944. Soil Sci. 58, 41 1-424. Elliott, I. L. 1971. Proc. Tech. Conf. N . Z . Fert. Mfr. Res. Ass., 13th, 1971 p. 93. Erickson, A. E., and Ellis, B. G. 1971. Mich., Agr. E r p . Sta., Bull. 31, 1-16. Ferguson, F. A. 1968. Environ. Sci. Technol. 2, 188-193. Fippin, E. 0. 1945. Soil Sci. 60, 223-239. Frink, C. R. 1967. Environ. Sci. Technol. 1, 425-428. Frink, C. R. 1971. I n t . Symp. Ident. Measure. Environ. Pollut., Sess. B . June 14 1971. Gardner, W. R. 1965. I n “Soil Nitrogen” (W. V. Bartholomew and F. E. Clark, eds.), Agron. Monogr. No. 10, pp. 550-572. Amer. SOC. Agron., Madison, Wisconsin. Gastuche, M. C., Fripiat, J. J., and Sokolski, S . 1963. Pedologie 13, 155-180. Gilbertson, C. B., McCalla, T. M., Ellis, R. J., Cross, 0. E., and Woods, W. R. 1970. Nebr., Agr. Expr Sta., Bull. 508, 1-23. Gorbunov, N. I., Dzydevich, G. S., and Tunik, B. M. 1961. Sov. Soil Sci. 11, 1252-1259. Gore, A. J. P. 1968. J. Ecol. 56, 4 8 3 4 9 5 . Goring, C. A. I., and Bartholomew, W. V. 1950. Soil Sci. SOC. Amer., Proc. 15, 1 89-1 95. Greaves, M. P., and Wilson, M. J. 1969. Soil Biol. & Biochem. 1, 37-43. Greenland, D. J. 1965. Soils Fert. 28, 415-425. Guy, H. P. 1970. In “Techniques of Water-Resources Investigations of the United States Geological Survey,” Book 3, Chapter CI, pp. 1-55. U.S. Dept. of the Interior, U.S. Govt. Printing Office, Washington, D.C. Guy, H. P., and Ferguson, G. E. 1970. J. Soil Water Conserv. 25, 217-221. Hanapel, R. J., Fuller, W. H., and Fox, R. H. 1964. Soil Sci. 97, 421-427. Harris, R. F., Ryden, J. C., and Syers, J. K. 1972. Agron. Abstr. p. 180. Hemwall, I. B. 1957. Advan. Agron. 9, 95-111. Hingston, F. J., Atkinson, R. J., Posner, A. M., and Quirk, J. P. 1969. Trans. Int. Congr. Soil Sci., 9th, 1968 Vol. 1, pp. 669-679. Hsu, P. H. 1964. Soil Sci. SOC. Amer., Proc. 28, 4 7 4 4 7 8 . Huffman, E. 0. 1969. Trans. I n t . Congr. Soil Sci., 9th 1968 Vol. 2, pp. 745-754. Jackson, M. L. 1963. Clays Clay Minerals, Proc. Nut. Conf. Clays Clay Minerals 11, 29-46. Jackson, M. L. 1969. Trans. Int. Congr. Soil Sci., 9th, 1968 Vol. 4, pp. 281-292. Jaworski, N. A., and Hetting, L. J. 1970. I n “Relationship of Agriculture to Soil and Water Pollution,” pp. 134-146. Cornell Univ. Press, Ithaca, New York. Johnson, H. P., and Moldenhauer, W. C. 1970. I n “Agricultural Practices and Water Quality” (T. L. Willrich and G. E. Smith, eds.), pp. 3-20. Iowa State Univ. Press, Ames. Johnston, W. R., Ittihadieh, F., Daum, R. M., and Pillsbury, A. F. 1965. Soil Sci. SOC.Amer., Proc. 29, 287-289. Jordan, D. 0. 1955. In “The Nucleic Acids” (E. Chargaff and J. N. Davidson, eds.), Vol. 1, pp. 447-492. Academic Press, New York. Juday, C., and Birge, E. A. 1931. Wis. Acad. Sci. Arts Lett. 26, 354-382. Kafkafi, U., Posner, A. M., and Quirk, J. P. 1967. Soil Sci. SOC. Amer., Proc. 31, 348-353. Keup, L. E. 1968. Water Res. 2, 373-386. Kittrick, J. A., and Jackson, M. L. 1956. J . Soil Sci. 7 , 81-89. Kluesener, J. 1972. Ph.D. Thesis, University of Wisconsin, Madison.


43

Knoblauch, H. C., Kolodny, L., and Brill, G. D. 1942. Soil Sci. 53, 369-378. Kunishi, H. M., Taylor, A. W., Heald, W. R., Gburek, W. J., and Weaver, R. N. 1972. 1. Agr. Food Chem. 20, 900-905. Lahav, N., and Bolt, G. H. 1963. Nufure (London) 200, 1342-1344. Langbein, W. B., and Iseri, K. T. 1960. U S . , Geol. Surv., Water-Supply Pup. 1541-A, 1-26. Larsen, J. E., Langston, R., and Warren, G. F. 1958. Soil Sci. SOC. Amer., Proc. 22, 558-560. Latterell, J. J., Holt, R. F., and Timmons, D. R. 1971. I. Soil. Water Conserv. 26, 21-24. Lee, G. F. 1970. “Eutrophication Information Program,” Occ. Pap. No. 1. University of Wisconsin, Madison. Lee, G. F., Beatty, M. T., Corey, R. B., Fruh, E. G., Holt, C. L. R.,Jr., Hunter, W., Lawton, G. W., Peterson, A. E., Schraufnagel, R. H., and Young, K. B. 1969. “Revised Report on the Nutrient Sources to Lake Mendota” Univ. of Wis. Report to Lake Mendota Problems Committee. Lindsay, W. L., and Stephenson, H. F. 1959. Soil Sci. SOC. Amer., Proc. 23, 12-18. McCarty, P. L. 1967. I . Amer. Water Works Ass. 59, 344-366. McKee, G. D., Parrish, L. P., Hirth, C. R., Mackenthun, K. M., and Keup, L. E. 1970. Water Sewage Works 117, 246-250. Machenthun, K. M. 1965. “Nitrogen and Phosphorus in Water.” U.S. Dept. of Health, Education and Welfare, Public Health Serv., Div. of Water Supply and Pollution Control, U.S. Govt. Printing Office, Washington, D.C. Mackenthun, K. M. 1968. I. Amer. Water Works Ass. 60, 1047-1054. Mackenthun, K. M., Keup, L. E., and Stewart, R. K. 1968. I. Water Pollirt. Control Fed. 40, 72-81. McKercher, R. B. 1969. Trans. Int. Congr. Soil Sci., 9th, 1968 Vol. 3, pp. 547-553. Martin, J. K. 1964. N.Z. J . Agr. Res. 7, 736-749. Massey, H. F., and Jackson, M. L. 1952. Soil Sci. SOC. Amer., Proc. 16, 353-356. Massey, H. F., Jackson, M. L., and Hayes, 0. E. 1953. Agron. I. 45, 543-547. Metzler, D. F., Culp, R. L. Staltenberg, H. A,, Woodward, R. L., Walton, G., Chang, S. L., Clarke, N. A., Palmer, C. M., and Middleton, F. M. 1958. 1. Amer. W a f e r Works Ass. 50, 1021-1057. Midgley, A. R., and Dunklee, D. E. 1945. Vt., Agr. E x p . Sta., Bull. 523, 1-19. Millar, C. E., and Turk, L. M. 1955. “Soil Fertility.” Wiley, New York. Millar, W. E., and Tash, J. C., 1967. Interim Report, Upper Klamath Lake Study, Oregon. Fed. Water Pollut. Contr. Admin. Publ. WP-20-8. Miller, R. B. 1961. N.Z. J . Sci. 4, 844-853. Minshall, N. E., Nichols, M. S., and Witzel, S. A. 1969. Water Resow. Res. 5, 706-7 13. Muljadi, D., Posner, A. M., and Quirk, J . P. 1966. I. Soil Sci. 17, 212-247. Neal, 0. R. 1944.1. Amer. SOC. Agron. 36, 601-607. Nelson, D. W., and Romkens, M. J. M, 1969. In “Relationship of Agriculture to Soil and Water Pollution,” pp. 215-225. Cornell Univ. Press, Ithaca, New York. Nykvist, N. 1959. A c f a Oecol. Scand. 10, 190-21 1 . Ohle, W. 1953. Vom Wasser 20, 11-23. Olsen, S. R., and Watanabe, F. S. 1957. Soil Sci. SOC. Amer., Proc. 21, 144-149. Ozanne, P. G . 1963. Trans. 1. Meet. Comm. IV and V , Int. SOC. Soil Sci., 1962 pp. 139-143.

44


Peperzak, P., Caldwell, A. G., Hunzicker, R. R., and Black, C. A. 1959. Soil Sci. 87, 293-302. Pierre, W. H., and Parker, F. W. 1927. Soil Sci. 24, 119-128. Pinck, L. A., Sherman, M. S., and Allison, F. A. 1941. Soil Sci. 51, 351-365. Rennie, D. A., and McKercher, R. B. 1959. Can. J. Soil Sci. 34, 64-75. Rogers, H. T. 1941. Soil Sci. SOC.Amer., Proc. 6, 263-271. Roth, C. B., Jackson, M. L., and Syers, J . K. 1969. Clays Clay Minera/s, Proc. Nat. C o n f . Clays Clay Minerals 17, 253-264. Russell, E. J., and Prescott, J . A. 1916. J . Agr. Sci. 8, 65-110. Ryden, J. C., Syers, J. K., and Harris, R. F. 1972a. J. Environ. Qua/. 1, 430-434. Ryden, J . C., Syers, J. K., and Harris, R. F. 1972b. J. Eniziron. Qua/. 1, 435-438. Ryden, J. C., Syers, J. K., and Harris, R. F. 1972c. Analyst 97, 903-908. Sagher, A., and Harris, R. F. 1972. Abstr., C o n f . Gt. Lakes Res., I S t h , 1972 p. 193. Saunders, W. M. H. 1965. N.Z. J . Agr. Res. 8, 30-57. Sawyer, C. N. 1947. J. N. E ~ i g l Water . Works Ass. 61, 109-127. Scarseth, G. D., and Chandler, W. V. 1938. J. Amer. SOC. Agron. 30, 361-374. Shen, M. J., and Rich, C. I. 1962. Soil Sci. Soc. Amer., Proc. 26, 33-36. Shipp, R. R., and Matelski, R. P. 1960. Soil Sci. SOC. Arner., Proc. 24, 450-452. Shukla, S . S., Syers, J. K., Williams, J. D. H., Armstrong, D. E., and Harris, R. F. 1971. Soil Sci. SOC. Amer., Proc. 35, 244-249. Sommers, L. E., Harris, R. F., Williams, J. D. H., Armstrong, D. E., and Syers, J. K. 1972. Soil Sci. SOC.Amer., Proc. 36, 51-55. Stewart, K. M., and Rohlich, G. A. 1967. Eutrophication-A Review. Report to the State Water Control Board, California. Stoltenberg, N. L., and White, J. L. 1953. Soil Sci. SOC.Amer., Proc. 17, 406-410. Stumm, W. 1964. U S . , Pub. Health Serv., Publ. 999-WP-15, 299-323. Sullivan, W. T., and Hullinger, D. L. 1969. Trans. 111. State Acad. Sci. 62, 198-217. Swanson, N. P., Mielke, L. N., and Lorrimor, J. C. 1970. I n “Relationship of Agriculture to Soil and Water Pollution,” pp. 226-232. Cornell Univ. Press, Ithaca, New York. Syers, J. K., and Walker, T. W. 1969. J . Soil Sci. 20, 318-324. Syers, J. K., Shah, R., and Walker, T. W. 1969. Soil Sci. 108, 283-289. Syers, J. K., Murdock, J. T., and Williams, J. D. H. 1970. Sod Sci. Plant Anal. 1, 57-62. Syers, J. K., Evans, T. D., Williams, J. D. H., and Murdock, J. T. 1971. Soil Sci. 112, 267-275. Syers, J. K., Smillie, G. W., and Williams, J. D. H. 1972. Soil Sci. Soc. Amer., Proc. 36, 20-25. Syers, J. K., Browman, M. G., Smillie, G. W., and Corey, R. B. 1973. Soil Sci. SOC. Amer., Proc. 37, 358-363. Sylvester, R. 0 . 1961. U S . , Pub. Health Serv., Publ. SEC-TR-W61-3, 80-87. Taylor, A. W. 1967. 3. Soil Water Conscrv. 5, 228-231. Taylor, A. W., and Kunishi, H. M. 1971. J . A g r . Food Chem. 19, 827-831. Taylor, A. W., Edwards, W. M., and Simpson, E. C. 1971. Water Resour. Res. 7, 81-89. Thomas, A. W., Carter, R. L., and Carreker, J. R. 1968. Trans. A S A E ( A m e r . SOC. Agr. Eng.) 11, 677--679 and 682. Tirnmons, D. R., Burwell, R. E., and Holt, R. F. 1968. Minn. Sci. 24, 16-18.


45

Timmons, D. R., Holt, R. F., and Latterell, J. J. 1970. Water Rasorrr. Res. 6, 1367-1 375. Twenhofel, W. M. 1950. “Principles of Sedimentation,” 2nd ed. McGraw-Hill, New York. Voelcker, A. 1874. J . Roy. Agr. SUC.Engl. [2] 10, 132-165. Vollenweider, R. A. 1968. “Scientific Fundamentals of the Eutrophication of Lakes and Flowing Waters with Particular Reference to Nitrogen and Phosphorus as Factors in Eutrophication.’’ OECD, Paris. Wang, W. C., and Brabec, D. J. 1969. J . Arner. Water Works Ass. 61, 460-464. Way, J . T. 1850. J . Roy Agr. SOC.EngI. 11, 313-379. Weibel, S. R. 1969. Z i t “Eutrophication: Causes, Consequences, Correctives,” pp. 383-403. Nat. Acad. Sci., Washington, D.C. Weibel, S. R., Anderson, R. J., and Woodward, R. L. 1964. J . Water Pollut. Contr. Fed. 36, 914-924. Weibel, S . R., Weinder, R. B., Cohen, J. M., and Christianson, A. G . 1966. J . Amer. Water Works Ass. 58, 1075-1084. Weidner, R. B., Christianson, A. G . , Weibel, S. R., and Robeck, G. G. 1969. J . Water Polliit. Contr. Fed. 41, 377-384. White, D. E., Hem, J . D., and Waring, G. A. 1963. U.S. Geol. Sun.’., Prof. Pap. 440F, Chapter F. White, E. 1972. N . Z . Ecol. SOC.Proc. 19, 163-172. White, R. W., and Beckett, P. H. T. 1964. Plnnr Soil 20, 1-15. Wier, D. R., and Black, C. A. 1968. Soil Sci. SOC.Arner., Proc. 32, 51-55. Williams, E. G . , and Saunders, W. M . H. 1956. J . Soil Sci. 7, 90-108. Williams, E. G., Scott, N. M., and McDonald, M. J. 1958. J . Sci. Food Agr. 9, 551-559. Williams, J . D. H., and Walker, T. W. 1969. Soil Sci. 107, 213-219. Williams, J. D. H., Syers, J. K., and Walker, T. W. 1967. Soil Sci. Soc. Amer., Proc. 31, 736-739. Williams, J. D. H., Syers, J. K., Walker, T. W., and Shah, R. 1969. Agrocliinzica 6, 491-501. Williams, J. D. H., Syers, J . K., Harris, R. F., and Armstrong, D. E. 1971a. Soil Sci. SOC.Anier., Proc. 35, 250-255. Williams, J. D. H., Syers, J. K., Armstrong, D. E., and Harris, R. F. 1971b. Soil Sci. SOC. Arner., Proc. 35, 556-561. Wischmeier, W. H., and Smith, D. D. 1965. U S , ,D r p . Agr., Handb. 282. Witzel, S . A,, Minshall, N., Nichols, M. S., and Wilke, J . 1969. Trans. A S A E ( A m r r . SOC. Agr. Eng.) 12, 338-341.


CRIMSON CLOVER W . E . Knight and E . A . Hollowell .

U S Department of Agriculture. Mississippi State. Mississippi. and U.S. Department of Agriculture. Beltsville. Maryland

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A . Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C . Economic Importance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A . Root. Stem. and Leaf ...................................... B. Flower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C . Pollination and Seed Development ........................... I11. Physiology ........................ ....................... A . Growth and Development . . . . . . . B. Flowering . . . . . . . . . . . . . . . . . . . C . Seed . . . . . . . . . . . . . . . . . ............................... IV . Culture ......................... A. Adaptation . . . . . . . . . . . . . . . . . . B. Soils and Soil Fertility . . . . . . . . C. Inoculation ............................................... D . Establishment . . . . . . . . . . . . ............................. E . Companion Grasses and Cro uences ...................... F . Weed Control ..................... .................... G . Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H. Insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I . Seed Production . . . . . . . . . . . . . . . . . . . . . . . . V. Utilization . . . . . . . . . . . ................................. A . Pasture . . . . . . . . . . ................................. B. Hay and Silage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Green Manure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D . Seed ..................................................... VI . Genetics and Cytology . . . . ................................. A . Cytology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Inheritance of Characters .............................. VII . Breeding . . . . . . . . . . . . . . . . A . Objectives . . . . . . . . . . . . . . . . . . B. Variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C . Seed Shattering . . . . . . . . . . . . . D . Seedling Vigor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Inbreeding and Hybridization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Cultivars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

48 48 48 49 50 50 50 51 52 52 54 55 57 57 57 59 59 61 62 63 63 64 65 65 66 66 67 68 68 68 69 69 70 70 70 70 71 72 73

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W. E. KNIGHT AND E. A. HOLLOWELL

I.

Introduction

A.

ORIGIN

Crimson clover, Trifolium incarnatum L., of the section Trifolium, belongs to the Leguminosae (Ascherson and Graebner, 1906-1910; Coombe, 1968; Zohary, 1970). Numerous botanists have recognized many varieties, based on wild populations. The authors believe, however, that these are nothing more than variations of morphological characteristics found in large populations of plants. Crimson clover is a winter-annual clover. It is native to Europe, where it was cultivated as a forage and green-manuring crop in Italy, France, Spain, Germany, Austria, and Great Britain during the eighteenth century. In 1818, this clover was introduced into the United States. By 1855, seed was widely distributed by the United States Patent Office (Kephart, 1920). This clover has been called “scarlet clover” because of the rich scarlet flowers. It is also known as “French clover,” “Italian clover,” “German clover,” “incarnate clover,” and “annual clover” (Westgate, 1913, 1914). Foury (1950) lists more than twenty common names by which crimson clover is known throughout the world.

B.

DISTRIBUTION

The genus Trifolium consists of some 250 described species of annual, and perennial forms that are widely distributed. Pieters and Hollowell (1937) listed crimson clover, Trifolium incarnatum L.; with red, T. pratense L.; alsike, T . hybridum L.; and white, T , repens L.; as one of the four Trifolium species of primary importance in the United States. Crimson clover is grown widely as a winter annual from the Gulf Coast region, except peninsular Florida, and as far northward as Maryland, southern Ohio, and Illinois. It spread rapidly throughout the southeastern states after 1880. By 1900, it was considered a good crop as far north as Kentucky. It also is grown in the Pacific Coast states and is an important seed crop in Western Oregon (Rampton, 1969; Williams et af., 1957; Williams and Elliott, 1960). If planted late in May or early in June, it can be grown as a summer annual in northern Maine (Westgate, 1924; Kephart, 1920) and is a promising crop for high altitudes. Initially, crimson clover was used as a winter cover and green manure crop (Duggar, 1897; von Horn, 1936; Westgate, 1914; Kephart, 1920). Since it grew during the off-season of the year, it was considered to be one of the most economical legumes for green-manuring (Duggar, 1897; Kephart, 1920).

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49

Before 1942, the largest acreage of crimson clover was located in Tennessee, Georgia, Alabama, Kentucky, and Oregon (Hollowell, 1943-1 947, 1947, 1950). After 1942, a rapid increase in use of crimson clover occurred. Contributing to this increase are: ( a ) the development of reseeding or volunteering varieties, ( b ) recognition of the requirements of crimson clover for substantial amounts of mineral fertilizers for rapid stand establishment and vigorous growth, (c) an appreciation of its value for winter grazing, and (d) an understanding of its need for thorough inoculation (Hollowell, 1951; Hollowell and Knight, 1962). C.

ECONOMIC IMPORTANCE

Crimson clover is probably the most important annual legume in the rapidly expanding winter grazing program of the South (Stewart and Boseck, 1947; Hollowell and Knight, 1962). One of the most important characteristics of crimson clover is its ability to grow rapidly during the fall and early spring when the land is not occupied by the ordinary summer-grown crops. It, therefore, fits well into cropping systems and sequences. Other characteristics that make crimson clover the most important winter-annual legume in the South are: ( a ) it will grow under a wide range of climatic and soil conditions; ( b ) it has many uses; ( c ) it produces large yields of easily harvested seed; and ( d ) it thrives in association with other crops (Hollowell, 1951 ;Hollowell and Knight, 1962). The total acreage of crimson clover is not known. The domestic disappearance of seed reached a peak in 1951 with 37,812,000 pounds of seed used in the United States. Since 1960, domestic use of seed has declined from an annual disappearance of 16,724,000 pounds to 10,116,000 pounds in 1970. Several factors contribute to this decline: ( a ) a sudden increase in seed losses in the mid 1950’s from clover seed weevils, ( b ) more than 60% of the crimson clover acreage was in reseeding cultivars that did not require annual reseeding, thus reducing demand for seed, (c) a decline in price of seed as seed production moved to the West and peracre yields of seed increased, and ( d ) an emphasis during the 1960’s on high per-acre yields of grass forage produced with mineral nitrogen. Since 1965, considerable emphasis has been placed on arrowleaf clover. This has resulted in a shift in acreage formerly in crimson clover to arrowleaf clover. Unless some of the hazards involved in the production of arrowleaf clover are overcome, crimson clover will continue to be the reliable standby in the winter-grazing program in the South (Kight and Wellhausen, 1968). Crimson clover has several advantages over arrowleaf, Trifolium vesiculosum Savi. (Beaty and Powell, 1969; Hoveland et al., 1569; Knight

50


et al., 1969). Crimson is ( a ) easier to establish, ( b ) it is easier to get effectively inoculated, (c) it will make more fall and winter forage if planted at the optimum time, (d) it reseeds more reliably under use, and (c) the seed is less expensive (Knight, 197 1 a,b) . II.

A.

Morphology

ROOT, STEM,

AND

LEAF

Crimson clover, T . incurnuturn L., has a central taproot, supported by many fibrous roots. Root development is influenced to a great extent by soil moisture and tilth. Under favorable soil-moisture conditions, seedlings make rapid growth, forming a dense crown or rosette type of leaf development. The leaves and stems resemble those of red clover, but are distinguished by the rounded tips of the leaves. The soft pubescent lower and median leaves usually have long petioles with cuneate-obovate emarginate leaflets. The leaflets of common crimson clover are essentially sessile. When crimson clover is inbred, considerable variation in leaf and stem morphology is observed (Knight, 1969b). Size, shape, and pubescence of stems and leaves vary greatly among different genotypes. Multifoliolate leaves, glabrous leaves, and petiolulate leaflet attachment were found to be characters recessive to trifoliolate leaves, pubescence, and sessile leaflet attachment (Knight, 1969b). Favilli (1952-1953) also reported a glabrous form of crimson clover. The number of stems depends greatly on stand density. In thin stands, plants tend to compensate by producing a larger rosette and more stems (Knight and Hollowell, 1959; Knight, 1967). Stem and petiole elongation are directly related to stand density (Knight and Hollowell, 1959). Earlier growth is produced from dense stands than from thin stands. B.

FLOWER

When daylength is more than 12 hours, erect hairy flower stems elongate, with many nodes and leaves. Growth is terminated by the formation of a pointed, conical flower head composed of 75-125 florets (Fig. 1 ) . The corolla is usually scarlet or deep red and extends beyond the calyx. The florets open in succession from the bottom to the top of the head. Reproductive parts of the flower consist of ten stamens and a simple pistil. One stamen is more or less separate, while the other nine stamens have fused filaments that form a tube surrounding the ovary. The legume, or pod, is included in the calyx and is usually one-seeded. The stigma extends

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FIG. 1. Flower head and leaflets of crimson clover.

beyond the stamens and is held under tension by the keel. The pollen is brought into contact with the stigma when the flower is tripped.

C . POLLINATION AND SEEDDEVELOPMENT Generally, crimson clover flowers are self-fertile, but not self-pollinating (Pieters and Hollowell, 1937). Bees in search of nectar, pollen, or both, trip the flowers and bring about pollination (Amos, 1951). The flowers produce considerable nectar available to all kinds of bees (Anonymous, 1971; Hollowell, 195 1; Hollowell and Knight, 1962). After pollination, fertilization takes place in about 18 hours, at which time the corolla wilts. The seed matures in about 24 to 30 days, and the plant dies.

52

W.

E. KNIGHT AND E.

A. HOLLOWELL

Although self-fertility is the general rule for crimson clover, self-sterile, or self-incompatible, plants were reported by James (1949) and were found in genetic studies in Mississippi (Knight, 1969a,b). Crimson clover is a highly cross-pollinated crop. James (1949) grew red and white-flowered crimson clover in close proximity and obtained 68.4% outcrossing. Similar results were obtained by Rogers ( 195 1 ) . White-flowered plants were used by Knight (1969b) to determine the effectiveness of hand pollination without emasculation. When red-flowered plants were crossed with white-flowered plants, cross-pollination varied from 54 to 86%, with an average of 75 % cross-pollination. Ill.

A.

Physiology

GROWTHAND DEVELOPMENT 1. Time of Seeding

Earliest growth of crimson clover is produced by planting annually on a well-prepared seedbed (Stewart and Boseck, 1947). If fall and winter growth is desired, the clover must be planted sufficiently early for strong plants to develop before the advent of cold weather. Naftel (1950) considered 6 weeks prior to the average date of first frost as the optimum planting time. At State College, Mississippi, crimson clover planted August 15 produced highest yields over a 6 year-period. Planting delayed until November 15 produced only 25 % as much dry forage as planting August 15. Although July planting has given early fall grazing at some locations, stand failures frequently result from severe virus infections. Moisture was the primary limiting factor to stand establishment and growth through November 1, when temperature became the critical element until about February 15. 2. Rate of Seeding

Knight and Hollowell (1959) found a close relationship between stand density and early growth (Fig. 2 ) . Crimson clover in dense stands produced earlier fall and winter growth and greater forage yields than clover in thin stands. In general, minimum soil temperature readings were higher and maximum temperature readings were lower under dense stands than under thin stands. Donnelly and Cope (1961) reported increased yields and earlier growth of crimson clover as seeding rate was increased from 10 to 30 pounds per acre. The seeding rate for crimson clover depends on the condition of the seedbed, the purpose for which the clover is grown, and the equipment used in seeding.

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53

Fro. 2. Comparative growth of crimson clover in spacings of % : inch (left) and 6 inches (right) on November 27, 1956 at the Mississippi Agricultural and Forestry Experiment Station.

3. Defoliation Management studies on crimson clover indicate that thick stands grow more rapidly from the start than thin stands, but do not necessarily produce highest seed yields (Knight, 1967; Knight and Hollowell, 1959; Rampton, 1969). Mowing is practiced in Oregon to remove excess growth and reduce lodging (Rampton, 1969). In Mississippi and in Oregon, early mowing had little influence on seed yields, but late mowing reduced plant recovery and seed yields (Knight and Hollowell, 1962; Rampton, 1969). Rampton ( 1969) found that mowing crimson clover decreased lodging, delayed flowering, and reduced the bulk of plant material for threshing. Mowing reduced seed size and increased the percentage of hard seed (Knight and Hollowell, 1962; Rampton, 1969). Knight and Hollowell (1962) concluded that crimson clover forage can be grazed until April without reducing total forage appreciably, and that regrowth will produce an adequate supply of seed to establish a volunteer stand. Donnelly and Cope (1961) recommended removal of grazing animals by April 1 in southern Alabama and April 15 in northern Alabama to allow reseeding. Overgrazing can prevent reseeding, because cattle will eat the seedheads if the stocking rate

54


is excessive. Crowder et al. (1955) determined the effect of clipping intensity on a mixture of Arlington oats, ryegrass, and crimson clover. Dry matter yields were greatest with an 8-week interval between clippings compared to yields with 2- and 4-week intervals. Mowing or grazing during the winter months reduced the incidence of crown rot, Sclerotinia trifoliorum Eriks (Knight and Hollowell, 1959; Knight, 1959). 4 . Moisture and Temperature

Crimson clover does not withstand either extreme cold or extreme heat. Its culture is therefore limited to regions with a long period of relatively mild, moist weather (Kephart, 1920). Reports from South Carolina indicate that crimson clover is killed by 10°F (Buie, 1929). However, preconditioning of the plants by cold weather usually prevents serious winter injury to crimson clover in the Southeast. In Alabama, Frontier crimson clover, the least winter hardy variety, survived temperatures of OOF during two winters (Hoveland et al., 1964). After seedlings become well established, crimson clover makes more growth at lower temperatures than most other clover species (Hollowell and Knight, 1962). B.

FLOWERING

1. Photoperiod and Temperature Photoperiodism, conditioned by temperature, occurs in crimson clover. Differences in maturity of crimson clover genotypes have been used to distinguish cultivars described as “early” and “late” (Westgate, 1913, 1914; Kephart, 1920). The vernalization requirement for several winter annual legumes was demonstrated by McKee ( 1935b). Moistened crimson clover seed, kept for 40 days at OOC, came into flower when subsequently planted in the greenhouse, while plants from untreated seed remained vegetative. Von Gliemeroth (1943) studied the effect of germination temperature and length of day on the development of crimson clover. He found that low germination temperature accelerated plant development, shortened the vegetative phase, caused earlier flowering and maturity, and accelerated formation of generative organs. In the same study, short daylength caused a markedly prolonged vegetative phase and intensive branching, which resulted in bushy plants. Crimson clover flower stems usually elongate when the length of day exceeds 12 hours (Hollowell, 1951; Hollowell and Knight, 1962). However, date of planting experiments conducted in the field at State College, Mississippi indicate that vernalization is required before stem elongation and flowering will occur. Crimson clover planted April 1, May 1 , and June 1 flowered in April of the following

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55

year. In greenhouse studies conducted by Knight and Hollowell (1958), date of flowering in crimson clover was greatly affected by temperature. Earliest flowering occurred when plants 6 weeks old were shifted from outside cold frames into a greenhouse with relatively high temperature. High night temperatures from germination to maturity inhibited flower production. Crimson clover flowered earlier as length of photoperiod increased. Long daylength imposed early in the life cycle of the plant produced smaller seed heads, fewer leaves, less branching, and greater height than did shorter photoperiods. A 30-minute break in the dark period was not effective in inducing flowering in crimson clover. 2. Defoliation

Rampton (1969) found that mowing crimson clover in the spring delayed flowering until favorable pollination conditions prevailed. Heavy defoliation late in the spring reduced head size and number of viable seeds per head and resulted in smaller seed with a higher percentage of hard seed (Knight and Hollowell, 1962; Rampton, 1969). Schmidt (1921) demonstrated the importance of seed size to rapid germination and growth. Excessive defoliation could cause weakened stands. C.

SEED

1, Germination The germination requirements of the seeds of many crop plants are critical in relation to moisture, aeration, light, and the interactions of these factors. Crimson clover has been considered an exception to most of these restrictions on germination, since the seeds of this winter annual crop produce successive volunteer stands through the summer whenever moisture is adequate to induce germination. As a rule, fresh crimson clover seed is of good viability, and failure to obtain a stand is not often caused by failure of the seed to germinate (Kephart, 1920). A germination of 90% in 48 hours is not uncommon (Smith, 1928). Toole and Hollowell (1939) reported that most seed of crimson clover will germinate when planted at any time during the summer. They found no significant difference in germination of crimson clover seed from 5 to 35OC. At 35"C, germination was slower than at 25OC and below. Fayemi (1957) found low seed viability in crimson clover seed at 6.7OC. In Alabama, Hoveland and Elkins ( 1965) obtained excellent germination of crimson clover at a constant temperature of 70°F, but under a temperature regime of 40°F for 8 hours and 70°F for 16 hours, germination of crimson clover was 42%. These results, which do not agree with those of Toole

56


and Hollowell (1939) and Knight (1965), suggest that seeds with low viability or pathogens may have been involved. Vaughn (1961) found a close relationship between rate of swelling and viability. Dead seeds and seeds with low viability swell quickly. Since crimson clover seeds germinate quickly, the need for adequate soil moisture at time of seeding is critical (Stitt, 1944). Seeding either immediately preceding or soon after a heavy rain increases the chance of SUCcessful stand establishment (McKee, 1935a). 2 . Impermeable Seedcoat

Before 1938, most crimson clover was of the common type with low levels of hard seed. Seeds of common crimson clover germinate rapidly after planting. Often, there may be enough moisture for germination, but not sufficient to keep the seedlings growing. The result is that seedlings die, and the stand is lost, Hard-seeded crimson clover cultivars were developed to avoid excessive early germination and to assure self-reseeding stands in the fall (Bennett, 1959; Hollowell, 1946). Elrod (1960) and Knight et al. (1964) found that, once high levels of hard seed had been attained by genetic selection, hardseededness persisted in the environment of the South. In California, Williams and Elliott (1960) found that seed coat impermeability of crimson clover declined rapidly during summer months after seed maturation, while rose clover, Trifolium hirtum All., maintained high levels of impermeable seed. The hard-seed content of varieties may vary from 30 to 75%. Apparently, hard-seed content is affected by environmental conditions while the seed is maturing. James (1949) concluded that impermeability of the seed was not inherited, unless the possible heritable factors were masked by environmental effects. Crimson clover seeds are easily scarified (Hollowell and Knight, 1962). Seeds of reseeding varieties frequently are scarified during harvesting and processing. Combine-harvested seed may contain less than 5 % hard seed when planted. 3 . Dormancy

Embryo dormancy may be defined as failure of fully imbibed and viable seed to germinate (Morley, 1961). Highly dormant seeds may not germinate in soil at high temperature even when moisture is adequate. They will remain viable despite several cycles of wetting and drying. Dormancy is considered an important ecological adaptation serving to diminish seed losses in some species. Knight (1965) reported dormancy induced by high temperature in crimson clover inbred lines. Incorporation of high temperature dormancy and

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57

hard seed into the same variety should reduce the hazard of stand losses in the summer when moisture is adequate but temperature is unfavorable for stand survival. 4 . Longevity

Seed longevity in crimson clover is greatly affected by harvest and processing conditions, moisture in the seed at harvest and storage, and by storage conditions (Ching et al., 1959a,b; Helmer et al., 1962; Lim, 1963). High quality seed stored under favorable conditions will retain viability for an indefinite time, whereas seed stored under ordinary warehouse conditions should not be used after two years (Ching, 1961, 1972).

IV.

A.

Culture

ADAPTATION

Crimson clover does well in the cool, humid weather that occurs in most of the South in winter (Hollowell, 1947). In the northernmost part of the region where crimson clover is grown as a winter annual, it is important to seed the crop not later than late in August (Fergus et al., 1938). When planted later in such areas as Kentucky, southern Missouri, and southern Ohio, the plants may not become well enough established to survive the winter. It is also important in these northerly areas to plant the crop in fertile soil and to grow adapted varieties.

B.

SOILS

AND

SOIL FERTILITY

Crimson clover thrives on both sandy and clay soils and is tolerant of medium soil acidity (Hollowell and Knight, 1962; Donnelly and Cope, 1961). It grows best on well-drained, fertile soils. Low or wet soils that are subject to overflow or soils with p o x internal drainage are not suited for this clover. Crimson clover will not grow on the calcareous soils or high-lime soils of the Black Belt of Mississippi and Alabama because of iron deficiency (Rogers, 1947). Crimson clovcr will produce satkfactory yields of forage on soils of medium fertility. However, on most soils, it is necessary to apply fertilizers before seeding and to make annual applications after reseeding cultivars have been established. Phosphate and potash fertilizers are the most important (Kephart, 1920; Naftel, 1942). Fertilizers containing nitrogen are not needed if plants are inoculated with an effective strain of nitrogen fixing bacteria. Where crimson clover is grown with permanent-grass sods which have received heavy nitrogen applications, attention must be given to pH

58


and potassium levels, particularly if hay crops are removed (Adams and Stelly, 1958; Adams and McCreery, 1959). Most of the soils on which crimson clover is grown are acid and need lime for satisfactory production (Moser, 1941; Naftel, 1942; Davis, 1949; Page and Paden, 1949; Stewart and Pearson, 1952; Donnelly and Cope, 1961). Although tolerant of more acidity than some other legumes, such as alfalfa, sweet clover, caley peas, and white clover, crimson responds to moderate lime applications on most soils having a pH of less than 5.7 (Fig. 3 ) . A soil test is the best method for determining the amount of lime needed. Generally, if the soil pH is less than 5.7, lime is needed and should be applied at the rate of about 1 ton per acre on sandy soils and 2 tons on heavy-textured soils (Adams, 1958). James and Bancroft (1951 ) used half-plants of crimson clover to demonstrate the need for calcium in the production of hard seed.

FIG. 3. Lime is needed on many soils for top crimson clover production. This photo, made May 7, 1959, in Tallassee County, Alabama, shows effect of lime. Area in the right background had not been limed since clover establishment in 1947. Area at left and in foreground received 2 tons of lime per acre in fall of 1958 (Donnelly and Cope, 1961).

It is commonly recognized that most legumes have a relatively high requirement for boron. In North Carolina and Alabama, the addition of borax gave increases of both hay and seed yields (Piland et al., 1944; Wear, 1957). Borax increased seed yield up to 529% over no borax on

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North Carolina soils. Response to borax depends on soil type (Hendricks, 1941; Davis, 1947-1948; Page and Paden, 1949; Wear, 1957). Experiments on sandy soils in Alabama showed large increases in seed yields from 10 pounds of borax. No response was obtained on two clay soils. Borax at 10 pounds per acre is commonly recommended for seed production, and this recommendation applies equally where reseeding is desired.

C. INOCULATION The importance of inoculation with effective Rhizobium was demonstrated in early work conducted at the Alabama Agricultural Experiment Station (Duggar, 1897, 1898, 1909, 1934). In these studies, hay yields were 0 and 761 pounds per acre on uninoculated soils and 4057 and 6100 pounds per acre on inoculated soils. Inoculum used in early experiments was imported from Germany. Later, soil was used from areas on which clover had been successfully grown. Scattering soil from a field where inoculated crimson clover has been grown is no longer recommended, because it may cause weed infestation and spread disease. Specific commercial Rhizobium cultures are available (Erdman, 1946; Burton and Allan, 1950). The nitrogen-fixing bacteria are dispersed in a carrier, usually peat soil. Inoculation is more effective if the seeds are moistened with a solution containing sugar, corn syrup, or molasses. The use of syrup or molasses sticks more of the culture to the seed and helps keep the bacteria alive in the soil for as long as 2-3 weeks (Erdman, 1959). While nitrogen-fixing bacteria are extensively distributed in agricultural soils, the effective strain may become diminished in soil where clover is not grown for several years. For this reason, proper seed inoculation is essential when planting crimson clover on new land or where clovers have not been recently grown (Donnelly and Cope, 1961). The small cost is repaid many times over in earlier and greater growth (Fig. 4 ) . D.

ESTABLISHMENT

Earliest growth of crimson clover is produced by planting annually on a firm but well-prepared seedbcd (Patterson et al., 1959; Donnelly and Cope, 1961; Hollowell, 1947). If the seedbed is loose, roots grow into air pockets between soil particles, dry out, and die. Best results with new plantings result when land is plowcd or disked 6 to 8 weeks before planting and fallowed (Donnelly and Copc, 1961). This controls weeds and conserves moisture for germinating the seed and maintaining the seedlings during fall droughts.

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W . E. KNIGHT AND E. A. HOLLOWELL

FIG.4. Crimson clover must be properly inoculated for healthy, vigorous growth. Plot at right was inoculated, that at left uninoculated. Photo was made January 3, 1952, at the Alabama Experiment Station Plant Breeding Unit, Tallassee, Alabama (Donnelly and Cope, 1961).

Seed can be planted with a cultipacker seeder, grain drill or broadcast seeder. About one-fourth inch is the correct depth (Moore, 1943). Ten pounds of seed per acre is sufficient if seedbed conditions are favorable and percentage germination is high. However, if grazing is desired, 20 to 30 pounds of seed per acre will provide earlier forage and grazing (Knight, 1959, 1967). To obtain reseeding stands in bermudagrass or other warm-season grasses, close grazing or mowing late in summer is necessary (Hollowell, 1947; Knight, 1967; Hoveland et al., 1971). If stands are mowed with a sicklebar mower, heavy grass residues should be removed. Warm-season grasses offer serious competition to young clover seedlings for light, moisture, and plant nutrients. Earliness of grazing is affected by the amount of such competition. Light disking before frost is often beneficial in reducing grass competition and in getting an early stand. However, Knight (1967) did not find disking a Coastal bermudagrass sod to be beneficial, provided the summer grass residue was removed. Crimson clover can be successfully introduced into dense grass sods in the establishment year by sod seeding (Coats, 1957). This method may be advantageous in some farming systems. It would, however, be more expensive than surface seeding after close clipping or burning to remove the excess grass, since clipping is also recommended before sod seeding.

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E. COMPANION GRASSESAND CROP SEQUENCES Increased yields and a longer grazing season can be obtained by planting crimson clover in mixtures with adapted winter-annual grasses. Bloat is less common in cattle on crimson than on white clover. The incidence of bloat can be greatly reduced by planting grass with crimson clover (Donnelly and Cope, 1961). When companion crops, such as rye, vetch, ryegrass, and fall-sown grains are seeded with crimson clover, the clover usually is seeded at two-thirds the normal rate and the companion crop at one-third to one-half the normal rate (Hollowell, 1947). Annual ryegrass and rescuegrass seedlings develop at about the same rate as crimson clover. Rye grows more rapidly in the fall than wheat, oats, or the annual grasses. Therefore, mixtures of rye, ryegrass, and crimson clover provide the longest grazing season (Patterson et al., 1959; Donnelly and Cope, 1961). Results of grazing studies in Mississippi indicate that crimson clover makes a profitable combination with ryegrass, oats, or wheat for beef production (Blount and Ashley, 1952; Gill and Coats, 1952, 1955, 1956). In 1952, winter pastures of ryegrass and crimson clover at Mississippi’s Brown Loam Branch Station produced the heaviest yield of beef and returned the greatest profit (Gill and Coats, 1952). Crimson clover has been seeded with excellent results on established stands of bermudagrass, dallisgrass, johnsongrass, and bahiagrass (Hollowell and Knight, 1962). In the lower South, one of the most productive combinations which approaches all-year grazing, is crimson clover and Coastal bermudagrass (Stephens and Hollowell, 1942; Preston, 1949). This mixture requires a minimum expense for maintenance compared to annually seeded forage crops and has been used successfully through 20 years (Kight and Wellhausen, 1968). Adams and Stelly (1962) estimated that 60% of the acreage of Coastal bermudagrass in the Piedmont of Alabama, Georgia, and South Carolina is seeded to crimson clover (Fig. 5 ) . In Mississippi, total forage production was 47% higher from the sequence of reseeding crimson clover-Coastal bermudagrass than from pure stands of Coastal bermudagrass fertilized with 200 pounds nitrogen per acre (Knight, 1970). This clover-grass sequence extended the grazing season 8-1 2 weeks, provided higher quality forage, and increased total production with minimum competitive effects between the legume and grass species. A legume mixture that often gives excellent results is 5 pounds of red clover and 10 pounds of crimson clover per acre (Hollowell, 1947). The crimson clover is usually predominant in the winter and spring, while the red clover continues growing in the summer after crimson clover dies. Crimson clover has been successfully grown with sericea lespedeza and to a lesser degree with kudzu (Hollowell and Knight, 1962). In Alabama,

62


FIG. 5. A good crop of crimson clover in Coastal bermudagrass. Crimson clover will provide good winter grazing, followed by a good yield of seed, and fits well into grazing sequences with other crops.

crimson clover and sericea provided more grazing over a 2-year period than any other combination of grasses and legumes (Stewart, 1948; Brackeen, 1948). This mixture provided grazing for 11 months of the year.

F. WEED CONTROL Crimson clover seed yields are frequently increased when weeds, including volunteer crop plants, are controlled. Seed quality is also improved by controlling weeds whose seed are difficult to remove from crimson clover seeds. Some weeds such as dock and sorrel (Rumex spp.) and wild onion ( A l lium spp.), whose seed are difficult to separate from crimson clover seed, cannot be selectively controlled in crimson clover. Thus, seed production should not be attempted in fields infested with these weeds (Anonymous, 1971). Winter-annual grasses, henbit (Lamium amplexicaule) , chickweed (Stellaria spp.), and volunteer small grains can be controlled by using 4-5 pounds per acre of isopropyl carbanilate (propham). Propham should be applied while the weeds are very young and small, but it should not be applied until the crimson clover has at least three leaves. In the Southern States, application of isopropyl m-chlorocarbanilate (chlorpropham) at 4 pounds per acre in granular form, when the crimson clover has at least four leaves, will control these same weeds. It, like propham, will not con-

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trol the weeds if they are much beyond the early stages of emergence when treated. In the Pacific Northwest, grasses arising from seed can be controlled in crimson clover seed fields by incorporating 3-4 pounds per acre of S-ethyl dipropylthiocarbamate (EPTC) in the soil before planting. Many broadleaf weeds, such as wild geranium (Geranium spp.), pepper-weeds (Lepidium spp. ), and plants of the genus Brassica (mustards, rape, turnips), can be controlled with [ (4-chloro-o-tolyl) oxylacetic acid (MCPA). MCPA should be applied at 0.10 to 0.13 pound per acre early in spring while both weeds and crimson clover are small. If treatment is delayed until the clover begins rapid upright growth, the weeds will not be controlled, and the crimson clover will be injured.

G.

DISEASES

Although crimson clover is attacked by several diseases, no one disease consistently causes great damage. The most widespread and serious disease is crown and stem rot, caused by Sclerotinia trifoliorum Eriks. This disease attacks during cool, wet weather (Wolf and 'Cromwell, 1919). Grazing during fall and winter destroys most of the initial infection and reduces subsequent spread of the disease (Knight and Hollowell, 1959; Knight, 1959). Crimson clover is highly susceptible to sooty blotch, Cymadothea trifolii Wolf; leaf and stem spot, Cercospora zebrina Pass.; and to viruses. Sooty blotch is a leaf-spot disease most in evidence at blooming. No great loss will occur if affected areas are mowed or grazed before severe leaf damage occurs.

H.

INSECTS

Several species of insects are destructive to crimson :lover in the seedling stage, but effective control measures are known for them (Donnelly and Cope, 1961) . The insects that normally cause the most injury in young clover are the fall armyworm, several cutworms, yellow-striped armyworm, the Hawaiian beet webworm, and several bean beetles. Two species of insects, the clover head weevil, Hypera meles (Fab.), and the lesser clover weevil, Hypera nigrirostris (Fab. ) , are responsible for seed losses and a reduction in the crimson clover acreage harvested for seed in the Southeast (Bass and Hays, 1961). The principal damage is caused by larvae feeding on the flowers, ovules, and growing seeds (Thomas and Parker, 1967; Tippens, 1958). After feeding, the larvae spin lacy cocoons attached to the clover head and pupate there until emergence

64


as adults (Stanley et al., 1970). Individual egg, larva, and pupa development times vary from 8 to 13, 10 to 20, and 5 to 10 days, respectively (Thomas and Parker, 1967; Machado, 1964). The first reports of effective chemical control of H. meles came from Georgia, where applications of granular insecticides gave excellent control of the clover head weevil and increased seed yields 67 to 112% in 1957 and 1958, respectively (Beckham, 1956; Tippens, 1958). In Alabama, good control of seed weevils was reported when granular insecticides were applied. Best seed-yield increases resulted from application of insecticides at the prebloom stage of growth (Reed et al., 1962; Donnelly and Cope, 1961; Hays, 1964). When infestation by seed weevils is severe, reseeding varieties may fail to produce sufficient seed for volunteer stands the following fall. I.

SEEDPRODUCTION

One of the reasons why crimson clover is so important as a winter annual legume in the South is its capacity to produce an abundance of seed even under relatively adverse conditions ( Hollowell, 1951; Hollowell and Knight, 1962). The tripping of the florets is essential for pollination and seed setting. Placing colonies of honeybees in or near to blooming fields is highly recommended for maximum seed production (Amos, 1951 ; Knight and Green, 1957; Blake, 1958). Two colonies of bees per acre are recommended. If the clover is well fertilized, soil moisture is adequate, and cattle are removed early to allow good growth, three colonies per acre will prove profitable. If the clover is not to be harvested for seed but is expected to reseed itself in the fall, bees are still needed to produce enough seed for the volunteer crop. With good stands and pollination, the seed set may range from 1000 to 1200 pounds per acre (Hollowell and Knight, 1962). The average harvested seed yield is about 250 pounds per acre. Serious seed losses often occur in harvesting. Timeliness in harvesting is important, as the seed shatters readily when mature. When seed is harvested from standing plants, the crop must be fully mature for best results. A greater risk is taken when this method is used, since one heavy rain or windstorm may cause extensive shattering losses (Hollowell, 1947). If the clover is to be picked up from the swath or windrow, the crop usually is cut when about three-fourths of the seed pods have turned a golden brown (Fig. 6 ) . Shattering losses may be minimized by cutting when the plants are damp with dew. The less the seed crop is handled when dry, the less the seed loss from shattering. Regardless of the harvesting method, the seed should be rough cleaned and dried as

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65

FIG.6. A combine with pickup attachment threshing a crimson clover seed crop that dried in the swath.

quickly as possible to prevent heating and browning, which usually reduces germination (Donnelly and Cope, 196 1) . V.

Utilization

A.

PASTURE

Crimson clover has been used for years to extend the grazing season, increase total forage production, improve forage quality, and make better use of land resources. The value of crimson clover as a grazing crop was recognized early in its use in the United States (Voorhees, 1894; Duggar,

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1909; Grantham, 1911; Westgate, 1914). An expanding livestock economy in the South and the development of reseeding crimson clover cultivars caused a rapid increase in the use of crimson clover for grazing in the 1940’s and 1950’s (Lowery, 1939, 1943; Hendricks, 1941; Lowery and Harbor, 1945; Stewart and Boseck, 1947; LaMaster, 1950; Holt el al., 1951;Langford, 1957). Animals grazing on crimson clover seldom bloat (Henson and Hollowell, 1960). However, animals that are hungry should not be placed in a fast growing field of clover. Bloat is less likely to occur on a mixture of clover and grass or grain than on clover alone. This legume will provide good winter grazing and later a seed crop (Hollowell, 1951; Hollowell and Knight, 1962). This profitable combination fits well into grazing sequences with other crops such as bermudagrass and bahiagrass (Knight and Hollowell, 1962). During winter, carrying capacity is considerably less than during the spring months when growth is most rapid. Four to six weeks before flowering, livestock can be removed from the clover, permitting the clover to make seed. B.

HAY AND SILAGE

The quality of crimson clover hay is highest if the crop is cut for hay at the early-bloom stage ( Hollowell, 1947). Voorhees ( 1894) concluded that crimson clover hay was superior to red clover, and Emery and Kilgore (1894) found it to be highly digestible and suitable as a feed in association with concentrates. Crimson clover is not often cut for hay, since it reaches hay stage in the spring during periods of frequent rains. Crimson clover cures slowly because of its high moisture content and the wet weather during the season of the year when it is harvested. A good stand will yield from one to 2.5 tons of dry hay per acre, depending on growth conditions and intensity of grazing. Mixtures with small grains produce higher yields and are less likely to lodge. Crimson clover in pure stands or in mixtures may be made into silage by methods used for other legumes. Since frequent rains may occur during hay harvest, this method of feed preservation is less hazardous. C.

GREENMANURE

In Alabama, Duggar (1897) recognized the potential value of crimson clover as a soil-improving crop and considered it the most important plant for improvement of cotton soils. A good growth of green manure will produce as much corn as 60-90 pounds of commercial nitrogen (Cope, 1955).

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67

With the development of relatively inexpensive sources of mineral nitrogen, the use of winter legumes for soil improvement has declined. However, crimson clover remains an important crop for use in many rotations. In pecan, peach, tung, and other orchards (Bregger, 1951; Donnelly and Cope, 1961; Hollowell and Knight, 1962), it may be allowed to reseed and does not have to be incorporated to make the nitrogen available to the trees (Fig. 7 ) . This clover was regarded more for its ornamental value than as a forage crop before its value for agricultural purposes was appreciated (Kephart, 1920). In, recent years, it has been used extensively for roadside stabilization and beautification throughout the southeastern United States. Frequently, bermudagrass or bahiagrass is grown in succession with the clover and receives 100-200 pounds of N per acre from the clover residue (Erdman, 1959; Knight, 1970).

FIG. 7. Crimson clover is well adapted for use in orchards for grazing and green manure. Here CHIEF crimson clover is shown growing in a 1-year-old tung orchard.

D.

SEED

Production of good seed yields is another reason for the importance of crimson clover. It is an important seed crop in Oregon and in the South (Donnelly and Cope, 1961; Rampton, 1969). Seed may be mechanically harvested in three ways: ( 1 ) Combined direct from standing plants; (2) cut with a mower and left in the swath or windrow to dry, then picked up and threshed with the combine; and ( 3 ) cut with a mower and left in the swath or windrow to dry, then hauled to a stationary huller or thresher.

68

W.

E. KNIGHT AND E. A. HOLLOWELL

Letting the crop dry in a swath or windrow permits earlier cutting, which reduces harvest shattering losses. Swathing or windrowing ,also reduces the risk of having seed shattered by strong winds or rain. Cutting and windrowing when the heads are damp and tough keeps shattering at a minimum. Because heads must be dry for direct combining, there is considerable shattering loss with this method of harvesting. Harvest when most of the hulls are light brown if the seed is stripped or cut with a mower. Wait until the hulls are dark brown for direct combining. Drying harvested seed usually is necessary in humid areas to lower moisture content to a safe level for storing. Drying may be done with hot-air driers, or seed may be thinly spread under shelter and frequently turned until dry enough to store. Remove trash and weed seed as soon as possible after harvest. Preharvest defoliation has not been very successful in humid areas. It is successful if the weather is dry at harvest time.

VI.

Genetics and Cytology

A. CYTOLOGY Crimson clover is a diploid annual with a generally accepted somatic chromosome complement of 2n = 14 (Britten, 1963; Favilli, 1952-1953; Hollowell and Knight, 1962; Pritchard, 1969; Wexelsen, 1928). Meiosis is regular, and the chromosomes pair as bivalents. None of the recent studies of the cytological behavior of crimson clover have agreed with Bleier ( 1925), who reported N = 8.

B. INHERITANCE OF CHARACTERS DeCillis ( 1914) reported success in selecting for various characteristics in crimson clover. The first efforts in crimson clover improvement in the United States were directed toward incorporating the hard-seed character into existing strains (Hollowell, 1946; Bennett, 1958, 1959; James, 1949; Rogers, 195 1 ) . This was successfully ' accomplished through the use of natural- and mass-selection techniques. James ( 1949) believed that the impermeability of crimson clover seed was not inherited, unless the possible heritable factors were masked by environmental factors. However, in Mississippi, efforts to isolate lines with a high percentage of hard seed were successful and indicated good heritability of this character (Bennett, 1958, 1959). Rogers (1951) also found selection for increasing hard seed percentages in crimson clover seed to be effective.

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Sandal (1955) described the inheritance of white flower color as a simply inherited recessive characteristic. He suggested the symbols Cr, cr for the alleles controlling flower color, the dominant gene being necessary for red flower color. Picard (1956) described the inheritance of several simply inherited mutant forms in crimson clover. He described variations between varieties and plants, but he did not report the inheritance of any of these characteristics. Inheritance of a male-sterile character was studied in the F,, F,, and F, by Knight (1969a). Sterility in this mutant was caused by the absence of anthers and was associated with multiple ovaries and absence of petals. Apparently, these characteristics are controlled by a single recessive gene pair with pleiotropic effects or closely linked genes. Inheritance of leaflet characteristics have also been studied in crimson clover (Picard, 1956, 1959; Knight, 1969b). Picard (1956) found albinism, absence of chlorophyll, and variations in flower colors. He suggested that a simply inherited two-unifoliolate-leaf character might be used as a genetic marker in seedling plants. Knight (1969a) determined that multifoliolate leaf, pubescent leaf, and petiolulate leaflet attachment were each determined by a simple recessive gene in the homozygous condition. In other studies, Knight and Lee (1971 ) found that a variegated flower color was controlled by two dominant genes; in the absence of either dominant gene, the flowers are white. A mutant sticky-leaf character was found to be controlled by a double recessive gene pair (Lee, 1969). Inheritance studies of crimson, deep pink, medium-pink, light-pink, lavender, and maroon flower colors indicated that crimson, deep- and medium-pink flower colors are under monogenic control (Sullivan, 1971; Sullivan et al., 1972). The remaining flower color mutants are under digenic control. Anthocyanins in color mutants of crimson clover were extracted and identified (Sullivan et al., 1972). All color mutants contained 3-glucoside and cyanidin 3-glucoside. The distinction between crimson and the varying pink forms was found to be caused by differences in concentration. Maroon flowers contained two additional pigments, cyanidin 3-sambubioside and an unidentified cyanadin 3-glucoside.

VII.

A.

Breeding

OBJECTIVES

Crimson clover breeding programs in the United States have been concerned with improvement and development of varieties to increase forage yields and reseeding ability. In addition to hard seed, breeding objectives

70

W. E.

KNIGHT AND E. A. HOLLOWELL

have involved seedling vigor, earlier fall growth, winterhardiness, resistance to seed shattering, and resistance to lodging. In spite of serious annual losses to insects and diseases, very little has been done to develop insectand disease-resistant varieties. B.

VARIABILITY

Crimson clover is less variable than most Trifolium species. Since this species is generally self-fertile, it is easy to inbreed and select for various characters within and among inbred lines. Upon inbreeding, wide differences are found for vigor and many plant characteristics. Vigor in some lines is reduced by inbreeding, until maintenance of the line is impractical. On the other hand, other plants lose very little vigor, and inbred lines can be maintained easily for a number of generations. Rogers (1951) suggested using these vigorous lines in a breeding and hybridization program.

C. SEEDSHATTERING Losses of crimson clover seed are severe when storms occur after the seed crop is ripe. In Mississippi, recurrent selection has been effective in obtaining genotypes with better seed retention and resistance to lodging.

D. SEEDLING VIGOR Seed size and seedling vigor are closely related in crimson clover. In 1962, a large-seeded crimson clover variety was released (Knight, 1963). In FRONTIER crimson clover, seedling vigor and early growth were associated with seed size, an indication that further improvement could be made for this characteristic by selecting large seed. At seven Alabama locations, the large-seeded FRONTIER variety exceeded the AUTAUGA variety by an average of 40% more dry forage in the fall and winter (Hoveland et al., 1964).

E. INBREEDING AND HYBRIDIZATION Crimson clover is easily inbred. Since the florets require tripping for pollination and seed set, the seedheads may be bagged in small cloth bags or the plants can be grown in an insect-free environment. Rolling the heads between the fingers effectively trips the florets. Seedheads should be rolled every few days as long as fresh florets are present on the head. Selection for general combining ability via the polycross method within

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71

and among selfed lines is effective in isolating superior lines for use in single and double-cross hybrids. Single- and double-cross hybrids can be made under saran-cloth bee cages or in isolated field crossing blocks. Lines chosen for insect or disease resistance could be effectively recombined by this method. Inbred lines selected for forage yield and combined in double-cross combinations have been equal to standard cultivars and in some cases superior in forage yield.

F. CULTIVARS The greatest differences between existing cultivars are time of maturity, percentage hard seed, and early fall growth. Before World War 11, all crimson clover was of the common type. More than half of the seed produced now is of the reseeding type. The term “reseeding” designates a type that produces good volunteer stands in the fall from seed shattered the previous spring. Fall volunteer stands are made possible by a hard seedcoat that delays germination from late spring, when the seed shatters, until fall. Common crimson clover is not a reseeding type. Five named varieties of the reseeding type are widely used. These are: DIXIE, AUBURN, AUTAUGA, CHIEF,and TALLADEGA. There are other reseeding strains less widely used. DIXIE,AUBURN, and AuTAUGA are early varieties-their seed matures about a week earlier than seed of CHIEF and TALLADEGA. They are also earlier than the common type. The early varieties make slightly more growth during the winter than the late varieties; the late varieties make more of their growth in the spring and can be grazed longer in the spring. DIXIEappears to be the most winter hardy crimson clover in the upper part of the South. A soft-seeded crimson clover variety named FRONTIER was released in 1962 by the Mississippi Agricultural Experiment Station in Cooperation with the Crops Research Division, USDA; likewise, a reseeding variety named TIBBEE was released in 1970 (Knight, 1963, 1972). FRONTIER and TIBBEE have the following characteristics : large seed size, superior seedling vigor, greater fall and winter growth, equal or superior forage and seed yields, and early maturity. The new varieties were derived from a plant introduction received from Italy in 1956 (PI-233,812). Under dry summer conditions such as occur in Oregon, shattered seed of common crimson behaves as the reseeding types. Volunteer stands from shattered seed occur with the advent of fall rains. Such seed is not of the reseeding type. The use of certified seed is the only way that consuming farmers can be assured of obtaining seed true to variety name as well as to avoid excessive numbers of noxious weed seeds.

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VIII.

Conclusions

The southeastern United States has the land and water resources for a thriving livestock economy. It is estimated that in Mississippi alone there are about 4,000,000 acres of improved permanent pasture. Of this total, only 1,000,000 acres has a legume growing in combination with grass. An abundance of high-quality forage with good seasonal distribution is the foundation for cattle profits. Economical production of this highquality forage is essential for the continued growth and success of the livestock industries. Labor and machinery costs involved in the production, handling, storage, and preservation of feed for livestock continually increase. Systems of year-round grazing that permit the animal to harvest most of the feed consumed should result in economical production of milk and beef. Through the years, reseeding crimson clover and Coastal bermudagrass has provided one of the most productive and economical systems. Generally, perennial grasses fertilized with heavy quantities of nitrogen produce more dry matter and total TDN/A than do pastures composed of legumes and grasses. However, the use of large quantities of nitrogen on grass pastures is questionable from the economic standpoint under many management systems. Yield alone does not necessarily make a practice economically efficient; but rather the amount of quality forage consumed and converted into beef and milk. There is universal acceptance of the fact that legume forage is highly digestible. This digestibility may range from 60 to 80% of digestible dry matter. Intake, by the animal, of legume or grass-legume mixtures is much greater than that of grass alone, even when the grass is fertilized heavily. Yields from crimson clover and grass mixtures usually compare favorably to grass alone fertilized with 100-200 pounds of nitrogen. In recent years, a renewed interest has developed in the utilization of annual clovers in pastures. Contributing to this has been an emphasis on use of idle acres, nitrate pollution, better-quality forage, and grazing systems for livestock. The economic advantage of clover-grass pastures has been demonstrated with both dairy and beef animals. Some of the less obvious, yet highly economic, benefits of clover in pastures involve animal health, milk flow, calf weaning weights, and conception rate. Economic analyses of grazing systems indicate that an increased emphasis will be placed in the future on the use of adapted clover varieties in grazing systems. Proper use and management of crimson clover in these systems should result in large economic gains to the livestock industry in the Southeast. This will require more widespread use of superior varieties already

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available, as well as others made available through future development. Economic gains would accrue to both beef and dairy farmers through an extended grazing season, increased forage production, increased forage quality, better utilization of land resources a stimulation of milk flow, higher calf weaning weights, and better calving percentages.

REFERENCES Adams, F. 1958. Ala., Agr. Exp. Sra., Bull. 301. Adams, W . E., and McCreery, R. A. 1959. Better Crops Plant Food 43(4). Adams, W. E., and Stelly, M. 1958. Agron. J. 50, 457459. Adams, W. E., and Stelly, M. 1962. J . Range Manage. 15, 84-87. Amos, J. M. 1951. Amer. Bee J. 91, 331-333. Anonymous. 1971. U.S., Dep. Agr., Leafl. 482. Ascherson, P., and Graebner, P. 1906-1910. Vol. 6, Part 2, p. 544. Englemann, Leipzig. Bass, M. H., and Hays, S. B. 1961. Ala., Agr. Exp. Sta., Highlights Agr. Res. 8, 1. Beaty, E. R., and Powell, J. D. 1969. J. Range Manage. 22, 36-39. Beckham, C. M. 1956. J . Econ. Entomol. 49, 542-544. Bennett, H. W. 1958. Miss. Farm Res. 21, 10. Bennett, H. W. 1959. Agron. J . 51, 15-16. Blake, G. H., Jr. 1958. 1. Econ. Entomol. 51, 523-527. Bleier, H. 1925. Jaltrb. Wiss.Bor. Pringslreini 64, 604-636. Blount, C. L., and Ashley, T. E. 1952. Miss. Farin Res. 15, 8. Brackeen, L. 0. 1948. Better Crops Plant Food 32, 17-18. Bregger, J. T. 1951. N.J. State Hort. Soc. N32,2408. Britten, E. J. 1963. Cytologia 28, 428-449. Buie, T. S. 1929. S. Car., Agr. Exp. Sta., Circ. 37. Burton, J . C., and Allan, 0. N. 1950. Soil Sci. Soc. Amer., Proc. 14, 191-195. Ching, T. M. 1961. Agron. J . 53, 6-8. Ching, T. M. 1972. Crop Sci. 12, 415-418. Ching, T. M., Parker, M. C., and Hill, D. D. 1959a. Agron. J. 51, 650-684. Ching, T. M., Taylor, H., and Jensen, L. A. 1959b. Proc. Assoc. Off. Seed Anal. 49, 167-1 72. Coats, R. E. 1957. Miss.,Agr. Exp. Sra., Bull. 554. Coombe, D. E. 1968. “Flora Europaea Organization,” Vol. 2. Cambridge Univ. Press, London and New York. Cope, J. T., Jr. 1955. AIa.# Agr. Exp. Sta., Higlrliglits Agr. Res. 2, 3. Crowder, L. V., Sell, 0. E., and Parker, E. M. 1955. Agvon. J . 47, 51-54. Davis, F. L. 1947-1948. Ala., Agr. Exp., Sra. Annu. Rep. 58/59. Davis, F. L. 1949. Agrorr. 1. 41, 368-374. DeCillis, E. 1914. Annu. Rep. Scuolo Sup. Agr. Portici [12] 2, 721-726. Donnelly, E. D., and Cope, J. T., Jr. 1961. Ala., Agr. Exp. Sta., Bull. 335. Duggar, J . F. 1897. Auburn Univ.(APZ), Agr. E x p . Sta., Bull. 87. Duggar, J . F. 1898. Auburn Univ. ( A P I ) ,Agr. Exp. Sta., Bull. 96. Duggar, J . F. 1909. Auburn Univ. ( A P I ) , Agr. E x p . Sta., Bull. 147.

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Duggar, J. F. 1934. J . Amer. SOC. Agron. 26, 919-923. Elrod, J. M. 1960. G a . , Agr. Exp. Sra., Mimeogr. Ser. [N. S.] 91. Emery, F. C., and Kilgore, B. W. 1894. N. Car., Agr. Exp. Sta., Bull. 97. Erdman, L. W. 1946. Soil Sci. SOC. Amer., Proc. 11, 255-259. Erdman, L. W. 1959. U S . , Dep. Agr., Farmers’ Bull. 2003. Favilli, R. 1952-1953. Univ. Pisa Inst. Agron. Gen. Colrivazioni Erbage, Exp. Rec. [N. S.] 6, 53-77. Fayemi, A. A. 1957. Agron. J . 49, 75-76. Fergus, E. N., Kenny, R., and Johnstone, W. C. 1938. Ky., Agr. Ex?. Circ. 318. Foury, A. 1950. “Les Cahiers de la Recherche Agronomique,” Vol. 3. Rabat, Morocco. Gill, J. B., and Coats, R. E. 1952. Miss. Farm Res. 15, 8. Gill, J. B., and Coats, R. E. 1955. Miss. Farm Res. 18, 7. Gill, J. B., and Coats, R. E. 1956. Miss. Farm Res. 19, 8. Grantham, A. E. 1911. Dela., Agr. Exp. Sra., Bull. 89. Hays, S . B. 1964. 1. Econ. Entomol. 58, 481-484. Helmer, J. C., Delouche, J. C., and Lienhard, M. 1962. Proc. Ass. Of. Seed Anal. 52, 154-161. Hendricks, H. E. 1941. Tenn., Agr. E x / . Spec. Circ. 146. Henson, P. R.,and Hollowell, E. A. 1960. US.,Dep. Agr., Farmers’ Bull. 2146. Hollowell, E. A. 1943-1947. Yearb. Agr. (US.Dep. A g r . ) pp. 427-432. Hollowell, E. A. 1946. US.,Dep. Agr. Mimeogr. BPIS & AE. Hollowell, E. A. 1947. U.S., Dep. Agr., Leap. 160. Hollowell, E. A. 1950. U.S., Dep. Agr., Mimeogr. BPIS & AE. Hollowell, E. A. 1951. In “Forages” (H. D. Hughes, M. E. Heath, and D. S. Metcalfe, eds.), 1st ed., pp. 206-214. Iowa State Coll. Press, Ames. Hollowell, E. A., and Knight, W. E. 1962. In “Forages” (H. D. Hughes, M. E. Health, and D. S. Metcalfe, eds.), 2nd ed., pp. 180-186. Iowa State Univ. Press, Ames. Holt, E. C., Potts, E. C., and Kapp, L. C. 1951. Tex., Agr. Exp. Sra., Progr. Rep. 1403, 1-5. Hoveland, C. S., and Elkins, D. M. 1965. Crop Sci. 5 , 244-246. Hoveland, C. S, Creel, J. M., and Webster, H. L. 1964. Ala., Agr. Exp. Sta., Highlights Agr. Res. 11. Hoveland, C. S., Carden, E. L., Buchanan, G. A., Evans, E. M., Anthony, W. B., Mayton, E. L., and Burgess, H. E. 1969. Ala., A g r . Exp. Sra., Bull. 396. Hoveland, C. S., Carden, E. L., Wilson, J. R., and Mott, P. A. 1971. A h . , Agr. Exp. Sta., Highlights Agr. Res. 18, 12. James, E. 1949. Agron. J . 41, 261-266. James, E., and Bancroft, T. A. 1951. Agron. J . 43, 96-98. Kephart, L. W. 1920. US.,Dep. Agr., Farmers’ Bull. 1142. Kight, T . G., and Wellhausen, H. W. 1968. Progr. Farmer No. 9, p. 20. Knight, W. E. 1959. Miss., Agr. Exp. Sta., Bull. 583. Knight, W. E. 1963. Crop Sci. 3, 460. Knight, W. E. 1965. Crop Sci. 5, 422-425. Knight, W. E. 1967. Agron. J. 59, 33-36. Knight, W. E. 1969a. Crop Sci. 9, 94-95. Knight, W. E. 1969b. Crop Sci. 9, 232-235. Knight, W. E. 1970. Agron. 1. 62, 773-775. Knight, W. E. 1971a. Miss. Farm Res. 34, 4. Knight, W. E. 1971b. Agron. J . 63, 418-420.

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Knight, W. E. 1972. Crop Sci. 12, 126. Knight, W. E., and Green, H. B. 1957. Miss. Farm Res. 20, 3. Knight, W. E., and Hollowell, E. A. 1958. Agron. 1. 50, 295-298. Knight, W. E., and Hollowell, E. A. 1959. Agron. I . 51, 73-76. Knight, W. E., and Hollowell, E. A. 1962. Crop Sci. 2, 124-127. Knight, W. E., and Lee, H. S. 1971. Agron. Abstr. 10. Knight, W. E., Donnelly, E. D., Elrod, J. M., and Hollowell, E. A. 1964. Crop Sci. 4, 190-193. Knight, W. E., Ahlrich, V. E., and Byrd, M. 1969. Crop Sci. 9, 393. LaMaster, J. P. 1950. S. Car., A g r . Exp. Sin., Bull. 380. Langford, W. R. 1957. Ala., A g r . Exp. Sta.. Higlrliglits Agr. Res. 4, 4. Lee, H. S. 1969. M. S. Thesis, Mississippi State University, Stale College, Mississippi. Lim, S. M. 1963. Thesis, Mississippi State University, State College, Mississippi. Lowery, J. C. 1939. Ala., Agr. Ext. Circ. 167. Lowery, J . C. 1943. Ala., Agr. Ext. Circ. 254. Lowery, J. C., and Harbor, A. R. 1945. A l a . , Agr. E x t . Circ. 312. Machado, W . C. 1964. M.S. Thesis, Louisiana State University, Baton Rouge. McKee, R. 1935a. I . A m e r . SOC.Agron. 27, 642-643. McKee, R. 1935b. U S . , Dep. Agr., Circ. 377. Moore, R. P. 1943. 1. A m e r . Soc Agron. 35, 370-381. Morley, F. H. W. 1951. Advan. Agron. 13,57-123. Moser, F. 1941. S. Car., Agr. Exp. Sta., Annir. Rep. p. 39. Naftel, J. A. 1942. 1. A m e r . Soc. Agron. 34, 975-985. Naftel, J. A. 1950. Better Crops Plant Food 34, 5. Page, N . R., and Paden, W. R. 1949.. Soil Sci. Soc. Amer., Proc. 14, 253-257. Patterson, R. M., Anthony, W. B., and Brown, W. L. 1959. Ala., A g r . E x p . Sta., Higlrliglits Agr. Res. 6, 3. Picard, J. 1956. Ann. Inst. Nut. Reck. Agron., Ser. B 6, 527-529. Picard, J . 1959. Ann. Inst. Nut. Rech. Agron., Ser. B 9, 319-331. Pieters, A. J., and Hollowell, E. A. 1937. Yearb. Agr. (U.S. Dep. A g r . ) pp. 1190-1214. Piland, J. R., Ireland, C . F., and Reisenauer, H. M. 1944. Soil Sci. 57, 75-84. Preston, J. B. 1949. Crops Soils 1, 32. Pritchard, A. J. 1969. Aust. I . Agr. Res. 20, 883-887. Rampton, H. H. 1969. Agron. J . 61, 92-95. Reed, J . K., Park, J, K., Hays, S. B., and Webb, B. K. 1962. S. Car., Agr. Exp. Sta., Circ. 134.

Rogers, T. H. 1947. J . A m e r . Soc. Agron. 39, 638-639. Rogers, T. H. 1951. Ph.D. Thesis, University of Minnesota, St. Paul. Sandal, P. C. 1955. Agron. J . 47, 147-148. Schmidt, D. 1921. N . I . , Agr. Exp. Sta. 42nd Annu. R e p . p. 3 3 3 . Smith, K. E. 1928. Proc. Ass. Off.Seed Anal. 19/20, 62-64. Stanley, R. L., Randolph, N. M., and Teetes, G. L. 1970. J . Econ. Entorno/. 63, 256-258. Stephens, J. S., and Hollowell, E. A. 1942. J . Arner. Soc. Agron. 34, 1057-1059. Stewart, F. 1948. Ala., A g r . E x p . Sta., Progr. Rep. Ser. 40. Stewart, F., and Boseck, J. 1947. Ala., Agr. E x p . Sta.. Progr. R e p . Ser. 9. Stewart, F., and Pearson, R. W. 1952. Agron. J . 44, 501-502. Stitt, R. E. 1944. J . Anzer. Soc. Agron. 36, 464-467. Sullivan, S. L. 1971. M.S. Thesis, Mississippi State University, State College, Mississippi.

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Sullivan, S. L., Baetcke, K. P., and Knight, W. E. 1972. Pkytochemisfry 11, 2525-2526. Thomas, J . G., and Parker, F. W. 1967. Ter., Agr. E r t . Ser., Entomol. Notes 8, 10. Tippens, H. H. 1958. J. Econ. Entornol. 51, 459-460. Toole, E. H., and Hollowell, E. A. 1939. 1. Amer. SOC. Agron. 31, 604-619. Vaughn, C. E. 1961. Miss., Agr. E r p . Sta., Inform.Sheet 313. von Gliemeroth, G. 1943. J. F. Landwirt. 89, 123-150. von Horn, A. 1936. Mitt. Landwirt. 51, 225-226. Voorhees, E. B. 1894. N.J., Agr. E r p . Sta., Bull. 100. Wear, J. I. 1957. Ala., Agr. Exp. Sta., Bull. 305. Westgate, J . M. 1913. U S . , Dep. Agr., Farmers’ Bull. 550. Westgate, J. M. 1914. U S . , Dep. Agr., Farmers’ Bull. 579. Westgate, J . M. 1924. U S . , Dep. Agr., Farmers’ Bull. 1411. Wexelsen, H . 1928. Univ. Calif. Agr. Sci. 2, 355-376. Williams, W. A., and Elliott, J. R. 1960. Ecology 41, 785-790. Williams, W. A., Love, R. M., and Berry, L. J. 1957. Calif., Agr. Exp. Sfa., Err. Ser. Circ. 458. Wolf, F. A., and Cromwell, R. 0. 1919. N . Car., Agr. Exp. Sta., Biill. 16. Zohary, M . 1970. “Flora of Turkey and the East Aegean Islands,” Vol. 3, p. 425. Edinburgh Univ. Press, Edinburgh.

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K . Baeumer and W A P Bakermans Faculty of Agriculture. University of Goettingen. Goettingen. Federal Republic of Germany. and Institute far Biologicol a n d Chemical Research of Field Crops and Herbage. Wageningen. The Netherlands

I . Introduction: The Concept of Zero-Tillage . . . . . . . . . . . . . . . . . . . . . . . . A Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Definition of Zero-Tillage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C . Motivation for Zero-Tillage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1 Comparison of Environmental Conditions in Tilled and Untilled Soils . . . A . Macroscopic Soil Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Soil Flora and Fauna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C Soil Organic Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D . Parameters of Soil Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E . Aeration and Soil Moisture . . . . . . . F. Soil Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G . Soil Erosion . . . . . . . . H Soil Trafficability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I. Nutrient Concentration and Distribution ....................... I11. Effects of Zero-Tillage on Plant Growth .................. A . Germination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Root Growth ...... ................ C . Nutrient Absor n ........................................ D. Crop Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... E. Changes in Weed Populations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Diseases and Pests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV . Crop Husbandry . . . .......................... A . Sowing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Weed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Pasture Renovation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Fertilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Crop Yields and Cropping Systems . . ....................... V . Evaluation of Zero-Tillage in Farming Systems ..................... A . Applicability of Zero-Tillage in Humid, Temperate Climate Regions B. Applicability of Zero-Tillage to Dryland Farming . . . . . . . . . . . . . . . . VI . Conclusion . . . .............................................. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Introduction: The Concept of Zero-Tillage

A.

HISTORICAL BACKGROUND

Moldboard ploughing or similar deep-tillage operations have for centuries been a feature of the more advanced systems of crop production. Nowadays, some farmers still consider it profitable to use the ever-increasing supply of more powerful tractors for ploughing deeper each year. Nonetheless, the concept of tillage requirements for crop production is changing rapidly. In 1927, Garber successfully ovcrsowed a legume into an unproductive grass sod without tillage using such simple techniques as close grazing or burning and heavy seed rates to manipulate the competition between the old sward and the surface sown forage species as well as the hooves of grazing animals to bring the seeds into close contact with the soil. This was an early demonstration of the essential features of zero-tillage, i.e., growing a crop with the least possible soil disturbance, which involves controlling unwanted vegetation by other than mechanical means. Realization of such a system became feasible in the 1950’s, when chemicals such as dalapon, amitrole, and atrazine, which can destroy the existing vegetation with relatively short or no residual effect on the crop to be established, were introduced. First used successfully in pasture renovation, the concept of zero-tillage received support as a consequence of the encouraging results obtained by mulch-farming practices in the United States where the till-plant system was developed for row crops in order to provide year-round protection of the soil from erosion and to minimize planting costs. This system can be regarded as a forerunner of zero-tillage which was initiated in the 1960’s and has since been used increasingly in the United States. For 1971, zerotillage production of maize, soybeans, sorghum, and cotton in the United States has been estimated at 438,600, 130,200, 22,900, and 2000 ha, respectively (D. M. Van Doren, personal communication, 1972). In Europe, the introduction of broad spectrum, nonresidual herbicides of the bipyridyl types opened new perspectives. Here, Great Britain takes the leading position with regard to zero-tilled acreage, which was doubled between 1970 and 1971. The census for zero-tillage production of fodder Brussicae, cereals and grassland in 1971 is recorded to be 19,600, 7500, and 4700 ha, respectively (J. T. Braunholtz, personal communication, 1972). B.

DEFINITION OF ZERO-TILLAGE

The term “conventional tillage” will be used here to designate the traditional tiIlage system, which typicaIIy begins with a primary deep tillage

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operation followed by some secondary tillage for seedbed preparation. Weed control after planting is effected by preemergence and/or postemergence cultivations and/or herbicide applications. The term “zero-tillage” is used to designate a tillage system in which mechanical soil manipulation is reduced to traffic and seedbed preparation only. It can be considered to be the most extreme form of minimum tillage, which, as a category of tillage systems, not only includes methods resulting in reduced tillage intensity but also the combined use of several implements in one operation, such as the plow-plant method. The term “direct-drilling” will be avoided in this review as it is used by horticulturists to mean sowing crops onto the final location instead of transplanting young plants grown as seedlings elsewhere. In zero-tillage studies, several tillage methods are usually compared with a standard conventional one. If in the following sections the term “zero tillage’’ is used, &hemost extreme form of zero-tillage examined has always been selected, if not otherwise stated.

C. MOTIVATION FOR ZERO-TILLAGE Soils are tilled to provide conditions suitable not only for optimum plant growth, but also for necessary field operations, e.g., planting and harvesting. But ncither the feasibility for, nor the advantage of, such a deep primary tillage operation is always given. An alternative to the conventional tillage system is most urgently needed where soils are subject to wind and water erosion, timing of tillage operations is too difficult, performance insufficient, and requirements of energy and labor too high. On slopes, bare, compactcd soils high in silt and fine sand content but low in organic matter content are exposed to soil erosion, especially when farmed continuously with a row cropping system. Only an improved soil structure such as is found under sod where organic matter is accumulated at the soil surface and aggregate stability is stimulated can reduce the risk of soil erosion. Very heavy, or shallow and stony soils are really marginal for arable farming undcr present economic conditions. Yet, as these occur in large areas in the world, methods must be developed to make arable crop production feasible under such conditions. The same question arises in the case of ameliorated, i.e., drained, peat soils. Repeated loosening and mixing of the top layer by tillage enhance the mineralization of organic matter; hence, the rapid loss of matter causes sinking of the soil surface so that the soil may soon become waterlogged again. Here too, zero-tillage may be preferable since it may involve a drastic reduction in the mineralization rate. Zero-tillage could also be of interest on medium- to fine-textured soils.

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Apart from reducing costs for tillage operations, zero-tillage may eventually alleviate some of the negative side effects of tillage and repeated heavy traffic on medium- to fine-textured soils. Tillage and traction, especially during a wet season and by the use of heavy implements, can result in formations of soil pans. When caused by deep cultivation, these pans are difficult to remove. It is thought that continued use of zero-tillage on arable land will ultimately result in a stable soil structure similar to that frequently found under a cover of permanent grass. This soil structure should provide the optimum conditions for both plant growth and the necessary traffic on the fields. Zero-tillage may be induced by the practical needs and aims of a farmer striving for more effective, less risky systems of crop production. Yet, as Kuipers (1970) pointed out, the possibility of growing field crops without tillage offers an excellent opportunity for tillage research to examine the simple but basic question whether soil tillage is really necessary and to what extent it is necessary under various edaphic, climatic, and economic conditions. Contrasting hypotheses as to whether weed control or soil tilth, as effected by tillage operations, are the predominant benefits of tillage, can now be tested.

II.

Comparison of Environmental Conditions in Tilled and Untilled Soils

A. MACROSCOPIC SOILSTRUCTURE In contrast to clean-tilled fields, a no-tilled site is covered always by some plant residue. On bare patches, some mosses, green algae or lichens may cover the zero-tilled soil. Especially under plant litter or a closed plant canopy, earthworms deposit their casts on top of the soil. Frequently, the earthworm burrows end underneath a “mitten.” This consists of little heaps composed of plant debris collected by earthworms as well as soil excreted by them. Compared to the rugged ground of a recently tilled field, the zero-tilled soil surface is relatively smooth and even. Minor height variations are caused by ruts and tilled strips, mole hills, vole burrows, and mice tracks. In general, undisturbed soil appears to be more dense and firm. More energy is needed to break up the soil into clods, especially in untilled soil with a higher clay content. Although naturally compacted soils are more homogeneous in structure than plowed soils, differently structured layers can be observed as well. The top layer, which is often no more than 5-15 mm thick, though it may be up to 50 mm thick, may have a crumbly and friable structure. This

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depends on the amount of mulch present, the activity of soil animals and the prevailing weather conditions. Where such a top layer of biologically stabilized crumb structure has been established, slaking of silt material and, consequently, formation of a dense crust are rarely observed on zero-tilled silty loams. The structure and size of deeper layers depend mainly on their soil texture and on the texture dependent reaction to changes in soil moisture and temperature. In 'hit soils, zero-tillage induces a platy structure (Bulfin, 1967). This type of frost structure is unstable in tilled soils owing to excessive water in the top layers during the thawing process; however, it normally remains visible throughout the year in zero-tilled silty loam soils. A polyhedric structure is typical for soils with a high clay content, low capillary water conductivity and distinct swelling and shrinking properties. Since swelling of the clay after remoistening will close every cleavage again if the soil has not been previously mechanically loosened, tilth induced by frost or drought may be a transient phenomenon in zero-tilled clay soils (Czeratzki, 1971).

B. SOIL FLORA AND FAUNA Changes in soil flora and fauna can be expected when zero-tillage practices are introduced. Suitable information is lacking, especially with regard to reactions of microorganisms to zero-tillage effects per se. Indirect evidence that zero-tillage changes microbial activity is derived from tests of cellulose decomposition under field conditions. On zero-tilled soils, higher decomposition rates than on plowed soils were observed by Herzog et al. (1969), whereas Bender and Adamczewski (1970) found the reverse. However, these results reflect more the prevailing soil conditions than possible changes in microbial populations. In two field experiments with continuous wheat in England (Corbett and Webb, 1970), the total number of nematodes was sometimes larger with zero-tillage, while migratory parasitic nematodes were usually less numerous on untilled soils. Small-sized species of nematodes seemed to be favored on naturally compacted soils; inadequate observations do not allow further conclusions. Although earthworms form the most conspicious group of soil-inhabiting animals, little information is available about the changes in weight and number of earthworms upon introduction of a no-tillage system. For sampling, all investigators used vermifuges, which do not allow complete recovery of existing worms; therefore, only relative values can be reported. In West Germany, Schwerdtle (1969) found on the average a 12-fold increase in number and a 16-fold increase in weight of earthworms collected

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on zero-tilled plots after three years’ cropping with corn. In England, Wilkinson (1967) reported less spectacular increases, e.g., on cereal stubble fields a mere 1.6-fold weight increase. On former leys and permanent pastures, he did not observe any difference in the weight of earthworm populations resulting from tillage treatments. On three sites of former permanent pastures in the Netherlands, we found about half as many earthworms on tilled as compared to untilled plots after seven years. More than in any changes in abundance of earthworm populations, the agronomist is interested in their activities which alter the ecological conditions of naturally compacted soils. On loamy sand in West Germany, Graff (1969) measured the rate of casting from September to May over a period of three years. On untilled, mulched plots, between 2 and 4.5 kg dry matter per m2, which is within the range of values encountered normally on old pastures (Evans and Guild, 1948), were deposited on the soil surface. Graff (1969) observed 20- to 25-fold increases in rate of casting on untilled plots as compared to turnplowed barley stubble. Normally, most earthworms deposit their castings in the soil, not on top of it. In compacted soils, however, most castings are deposited on the soil surface. Still, even on zero-tilled soils, considerable soil mixing is to be expected. Earthworm tunnels which open to the soil surface may influence the rate of water infiltration. Since small tunnels are difficult to distinguish from soil cracks, the number of earthworm “mittens” may serve as a first approximation. On untilled cereal stubble fields, we found an average of 55 mittens per m2 soil surface. One centimeter below the soil surface, an average of 68 tunnels (diameter 2-10 mm) was observed on no-tilled stubble as compared to 15 on plowed stubble (W. Ehlers, personal communication, 1972). Moles are predators of earthworms and increase in number when fields are left undisturbed for a prolonged period. In Switzerland, Vez (1969) counted 10 to 12 molehills per are on zero-tilled plots as compared to 1 to 2 on conventionally tilled plots. Similar differences can be observed in burrows of voles and mice; these have not yet been quantified.

C. SOILORGANIC MATTER With zero-tillage, plant residues remain on the soil surface. This is essential when soil erosion limits successful farming. Where erosion presents no problem, a mulch cover may be desirable to create a favorable soil tilth. The highest permissible level of mulch is determined by conditions governing the effective performance of drilling and weed control operations. In the Netherlands, 3000 to 4000 kg straw (dry matter) per hectare can remain on the ground only when it is chopped up into small pieces

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83

and evenly distributed. For dryland farming, an amount of 3500 kg straw per hectare at harvest is considered to provide adequate soil protection from erosion without presenting problems with seeding, weed control and soil fertility. Brown and Dickey (1970) determined losses of wheat straw buried in the soil, placed on the soil surface and exposed above the soil surface to simulate a standing stubble. At two locations in Montana, they found that the rate of dissipation increased with greater contact between soil and plant material and decreased when rising amounts of straw were applied. Lower mean annual temperature, though in combination with higher precipitation, retarded straw decompositibn. During the first 3 months of exposure, the weight of above-surface and on-surface straw increased by as much as 1 3 % due to an accumulation of soil particles inside the hollow straws. Some soil probably is moved by wind or raindrop splash and, near the ground, by the activities of soil-inhabiting animals. The experiment of Brown and Dickey was begun in May. After 18 months’ exposure, only 22 to 40% losses were measured on and above soil as compared to 93 to 98% buried in the soil. In the more humid climate of Germany, higher rates of wheat straw decomposition were observed, e.g., 40% during the period September through July (K. W. Becker, personal communication, 1972). The weight of adhering soil particles equaled the amount of straw remaining after 1 1 months’ exposures. These figures indicate that a slow surface accumulation of straw residue can occur if the above-mentioned process of incorporation into the soil is not effective. Plant residues with higher N content decompose more rapidly. Cornstalks applied to the surface of a cornfield in Iowa in May lost 50% of the initial weight after 20 weeks’ exposure (Parker, 1962). Sugar beet tops left on the ground in Western Europe are completely disintegrated by July. Hence, leafy and succulent plant material presents no problems in mulch management. In Fig. 1, some results of trials on former grassland and recently plowed soils concerning the distribution of organic matter in soil are summarized. No differences in organic matter concentration were found in regularly plowed soil layers. In undisturbed soils, the concentration was highest near the soil surface and declined steadily to subsoil values below those on conventionally tilled soils. The gradient of organic matter content was more pronounced on former grassland soils, where zero-tillage presumably preserved the original distribution of organic matter. No thorough investigation has yet been published as to whether zerotillage results in an accumulation of total soil organic matter. Available

84

K. B A E U M E R A N D W. A. P. B A K E R M A N S organic matter ( % . d r y s o i l ) 1

30i

2

3

4 5

arable l a n d (16)

6

7

N ( '/. )

8 9 1011

grassland

(22)

arable land

(16)

FIG.1. Average distribution of organic matter and N in tilled (-) and untilled (- - - ) soil. (From Bakermans and De Wit, 1970, grassland; Buhtz et al., 1970, arable land.)

data from long-term trials (Moschler et al., 1972; Buhtz el al., 1970) suggest that zero-tillage increases the total organic matter of the soil. Whether the observed differences in accumulated organic matter are caused by restricted decomposition andlor higher production of organic matter on zero-tilled soils is not yet known. As compared to a tilled chernozem soil in East Germany, concentrations of CO, in the atmosphere near the soil surface were lower on untilled soil (Buhtz, 1972). These observations suggest a reduced rate of mineralization in naturally compacted soils.

D. PARAMETERS OF SOIL STRUCTURE With zero-tillage, soils are loosened only locally and superficially; yet they have to bear the normal load of traffic in the field. Hence natural consolidation and mechanical compaction will cause a denser packing of zero-tilled topsoils. The average decrease in total porosity was found to vary between 0 and 6% (v/v) (Czeratzki and Ruhm, 1971; Herzog and Bosse, 1969; Vez and Vullioud, 1971a,b). A few exceptions were noted on heavy river clay rich in organic matter (Van Ouwerkerk and Boone, 1970), on two sites with silt loam-chernozem soil (Buhtz et al., 1970) and on silty clay (Bachthaler, 1971) where values of total porosity were lower on tilled than on zero-tilled plots, probably as a result of compaction caused by tillage operations. In general, the differences in total porosity were greatest in the soil layer which is loosened by plowing, but not compacted by seedbed preparation and cultivation ( 10-1 8 cm) . In deeper soil layers, the differences .tended to diminish. Near the soil surface, they varied with the effects of tillage operations, weather, and biological activities. Mean values of total pore space average over sampling dates, crops,

85

ZERO-TILLAGE

and locations, eliminate extreme values, which may be decisive for plant growth and farming operations in critical situations. The lowest sampling means of the porosity data published were found with values near 35 and 38% (v/v) on untilled plots on sandy soil (22-27 cm) and clay soil (15-20 cm), respectively (Czeratzki and Ruhm, 1971). The very high density of the soil layer 22-27 cm on sand merits special attention, as it is probably induced by mechanical compaction and perhaps by downward displacement of finer soil particles, which by turnplowing are redistributed to upper soil layers. This could be proved by particle size and pore size distribution analysis, but no information is yet available. The observed minimum values did not mark the final stage of soil density on zero-tilled soils. At subsequent dates, porosity increased again, especially on stable soils with medium to high clay content. The above-stated lower levels of pore space were reached within two to three years of zerotillage, after which time seasonal fluctuations of total porosity tended to be smaller as compared with conventionally tilled soils (Van Ouwerkerk and Boone, 1970). Similar results are shown in Fig. 2, which contains a time series of porosity measurements in the top 2-6 cm layer of arable silt loam soil derived from loess (Ehlers, 1973). Zero-tillage resulted in a smaller total porosity but also in reduced variability of- the sampling means; consetotal porosity

40

40 20 1s h

>

10

$ 5

- 0

10

5

medium pores

0

3

15 10

- 30rm

sma II pores 0.2 - 3.0p m

5

0 10 5

0 oats

-*

un ti lied

radish

rotabated

cultivated

tilled

FIG. 2. Changes of total pore space and pore size distribution with time at a depth of 2-6 cm on tilled and untilled silt loam soil. (From Ehlers, 1973.)

86


quently, homogeneity in time increased in naturally compacted soils. The remaining fluctuations of total porosity presumably result from the combined effect of seasonal changes in climate and soil cover on the activities of soil flora and fauna. The example in Fig. 2 shows further that changes in total porosity were accompanied by concomitant changes in other pore size fractions. It can be concluded, therefore, that mechanical loosening effects mainly the fraction of large pores and that dense parts of the soil remain more or less unchanged. As compared to the fraction of large pores, the other pore size fractions fluctuated to a smaller extent.

5

Y

30 ..

20

10

0

p o r e space

('I-vlv)

60

10

20

40

30

10

;

20

0

30 1

pore size

A

-F

3

P

N I W

w

W

0

I

I

w

0)

o

O

"

W

m o

o z

F

3

=

z A

-F

3

W

R

W

I

I

W

O I

w

m

0

0

:! 0--.

-Fn 3

FIG. 3. Changes of total pore space and pore size distribution with depth on tilled (right panel) and untilled (left panel) silt loam soil. (From W. Ehlers, personal communication, 1972.)

Figure 3 shows the vertical pore size distribution of a silt loam soil (W. Ehlers, personal communication, 1972). Although in this case the total porosity and the fraction of large pores did not differ much between zerotilled and conventionally tilled soils, the pattern of porosity reveals an important difference: on undisturbed soil, the relative space occupied by each pore size fraction varied less than on the ploughed soil, where the layers at 0 to 15 cm and 25 to 30 cm were compacted as compared to the layer at 15 to 20 cm. The compaction at 25-30 cm is presumably caused by pressure and smearing actions during plowing. It resulted not only in an absolute reduction in large and medium pores-based on volume as well also in a relative increase in small and very small as on weight-but pores-based on volume only, as discussed by Ehlers (1973). Untilled soil, though generally denser, may also exhibit more structural homogeneity in space as compared to conventionally tilled soils. A relatively higher amount of smaller pores, but greater homogeneity in time as well as in space are thus the dominant changes in porosity when

ZERO-TILLAGE

87

a soil remains untilled for a long period. Another feature may be connected with the continuity of pores. Since earthworm tunnels can be regarded as primarily continuous pores, an estimate of the relative pore space occupied by them may serve as a first approximation. Figure 4 shows that the space occupied by large pores with presumably uninterrupted connections to the atmosphere is more than doubled near the soil surface and in the top 20 cm of zero-tilled soil as compared to plowed soil (W. Ehlers, personal communication, 1972) . p o r e space (% v/v)

o

0.2

0.4

0.6

ae

1.0

FIG. 4. Pore space occupied by rainworm tunnels on tilled (O---O) and untilled (0-0) silt loam soil (From W. Ehlers, personal communications, 1972.)

701 ao

Other composite parameters of soil structure are resistance to penetration and shear stress, which are highly dependent on texture, soil moisture, and porosity. In general, larger resistance to a cone-shaped probe forced into the soil was observed on zero-tilled soil (Buhtz et al., 1970). On sand soil, J. M. Houben (personal communication, 1972) found no rooting when penetrometer resistance exceeded 40 kg/cm2. In one case, we observed that continuous zero-tillage on sandy soil produced a comparable compaction in layers between 5 and 30 cm.

E.

AERATION AND SOILMOISTURE

Soil aeration depends on porosity and water content. Hence when a soil is water saturated to field capacity (soil moisture tension: 0.1 bar A p F 2), the extent of the remaining pore space filled with air (air capacity) may be critical for the maintenance of soil aeration. A minimum volume of 10% is thought to be necessary for adequate gas exchange between the soil air and the free atmosphere. Though zero-tillage generally causes a decrease of large, mostly air filled pores (diameter > 30 pm) and thus reduced aeration, air capacity at p F

88


2 was observed only on medium- to heavy-textured soil to be below 10% (v/v) (Van Ouwerkerk and Boone, 1970; Czeratzki and Ruhm, 1971; W. Ehlers, personal communication, 1972). Impeded aeration, if caused by zero-tillage, may provide a serious objection to the application of this system on heavy soil in humid regions. With regard to this point, an evaluation of large, continuous pores, such as earthworm tunnels, would be of interest. The observed relative increase in the amount of medium to small pores caused by zero-tillage has consequences for the water-holding capacity of the soil. Plowing up grassland results in the redistribution of organic matter ( Fig. 1 ) ; zero-tilled sod retains its original accumulation of organic matter near the soil surface. Water-holding capacity is related to organic matter content, especially on sandy soils; this was confirmed by Van Ouwerkerk and Boone (1970), who found that water content at p F 2 changed more in conjunction with organic matter content than with soil porosity. Hence, in the top 6 cm of the zero-tilled soil, a higher water content at p F 2 was found than in the plowed soil, whereas the reverse was true in the layer at 11-1 6 cm. Thus, beginning with a permanent pasture, changes in soil behavior caused by different tillage systems cannot be ascribed solely to differences in porosity. On arable land, the situation is less complicated since waterholding capacity generally increases with increasing pore space of an equivalent diameter Na+ > Li+ > Na+ > Li+ Rhf > K+ > Cs+ > Na+ > 12 K+ > Rb+ > Cs+ > Na+ > Li+ K+ > Rh+ > Na+ > CY+> Li+ K+ > Na+ > Rb+ > Cs+ > Li+ Na+ > K+ > Rhf > Cs+ > Li+ Na+ > K+ > Rb+ > Li+ > Cs+ Na+ > K+ > Li+ > Rb+ > Cs+ Na+ > Li+ > K+ > RIP > Cs+ Li+ > Na+ > K+ > Rb+ > Cs+ Cs+ > Rb+ > K+ Rbf > Cs+ > K+

Sequence I is in the order of increasing apparent hydrated size and Sequence XI is in the order of increasing nonhydrated size. In each of the intermediate sequences, one pair of cations shifts positions. The basis for these selectivity patterns is the relative free energy differences between ion: site and ion :water electrostatic interactions. Thus, the cation preferred by a specific negative site will be that cation which experiences the greatest decrease in free energy when its nearest neighbor becomes the negative site rather than water. When the negative site has a very strong electric field strength, the free energy differences between ion :site and ion: water interactions is such that the cation with the smallest ionic radius will be preferred and thus the selectivity pattern will be that shown in Sequence XI. But, if the negative site has a weak electric field strength the free energy differences between ion:site and ion:watcr interactions is such that the largest nonhydrated cation, which has the lowest free energy of hydration, will be preferred and the order of selectivity will be that shown in Sequence I. Thus, when the electric field strength of the negative site is very weak, Sequence I is preferred, and as the electric field strength of the negative site increases, the order of specificity of ion binding progressively shifts until at very high electric field strengths, Sequence XI is the order of preference. Eisenman also showed that the selectivity for H+ relative to the

186

T. K. HODGES

alkali ions is also dependent on the field strength of the binding site. At high field strengths, H+ is preferred over the alkali ions and at low field strengths, the alkali ions are preferred over H+. The basis for selective ion transport by plants is almost sure to reside in the electric field strength of the ion binding sites and in shifts in the electrical strength of the sites. For example, in barley roots, the preference for K+ over Na+ at low external concentrations and the reverse at high external concentrations (Epstein, 1961; Rains and Epstein, 1967a,b) indicates that the field strength of the binding site increases as the external ion concentration increases. A change in the field strength of the binding sites could be brought about by conformational changes in carrier subunits as described previously. The shift from a low to a high field strength, with sequential ion binding, would also bring about a continually increasing preference for Ht by the binding sites. This could account for the continually decreasing affinity, or increase in apparent K,, for the alkali cations as the external concentration increases. There are numerous reports for the selectivity of ion absorption by plants where two or three of the alkali cations have been considered (Collander, 1941; Fried and Broeshart, 1967), but very few where all five of the alkali cations have been studied. Steward and Mott (1970) reported an order of alkali cation preference by carrot cells that corresponds to Sequence VI. In a very thorough study, Jacobson et al. (1960) determined the effect of both pH and Ca2+on the absorption of the alkali ions by barley roots. In the presence of Ca2+and at pH 7 the preferred sequence was Kt > Rb+ > Na’ > Cst > Lit (i.e., Sequence V ) . As the pH decreased to 3, in the presence of Ca2+,a shift occurred such that the preferred sequence was Rb+ > K > Cs’ > Nat > Li+ (i.e., Sequence 111). It was not possible to determine from their figures whether Sequence IV occurred. This effect of H+ on changing the selectivity pattern of transport is analogous to the effect of Ht on the selectivity pattern of alkali ion binding to glass electrodes (see Diamond and Wright, 1969). The effect of increasing the proton concentration is to reduce the “effective” negative charge strength of the binding site. The effect of Ca2+on the selectivity is complex because of its association with various negative charges, but it too appeared to alter the “effective” field strength of the transport sites since selectivity was shifted from the higher to the lower sequences in barley roots (Jacobson et al., 1960). The CaZt-induced specificity for Kt over Nat at low external concentrations (Epstein, 1973) would also be consistent with this interpretation. An alteration in the “effective” field strength by Ca2+might also be the basis for the anomalous effect of Ca2+on Kt absorption in barley and wheat roots (Hiatt, 1970a,b) as well as the promotive effect of CaZt on K+ transport under some conditions (Viets, 1944; Overstreet et al.,


187

1952; Kahn and Hanson, 1957; Epstein, 196 1 ), and its inhibition in others (Handley et al., 1965; Elzam and Hodges, 1967). Eisenman ( 1965) has further shown that halide specificity by fixed positive charges is also governed by the relative free energy differences between ion:site and ion: water interactions. Of the 24 possible selectivity sequences, only seven occur commonly. Diamond and Wright (1969) have cited some deviations from the main seven orders and discuss why these sometimes occur. The normal seven are as follows: I

I1

I11 IV V VI

VII

I- > Br- > CIBr- > I- > C1Br- > CI- > ICI- > Rr- > IC1- > Br- > FCI- > F- > Br-

F- > CI- > Rr-

> F> F> F>P > I> I> J-

A site with a very strong electric field would prefer the ion having the smallest nonhydrated radius, F-, and Sequence VII would be the order of specificity. A very low field strength site would prefer the most nonhydrated ion, I-, and Sequence I would be preferred. In roots, C1- is generally absorbed at about the same rate as Br- (Epstein, 1953; Boszormenyi and Cseh, 1961, 1964) or slightly faster (Elzam and Epstein, 1965) and both are absorbed more rapidly than either F(Venkateswarlu et al., 1965) or I- (Boszormenyi and Cseh, 1964). Probably one of the sequences from I1 to V correctly describes the normal selectivity pattern for halide absorption by roots. Whether the ion concentration, pH or Ca+affect the selectivity sequence is unknown. The concept of a single cation carrier and a single anion carrier is supported by observations that the total cation or total anion absorption is generally constant from salt solutions that vary in the proportions of the cations or anions (Bear and Prince, 1945; Jacobson et al., 1960; Jackson and Stief, 1965; Hiatt, 1968, 1969, 1970a,b; Pitman et al., 1968). The clearest example of this is for K and Na' absorpion by barley roots (Jackson and Stief, 1965). They found the combined rates of K' and Na' transport were constant even though the individual rates of K+ and Na+ transport were different. These experiments were conducted in the absence of Ca2+.However, Hiatt ( 1970a,b) obtained virtually the same results when Ca2+was present. A single carrier for cation influx would not account for the active efflux of cations, such as sodium (see Section 111), unless the carrier functions in an exchange manner. There is evidence that cation influx is in exchange for H i in low salt roots (Jackson and Adams, 1963; Jacobson et al., 1950; Pitman, 1969) and that a Kt/Kt exchange becomes more prominent as

188

T. K. HODGES

salt saturation is approached (Pitman, 1969; ,also see Poole, 1969). Active sodium efflux might be accomplished by a general cation carrier if exchange is involved and if the carrier has a high affinity for Na+ when the site(s) faces the cytoplasm. All that would be needed to accomplish an active Na+ efflux would be for the carrier site, when facing the cytoplasm, to have a sufficiently high electric field strength that Na+ is preferred over the other cytoplasmic ions. Jeschke (1970) and Pitman and Saddler (1967) have presented evidence that a K+ influx-Na+ efflux does occur in barley roots. This type of exchange, however, is not as specific as it PlOIlnO

MambrOM Li < Csc No< Rb c K Hiqh Ccncentration Li c Csc Rb.r K c Na

I

I CATION CARRIER H+. No'

,TF

OH-, HCO;

ANION CARRIER

FI < I c Br < CI

FIG.4. A model depicting a single cation exchange carrier and a single anion exchange carrier in the plasma membrane of root cells.

is for the active K+/Na+ exchange reaction of mammalian cells (Skou, 1965). This is also evident from the failure of ouabain to inhibit ion fluxes in plants (Hodges, 1966; Cram, 1968b). Cram (1968b) did observe a slight inhibition of Na+ efflux by ouabain, but there was no evidence that the Na+ efflux was tightly coupled to K influx. Thus, energy-dependent cation exchange does occur, but it is not highly specific. It is suggested, however, that it could have sufficient specificity to account for the active Na+efflux at the plasma membrane. A model depicting a single cation carrier and a single anion carrier is shown in Fig. 4. Both carriers are considered to carry out an energy-dependent exchange of external for internal ions. The approximate order of specificity for the alkali cations at low and high external concentrations is shown. Also, the apparent order of halide specificity by the anion carrier is shown. As discussed above, there is evidence for energy-dependent ca-


189

tion exchange. However, evidence for energy-dependent anion exchange is limited. A suggestion of the latter comes from studies that show changes in organic acid levels in the cell when the absorption rates of cations and anions are different (Ulrich, 1941; Jacobson and Ordin, 1954; Hiatt and Hendricks, 1967; Hiatt, 1967a,b). Thus, when inorganic anion absorption exceeds inorganic cation absorption, an anion/HCO,- exchange is a strong possibility. Also, as will be discussed in Section VI, a HC0,- influx coupled to a OH- efflux on the anion carrier is suggested when cation absorption exceeds anion absorption. Finally, the exchange-diffusion of C1- reported by Cram (1968a) and Cram and Laties (1971) might represent a manifestation of the anion exchange carrier. One of the strongest arguments against a single carrier for cations is the shift in preference for Na+ and K+ as a function of aging in stem tissue (Rains, 1969; Rains and Floyd, 1970) and in red beet tissue (Poole, 1791a,b). At low external concentrations freshly cut bean stem slices transport Na+ much more rapidly than K+, but after aging 20 hours in CaS04, K+ is transported more rapidly than Na'. A similar specificity occurs at high concentrations, but it is not as pronounced. Rains (1 969) and Rains and Floyd (1970) interpret these changes in transport specificity as evidence for the development of a K' carrier that is independent of the Na+ carrier. A similar phenomenon occurs in the beet tissue (Poole, 1971a,b). K+ transport is more rapid than Na+ transport in slices aged for 1 day, whereas Na' transport is more rapid than K+ in slices aged for 6 to 7 days. Poole also interprets these data as indicating two separate carriers. In both tissues, however, an altered specificity of the same carrier could conceivably account for the results. Many metabolic changes occur during the washing (aging) period, and alterations in the membrane lipids or proteins could alter the molecular environment of the carrier to such an extent that the charge density or field strength of the binding sites would undoubtedly be altered. Because such a change would alter ion selectivity, one carrier could probably account for the results. The model of ion transport proposed here (Fig. 4 ) represents the combination of two widely different concepts, the negative cooperativity concept of Koshland (1970) and the selectivity concept of Eisenman (1961, 1962). Together they account for most, if not all, of the kinetic and selectivity aspects of ion transport in plants.

V.

Energetics of Ion Transport

Aerobic conditions are essential for nutrient absorption by roots. This has been shown by the pioneering investigations of Steward (1932),

190

T. K. HODGES

Lundegirdh (1934), and Hoagland and Broyer (1936). These studies were followed by demonstrations of parallels between aerobic respiration and nutrient absorption (Lundegdrdh and Burstrom, 1933; Ulrich, 1941; Vlamis and Davis, 1944; Robertson and Turner, 1945; Robertson and Wilkins, 1948). Finally, the finding that respiratory poisons. such as cyanide, azide, and carbon monoxide inhibited ion absorption clearly established the requirement of aerobic respiration for nutrient absorption by plant roots (Ordin and Jacobson, 1955). The precise manner in which aerobic respiration is coupled to ion transport is still uncertain, but many significant observations and interpretations have been made. The first detailed interpretation of the link or couple between respiration and ion transport was presented by Lundegdrdh and Burstrom (1933). Lundegdrdh’s concept has been discussed at length (Lundegirdh, 1939, 1945, 1955), and only the major features will be consided here. In essence, he postulated that during the oxidation of reduced compounds, electrons were transferred through a chain of cytochromes and associated with this electron flow was a reversed flow of anions along the cytochrome chain. Cations were considered to enter cells passively in order to maintain electrical neutrality. Lundegirdh’s concept met with disfavor when it was realized that the cytochrome chain was localized in mitochondria and not in the plasma membrane. The concept was also inconsistent with the finding that the phosphorylation uncoupler, 2,4-dinitrophenol (DNP), which eliminates the formation of ATP but does not inhibit electron transfer through the cytochrome chain, is a powerful inhibitor of ion absorption (Robertson et al., 1951 ) . The latter finding was taken as evidence that ATP was the intermediary link between aerobic respiration and ion transport. This view was further supported by subsequent findings that arsenate (Ordin and Jacobson, 1955; Higinbotham, 1959; Weigl, 1963, 1964) and oligomycin (Hodges, 1966; Jacoby, 1966; Bledsoe et al., 1969), which interfere with ATP formation, were also potent inhibitors of ion absorption. Thus, it would appear that ATP is the actual energy source for ion transport; however, some observations indicate that this may not always be so. Evidence discounting ATP as the source of energy for ion transport has come from tissues other than roots with the exception of storage root tissue. With aged root tissue of carrots (Atkinson et al., 1966) and beets (Polya and Atkinson, 1969), various inhibitors such as a nitrogen atmosphere, uncouplers and ethionine (an ATP-trapping agent) did not affect ion absorption and the levels of ATP in the tissue in parallel. Thus, the corelation that one would expect if ATP were the energy source did not exist, and these authors concluded that electron transfer reactions, rather than ATP, were involved in ion absorption. This interpretation assumes


191

that there are no cellular pools or compartments of ATP, i.e., all the ATP in the cell is considered to be available for transport. This may not be so, and one must question whether total tissue levels of ATP would be expected to show correlations with ion transport rates. However, Cram (1969a) has also shown that C1- influx across the plasma membrane of carrot cells is not inhibited by either carbonyl cyanide rn-chlorophenyl hydrazone (CCCP) or oligomycin but is inhibited by anaerobiosis. In comparing these results to those of Atkinson et al. (1966) concerning ATP levels in the tissue, Cram also concluded that active C1- influx at the plasma membrane was more closely linked to redox reactions than to ATP per se. At the tonoplast, CCCP, oligomycin and anaerobiosis all inhibited C1- influx from cytoplasm to vacuole which would appear to implicate ATP. However, a longer time was required for oligomycin to inhibit C1influx than to bring about a decrease in tissue levels of ATP (Atkinson el al., 1966), and Cram suggested that a high energy intermediate (presumably of mitochondria1 respiration) rather than ATP was the likely energy source for C1- transport at the tonoplast. Light stimulates ion transport in green tissue (leaves and algae). Presumably the energy for ion absorption is derived from some aspect of photosynthesis (in addition to respiration). In these systems there have been claims that ATP is the energy source for ion transport (mostly for cation transport) (Rains, 1968; Nobel, 1969, 1970) as well as claims that electron transfer reactions are more closely linked to ion transport (mostly for anion transport) (MacRobbie, 1965, 1966; Raven, 1967, 1969; Nobel, 1969). In segments of corn leaf tissue, Rains (1967, 1968) showed that the light-stimulated K+ transport was not inhibited by 3- ( 3,4-dichlorophenyl) -1 ,I-dimethylurea (DCMU) . This indicated that photosystem 11, which is inhibited by DCMU, was not necessary for K+ transport. This result, along with the effect of other inhibitors, led Rains to conclude that ATP produced by cyclic photophosphorylation (photosystem I) provided the energy for K+ transport. In pea leaf tissue, Nobel (1969) showed that light stimulated both K+ and C1- absorption. Bicarbonate ions markedly enhanced the light-driven K+ absorption and this was thought to be due primarily to the absorption of HC0,- (although this was not measured) down a gradient which resulted from light-stimulated CO, fixation; K+ presumably entered passively as an electrical balance for the negatively charged HC0,-. Absorption of K+ was inhibited by the phosphorylation uncoupler, p-trifluoromethoxy carbonyl cyanide phenylhydrazone (tri-F1CCP). Nobel suggested this could be due to a diminished supply of ATP and a consequent curtailment of CO, fixation which would in turn diminish the gradient for HC0,- and thus the driving force for K+ entry. This interpretation could also account for the results obtained with corn leaves by

192

T. K. HODGES

Rains (1968). Chloride transport in pea leaves was little affected by the uncoupler tri-F1-CCP, indicating that ATP was not the energy source for C1- transport. DCMU did inhibit C1- absorption, and it was concluded that C1- transport was somehow closely coupled to the noncyclic electron transfer reactions. Thus, with regard to light-driven ion transport in leaf tissue, K+ transport appears to depend on ATP, either directly or indirectly, whereas C1- transport may be more closely linked to the photosynthetic electron transfer reactions. The energetics of ion transport in several algae have been carefully reviewed by MacRobbie (19,70), and I will only summarize the main points. In the two species studied most thoroughly (Nitella translucens and Hydrodictyon africanurn), ATP seems to be the energy source for cation transport but not for anion transport (MacRobbie, 1965, 1966; Raven, 1967, 1968, 1969). In both organisms, K and C1- are actively pumped across the plasma membrane and Na+ is actively extruded. The active fluxes of all 3 ions are stimulated by light, and K+ influx is coupled to Na+ eflux. The Kt- Na+ exchange in the algal cells is inhibited by ouabain which also inhibits the active K+ - Na-bexchange in animal cells (Skou, 1965). Also, this exchange is inhibited by uncouplers (DNP, CCCP) and phosphorylation inhibitors (phlorizin and Dio 9). All these results implicate ATP in K+ influx and Na+ efflux. Influx of C1-, on the other hand, was less sensitive to the uncouplers and phosphorylation inhibitors than either Kt influx or CO, fixation. Additionally, the blocking of photosystem I1 with DCMU inhibited C1- influx. Finally, it was shown in experiments with red and far-red light (Raven, 1967) that cation transport could be driven solely by photosystem I but C1- transport required both photosystems I and 11. From these results it was concluded (see MacRobbie, 1970) that C1- influx required something other than ATP; i.e., it was either coupled directly to electron transfer reactions or was dependent on some reduced product. From the foregoing discussions, it seems justifiable to conclude that ATP serves as the energy source for cation transport, but in certain tissues, especially those having a photosynthetic capability, a separate energy source is used for anion transport. In roots, there is no evidence that cation and anion transport require different energy sources. On the basis of the arsenate and oligomycin inhibition of both cation and anion transport, the energy source would appear to be ATP; however, the contrasting results with the fleshy tissues of beet and carrot indicate that a separate or alternative energy source may be involved. Additional evidence that ATP is the energy source for ion transport in roots is provided by the finding that the plasma membrane and tonoplast


193

of root cells have ATPase activity. This has been shown histochemically by Poux (1967) and Hall (1969, 1971a) and we (Hodges et al., 1972; Hodges and Leonard, 1973) have shown that isolated plasma membranes of oat roots possess an ion-stimulated ATPase. The association of this enzyme with the plasma membrane is especially interesting since it could represents the mechanism of energy coupling between ATP and ion transport. It is well documented that a plasma membrane-bound ATPase participates in the coupled transport of Na+ and K+ in animal cells (Skou, 1965). In roots of cereals we have obtained considerable evidence that the ATPase mentioned above is involved in ion absorption. It was first shown that crude preparations of membranes, i.e., differential centrifugation fractions, contained ATPase activity that required Mg", but was further stimulated by a variety of monovalent salts, such as KCl, NaCl, K,SO, (Fisher and Hodges, 1969). Activation of the ATPase by monovalent salts depends on Mg"; in the absence of Mg2+,the monovalent salts are ineffective. Mn2+will substitute for the Mg" requirement, but other divalent cations are less effective (Leonard and Hodges, 1973). Fisher et al. (1970) have shown that the component of the ATPase activated by K+ or Rb+ is highly correlated with K+ or Rb+ absorption in roots of 4 plznt species (Fig. 5 ) . These results suggest that the ATPase is involved in ion absorption. In subsequent studies it was found that the ATPase that is stimulated by monovalent ions is associated with the plasma membranes of oat root cells (Hodges et al., 1972; Hodges and Leonard, 1973; Leonard et al., 1973). The ATPase is very specific; other nucleoside triphosphates are hydrolyzed at less than 5 % of the rate of ATP. In experiments using the purified plasma membranes (Leonard and Hodges, 1973), the ATPase was found to be primarily activated by cations with a selectivity preference at 50 mM salt concentrations of K+ > Rb+ > Na+ > Cs+ > Li+ (Sequence V in the Eisenman selectivity scheme). Preliminary studies indicate this to be the order of specificity for transport at 50 mM concentrations in oat roots (Fisher, 1969; H. Sze and T. K. Hodges, unpublished). Organic cations (tris, choline, and tetramethyl ammonium ions) are also capable of activating the ATPase, but they were less effective than the inorganic cations (Leonard and Hodges, 1973). Ratner and Jacoby (1973) have recently reported that organic cations are as effective as the inorganic cations in activating ATPases. This result was probably due, however, to their use of crude membrane preparations which are known to contain several different membrane-bound ATPases (Hodges and Leonard, 1973; Leonard et al., 1973). In addition, the pH of their ATPase assays was 8.2, but the pH optimum for K' stimulation of the plasma membrane ATPase is 6.5 (Leon-

T. K. HODGES

194

ard and Hodges, 1973). Whether the organic cation activation of the plasma membrane ATPase (Leonard and Hodges,, 1973) is related to organic cation absorption is unknown. The kinetics of the plasma membrane ATPase have been determined for ATP, Mg2+,and K+ (Leonard and Hodges, 1973). ATP and Mg2+activation of the ATPase exhibited typical Michaelis-Menten kinetics, and the K , values were 0.38 and 0.84 mM, respectively. K+ activation of the 33

0

7

Rb* or K' CONCENTRATION (mM)

FIG. 5 . Upper: Influx of K+ (oats) or Rb' (barley, wheat, corn) into roots as a function of the external concentration of K+ or Rb'. Lower: K+ or Rb+-stimulated ATPase activity of membrane fractions obtained from roots of the various species as a function of K' or Rb' concentration A-A, barley; 0-0, oats; X-X, wheat; 0-0, maize. (From Fisher and Hodges, 1969.)

ATPase was not typical of Michaelis-Menten kinetics since a saturation did not occur for K+ concentrations from 0.01 to 100 mM (see Fig. 6). Instead, the K, for K+ continually increased as the K concentration increased, and, as discussed in Section IV, these kinetics are described as negative cooperativity (Koshland, 1970). The different Km's for K+ do not appear to result from several ATPases having different affinities for K+ since the affinities for both Mg2+and ATP did not vary; the latter might be expected if different enzymes were involved. The similarity in the kinetics of K+ activation of the plasma membrane ATPase of oat roots and

195


K+ absorption by oat roots is apparent when one compares Figs. 3 and 6. In view of the negative cooperativity kinetics for K+ activation of the ATPase, the enzyme should consist of interacting subunits according to the Koshland model. We have not determined whether this is the case, but it is interesting that the ATPase of mitochondria consists of at least 11 subunits (MacLennan, 1970) and the ATPase of chloroplasts have been shown to consist of five subunits (Nelson et al., 1973).

I

0

I

10

I

I

20

30

I

40

50

Velocity KCI (mM)

FIG. 6. Eadie, Hofstee plot of the K+-stimulatedATPase activity of plasma membranes obtained from oat roots. (From Leonard and Hodges, 1973.)

In the last 10 years there have been several other reports of ATPase activity in cell free homogenates of plant tissue (Brown and Altschul, 1964; Brown et al., 1965; Dodds and Ellis, 1966; Gruener and Neumann, 1967; Neumann and Gruener, 1967; Atkinson and Polya, 1967; Hall and Butt, 1969; Hansson and Kylin, 1969; Horowitz and Waisel, 1970; Kylin and Gee, 1970; Sexton and Sutcliffe, 1969; Hall, 1971b; Leonard and Hanson, 1972; Ratner and Jacoby, 1973). Of these reports, only the ones by Leonard and Hanson (1972), Hansson and Kylin (1969), and Kylin and Gee (1970) seem to illustrate a potential role of the ATPase in transport, whereas the paper by Ratner and Jacoby (1973) claims that the

196

T. K. HODGES

enzyme is not involved in absorption. Leonard and Hanson (-1972) found that the K+-stimulated ATPase of a membrane fraction of corn roots increased as a result of washing (aging) the roots and that this correlated with an increased capacity for ion absorption. However, the increased ATPase activity caused by washing was not as great as the increased ion absorption rates, so it is not clear whether the two phenomena were related. Hansson and Kylin (1969) and Kylin and Gee (1970) report that the ATPase activity of membrane fractions of sugar beet and mangrove is stimulated more by combinations of Nat and K+ than by either ion alone. This is an interesting finding because it is similar to that reported for animal cells (Skou, 1965); however, it is not clear whether the plant enzyme studied by Kylin and associates is involved in transport. It should be pointed out here that there are at least 5 membrane-associated ATPases in oat root cells (Hodges and Leonard, 1973; Leonard et al., 1973), and presumably this would be true for the beet and mangrove cells. Thus, the possibility exists that the Na+ Kt stimulated ATPase activity found in Kylin's studies was due to two different membrane-bound ATPases. With the ATPase of the purified plasma membrane fraction of oat roots, various combinations of K+ and Na+ were not significantly better than either ion separately (Leonard and Hodges, 1973). It may also be relevant that oats are glycophytes whereas both the sugar beet and mangrove are halophytes, and Horowitz and Waisel ( 1 970) have obtained evidence that the Na+ stimulated ATPase activity of membrane fractions from these two groups of plants is somewhat different. The disparity between organic cation stimulation of ATPase activity and cation absorption by roots led Ratner and Jacoby (1973) to question the role of an ATPase in ion absorption. However, the basis for this view is questionable since their studies involved the use of impure membrane preparations, and an assay pH that may have discriminated against the plasma membrane ATPase. Although reservations have been expressed (Ratner and Jacoby, 1973), the evidence that an ATPase functions in ion absorption by roots can be summarized as follows: (1 ) a high correlation exists betwen K+ or Rb+ absorption and the K+ or Rb+ stimulated ATPase activity in roots of 4 plant species (Fisher and Hodges, 1969; Fisher et al., 1970). ( 2 ) The ATPase is cation-activated, and it is specific for ATP (Hodges et al., 1972; Leonard and Hodges, 1973). ( 3 ) The ATPase is on the plasma membrane (Hodges et al., 1972). (4) The kinetics of K+ activation of the plasma membrane ATPase is virtually identical to the kinetics of K+ absorption by oat roots (Leonard and Hodges, 1973). Taken together, these results strongly support the concept that ATP provides the energy for transport and that the plasma membrane ATPase is responsible for the energy coupling involved in ion transport.

+


197

The mechanism of energy transfer, via an ATPase, to ion transport has been most thoroughly studied in mitochondria (Vasington and Murphy, 1962; Hanson and Hodges, 1967; Moore, 1971) and chloroplasts (Packer et al., 1970; Dilley, 1971). In these organelles, it has been shown that either ATP or the electron transfer reactions can provide the energy for ion transport (Hodges and Hanson, 1965; Elzam and Hodges, 1968; Packer et al., 1970). With either energy source a common high energy intermediate state is produced which is very closely coupled to ion transport. The formation of the high energy state can be depicted as follows: Uncouplers

Electron transport P hospho ry lat ion inhibitors

Electron transport inhibitors Ion transport

Ion transport driven by electron transport reactions is inhibited by substances such as antimycin A or cyanide, but not by phosphorylation inhibitors, such as oligomycin or phlorizin. When AT? is the driving force for transport, the phosphorylation inhibitors a-e effective in blocking ion transport but the electron transport inhibitors are ineffective. Uncouplers such as DNP or CCCP inhibit ion transport when either electron transport or ATP is the energy source, and this is generally believed to be due to the hydrolysis or dissipation of the intermediate high energy state. The high energy intermediate state is believed by some to be a proton gradient across the membrane (Mitchell, 1961, 1966; Robertson, 1960, 1968), a specific compound having a high free energy of hydrolysis (Slater, 1953; Chance and Williams, 1955), a specific conformational state of the membrane or proteins within the membrane (Dilley, 1971; Green et al., 1968), or a combination of these alternatives (Hanson et al., 1972). Presently, it is uncertain which of these concepts is correct (Slater, 1971); however, the proton gradient has received the most support with respect to how it might function in driving ion transport (Mitchell, 1966, 1968; Robertson, 1968; Chappell and Haarhoff, 1967). The formation of a proton gradient across a membrane by the action of an ATPase has been discussed by Mitchell ( 1961, 1966, 1968). In essence, the 2 sides of the membrane are thought to be sufficiently different that the components of water (H+ and OH- or 2H+ and 0 2 - ) involved in the hydrolysis of ATP are believed to approach the ATPase

T. K. HODGES

198

from different sides of the membrane (Mitchell, 1961). In its simplest form, this leaves a H+ on one side of the membrane and an OH- on the other. Depending on the reactive elements of H,O, the magnitude of the pH gradient generated by ATP hydrolysis might be different, e.g., Mitchell's ATPase I and I1 (1966, 1968). The main feature, however, is that a pH and charge gradient is created across the membrane, and, if the membrane is impermeable to H+ and OH-, this gradient represents a form of energy which can be coupled to endergonic processes. Mitchell (1366, 1968) suggests that the transport of K+,Ca2+,phosphate, organic acids, ADP, and ATP could be coupled to either H+ or OH- fluxes across the membrane with the mediation of carriers (or porters in his terminology). Chappell and Haarhoff (1967) have obtained evidence for these exchange reactions in mitochondria. The plasma membrane ATPase could conceivably be coupled to ion transport in a manner similar to that observed for ATPase-linked ion transport in the organelles. As mentioned previously, the exact nature of the high energy intermediate state resulting from ATP hydrolysis is uncertain, but the proton gradient concept of Mitchell (1968) can most readily account for a variety of associated ion fluxes. Accordingly, it is proposed in the next section that the monovalent c'ation-activated ATPase of the plasma membrane gives rise to a pH and charge gradient across the plasma membrane which serves to drive anion fluxes across the membrane.

VI.

Proposed Model for Ion Absorption by Roots

In an attempt to reconcile the various observations discussed here, a model is proposed (Fig. 7) that could account for both cation and anion absorption by root cells. This model consists of two main features: (1) A cation-activated A TPase in the plasma membrane. This ATPase brings about the exchange of cations across the membrane, and it generates a charge and pH gradient across the membrane. ( 2 ) An anion carrier that brings about the exchange of anions across the plasma membrane. The internal anions can be either OH- resulting from the pH gradient established by the ATPase or HC0,- ions resulting from aerobic respiration. The ATPase is activated by monovalent cations, and the kinetics of this activation are consistent with the concept that the enzyme consists of subunits which interact (Leonard and Hodges, 1973). Cation activation of the enzyme involves the exchange of an alkali cation for H+ which brings about a conformational change of the cation-subunit complex (Koshland, 1970). This conformational change could move the cation from the outside surface of the membrane to the inside surface of the membrane where it


199

would, in time, exchange for cytoplasmic H'. The initial conformational change of the cation-subunit complex alters or distorts the other subunits of the ATPase such that their electrical field strength is increased (Eisenman, 1962). An increased field strength would increase the affinity of the subunit for H , thus decreasing the overall affinity for the alkali cations. As a consequence of the higher field strength of the binding sites, the preference for the alkali ions is shifted. This accounts for the observed change in selecOutside

Plasma Membrane

Cytoplasm

No, K *

FIG. 7. A hypothetical model depicting how inorganic cations and anions are transported across the plasma membrane into root cells. See text for explanation.

tivity of ion transport as the external concentration is increased (see Section IV) . The ATPase can also give rise to a pH and charge gradient across the membrane (Mitchell, 1961) which represents an intermediate conservation of the energy originally present in ATP. Since a charge separation occurs, the ATPase contributes directly to the electrical potential and is therefore electrogenic. Inhibition of the ATPase by shutting off the supply of ATP or by direct inhibition would cause the electrical potential to fall abruptIy. This accounts for the view that H+ is actively secreted by roots (Pitman, 1970), and for the evidence that ion transport is electrogenic (see Section II,3).

200

T. K. HODGES

The anion carrier exchanges internal OH- for external anions, and this collapses the pH gradient. Thus, inward anion transport uses the energy which was temporarily conserved in the pH gradient. According to the model, the ATPase is directly responsible for cation transport and indirectly responsible for anion transport. However, the anion carrier depends on an internal anion such as OH- or HC0,- and is not necessarily dependent on ATP. Any intracellular reaction that generates excess OH- or HC0,- could drive the anion carrier. For example, the light-driven C1- transporr in green tissue or cells may be a Cl-/OHexchange with the OH- being generated by the chloroplast redox reactions. This possibility is supported by studies which show a light stimulated HC0,-/OH- exchange in green algae (Smith, 1970; Raven, 1970; Lucas and Smith, 1973). In roots, anion/HCO,- and HC0,-/OH- exchanges are probable. This is based on the increase or decrease in organic acids which accompany excess cation or excess anion uptake (Ulrich, 1941; Jacobson and Ordin, 1954; Hurd, 1958; Hurd and Sutcliffe, 1957; Torii and Laties, 1966b; Hiatt, 1967a,b; Pitman, 1970; Zioni et al., 1971). For example, when cation absorption exceeds anion absorption a HC0,-/OH- exchange is likely. When anion absorption exceeds cation absorption an anion/ HC0,- exchange is likely. In the latter situation, the HC0,- would be generated by breakdown of organic acids, and thus anion entry would not be driven by ATP. Direct evidence for this has not been obtained, but one would predict that uncouplers of phosphorylation should have less effect on anion transport than on cation transport. This is true for photosynthetic tissue (see Section V ) , but similar comparisons have not been made with roots. The important point here is that cation transport depends on ATP and the plasma membrane ATPase. But, the anion carrier, and thus anion influx, is driven by internal anions, which can be generated by the action of the ATPase, breakdown of organic acids or, in the case of green tissue, by OH- ions produced by chloroplasts. The relationship between ion fluxes and membrane electrical potentials also deserves further comment. Na+ appears to be actively secreted at the plasma membrane, and K+ is generally close to electrochemical equilibrium (see Section 111). In this model, ATP hydrolysis, via the ATPase, contributes directly to the membrane potential. Since cations activate the ATPase, one would expect to find a close relationship between the electrical potential difference generated by the ATPase and cation transport, and this generally is what is observed for K+, but not for Na+. The basis for active Na+ efflux could reside in the ion binding sites on the ATPase having a high electric field strength following the ion-induced conformational change. A site having a high field strength prefers either H+ or Na+ over K+ (see section IV). Whether a K+/Na+exchange on the binding site at


20 1

the cytoplasmic side of the membrane would induce the carrier to return to its original conformation is unknown, but if this occurred, it could account for Na+ being actively transported back across the membrane. This might also be the basis for the plant ATPase being slightly stimulated by combinations of Na' and K (Hansson and Kylin, 1969; Kylin and Gee, 1970). This model is admittedly speculative, but it is based on the characteristics of ion absorption by a variety of plant organs, tissues, and organelles. In addition, it combines the concepts of negative cooperativity kinetics (Koshland, 1970), the thermodynamic basis for selective ion binding by charged sites (Eisenman, 1962; Diamond and Wright, 1969), an ATPase generated proton gradient (Mitchell, 1961, 1966), and an anion exchange carrier (Mitchell, 1968). Thus, the model represents the integration of several different concepts with a variety of experimental observations. It is, however, a hypothetical model, and many of its features need to be critically evaluated.

VII.

'

Summary

Major advances are being made toward elucidating the mechanism of nutrient absorption by roots. Some of these are as follows: 1. It is now possible to estimate the concentrations of ions in the cytoplasm and vacuoles of root cells. The bidirectional fluxes of ions across both the plasma membrane and tonoplast can be determined. A knowledge of these parameters is permitting the electrophysiologist to evaluate the driving forces responsible for ion movements into and out of root cells. 2. Kinetics of ion absorption by roots is similar to the kinetics of enzyme catalyzed reactions. This is providing insight into the nature of ion carriers. It is suggested here that an ion carrier consists of several subunits, and it is the interaction of these subunits that is responsible for the observed decrease in ion affinities as the external ion concentration is increased. 3. The selectivity of ion absorption by roots is similar to the selectivity of ion binding to glass electrodes. The basis for the latter is the electrical field strength of the binding sites. It is suggested here that variations in the field strength of binding sites on the carriers are responsible for the selectivity of ion absorption by roots. The field strength of the binding sites may be governed by the interaction of carrier subunits, as well as by the molecular environment of the ion carrier, e.g., the lipid and protein composition of the membrane. 4. Aerobic respiration provides the energy for ion absorption by roots.

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ATP appears to be the primary energy source for absorption, and an ATPase in the plasma membrane may represent the energy transducing agent between ATP and transport. The possibility of the ATPase being a cation carrier is considered. The possibility of anion absorption being coupled to HC0,- and/or OH- efflux, via an anion exchange-carrier, is suggested. 5 . The plasma membrane of root cells has only recently been isolated. This should facilitate the isolation and identification of ion carriers. In this paper I have primarily concentrated on reconciling the mechanism of ion transport. Hopefully, the ideas and concepts that have emerged will lead to a better understanding of fertilizer usage by crops, the effects of moisture or temperature stresses on nutrient absorption, the unique ability of certain plants to thrive in saline areas, and plant growth in general. ACKNOWLEDGMENTS

I especially wish to thank Drs. Charles E. Bracker, Richard A. Dilley, and D. James Morrt for reading the manuscript and making many helpful suggestions. REFERENCES Atkinson, M. R., and Polya, G. M. 1967. Aust. I . Biol. Sci. 20, 1069-1086. Atkinson, M. R., and Polya, G. M. 1968. Aust. J . Biol. Sci. 21, 409-420. Atkinson, M. R., Eckerrnan, G., Grant, M., and Robertson, R. N. 1966. Proc. Nut. Acad. Sci. US.55, 560-564. Bange, ,G. G. J., and Meijer, C. L. C. 1966. Acta Bot. Neer. 15, 434-450. Barber, S. A. 1966. Nature (London) 212, 638-640. Barr, C. E. 1965. J. Gen. Physiol. 49, 181-197. Bear, F. E., and Prince, A. L. 1945. J . Amer. SOC. Agron. 37, 217-222. Bledsoe, C., Cole, C. V., and Ross, C. 1969. Plant Physiol. 44, 1040-1044. Blinks, L. R. 1935. J . Gen. Physiol. 18, 409-420. Boszormenyi, Z., and Cseh, E. 1961. Physiol. Plant. 14, 242-252. Boszormenyi, Z., and Cseh, E. 1964. Physiol. Plant. 17, 81-90. Bowling, D. J. F., and Ansari, A. Q . 1971. Planta 98,323-329. Bowling, D. J. F., and Ansari, A. Q. 1972. J. Exp. Bot. 23, 241-246. Briggs, G. E., and Robertson, R. N. 1957. Annu. Rev. Plant Physiol. 8 , 11-32. Briggs, G. E., Hope, A. B., and Robertson, R. N. 1961. “Electrolytes and Plant Cells.” Blackwell, Oxford. Brown, H. D., and Altschul, A. M. 1964. Biochern. Biophys. Res. Commun. 15, 479-483. Brown, H. D., Neucere, N. J., Altschul, A. M., and Evans, W. J. 1965. Life Sci. 4, 1439-1447. Butler, G . W. 1953. Physiol. Planf. 6, 617-635. Cereijido, M., and Rotunno, C. A. 1970. “Introduction to the Study of Biological Membranes.” Gordon & Breach, New York. Chance, B., and Williams, G. R. 1955. J. Biol. Cliem. 217, 383-407. Chappell, J. B., and Haarhoff, K. N. 1967. In “Biochemistry of Mitochondria” (E. C. Slater, Z. Kaniuga, and L. Wojtczak, eds.), p. 75. Academic Press, New York.


203

Collander, R. 1941. Plant Physiol. 16, 691-720. Collander, R. 1959. I n “Plant Physiology” (F. C. Steward, ed.), Vol. 2, pp. 3-103. Academic Press, New York. Coster, H. G. L. 1966. Aust. J. Biol. Sci. 19, 545-554. Cram, W. J. 1968a. Biochim. Biophys. Acta 163, 339-353. Cram, W. J. 1968b. J. Ewp. Bor. 19, 611-616. Cram, W. J. 1969a. Biochim. Biophys. Acta 173, 213-222. Cram, W. J., and Laties, G. G. 1971. Aust. J . Biol. Sci. 24, 633-646. Dainty, J . 1962. Annu. R e v . Plant Physiol. 13, 379-402. Diamond, J. M., and Solomon, A. K. 1959. J . G e n . Physiol. 42, 1105-1121, Diamond, J. M., and Wright, E. M. 1969. Annu. R e v . Physiol. 31, 581-646. Dilley, R. A. 1971. Curr. Top. Bioenerg. 4, 237-271, Dodds, J. J. A., and Ellis, R. J. 1966. Biochem. ,I. 101, 31. Dunlop, J., and Bowling, D. J. F. 1971. J . Exp. Bot. 22, 445-452. Eadie, G. S. 1942. J . Biol. Chem. 146, 85-93. Eisenman, G. 1961. I n “Membrane Transport and Metabolism” (A. Klienzeller and A. Kotyk, eds.), p. 163. Academic Press, New York. Eisenman, G. 1962. Biophys. 1. 2, 259-323. Eisenman, G. 1965. In!. Congr. Physiol. Sci. [Proc.], 23rd pp. 489-506. Elzam, 0. E., and Epstein, E. 1965. Plant Physiol. 40, 620-624. Elzam, 0. E., and Hodges, T. K. 1967. Plant Physiol. 42, 1483-1488. Elzam, 0. E., and Hodges, T. K. 1968. Plant Physiol. 43, 1108-1114. Elzam, 0. E., Rains, D. W., and Epstein, E. 1964. Biochem. Biophys. Res. Commrtn. 15, 273-276. Epstein, E. 1953. Nature (London) 171, 83-84. Epstein, E. 1955. Plant Physiol. 30, 529-535. Epstein, E. 1961. Plant Physiol. 36, 437-444. Epstein, E. 1966. Nature ( L o n d o n ) 212, 1324-1327. Epstein, E. 1968. Experientia 24, 616-617. Epstein, E. 1972a. “Mineral Nutrition of Plants: Principles and Perspectives.” Wiley, New York. Epstein, E. 1972b. N e w Phytol. 71, 873-874. Epstein, E. 1973. I n t . R e v . Cytol. 34, 123-168. Epstein, E., and Hagen, C. E. 1952. Planr Physiol. 27, 457-474. Epstein, E., and Rains, D. W. 1965. Proc. Nut. Acad. Sci. US.53, 1320-1324. Epstein, E., Rains, D. W., and Elzam, 0. E. 1963. Proc. Nut. A c a d . Sci. US. 49, 684-692. Etherton, B. 1963. Plant Physiol. 38, 581-585. Etherton, B. 1967. Plant Physiol. 42, 685-690. Etherton, B. 1968. Plant Physiol. 43, 838-840. Etherton, B., and Higinbotham, N. 1960. Science 131, 409-410. Fisher, J. D. 1969. Ph.D. Dissertation, University of Illinois, Urbana. Fisher, J., and Hodges, T. K. 1969. Plant Physiol. 44, 385-395. Fisher, J . D., Hansen, D., and Hodges, T. K. 1970. Plant Physiol. 46, 812-814. Foote, B., and Hanson, J. B. 1964. Plant Physiol. 39, 450-460. Fried, M., and Broeshart, H. 1967. “The Soil-Plant System in Relation to Inorganic Nutrition.” Academic Press, New York. Fried, M., and Noggle, J. C. 1958. Plant Physiol. 33, 139-144. Fried, M., and Shapiro, R. E. 1961. Annu. R e v . Plant Physiol. 12, 91-112. Fried, M., Oberlander, H. E., and Noggle, J. C. 1961. Plant Physiol. 36, 183-191.

204

T. K. HODGES

Gauch, H. G. 1972. “Inorganic Plant Nutrition.” Dowden, Hutchinson & Ross, Inc., Stroudsburg, Pennsylvania. Gerhart, J. C., and Pardee, A. B. 1962. J. Biol. C k e m . 237, 891-896. Gerson, D. F., and Poole, R. J. 1972. Plant Physiol. 50, 603-607. Goldman, D. E. 1943. J . Gen. Physiol. 27, 37-60. Green, D. E., Asai, J., Harris, R. A,, and Penniston, J. T. 1968. Arch. Biochem. Biophys. 125, 684-705. Gruener, N., and Neumann, J. 1967. Physiol. Plant. 19, 678-682. Hagen, C. E., and Hopkins, H. T. 1955. Plant Physiol. 30, 193-199. Hagen, C. E., Leggett, J. E., and Jackson, P. C. 1957. Proc. Nat. Acad. Sci. U.S. 44, 496-503. Hall, J. L. 1969. PIanta 85, 105-107. Hall, J. L. 1971a. J . Microsc. (Paris) 93, 219-225. Hall, J. L. 1971b. J . Exp. Bot. 22, 800-808. Hall, J. L., and Butt, V. S. 1969. J. Exp. Bot. 20, 751-762. Handley, R., Metwally, A., and Overstreet, R. 1965. Plant Physiol. 40, 513-520. Hanson, J. B., and Hodges, T. K. 1967. Curr. T o p . Bioenerg. 2, 65-98. Hanson, J. B., Bertagnolli, B. L., and Shepherd, W. D. 1972. Plant Physiol. 50, 347-354. Hansson, G., and Kylin, A. 1969. 2. Pflanzenphysiol. 60, 270-275. Hiatt, A. J. 1967a. 2. Pflanzenphysiol. 56, 233-245. Hiatt, A. J. 1967b. Plant Physiol. 42, 294-298. Hiatt, A. J. 1968. Plant Physiol. 43, 893-901. Hiatt, A. J. 1969. Plant Physiol. 44, 1528-1532. Hiatt, A. J. 1970a. Plant Physiol. 45, 408-410. Hiatt, A. J. 1970b. Plant Physiol. 45, 411-414. Hiatt, A. J., and Hendricks, S. B. 1967. Z Pflanzenphysiol. 56, 220-232. Higinbotham, N. 1959. Plant Physiol. 34, 645-650. Higinbotham, N. 1970. Amer. Zoo/. 10, 393-403. Higinbotham, N. 1973. Annu. Rev. Plant Physiol. (in press). Higinbotham, N., Etherton, B., and Foster, R.J. 1964. Plant Physiol. 39, 196-203. Higinbotham, N., Etherton, B., and Foster, R.J. 1967. Plant Physiol. 42, 37-46. Higinbotham, N., Graves, J. S., and Davis, R. F. 1970. J . Membrane Biol. 3, 210-222. Hill, A. V. 1910. 1. Physiol. (London) 40, IV-VII. Hoagland, D. R., and Broyer, T. C. 1936. Plant Physiol. 11, 471-507. Hoagland, D. R., and Davis, A. R. 1929. Protoplasma 6,610-626. Hodges, T. K. 1966. Nature (London) 209,425-426. Hodges, T. K., and Hanson, J. B. 1965. Plant Physiol. 40, 101-109. Hodges, T. K., and Leonard, R. T. 1973. In “Methods in Enzymology” (S. P. Colowick and N. 0. Kaplan, eds.). Academic Press. New York (in press). Hodges, T. K., and Vaadia, Y . 1964 Plant Physiol. 37, 490-494. Hodges, T. K., Leonard, R. T., Bracker, C. E., and Keenan, T. W. 1972. Proc. Nut. Acad. Sci. U.S. 69, 3307-3311. Hofstee, B. H. J. 1952. Science 116, 329-331. Hope, A. B. 1965. Aust. J . Biol. Sci. 18, 789-801. Hope, A. B. 1971. “Ion Transport and Membranes, A Biophysical Outline.” Butterworth, London. Hope, A. B., and Stevens, P. G. 1952. Aust. J . Sci. Res., Ser. B 5, 335-343. Horowitz, C. T., and Waisel, Y . 1970. Experientia 26, 941-942.


205

Hurd, R. G. 1958. J. Exp. Bot. 9, 159-174. Hurd, R. G., and Sutcliffe, J. F. 1957. Nature ( London) 180, 233-235. Jackson, P. C., and Adams, H. R. 1963. J . Gen. Physiol. 46, 369-386. Jackson, P. C., and Stief, K. J. 1965. J . Gen. Physiol. 48, 601-616. Jacobson, L., and Ordin, L. 1954. Plant Physiol. 29, 70-75. Jacobson, L., Overstreet, R., King, H. M., and Handley, R. 1950. Plant Pliysiol. 25, 639-647. Jacobson, L., Moore, D. P., and Hannapel, R. J. 1960. Plant Physiol. 35, 352-358. Jacoby, B. 1966. Plant Cell Physiol. 7 , 307-311. Jennings, D. H. 1963. “The Absorption of Solutes by Plant Cells.” Iowa State Univ. Press, Ames. Jeschke, W . D. 1970. Planta 94, 240-245. Johansen, C., Edwards, D. G., and Loneragan, J. F. 1970. Plant Physiol. 45, 601-603. Kahn, J. S., and Hanson, J. B. 1957. Plant Physiol. 32, 312-316. Kedem, 0. 1961. I n “Membrane Transport and Metabolism” (A. Kleinzeller and A. Kotyk, eds.), pp. 87-93. Academic Press, New York. Kitasato, H. 1968. J . Gen. Physiol. 52, 60-87. Koshland, D. E. 1969. Curr. Top. Cell. Regul. 1, 1. Koshland, D. E. 1970. I n “The Enzymes” (P. D. Boyer, ed.), 3rd ed., Vol. 1, pp. 341-396. Academic Press, New York. Kylin, A., and Gee, R. 1970. Plant Physiol. 45, 169-172. Laties, G. G. 1969. Annu. Rev. Plant Physiol. 20, 89-116. Leggett, J. E., and Epstein, E. 1956. Plant Plrysiol. 33, 222-226. Leonard, R. T., and Hanson, J. B. 1972. Plant Physiol. 49,436-440. Leonard, R. T., and Hodges, T. K. 1973. Plant Physiol. 52, 6-12. Leonard, R. T., Hansen, D., and Hodges, T. K. 1973. Plant Physiol. 51, 749-754. Levitzki, A., and Koshland, D. E. 1969. Proc. Nut. Acad. Sci. U S . 62, 1121-1128. Lineweaver, H., and Burk, D. 1934. J. Amer. Chem. SOC. 56, 658-666. Lucas, W. J., and Smith, F. A. 1973. J . Erp. Bot. 24, 1-14. Lund, E. J. 1928. I . Exp. Zool. 51, 265-290. Lundegirdh, H. 1934. Annu. Rev. Bioclrern. 3, 485-500. Lundeglrdh, H. 1939. Nature ( London) 143, 203-204. Lundeglrdh, H. 1945. A r k . Bot. 32A, 1-139. LundegHrdh, H. 1955. Annu. Rev. Plant Physiol. 6, 1-24. LundegHrdh, H., and Burstrom, H. 1933. Planta 18, 683-699. Luttge, U., and Laties, G. G. 1966. Plant Physiol. 41, 1531-1539. Luttge, U., and Laties, G. G. 1967. Plant Physiol. 42, 181-185. MacDonald, I. R., DeKock, P. C., and Knight, A. H. 1960. Physiol. Plant. 13, 76-89. Macklon, A. E. S., and Higinbotham, N., 1970. Plant Pliysiol. 45, 133-138. MacLennan, D. H. 1970. I n “Current Topics in Membranes and Transport” (F. Bronner and A. Kleinzeller, eds.), Vol. 1, pp. 177-232. Academic Press, New York. MacRobbie, E. A. C. 1965 Biochim. Biophys. Acta 794, 64-73. MacRobbie, E. A. C. 1966. Aust. J . Biol. Sci. 19, 363-370. MacRobbie, E. A. C. 1969. J. Exp. Bor. 20, 236-256. MacRobbie, E. A. C. 1970. Quart. Rev. Biophys. 3, 251-293. MacRobbie, E. A. C., 1971. Annu. Rev. Plant Physiol. 22, 75-96. MacRobbie, E. A. C., and Dainty, J. 1958a. J . Gen. Physiol. 43, 335-353. MacRobbie, E. A. C., and Dainty, J. 1958b. Physiol. Planr. 11, 782-801.

206

T. K. HODGES

Marinos, N. G. 1962. Amer. J . Bot. 49, 834-841. Mitchell, P. 1961. Nature (London) 191, 144-148. Mitchell, P. 1966. Biol. Rev. 41, 445-502. Mitchell, P. 1968. “Chemiosrnotic Coupling and Energy Transduction.” Glynn Res. Ltd., Bodmin, Cornwall, England. Monod, J., Changeux, J. P., and Jacob, F. 1963. J. Mol. Biol. 6, 306-329. Moore, C. L. 1971. Curr. Top. Bioenerg. 4, 191-236. Neirinckx, L. J. A., and Bange, G. G. T. 1971. Acta Bot. Neer. 20,481-488. Nelson, N., Deters, D. W., Nelson, H., and Racker, E. 1973. 1. Biol. Chem. 248, 2049-2055. Nerncek, O., Sigler, K., and Kleinzeller, A. 1966. Biochim. Biophys. Acta 126, 73-80. Neurnann, J., and Gruener, N. 1967. Zsr. J . Chem. 6, 107. Nissen, P. 1971. Physiol. Plant. 24, 315-324. Nobel, P. 1969. Plant Cell Physiol. 10, 597-605. Nobel, P. 1970. Plant Physiol. 46, 491-493. Noggle, J. C., and Fried, M. 1960. Soil Sci. SOC.Amer., Proc. 24, 33-35. Ordin, L., and Jacobson, L. 1955. Plant Physiol. 30, 21-27. Osterhout, W. J. V. 1935. Proc. Nut. Acad. Sci. U.S. 21, 125-132. Overstreet, R., Jacobson, L., and Handley, R. 1952. Plant Physiol. 27, 583-590. Packer, L., Murakami, S., and Mehard, C. W. 1970. Annu. Rev. Plant Physiol. 21, 271-304. Pallaghy, C. K., and Scott, B. I. H. 1969. Aust. J . Biol. Sci. 22, 585-600. Pallaghy, C. K., Luttge, U., and von Willert, K. 1970. 2. Pganzenphysiol. 62, 51-57. Penth, B., and Weigl, J. 1969. 2. Naturforsch. B 24, 342-348. Pierce, W. S.,and Higinbotham, N. 1970. Plant Physiol. 46, 666-673. Pitman, M. G. 1963. Aust. J . Biol. Sci. 16, 647-668. Pitrnan, M. G. 1969. Plant Physiol. 44, 1233-1240. Pitrnan, M. G. 1970. Plant Physiol. 45, 787-790. Pitrnan, M. G. 1971. Aust. 1. Biol. Sci. 24, 407-421. Pitrnan, M. G., and Saddler, H. D. W. 1967. Proc. Nut. Acad. Sci. US.57, 44-49. Pitman, M. G., Courtice, A. C.,and Lee, B. 1968. Aust. J. Biol. Sci. 21, 871-881. Polya, G. M., and Atkinson, M. R. 1969. Aust. J. Biol. Sci. 22, 573-584. Poole, R. I. 1966. J . Cen. Physiol. 49, 551-563. Poole, R. J. 1969. Plant Physiol. 44,485-490. Poole, R. J. 1971a. Plant Physiol. 47, 731-734. Poole, R. J. 1971b. Plant Physiol. 47, 735-139. Poux, N. 1967. J . Microsc. (Paris) 6, 1043-1058. Rains, D. W. 1967. Science 156, 1382-1383. Rains, D. W. 1968. Plant Physiol. 43, 394-400. Rains, D. W. 1969. Plant Physiol. 44, 547-554. Rains, D. W., and Epstein, E. 1967a. Plant Physiol. 42, 314-318. Rains, D. W., and Epstein, E. 1967b. Plant Physiol. 42, 319-323. Rains, D. W., and Floyd, R. A. 1970. Plant Physiol. 46, 93-98. Ratner, A., and Jacoby, B. 1973. J. Exp. Bot. 24, 231-238. Raven, J. A. 1967. J . Cen. Physiol. 50, 1627-1640. Raven, J. A. 1968. J . Exp. Bot. 19, 233-253. Raven, J. A. 1969. New Phytol. 68, 1089-1113. Raven, J. A. 1970. Biol. Rev. Cambridge Phil. SOC. 45, 167-221.


207

Robertson, R. N. 1960. Biol. Rev. Cambridge Phil. SOC.35, 321-264. Robertson, R. N. 1968. Cambridge Monogr. Exp. Biol. No. 15. Robertson, R. N., and Turner, J. S. 1945. Aust. J . Exp. Biol. 23, 63-73. Robertson, R. N., and Wilkins, M. J. 1948. Aust. J . Sci. Res. B 1, 17-37. Robertson, R. N., Wilkins, M. J., and Weeks, D. C. 1951. Airst. J . Sci. Res. B 4, 248-264. Rosene, H. F., and Lund, E. J. 1953. In “Growth and Differentiation in Plants” (W. E. Loomis, ed.), pp. 219-252. Iowa State COIL Press, Ames. Saddler, H. D. W. 1970. J . Gen. Physiol. 55, 802-821. Scott, B. I. H., Gulline, H., and Pallaghy, C. K. 1968. Arcst. I . Biol. Sci. 21, 185-200. Sexton, R., and Sutcliffe, J. F. 1969. Ann. Bor. (London) [N.S.] 33, 683-694. Skou, J . C. 1965. Physiol. Rev. 45, 596-617. Slater, E. C. 1953. Nature (London) 172, 975-982. Slater, E. C. 1971. Quart. Rev. Biophys. 4, 35-72. Slayman, C . L. 1970. Amer. Zool. 10, 377-392. Smith, F. A. 1970. New Phytol. 69, 903-917. Spanswick, R. M. 1970. J. Membrane Biol. 2, 59-70. Steward, F. C. 1932. Protoplasma 15, 29-58. Steward, F. C., and Mott, R. L. 1970. Int. Rev. Cyrol. 28, 275-310. Sutcliffe, J. F. 1962. “Mineral Salts Absorption in Plants.” Pergamon, Oxford. Teorell, T. 1949. Arch. Sci. Physiol. 3, 205-219. Thomas, R. C. 1972. Physiol. Rev. 52, 563-594. Torii, K., and Laties, G. G. 1966a. Plant Physiol. 41, 863-870. Torii, K., and Laties, G. G . 196613. Plant Cell Physiol. 7, 395-403. Ulrich, A. 1941. Amer. 1. Bot. 28, 526-537. Ussing, H. H. 1949. Acta Physiol. Scand. 17, 43-56. van den Honert, T. H. 1937. Naturcrk. Tijdschr. Ned. Indie 97, 150-162. Vasington, F. D., and Murphy, J. V. 1962. J. Biol. Chem. 237, 2670-2677. Venkateswarlu, P., Armstrong, W. D., and Singer, L. 1965. Plant Physiol. 40, 255-261. Viets, F. G. 1944. Plant Physiol. 19, 466-480. Vlamis, J., and Davis, A. R. 1944. Plant Physiol. 19, 33-51. Vorobiev, L. N. 1967. Nature (London) 216, 1325-1327. Weigl, J. 1963. Planta 60, 307-321. Weigl, J. 1964. Z. Naturforsch. B 19, 646-648. Weigl, J. 1967. Planta 75, 327-342. Weigl, J. 1970. Z. Naturforsch. B 25, 96-100. Weigl, J. 1971. Z. Pflanzenphysiol. 64, 77-79. Welch, R. M., and Epstein, E. 1968. Proc. Nut. Acad. Sci. U S . 61, 447-453. Welch, R. M., and Epstein, E. 1969. Plant Physiol. 44, 301-304. Zioni, A. B., Vaadia, Y., and Lips, S. H. 1971. Physiol. Plant. 24, 288-290.


LODGING IN WHEAT, BARLEY, AND OATS: THE PHENOMENON, ITS CAUSES, AND PREVENTIVE MEASURES Moshe J. Pinthus The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot, Israel

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

........................... ........ A. Stem Lodging and Root Lodging . . . . . . . . . . . . . . . .................... B. Mechanical Aspects of Lodging . . . . . . C. Recovery from Lodging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. Description and Causes

111.

IV.

V.

VI.

VII.

...

D. Lodging Caused by Foot-Rot or Root-Rot Diseases . . . . . . . . . . . . . . E. Lodging of Insect-Attacked Culms . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of Lodging on Crop Development and Yield . ......................... A. Methods of Investigation . . B. Effects on Grain Yield . .:. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................... C. Effects on Grain Quality . . . . . . . D. Effects on Culm Development and Tillering . . . . . . . . . . . . . . . . E. Physiological Effects of Lodging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Impact of Lodging on Grain Harvest . . . . . . . . . . . . . . . . . . . . . . . . G. Incidence of Diseases in Lodging Crops . . . . . . . . Plant Characters Associated with Lodg A. Culm Characters .................................. B. Root and Crown Characters . . . . . . . ................. .................................. C. Mechanical Prope D. Other Characters . . . . . . . . . . . . . Environmental and Agronomic Factor A. Light and Temperature . . . . . . . . B. Nitrogen Supply . . . . . . . . . . . . . C. Phosphorus, Potassium, and Trace Elements . . . . . . . . . . . . . . . . . . D. Moisture Supply and Soil Aeration . . . . . . . . . . . . . . . . . . . . . . . . . . E. Crop Rotation and Tillage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Synergistic Effects . . . . . . . . . . ............. Prevention of Lodging . . . . . . . . . . . ............. ....................... A. Cultural Practices . . . . . . . . . . . . B. Application of 2-Chloroethyl T r C. Application of Herbicides and Other Che Breeding for Lodging Resistance . . . . . . . . . . . . . . . . ............ A. Evaluation of Lodging Resistance . . . . . . B. Inheritance of Lodging Resistance and Associated Characters . . . . . . C. Achievements and Prospects of Breeding . . . . . . . . . . . . . . . . . . 209

210 21 1 21 1 213 216 216 217 217 217 220 221 222 222 223

224 226 228

235 236 236 237 238 238 246

210 VIII.

MOSHE J. PINTHUS

Increased Exploitation of Yield-Promoting Factors Due to the Prevention of Lodging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

I.

Introduction

Lodging was long ago recognized to be a severe malady of small grains in most parts of the world. Previous rather comprehensive reviews of this subject were published by Dorofeev and Ponomarev (1970) and by Kohli and Mukherjee ( 1966), Lodging may damage grain yield directly by interfering with dry matter accumulation, and reduce the yield indirectly owing to the difficulties that it imposes on harvest. Lodging may also adversely affect grain quality (Section 111). The severity of lodging and the extent of the losses resulting from it depend on the crop’s environment (see Section V) and on the growth stage at which lodging occurs (Section 111, B, 1) . Generally speaking, favorable growing conditions, promoting crop development and grain yield, will evoke lodging and increase its severity. Consequently, lodging should be regarded as an “abundance disease” which restricts the exploitation of otherwise yield-promoting factors. The conjunction of lodging with rather high yields may result in a serious underestimate of its economic importance. However, the extensive experimentation during the last decade with the lodging-inhibiting chemical 2-chloroethyl trimethylammonium chloride (CCC) indicates that the losses due to lodging may often amount to up to 30% of the grain yield. Considering also other estimates on the frequency and severity of lodging (Ansiaux, 1969; Moore, 1949; Paleev, 1953), it may be concluded that in regions where high grain yields are obtained, the damage due to lodging is at least as great as that due to cryptogamic diseases and insect pests. The past decade should be credited with two outstanding achievements in the control of lodging: The release of short-strawed varieties and the introduction of CCC (Sections VII and VI, B, respectively). However, neither the new varieties nor the application of CCC has eliminated the problem of lodging but has rather concentrated it into a narrower range of conditions and transferred it to a higher level of yields. Another aspect of lodging is its occurrence in eyespot-infested fields (Section 11, D) . The severity of this problem has grown recently in different parts of the world, following intensification of wheat production and its increased part in the crop rotation (Manning, 1967; Mielke, 1970).

LODGING I N WHEAT, BARLEY, AND OATS

21 1

Complete losses of crops were reported by Witchalls (1970) from southern New Zealand. The dependence of lodging on a great number of environmental factors as well as on numerous plant characters warrants a detailed examination of the phenomenon and its control. This is attempted in the present review. II.

Description and Causes

Lodging is the state of permanent displacement of the stems from their upright position. It is induced by external forces exerted by wind, rain, or hail. It may culminate in the plants being laid flat on the ground, and sometimes involve breakage of the stems. Although bending at the base of the peduncle has also been considered as lodging (Patterson et al., 1957), cereal culms generally lean over at their bases. The culms may remain straight throughout their length or become curved in various forms (Grafius and Brown, 1954). Lodging is often not distributed uniformly throughout an affected field but may be scattered over certain sections or spots. The degree of lodging, i.e., the degree at which the culms lean from the perpendicular, may also vary at different places within the field. The prevalence, together with the degree, determine the severity of lodging. A.

STEM LODGING AND ROOTLODGING

Stem lodging follows bending or breaking of the lower culm internodes, whereas root lodging refers to straight and intact culms leaning from the crown, involving a certain disturbance of the root system. Stem lodging may be caused by hail or by previous damage of the culms by insects or by foot rot, but its occurrence is induced mainly by storm. Stem lodging is restricted to plants that are held tightly by a dry and hard upper soil layer. In moist soil the roots and crowns will yield to the torque created by the wind, and root lodging will develop. In this case cracks parallel to the planting rows, on the side opposite to lodging, can sometimes be observed after the soil has dried again (Fig. 1). Experiments in wind tunnels (Bauer, 1964; Udagawa and Oda, 1967) indicate that in order to cause stem lodging high wind velocities (15-30 m/sec) , which correspond to strong wind or gale, are necessary. Moreover, even under such conditions, as long as the intact culms are moist and turgid, they will rarely break. Consequently, lodging due to fracture of the culms is to be expected only of senescent plants after ripeness (Grafius et al., 1955).

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If the upper soil layer is softened by rain or irrigation, the anchorage of the plants is weakened, and even a light breeze may exert a sufficient torque to induce lodging (Udagawa and Oda, 1967). Rain or sprinkler irrigation may promote lodging also by wetting the plants and thus adding

FIG.1. Soil-crack following root lodging of wheat.

to their weight, which in turn increases the torque. Sprinkler irrigationinduced lodging in nascent stage is illustrated in Fig. 2. The decrease in lodging resistance due to wetting of the upper soil layer can be demonstrated by means of a chain hooked to the head of a plant grown in dry soil. When the soil around the plant is watered, a gradual drop in the number of chain links supported by the culm can be observed. In our own (unpublished) experiments, this drop amounted to about half the initial value before watering.


213

It is concluded that root lodging is the predominant type of lodging occurring during the crucial growth stages (Section 111, B, 1 ) and that rain and irrigation, which moisten the soil and thus loosen the anchorage of the plants, are its main causal agents.

B. MECHANICAL ASPECTSOF LODGING The cereal plant is anchored in the ground by its root system, its crown, and by the lower portions (5-30 mm) of the first elongating internodes

FIG. 2. Lodging of sprinkler-irrigated wheat in nascent state (Tirat Zevi, Israel, 1965).

of its culms (main shoot and tillers) which are embedded in the soil. Provided that their anchorage and culms are undamaged, plants are able to support their own weight as long as they are not affected by external forces. However, the plant is subjected to wind, rain, and hail, which exert forces ( p ) operating perpendicularly to the culms, thus inducing a torque which causes bending. Once a culm has been drawn out of its vertical position, the weight of the shoot to which it belongs operates as a force ( f ) which will increase the torque. Moreover, this force will grow as bending proceeds. The external factors which evoke p , especially wind, act predominantly on the head of the plant. Therefore, the torque will affect the whole culm

214

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and increase gradually from the top down to the basal portion, near the ground, where the lever attains its greatest value. Consequently, the properties of the basal region of the culm are decisive for bending. Since the nodes are too rigid to enable bending, this will occur in the internodes, which will permit more bending the longer they are. The total torque ( T ) will be

where 1, and I , are the levers of the forces p and f, respectively; 1, may be assumed to equal the height of the erect plant up to the center of the head; I , is related to the center of gravity of the shoot which will be located near, below or above the base of the head. The exact location changes somewhat according to the development of the plant. During the period of intensive filling of the grain, after stem elongation has been completed, the center of gravity will move upward, whereas during the later stages of ripening, when the kernels are drying, it will move downward. Average percentages of total shoot weight at the dead-ripe stage were reported by Hancock and Smith (1963) as 42.1, 47.5, and 43.6 for regular wheat, barley, and oat heads, respectively. In short-straw varieties these percentages will, of course, be higher. It follows that the length of either lever, 1, or 12, is rather similar to that of the culms. Therefore, the crucial effect of plant height on the torque T is obvious. The torque T is resisted by the anchorage of the culm in the soil and by the bending-resistance moment of its aboveground internodes. The interrelationship of soil, underground plant parts, and aboveground parts, and their individual and combined responses to external forces, constitute a complex system. The mechanical aspects of this complete system have, apparently, not yet been investigated. However, as far as stem lodging is concerned, the mechanical analysis may be restricted to the response to external forces of undamaged culms which are anchored in the soil firmly enough not to permit wind or rain to cause any shift of their underground parts. Such culms may be considered as similar to cylindrical rods which are fastened at one end and will be treated accordingly hereafter, following some principles of statics (Timoshenko, 1940). The highest bending-resistance moment of the culm should be regarded as the straw strength, which is, unfortunately, often confused with lodging resistance. The extent of bending increases with the torque T . However, up to a certain limit it is reversible, and the plant will resume its upright position as soon as the forces which have induced the bending moment cease to operate. Beyond this limit bending cannot be restored and lodging occurs.

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215

The property of the plant to return to its original position after bending, conforms with the definition of elasticity. In the region of bending, stresses (S) originate. These stresses evoke deformations, the summation of which is expressed by the removal of the lower part of the culm from the vertical, as well as by the lowering of the head. At the peak of reversible bending, the stresses at each point reach the elastic limit. Up to the elastic limit the deformation increases proportionally to the stress. Beyond the elastic limit the yield point is reached and from this point the deformation may increase greatly for very little increase in stress. The magnitude of the elastic limit of the straw of a certain crop will affect the percentage of lodging plants ( = prevalence of lodging), whereas the degree of lodging will depend on the yield point. Stresses smaller than the elastic limit may affect lodging only indirectly, through their effects on culm displacement which affects the force f . Straw strength may be estimated by the torque which will cause stresses of the same magnitude as the elastic limit of the straw. It is dependent on the value of the elastic limit as well as on the rate of increase of the stresses with the torque T . The relation between torque and stress for a cylindrical rod is

T

=

S(&Z/d)

where I is the moment of inertia depending on the shape of the cross section of the rod and d is its diameter. For a solid cylinder I = ( a d 4 / 6 4 ) , whereas for a hollow cylinder with inner diameter d,, I = [a(d4- d,')1/641. Consequently, increased values of culm diameter, culm-wall thickness, and elastic limit may all promote straw strength. The deformation evoked by S will be inversely proportional to E, which is Young's modulus of elasticity for the material of the rod. Consequently, the deformation originating from the torque T will be inversely proportional to the product ZE. This product is called the flexural rigidity of the rod, which should correspond to straw stiffness of cereal plants. The latter term has, unfortunately, sometimes been confused with straw strength and even with lodging resistance. It should be emphasized that culms may be stiff and nevertheless weak because of a low elastic limit. Moreover, high straw stiffness may contribute to straw strength only to the extent that it results from a high moment of inertia ( I ) rather than from a high modulus (El. Very stiff straw, in which no deformations may occur, will transfer the torque which operates on it to the underground plant parts and may thus promote root lodging, if soil conditions are appropriate. On the other hand, reduced stiffness will increase the swing movements of the culm, which may be induced by a light breeze. This, presumably, may promote root

216

MOSHE J. PINTHUS

lodging due to the effects of these movements on the adhesion of the roots to the soil. The analysis of lodging according to the concepts and laws of elasticity has been attempted by Hashimoto (1963), Hozyo (1969), Oda et al. (1966), and Ustum and Hungerbuhler (1968).

c.

RECOVERY FROM LODGING

Cereal culms which have not yet completed their internode elongation may recover from lodging. The upper internodes of lodged culms may resume their erect growth after an upward bending at one or at several of the lower nodes. This bending is induced through geotropic stimulus (Percival, 1921 ) and is performed through elongation of node cells and sheath cells on that side of the node which is turned to the ground (Dudinskii, 1970). The regained upward growth of young internodes, which had only started to elongate when lodging occurred, may be attributed to the activity of their basal intercalary meristem (Dudinskii and Mikolenko, 1970). Recovered lodging can sometimes contribute to the prevention of later lodging, due to its height-reducing effect. Furthermore, in certain cases, adventitious roots develop at the lowest bent node and this strengthens the anchorage of the plant. D.

LODGING CAUSED BY FOOT-ROTOR ROOT-ROTDISEASES

Root rots and foot rots weaken the anchorage of plants and affect the lower stem internodes, thus promoting lodging. The lodging of infested plants is characterized by the culms leaning or lying in disorder, whereas the stem or root lodging discussed above occur in a uniform direction throughout the field. The main root-rot disease which may induce lodging of cereals is “takeall,” caused by Ophiobolus graminis, which has been investigated extensively by Nilsson (1 969). However, the most serious and widespread diseas.e resulting in lodging is the “eyespot” foot rot caused by Cercosporella herpotrichoides. On plants infested with this fungus, brown elliptical lesions develop on the sheaths of the lower leaves and on the lower culm internodes. The fungus may penetrate deeply into the culms, and the lesions may eventually girdle them near ground level. Eyespot is found on all three cereals under consideration, but wheat is the most severely affected one, and the relation between lodging of this crop and eyespot has been investigated extensively (Bauer, 1963; Bock-


217

mann, 1964; Glynne, 1963; Gregory, 1959; Manning, 1967; Mielke, 1970). Eyespot infection results from infested stubble of a previous crop and therefore its occurrence increases with the frequency of these cereals in the crop rotation. The spread of the disease and its severity are promoted under conditions of high humidity and a temperature range of 5-1OoC (Mielke, 1970). Cool and damp weather during the period of stem elongation is favorable for the disease and therefore it is most frequent on winter wheat in northwestern Europe. In the United States, eyespot has become a serious disease of semidwarf wheat grown at high fertility level because of its adaptation to the microclimate prevailing within the dense canopy of such wheat (Briggle and Vogel, 1968). The effects of most cultural practices as well as those of various culm characters on the incidence of eyespot, are similar to their effects on lodging. Moreover, infection by eyespot is often increased by the conditions prevailing in a lodged crop. Therefore, in many cases it is difficult to decide whether the correlation between eyespot and lodging should be attributed to the effect of eyespot on lodging, the effect of lodging on eyespot, or the effect of a certain plant character or environmental factor influencing both maladies similarly. E.

LODGING O F INSECT-ATTACKED

CULMS

Insect-induced lodging rarely affects great areas of a field but it occurs frequently to individual culms distributed throughout the field. It concerns wheat primarily and is caused mainly by the Hessian fly (Mayetida destructor Say) and by sawflies (Cephus cinctus Nort. and C . pygmaeus L.). The maggots of the Hessian fly girdle the culms at their bases by scratching the leaf sheaths and internodes, whereas the larvae of the sawfly bore within the stem. The occurrence of these insects has been restricted considerably due to adequate cultural practices and the use of resistant varieties (Dahms, 1967). Ill.

Effects of lodging on Crop Development and Yield

A.

METHODSOF

INVESTIGATION

The effects of lodging on the crop are confounded with the effects on the crop of the factors causing lodging. No completely satisfactory method has been found to distinguish between these effects and to isolate the effects of lodging. The main methods which have been applied are outlined below. In spite of the shortcomings of each one of them, the results obtained

218

MOSHE J. PINTHUS

through them in many experiments fall into line with each other and enable rather definite conclusions to be reached with regard to certain effects. 1 . Comparison between Samples from Lodged and from Standing Areas within the Same Plot

This method has been used in early work (e.g., Welton and Morris, 1931) as well as in more recent studies (Das et al., 1966; Miladinovic, 1959; Mulder, 1954; Syme, 1968). It is based on the assumption that the variations in lodging within the plots have not been caused by factors that may per se affect the crop characters under investigation. In most cases such an assumption will not be justified. 2 . Comparison of Lodging Plants in Untreated Plots with Erect Plants in CCC-Treated Plots A vast number of field experiments with 2-chloroethyl trimethylammonium chloride (CCC) on cereals, primarily wheat, have been conducted throughout the world since 1961 and up to the present day. The reports on these experiments provide much information on various plant characters, particularly grain yield, from lodged and from standing plots. However, many plant characters, including grain yield, are affected by CCC also in the absence of lodging (Humphries, 196Sa). Moreover, strong CCC x environment interaction effects should also be considered. Some characters, e.g., those determining baking quality of wheat grain, are hardly affected by CCC (Humphries, 1968a) and their response to lodging can be ascertained by the method under consideration. This method also enables us to deduce the effects of lodging on characters for which reliable estimates on their response to CCC, in the absence of lodgig, are available, e.g., elongation of culm internodes. Moreover, considering information on the effect of CCC on the grain yield of wheat, in the absence of lodging (Humphries, 1968b; Lowe and Carter, 1972; Pinthus and Rudich, 1967; Primost, 1968; Schultz, 1971; Sturm and Jung, 1964; Zadontsev et al., 1969), it can be assumed that this effect will in most cases be slight or account for an increase of up to 15%. Consequently, differences in grain yield beyond this limit, between standing treated plots and lodging untreated plots, can be attributed to the effect of lodging on grain yield. 3. Artificially Prevented Lodging

Within a crop which is liable to lodge, the plants in some plots are supported to prevent lodging. This method has been applied by Mulder


219

(1954) and by Zadontsev et al. (1969). Theoretically, this is the most sound method. However, its restriction to rather small plots reduces its practical usefulness. 4 . Artifically Induced Lodging

In order to be effective this method should be accompanied by supporting the plants in control plots in order to prevent their lodging. Harrington and Waywell ( 1950) induced lodging by exposing plots individually to a strong controlled wind produced by an airplane propeller. A movable chamber in which field plots can be subjected artificially to various rainfall and wind intensities was constructed and applied by LekeS and ZeniSEeva (1962) to induce lodging. A mobilc wind tunnel for the same purpose was described by Udagawa and Oda (1967). Lodging induced by these techniques is most similar to that occurring in nature, but their application is too cumbersome. Spraying plants after anthesis with heteroauxin was used to induce lodging by Petinov and Urmantsev (1964). This technique will most probably cause effects on plant characters in addition to lodging. Lodging was accomplished by Laude and Pauli (1956) through bending and pinching the culms between the fingers. The results obtained by this technique, which involves mechanical injury, can pertain only to stem lodging following breakage of the culms. Sisler and Olson ( 195 1 ) induced lodging by pushing the plants down with a long board following irrigation of the plot. This was done gently and with care to avoid damaging the culms or the roots. Consequently, lodging accomplished by this technique resembles root lodging. The most frequently applied technique was devised by Sisler and Olson ( 195 1 ) and used by them to induce lodging in barley. It was subsequently applied with this crop by Day (1957) and by Day and Dickson (1958), with oats by Norden and Frey (1958) and Pendleton (1954), and with wheat by Jankovid (1966a) and Weibel and Pendleton (1964). In this technique the plants are allowed to grow up through a wire netting which is installed for each plot 30-60 cm above the ground. Lodging is obtained by moving the wires of the respective plots horizontally; in the control plots it is prevented by the support of the intact wire. The obvious advantages to this technique are the possibilities of inducing lodging at different degrees from the perpendicular and at various growth stages, as well as its maintaining the erect position of the control plants. The disadvantages are some damage to the plants, which can hardly be avoided, and the difficulty of applying this technique to large field plots.

MOSHE J. PINTHUS

220

B.

EFFECTSON GRAINYIELD

1 . Degree of Lodging and Growth Stage at Which I t Occurs

The effect of lodging on grain yield is dependent on its severity and on the time of its occurrence. Early lodging, during the period of intensive stem elongation, may hardly affect grain yield because of the rapid recovery, which will restore the upright growth of the plant prior to heading. Culm breakage at this stage, to which should be ascribed the yield reduction from lodging in Laude and Pauli's (1956) experiment, is not to be expected under natural field conditions. Lodging close to maturity cannot affect grain yield directly but may cause losses due to its interference with harvest. Heading and early grain debelopment are obviously the most crucial stages. Artificially induced lodging at heading reduced grain yield by 27-40%, whereas the yield reduction due to lodging at about the softdough stage surpassed 24% only at one location (Table I ) . TABLE I Reduction of Grain Yield Due to Artificially Induced 90' Lodging at Two Growth Stages Lodging induced at:

Crop

Location

Winter wheat Winter wheat

Kansas Illinois

Spring barley Fall-sown barley Oats Oats

Manitoba Arizona Iowa Illinois

Heading

15-20 Days after heading

Reference

(%I

(%I

Laude and Pauli (1956) Weibel and Pendleton (1964) Sisler and Olsoii (1951) Day (1957) Norden and Frey (1958) Pendleton (1954)

27 31

22 20

34 40 36 37

24

39 23

17

In other experiments reported by Sisler and Olson (1951) and in those of Jancovid ( 19866a), lodging at heading reduced grain yield by 65 % , perhaps because in these experiments plants were forced to remain flat on the ground. Artificially induced lodging at 45" caused one-fourth to onehalf the reduction of that at 90" (Day, 1957; Norden and Frey, 1958; Pendleton, 1954; Sisler and Olson, 1951) .

LODGING IN

WHEAT, BARLEY,

AND OATS

22 1

Comparison of the yields in lodged plots with those of supported plants, in experiments in the Netherlands (Mulder, 1954) showed that reductions of 4-33%, 4-22%, and 0-31 % for spring wheat, barley, and oats, respectively, were obtained, depending on the severity and time of occurrence of lodging. As mentioned above, experiments with CCC may also supply information on the effect of lodging on the grain yield of wheat. In these experiments, which were conducted primarily in Europe, the greatest increase in grain yield from treated plots was 82% (Humphries, 1968a). However, an estimated increase of up to 40% would be more realistic. The effect of the growth stage at which lodging occurs may also be deduced from some of the CCC experiments. This is illustrated by some of our own (unpublished) results from an experiment with F.A. 8193 wheat at two locations in Israel, where severe lodging was effectively controlled by CCC. The grain yield at one location, where lodging started 3 days after heading, was 3130 kg/ha in control plots and 4400 kg/ha in CCC-treated plots; at another location, where lodging started 20 days after heading, the yield both in treated and in control plots was 4500 kg/ha. 2. E#ect on Grain-Yield Components

The above cited reports on artificially induced lodging, as well as Mulder’s experiments, indicate that lodging at heading affects both the number of kernels per head and the individual kernel weight. Lodging that occurs later affects primarily kernel weight. The increase in wheat yield from plots in which lodging had been prevented by the application of CCC, was associated in many cases with an increase in the number of kernels per spike, whereas kernel weight was only rarely, and then slightly, affected (Humphries, 1968a; Lowe and Carter, 1972; Martin, 1968). c.

EFFECTSON GRAINQUALITY

Lodging may cause shriveling of the grain and reduce its test weight (bushel or hectoliter weight). In most of the experiments in which lodging was artificially induced or prevented, a similar reduction in test weight was found amounting to about 8 % for wheat and barley and 15% for oats. In these experiments as well as in others (Gately, 1968; Hirano et al., 1970; Miladinovic, 1959), the N (or protein) content of the grains from lodged plots exceeded that from standing plots by 3-20%. Lodging may reduce milling quality of wheat (Hirano et al., 1970) whereas its effect on baking quality seems to be negligible and may sometimes even be advantageous (Miladinovic, 1959). Lodging, however, ad-

222

MOSHE J. PINTHUS

versely affects the malting quality of barley (Coenradie and Wilten, 1961; Day and Dickson, 1958). Sprouting in the heads has also been found to occur more frequently in lodged than in standing crops (Kivi, 1961 ) .

D. EFFECTS ON

CULM

DEVELOPMENT AND TILLERING

The elongation of the two upper culm-internodes, which is not completed until 5-10 days after heading, can be affected by lodging which occurs up to this period. Thus, although CCC reduces internode elongation, the two upper culm internodes of erect treated plants have often been found to be longer than those from lodged untreated plants (see, e.g., Pinthus and Halevy, 1965). Since these internodes comprise about twothirds of the total culm length, any interference with their development may affect straw yield considerably. Straw yield was indeed as much as 25 and 21% lower for lodged wheat and oat plants, respectively, than for supported plants (Mulder, 1954). Lodging may sometimes promote the development of late tillers, presumably because of the reduction in the competition for minerals and carbohydrates by the lodging culms. However, these tillers rarely attain normal growth. E.

PHYSIOLOGICAL EFFECTSOF LODGING

The most obvious effect of lodging on the plant’s physiological processes is its interference with carbohydrate assimilation (Mulder, 1954). This results from a large part of the foliage and other photosynthesizing parts being shaded by plants which are leaning or lying on top of them. The heads of low-lying plants in a lodging crop may sometimes be completely empty, whereas those of the plants lying on top develop normal grain. The reduced carbohydrate assimilation will, of course, affect primarily their accumulation in the grains, but, depending on the time of lodging, may affect any process or plant part demanding carbohydrates during that time. The protein in cereal grain originates primarily from nitrogen which has accumulated in the foliage prior to heading. Therefore, its absolute amount in the kernels is hardly affected by lodging, which occurs at heading or thereafter. Consequently, the percentage of N, or protein, in the grain of lodged plants may rise due to the decrease in carbohydrate accumulation. Lodging which involves culm breakage will also interfere with the translocation of carbohydrates and of minerals (Hashimoto, 1959; Pauli and Laude, 1959). In this case the absolute content of N and other minerals in the grain may also be reduced if lodging occurs during heading or early grain development.

LODGING I N

F.

IMPACT

OF

WHEAT, BARLEY,

AND OATS

223

LODGING ON GRAINHARVEST

Very few data are available on the quantitative effects of lodging on combine harvesting. Considering the report of Baumgartner ( 1969), it may be concluded that, in a lodged crop, harvest capacity can be reduced by up to 25% and the loss of unthreshed heads may be doubled. The moisture content of lodged grain will be higher than of unlodged grain, which also interferes with the harvest and may increase the expenses for grain drying by 30%.

G.

INCIDENCE OF

DISEASESI N LODGING CROPS

Some environmental factors and several plant characters which promote lodging also improve the growing conditions for rots and leaf diseases. Moreover, these diseases are often favored by the microclimate prevailing within a lodged crop. These facts have been recognized by various workers (e.g., Bauer, 1963; Mulder, 1954; Weibel and Pendleton, 1964), but no relevant data seem to be available. The eyespot disease, which itself may cause lodging, seems to be enhanced by the conditions within a lodged crop. Its reduction due to the application of CCC can be attributed partly to the control of lodging by this chemical (Bockmann, 1968).

IV.

Plant Characters Associated with lodging

Culm length and the shape of the head affect the magnitude of the lodging-inducing torque whereas the plant’s resistance to the torque is dependent on various other characters. The information on the association of these characters with lodging is derived predominantly from the study of varieties or lines differing in lodging resistance. In evaluating this information, which is often contradictory, the following points should be considered: First, the reliability of the assessment of lodging resistance, considering the strong variety x environment interaction effects on lodging (Section VII, A, 1 ). Moreover, in certain cases lodging assessments have been based on mechanical properties rather than on direct field observations. Second, varietal differences in lodging are accompanied by differences in many other characters which may or may not be correlated with each other. Partial correlation and path-coefficient analyses can be useful in this respect. Third, interaction effects of variety x different characters on lodging cast much doubt on the relevance of studies performed on a small number of varieties. Similarly, the association with characters which are

MOSHE J. PINTHUS

224

strongly affected by variety x environment interaction effects must be based on extensive tests of these characters. Finally, a high correlation between a certain plant character and lodging does not necessarily imply a causal relationship. This reservation also holds for the information obtained through comparison of lodged and erect plants in the same plot. A.

CULMCHARACTERS 1 . Length

Culm length, which comprises the lever of the lodging-inducing torque, is obviously associated with lodging. Nevertheless, in many of the investigations in which no dwarf or semidwarf varieties were included, no marked correlation between these traits was ascertained (Baier, 1965; Rodger, 1956; Zimina, 1968). This may be ascribed to the occurrence of lodging prior to complete culm elongation as well as to culm length X maturity interaction effects. An early, short-strawed variety close to maturity will be taller and more prone to lodging than a late, long-strawed variety, which at that time has attained only the late boot or heading stage. With regard to lodging at heading, the length of the three or four lowest internodes is of greater effect than that of the two uppermost internodes, which, although comprising about two-thirds of the final culm length, have not yet completed their elongation at this stage. 2 . Basal Internodes From the mechanics of lodging it is apparent that the properties of the basal culm internodes should affect lodging resistance. Some durum and rivet wheats have solid stems, and this character has also been bred into certain common wheat varieties in order to achieve sawfly-resistance (Dahms, 1967). However, in most wheat varieties, as well as in barley and oats, the internodes are hollow. Therefore, their flexural rigidity is greatly dependent on both diameter and wall thickness (Section 11, B ) . Varietal differences in lodging resistance were indeed found to be significantly positively associated with the diameter and wall thickness of the basal internodes-primarily the second one-in many studies (Hamilton, 1941; Hansel, 1957; Jellum, 1962; Mukherjee et al., 1967; Multamaki, 1962; Oda et al., 1966; Sechier, 1961). In other studies, marked positive correlations were established between these characters and culm bending or breakage (Bhamonchant and Patterson, 1964; Hancock and Smith, 1963; Norden and Frey, 1959). The coefficients for the correlations which were found between these culm characters and lodging, bending or breakage, rarely exceeded the value of 0.7.


225

The association between lodging and the diameter and wall thickness of the basal internodes is also evident from comparisons between lodged and erect plants of the same variety in the same plot (Das et al., 1966; Mulder, 1954). The diameter of the basal internodes was found to be closely correlated with the number of coronal roots (Hamilton, 1951; Hansel, 1957). Its association with lodging resistance may therefore be attributed in part to the relation between lodging and root development. The length of the basal internodes has been rather closely correlated with lodging of barley (Baier, 1965). Moreover, increased internode elongation, due to cell elongation rather than cell division, is usually accompanied by reduction in diameter and wall thickness. Consequently, the length :diameter ratio has been found to be distinctly correlated ( r = 0.71-0.78) with lodging (Baier, 1965). Culm density, i.e., dry weight per unit length of culm measured at the base of the plant, is, of course, dependent on the diameter and wall thickness. It was found by Atkins (1938) to be closely correlated with breaking strength of wheat. Its correlation with lodging resistance in the field, though significant, was rather low ( r = 0.4-0.6). Obviously, this character may affect mainly stem lodging rather than root lodging. 3 . Anatomical Structure The anatomic structure, as well as the chemical compostion, should affect the modulus of elasticity of the straw ( E ) and through it, the flexural rigidity of the culm. The relationship between lodging and culm anatomy, and in particular that of the basal internodes, of all cereal crops, has been investigated extensively. Results of early work have been summarized by Esteves (1952) and Ramaswamy (1963). More recent studies have been carried out primarily in eastern Europe (e.g., MiliEa et al., 1966 NBtr, 1964; Strutsovskaya, 1968). Significant differences between extremely lodging-resistant and susceptible varieties have been found in most studies. However, consistent relationships, relcvant to a complete array of varieties differing in lodging resistance, have not been ascertained. The most marked and significant anatomical feature related to lodging resistance was a great number of vascular bundles. The results regarding the width of the sclerenchyma layer are contradictory. This may be due to differences in the quantity of assimilating parenchyma embedded in this layer, which was found to be negatively correlated with lodging resistance (Skucifiska, 1965). The above mentioned relationships between anatomical features and lodging resistance may be partly ascribed to the effect of lignification on culm rigidity. A significant positive relationship between lodging resistance and the proportion of the

226

MOSHE J. PINTHUS

lignified tissues in the cross section of the basal internodes was found by Multamaki (1962) for oats and for barley. The changes in lignification throughout the growing period (Heyland, 1956) presumably contribute to the inconsistent relationships between lodging and culm anatomy.

4 . Chemical Composition Early work attributed culm rigidity to a high content of silicia, but later investigations disproved this hypothesis (Heyland, 1959; Ramaswamy, 1963). Cellulose and lignin contents in the basal internodes have been found to be associated with lodging resistance in certain cases. However, the results are inconsistent and sometimes even contradictory (Heyland, 1959; Ramaswamy, 1963). Spahr (1960) found that in barley a high cellulose content in the lower two-thirds of the culm was associated with lodging resistance. Recently, Galkovskaya and Baltaga (1970) reported on a high content of cellulose, hemicellulose, and lignin in the culms of lodgingresistant winter wheat strains. The cellulose:lignin ratio in the lower part of the culm was foynd to be associated with lodging in wheat (Heyland, 1959) and barley (Skopik, 1969). B. ROOTAND

CROWN CHARACTERS

The qualities of the root system affect the anchorage of the plant in the soil and therefore are of major importance in determining resistance to root lodging. The association of various root and crown characters with lodging of the different cereal crops has generally been accepted (Troughton, 1962). Many studies indicate the relationship between lodging resistance and a vigorous root system in the upper soil layer (Hamilton, 1951; Hurd, 1964; Maas, 1970; Percival, 192 1 ; Pinthus, 1967a; Sechler, 196 1 ) . Visual daerences between the root systems of extremely lodging-resistant and lodging-susceptiblevarieties are obvious (Fig. 3). Numerical ratings according to the visual appearance have been used for the assessment of root development (Hamilton, 1951; Maas, 1970; Pinthus, 1967a). Through such assessments, as well as determinations of root volume (Sechler, 196 1 ) , relationships were established between root development and lodging resistance for varieties differing greatly in these respects. The dry weight of the roots seems to be a rather poor parameter of the development of the root system of cereals because it does not represent its extension and its surface area. Moreover, its determination is subject to great experimental errors (Troughton, 1962). A relationship between


227

FIG.3. The root systems of a lodging-resistant wheat variety (SELKIRK) and a susceptible variety (CCC 10).

228

MOSHE J. PINTHUS

this character and lodging was found in certain studies (Strutsovskaya, 1968), but not in others (Spahr, 1960). Significant correlations, ranging from 0.4 to 0.9, have been found between lodging resistance and the number of coronal roots per plant or per tiller (Bauer, 1964; Hansel, 1957; Harrington and Waywell, 1950; Multamaki, 1962). A positive relationship between these characters has also been found by Sechler (1 961 ) . However, the number of coronal roots per plant is strongly affected by environmental factors whereas varietal differences within the same species and maturity class are rather slight (Pinthus, 1969). Positive relationships between lodging resistance and coronal root diameter were found for the different‘species when a limited number of varieties differing greatly in lodging resistance were compared (Dorofeev, 1959; Sechler, 196 1 ; Wag, 1970). The roots of lodging-resistant varieties of barley had greater tensile strength, as indicated by a higher breaking point, than susceptible varieties (Spahr, 1960). Sechler (1961) found a close association between the length of the root crown and lodging resistance in oats. Anatomical root characters were also investigated: Dorofeev ( 1959), examining one lodging-resistant and one susceptible variety each of durum, turgidurn, aestivum, and compactum wheat, found thicker cell walls and a larger diameter of the sclerenchymatic layer in the lodgingresistant varieties. A consistent and rather high correlation (0.8) was established in wheat between lodging resistance and the spread of the coronal roots, expressed as the angle from the perpendicular at which these roots penetrate the ground (Pinthus, 1967a). This relationship seems to be of particular significance, since it was found for varieties which were similar with regard to other root and crown characters as well as culm diameter.

C. MECHANICAL PROPERTIES 1 . Straw Stiflness and Straw Strength Straw stiffness refers to the flexural rigidity of the culm (Section 11, B). It has been estimated by several methods: Measurement with a spring balance of the force required to pull horizontally a certain number of culms, growing close together in the field, to a reclining position at a certain angle (Multamaki, 1962; Oda et af., 1966); determination of the “buckling load,” which is the force required to bend an internode, fastened at the node, from an inclination of 30° to 90° (Watson and French, 1971 ) ; determination of the angle at which culms, in the field or in pots, are bent by a certain load (Baier, 1965); and the “snap test” by which plants in

229


the field are graded according to the force required to pull a handful of culms to a reclining position, and according to their resilience (Murphy et al., 1958). The snap test is apparently the most widely used method (Frey et al., 1960; Hess and Shands, 1966). Coefficients of correlation between culm rigidity and lodging resistance varied from 0.33 to 0.98 in different studies (Table 11). TABLE I1 Coefficients of Correlation between Lodging Resistance and Straw Stiffness, and Lodging Resistance and Breaking Strength Correlated characters

Crop

Lodging resistance and straw stiffness

Spring wheat Spring wheat Barley Barley Barley Oats Oats Oats Winter wheat Winter wheat Spring wheat Barley Barley Oats

Lodging resistance and breaking strength

Coefficient 0.33 0.7-0.8 0.6-0.7 0.66 0.98 0.19 0.80 0.4-0.8

0.3-0.6 0.5-0.7 0.55 0.81 0 .60 0.10

(7)

Reference Multamaki (1962) Oda el al. (1966) Oda et al. (1966) Baier (1965) Multamaki (1962) Multamilki (1962) Murphy el al. (1958) Hess and Shands (1966) Salmon (1931) Atkins (1937) Multamkki (1962) Multamaki (1962) Baier (1965) Multamaki (1962)

Since flexural rigidity of the culm is a product of its moment of inertia ( I ) and its modulus of elasticity (E), a high value for it may be due to both E and I. Oda et al. (1966) found that in barley it was due primarily to high I, whereas in wheat it originated mainly from a high E. High flexural rigidity may contribute to lodging resistance through its effect on straw strength, i.e., the highest bending moment that the culm per se is capable of resisting (Section 11, B ) . This culm property is estimated when the lodging resistance factor, cLr (Grafius and Brown, 1954), or the load bearing capacity, LBC (Miller and Anderson, 1963), is determined (Section VII, A, 3 ). 2 . Breaking Strength Breaking strength refers to the force required to break a section of certain length of the basal culm internodes. It has been studied extensively with all three cereals, various types of instrumentation being applied. Its correlation with lodging resistance has been found to vary considerably

230

MOSHE J. PINTHUS

(Table 11). However, when comparing lodging-resistant and susceptible varieties, the former were generally found to have higher breaking strength than the latter, although within each group rather wide ranges were encountered. This may be illustrated by the results reported by Strutsovkaya (1966) : The breaking strength of 222 lodging-resistant wheat varieties ranged from 1000 to 2800 g, whereas that of 148 susceptible varieties ranged from 300 to 800 g. Similar results were obtained with barley by Khramysheva (1970), and they are in line with many other studies with all three cereals (e.g., Hancock and Smith, 1963; Oda et al., 1966; Zimina, 1968). Breaking strength changes during the period from heading to maturity (Bartel, 1937) and therefore the values obtained will depend on the growth stage of the plants at the time of testing. The same applies also to some other characters, associated with lodging, which have been shown to vary during the course of plant development, e.g., chemical composition (Heyland, 1959) and straw strength (Jellum, 1962). It is obvious that breaking strength should be associated with stem lodging following fracture of the culms. However, breaking strength may also indicate lodging resistance because of its relation to the elastic limitwhich affects stem lodging, and to flexural rigidity-which is associated with both stem lodging and root lodging. 3 . Root Pulling Resistance

This resistance is the vertical force required to pull out of the soil a certain number of plants, and is expressed as force per culm or per plant. It has been investigated extensively with corn. Harrington and Waywell (1950) have investigated it with wheat, barley, and oats and have found no close correlation between it and lodging. It was found to be closely related to lodging resistance of wheat by Surganova (1967), and of barley by ZeniSEeva (1968). Our own (unpublished) work on this subject with wheat, in Minnesota and in Israel, established marked and significant differences for this character between extremely lodging-resistant and susceptible varieties. However, no correlation was found between it and the other varieties. These inconsistent results, as well as those regarding the relationship between lodging and some other characters, may be due partly to interaction effects between the variety and the conditions under which the character is tested, as, for instance, soil moisture in the present case. No effects of such interactions should be expected in the case of relationships between lodging and characters which can be determined under standard conditions, e.g, breaking strength.


23 1

D. OTHERCHARACTERS Head density and shape may affect lodging through their effect on the area that the head subjects to the wind (Grafius, 1958; Udagawa and Oda, 1967). Patterson et al. (1964) found that in oats the more dense panicle was associated with greater bending resistance. In other studies with oats (Hess and Shands, 1966), panicle density and lodging were closely correlated in progenies of some crosses, but not at all in others. The nodding angle, i.e., the angle between the head and the continuation of the culm, of all three cereals, was investigated by Hancock and Smith (1963). It may affect lodging through its effect on the torque-inducing force of the head. Awnedness may also affect lodging, through the accumulation of rain drops on the awns., thereby increasing the weight of the head. Flag-leaf shape may also affect the subjection of the plant to the wind during the critical period for lodging. Its association with lodging resistance of wheat and barley has been investigated by Kyzlasov (1969) and by Vikitenko ( 1968), respectively. An association between lodging and maturity has been established in certain cases (Mukherjee et al., 1967; Vikitenko, 1968). However, the effect of this character on lodging varies according to the time of the onset of lodging and is therefore not reliable. The association of profuse tillering with lodging resistance which is sometimes observed (Vikitenko, 1968), should be ascribed to the increase in coronal roots due to tillering (Pinthus, 1969). Certain information indicates that varietal differences in lodging resistance are associated with differences in the content of *growth-promoting or inhibiting substances (Petinov and Prusakova, 1965; Prusakova, 1964; Turkova and Suan, 1966). This seems to be in accordance with the effect of internode elongation on lodging.

V.

Environmental and Agronomic Factors Affecting Lodging

Lodging is affected very strongly by environmental conditions. Any effect on any one of the various plant characters associated with lodging will affect it to some extent. The most remarkable effects on lodging will be those exerted through the characters that are most prone to environmental effects, namely, the structure of basal culm internodes and total culm length. The spread of the coronal root system in the upper soil layer is much less influenced by environmental factors, within a considerable range, but is affected strongly by extreme conditions. Regarding the two traits mentioned above, any factor increasing elongation of internodes, particularly that of the basal ones, will promote susceptibility to lodging.

23 2

MOSHE J. PINTHUS

Increased internode elongation may be due to cell division as well as to cell elongation. Both are subject to considerable environmental effects which affect the growth-regulating mechanism within the plant. However, the transverse growth of the subapical tissues which would increase culm diameter and wall thickness, is usually quite limited (Sachs, 1965). Cell number in the transverse direction is already complete before the onset of internode elongation. Moreover, there exists an inverse relationship between the rate of cell elongation and transverse growth (Sachs, 1965). Consequently, an increased length of the internodes will be accompanied by a reduction in their diameter and wall thickness. This, together with the increased hydration which accompanies cell elongation, results in a conspicuous reduction in dry weight per unit length of culm, which has often been associated with lodging susceptibility. A.

LIGHTAND TEMPERATURE

Light intensity is a decisive factor in internode elongation. It also controls the balance between longitudinal and transverse development of vascular tissues. High intensities block the action of natural gibberellin which promotes both division an elongation of cells (Sachs, 1965). Consequently, low light intensity promotes internode elongation and reduces culm-wall thickness. It will also reduce carbohydrate assimilation, which may interfere with cell wall development and lignification (Percival, 1921). Furthermore, root growth may also be depressed by low light intensity (Campbell and Read, 1968). The effect of light intensity on cereal culm internodes has been investigated in field and pot experiments where illumination was controlled by shading. Shading resulted in an up to 25% increase in internode length (Carles et al., 1960; Holmes et al., 1960; Mulder, 1954). Culm diameter and wall thickness of oats were reduced (Mulder, 1954), solidness of the lower internodes of wheat was decreased (Holmes et al., 1960), and the bending resistance of barley culms was lowered (Hozyo and Oda, 1965). Artificial shading of field plots, which reduced light intensity during the period of elongation of the 2-4 lowest internodes by 35-75%, promoted lodging of wheat (Holmes et al., 1960; Welton and Morris, 1931) and of barley (Coenradie and Wilten, 1962; Wilten and Coenradie, 1958, 1959). The effect of light intensity on lodging is evident from numerous studies of plant density (Section VI, A, 3). In dense stands light interception is reduced, which affects the lower culm internodes and promotes lodging. It should be pointed out that the effect of shading, caused by dense stands,


23 3

may also be exerted by infestation with weeds, which does indeed promote lodging in certain cases. Very conspicuous effects of plant populations from 50 to 1600 plants per m2, on culm elongation of barley, were reported by Kirby and Faris (1970) and attributed to the effects of light on plant gibberellin. Internode elongation may presumably be affected directly by the temperature prevailing during the pertinent growth period. A significant correlation was found between the culm length of barley and the temperature during the period from seedling emergence to heading (Pasela, 1967). An increase in temperature, however, may also promote tillering (Nanda et al., 1959). This, in turn, will increase the density of the foliage, which may reduce light interception and thus affect the lower culm internodes. Another indirect effect on the promotion of internode elongation through increased temperatures may be due to its effect on the release of soil nitrogen.

B.

NITROGEN SUPPLY

The promotion of lodging due to abundant nitrogen supply is well known and has been established in many studies with various cereal crops (e.g., Bremner, 1969; Dilz, 1967; Morey et al., 1970; Mulder, 1954; Nilsson, 1972). Usually, at high nitrogen levels there is a reduction in grain yield. In most cases it may be attributed to lodging, although it should be kept in mind that lodging is not the only factor limiting yield response to high nitrogen levels (Fiddian, 1970). It is of special significance that high N levels are conducive to lodging also of semidwarf varieties. This has been reported, so far, for common wheat (Asana and Chattopadhyay, 1970; Hadiiselimovik, 1969; Sage, 1970; Sharma et al., 1970), durum wheat (Scarascia Mugnozza et al., 1965), and barley (Sage, 1970). In most cases lodging of these varieties at high N levels was accompanied by a reduction in grain yield. Lodging and reduction in grain yield of the semidwarf varieties commence at higher N levels and seem to proceed more moderately than in the case of tall varieties. Similarly, at those N levels which have been tested so far, no complete lodging of the semidwarf varieties has been encountered. Considering the information from the above-cited sources and others (e.g., Sillampaa, 1971), as well as recent local experience (Weiss, 1972), we attempted to demonstrate the comparative response of tall and semidwarf varieties to nitrogen supply (Fig. 4 ) . The effect of nitrogen on lodging should be ascribed primarily to its effect on the basal culm internodes. The results presented by Mulder ( 1954) indicate that nitrogen affected all the morphologic and anatomic culm characters associated with lodging. An increase of 10-25% in the

234

MOSHE J. PINTHUS

length of the three lowest internodes due to high N level was observed in the various crops (Carles et al., 1960; Friichtenicht, 1965). Enhanced culm elongation following application of high N rates has been found also in semidwarf varieties of wheat (Koltay, 1968; Woodward, 1966) and of barley (Lovato and Venturi, 1968). High N rates may also bring about restrictions in the development of the coronal roots (Mulder, 1954). In this respect it is of special interest that root anchorage of a semidwarf wheat variety was found to be weakened due to application of high N rates r

t

a

.-)I .-C

e

0

, -Toll

Mox.r i-

/

/

- 1

/ Sernidworf

2

----

/

Nitrogen supply

-

FIG. 4. The effect of increasing nitrogen supply on lodging and grain yield of tall and semidwarf cereal varieties.

(Berlyand-Kozhevnikov et al., 1968). However, the information concerning the effects of N supply on the root growth of cereals is somewhat contradictory (Troughton, 1962). In general, it may be concluded that its effect is less on root growth than on shoot growth and therefore increased N supply will always result in an increased shoot:root ratio, which is conducive to lodging. An increase in nitrogen supply may affect the basal culm internodes also through the promotion of plant canopy development, which reduces light interception. The interaction effects of nitrogen and shading on the basal internodes have been investigated by Mulder (1954) and by Carles etal. (1960). The promotion of plant canopy and the weakening of the basal internodes due to increased nitrogen supply have been found to enhance the


235

incidence of eyespot disease (Bauer, 1963). This,in turn, may also promote lodging. In other studies, however, eyespot-induced lodging of wheat was reduced by the application of N fertilizer (Bockmann, 1964).

c.

PHOSPHORUS, POTASSIUM, AND

TRACEELEMENTS

The effect of these elements on lodging are less pronounced and less consistent than those of nitrogen. In evaluating their effects, differentiation should be made between those that originate in the repair of deficiencies and those which are due to additional supply. The former may improve lodging resistance because deficient plants in many cases suffer from poorly developed culm walls or crown roots; such effects of P and K deficiencies are evident from Casserly’s (1957) studies of lodging in oats. In many experiments no effects on lodging, or only very slight ones, were exerted following the application of either P or K (Chapman and Mason, 1969; Hernes, 1965; Morey et al., 1970; Raheja and Misra, 1955). An increased supply of phosphorus has been found to promote lodging of wheat (Miller and Anderson, 1963; Mulder, 1954; Pyatpgin and Semikhov, 1967; Shrivastava and Yawalkar, 1960) and of oats (Mulder, 1954). Reduced breaking strength of the culms was found by Miller and Anderson ( 1963), whereas increased breaking strength of the roots was reported by Spahr (1960). Increases in the length and diameter of the basal internodes of wheat, following increased P application, were reported by Skorda (1970). Based on the findings that phosphorus increases the N content and decreases the lignin content of wheat culms, Miller and Anderson ( 1965) suggested that it may promote lodging due to its enhancing of the nitrogen effect and by “reducing the ratio of mechanical tissues to proteinaceous ones.’’ An increased supply of potassium has been found to reduce lodging (Shrivastava and Yawalkar, 1960; Wahhab and Ali, 1962). It has also been found to reduce elongation of the lower culm internodes and to increase their diameter (Shrivastava and Yawalkar, 1960; Wahhab and Ali, 1962) Koch, 1969). Increased wall thickness and number of vascular bundles was also found (Wahhab and Ali, 1962). Following applications of K, an increase in culm rigidity, due to the modulus of elasticity (E), and in straw strength, was also reported LHashimoto (1959) and Koch (1969), respectively]. Nightingale (1943) ascribes the culm-strengthening effect of K to its positive effect on carbohydrate synthesis and states that “potassium is frequently recorded as favoring the development of thick cell walls and stiff straw, but in perhaps as many cases this element is reported as having the opposite effect.”

236

MOSHE J. PINTHUS

Koval’skii and Maslyanaya (1969) claim that lodging of cereals grown on peat soil may be caused by Cu deficiency. Experiments in Germany with wheat, barley, oats and rye indicated that on Cu-deficient soils, receiving high N-dressings, the application of copper resulted in the reduction of lodging and subsequent increase in grain yield (Vetter and Teichmann, 1968). The application of manganese to barley grown on peat soil was reported to increase the breaking strength of its two lowest culm internodes (Loiko, 1968). However, Bachthaler (1969) did not find any effect on lodging resistance of winter wheat, spring wheat, or barley through the application of copper, manganese, or boron. SUPPLY D. MOISTURE#

AND

SOIL AERATION

Abundant moisture supply may be conducive to lodging due to its promoting effect on culm elongation; it may also increase the incidence of eyespot, and-when the surplus moisture is in the upper soil layerweaken the anchorage of the root system. On the other hand, dryness of the upper layer may restrict the development of the coronal-root system and thus promote lodging (Harlan, 1957). The lodging of spring wheat stressed for moisture at the onset of culm elongation, in trials at Tucson, Arizona, was attributed to the interference with normal development of upper crown roots (Day and Intalap, 1970). The interference of dryness in the upper soil-layer with coronal-root formation was also reported by Boatwright and Ferguson (1967) and by Ferguson and Boatwright (1968). Furthermore, lodging on clay soils under dry conditions may be evoked by the cracking of the soil, which damages the roots. This has been observed with wheat in Canada (Hurd, 1964). Poor soil aeration may increase susceptibility to lodging due to the effects of respiration inhibition on changes of metabolism which promote cell elongation (Turkova et al., 1965). It may, presumably, increase lodging also through its harmful effect on root development (Troughton, 1962). The promotion of lodging due to poor aeration and high moisture content of the soil is especially evident in waterlogged fields and in fen soils. Soil aeration and soil structure, however, also affect nitrogen availability, which in turn affects lodging, and therefore the effects of these factors on lodging are not clear cut (Mulder, 1954). E.

CROP ROTATION AND TILLAGE

The main effect of crop rotation on lodging is exerted through its effects on the incidence of eyespot, which concerns primarily wheat in western


237

and central Europe. A close sequence of wheat and other cereals on which the disease can survive will promote its incidence (Glynne, 1963; Lelley, 1965). Green manuring and underplowing of the stubble have been reported to reduce eyespot-induced lodging (Grootenhuis, 1968). Other effects of crop rotation on lodging are probably due to its effects on soil fertility and, in particular, on nitrogen availability. In this respect the effects of the fertilizers applied to the preceding crop may be greater than those of the crop itself. Thus, lodging of barley was found to be more frequent and severe following root crops, alfalfa or well-fertilized grass than after a grain crop (Beaven, 1947; Dyke, 1967; Gately, 1968; Widdowson and Penny, 1970). Only little information is available on the effects of tillage practices on lodging. More lodging of spring wheat was found on plowed land (at Rothamsted, England) than after slit seeding into an unplowed grass sward (Hull, 1967). In Czechoslovakia, Kopeckg (1970b) found that subsoiling increased lodging of barley over that obtained on a regularly prepared seed bed, whereas rolling after sowing decreased it (Kopeckp, 1970a). A similar effect of rolling was found in Norway (Njes, 1962). These effects may perhaps be attributed to the impact of the respective seed bed preparation on nitrification in the upper soil layer and subsequent N availability to the crop. Thus, subsoiling may have increased nitrification whereas rolling may have reduced it.

F. SYNERGISTIC EFFECTS The interaction of lodging-promoting factors is apparently of a synergistic nature. Thus, increased nitrogen supply may promote lodging more under irrigated than under dryland conditions and, similarly, more in dense than in sparse stands. This may be illustrated by our (unpublished) observations in a field trial conducted by Dr. Z. Karchi (Table 111). TABLE I11 Lodging Rates (0 = No Lodging; 4 = Complete Lodging) of Wheat as Affected by Plant Density and N Application (En nor, Israel, 1965) Basic dressing of N (kg/ha) Plants per m*

0

120

50 100 150

1 .o 1.2 1.7 0.20

1.6 2.5 5.8 0.20

SE

238

MOSHE J. PINTHUS VI.

A.

Prevention of lodging

CULTURAL PRACTICES

From the information presented above it is apparent that the main lodging-promoting factors are abundant moisture and nitrogen supply, dense stand, and warm temperature. All these factors, however, are also favorable to grain yield production, Therefore, cultural measures to control lodging must aim at the achievement of an equilibrium between yield promotion and lodging prevention. Factors affecting the incidence of eyespot should also be considered. 1 . Date of Sowing

The probability of the plants being at a growth stage particularly susceptible to lodging, during a period of high frequency of lodging-inducing factors, may sometimes be reduced by a suitable sowing date. Furthermore, the sowing date may affect lodging through its effects on tillering and on the period during which stem elongation will take place. Early fall-sowing of winter wheat will prolong the tillering period; and has been found to increase lodging (Hanley et al., 1961; Vez, 1968), presumably because it encourages profuse vegetative growth. Late sowing reduced lodging also because it decreased the incidence of eyespot (Vez, 1968). On the other hand, lodging of early-sown crops has sometimes been less than that of later-sown crops (Henriksen, 1961), which may perhaps be ascribed to a better developed coronal root system resulting from increased tillering. In Mediterranean and other warm regions, where spring-type varieties of cereals are grown in winter, late sowing may reduce the tendency to lodge. This was demonstrated by the results obtained with irrigated barley in Arizona (Day and Thompson, 1970), and it has been recommended to farmers in Israel. Tillering, as well as the elongation of the lower culm internodes of late-sown crops, in these regions, will occur at lower ambient temperature and will therefore be restricted. This, and in particular the restriction of the elongation of the basal culm internodes, may prevent lodging. Spring-sown cereals will enjoy warmer temperature during the periods of tillering and shooting when sown later. In this case early sowing may contribute to the prevention of lodging due to a certain retardation of growth (Rodger, 1956), as found with oats in Scotland (Bain and Morrison, 1961). On the other hand, late-sown plants also enjoy a longer daylength and will therefore reach the stage of head initiation sooner, which in turn may restrict tillering and the number of elongating stem internodes.


239

This may have been the reason for the reduction in lodging of late-sown barley in Belgium (Froidment, 1968). It is concluded that adopting a suitable sowing date may contribute to the prevention of lodging. The application of this measure will, of course, be restricted to those cases in which it has no negative effect on grain yield. 2 . Depth of Sowing and Row Orientation Deep sowing increases the depth at which the root crown is located (Chambers, 1963; Foltjh and Mikala, 1971; Percival, 1921) and also its length (Table IV). This may strengthen the anchorage of the plants in the soil and thus increase their lodging resistance. However, because of increased epicotyl elongation at deep sowing, the depth of the root crown TABLE I V Effects of Sowing Depth on the Root Crown of Common Wheat (Averaged over 10 Varieties Tested a t Rehovot, Israel, in 1964)

Sowing depth (cm) 4 10 18

SE

Length of epicotyl (mm)

Length of crown

3 34 69 2.9

29 34 44 1.3

(mm)

Location of crown below soil surface (mm) 8-37 32-66 47-91

does not reach the depth of sowing (Table IV). Therefore, it seems that within the range of practicable variations in sowing depth, the effects on the root crown may be rather small. Nevertheless, deeper sowing has, indeed, been found to increase lodging resistance of barley (Socittt d’Enccouragement de la Culture des Orges de Brasserie et des Houblons en France; Rapports sur la campagne 1959). Sowing in drill rows in a direction parallel to that of the prevailing strong winds may reduce the incidence of stem lodging. This should also be taken into account while the effects are considered of plant-row direction on yield due to their influence on light interception. 3. Spacing

Numerous studies, with all three cereals and in all parts of the world, indicate that lodging may be prevented or reduced by a decrease in plant density accomplished by a reduced seeding rate (e.g., Bengtsson and Ohlsson, 1965; Furrer and Stauffer, 1970; JevtiC, 1971; Kirby, 1967;

240

MOSHE J. PINTHUS

Lowe and Carter, 1972; Nelson and Roberts, 1961). Up to a certain seeding rate, plant density will be compensated by tillering, resulting in a rather constant shoot density. In this situation lodging resistance will benefit from low seeding rates due to the promotion, through tillering, of coronal-root formation. Beyond this rate, lodging resistance will be affected by the shading effects of plant density. The beneficial effect of low seeding rates on lodging resistance applies also to eyespot-induced lodging (Salt, 1955; Witchalls and Hawke, 1970). The prevention of lodging through a decreased seeding rate must, however, be restricted to those levels where no reduction in grain yield is to be expected in response. Moreover, the effect of plant population on grain yield is of particular significance under fertile conditions conducive to maximum yields which may be challenged by lodging. [Regarding the relationships between plant population and yield, two reviews should be consulted: Holliday (1960), and Willey and Heath ( 1969) .] Reduction in the effect of shading and concurrent maintenance of high plant population may be obtained by decreasing interrow spacing. Narrower spacing, without any change in the seeding rate, was indeed found to reduce the length and increase the diameter and wall thickness of basal culm internodes of wheat (Furrer and Stauffer, 1970; Watson and French, 1971). It also reduced lodging of wheat (Furrer and Stauffer, 1970; Humphries and Bond, 1969), and of barley (Delhaye, 1971; JevtiC, 1971), and the incidence of eyespot (Furrer and Stauffer, 1970). No effect of row spacing on lodging of wheat was obtained by Kinra et al. (1963), but in their experiments lodging was rather slight. In most of the abovecited experiments, the narrower spacing between rows, usually within the range of 8-25 cm, resulted in a certain yield increase (up to 10%) which may have been due to the reduction in lodging. Furthermore, in a review on “the effect of row width on the yield of cereals,” Holliday (1963) shows that reduced spacing (

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