Advances in Agronomy, Volume 53

Advisory Board Martin Alexander

Eugene J. Kamprath

Cornell University

North Carolina State University

Kenneth J. Frey

Larry P. Wilding

Iowa State University

Texas A&M University

Prepared in cooperation with the

American Society of Agronomy Monographs Committee P. S. Baenziger J. Bartels J. N. Bigham L. P. Bush

M. A. Tabatabai, Chairman R. N. Carrow W. T. Frankenberger, Jr. D. M. Kral S. E. Lingle

G. A. Peterson D. E. Roiston D. E. Stott J. W. Stucki

D V A N C E S I N

ono V O L U M5E3 Edited by

Donald L. Sparks Department of Plant and Soil Sciences University of Delaware Newark, Delaware

ACADEMIC PRESS San Diego New York Boston London Sydney Tokyo Toronto

This book is printed on acid-free paper. @ Copyright 0 1994 by ACADEMIC PRESS, INC. All Rights Reserved. No part of this publication may be reproduced or transmitted 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. A Division of Harcourt Brace & Company 525 B Street, Suite 1900, San Diego, California 92101-4495 United Kingdom Edition published by Academic Press Limited 24-28 Oval Road, London NWI 7DX

International Standard Serial Number: 0065-2 I 13 International Standard Book Number: 0-1 2-000753-3

PRINTED IN THE UNITEDSTATES OF AMERlCA 94 95 9 6 9 7 98 9 9 B B 9 8 7 6

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Contents CONTRIBUTORS ........................................... PREFACE.................................................

vii ix

CROPROTATIONSFOR THE 2 1 s CENTURY ~ D . L . Karlen. G. E. Varvel. D . G. Bullock. and R . M . Cruse I. Origin of Crop Rotations ................................... I1. 2 0 t h Century Crop Rotations ................................ I11. Agronomic Impacts of Crop Rotation ......................... IV. Soil Quality Effects ........................................ V. Biological Diversity ......................................... VI. Economics of Crop Rotation ................................ VII . Policy Impacts on Crop Rotations ............................ VIII . Summary and Conclusions .................................. References ................................................

2 5 11 22 30 32 33 36 37

ROLEOF DISSOLUTION AND PRECIPITATION OF MINERALS INCONTROLLING SOLUBLE ALUMINUMIN ACIDICSOILS G. S . P. Ritchie I . Introduction .............................................. I1 A Framework for Understanding Mineral Dissolution and Precipitation in Soils ....................................... I11. Factors Affecting Dissolution and Precipitation of AluminumContaining Minerals ....................................... Iv. Modeling Soluble Aluminum ................................ V. Aluminum in Acidic Soils: Principles and Practicalities .......... References ................................................

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47 50

51 64 77 80

CONTENTS

vi

MANAGINGPLANTNUTRIENTS FOR OPTIMUM WATERUSEEFFICIENCY AND WATER CONSERVATION Jessica G. Davis I. Introduction .............................................. 11. Conserving Water Supply by Optimizing Water Use Efficiency . . . 111. Conserving Water Quality through Nutrient Management . . . . . . . Iv Needs for Further Research ................................. References ................................................

INTERPARTICLE FORCES: A BASIS FOR THE INTERPRETATION OF SOILPHYSICAL BEHAVIOR J. P. Quirk Introduction .............................................. Interparticle Forces ........................................ Soil Water Relations: Swelling and Shrinkage . . . . . . . . . . . . . . . . . .

85 86 92 108 109

Iv. Swelling of Sodium Clays ................................... v. Swelling of Calcium Clays .................................. VI. Surface Area and Pore Size .................................. VII . Water Stability of Soil Aggregates ............................

VIII. Sodic Soils and the Threshold Concentration Concept . . . . . . . . . . Ix. Concluding Remarks ....................................... References ................................................

122 124 143 146 152 161 166 169 176 177

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

185

I. 11. 111.

Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin.

D. G. BULLOCK (l), Department ofAgronomy, University of Illinois, Urbana, Illinois 61801 R. M. CRUSE (I), Department of Agronomy, Iowa State University,Ames, Iowa 5001 I JESSICA G. DAVIS (85), Department of Crop and Soil Sciences, University of Georgia, Coastal Plain Experiment Station, Tifton, Georgia 3 1 793 D. L. KARLEN (I), National Soil Tilth Laboratory, United States Department of Agriculture, Agricultural Research Service, Ames, Iowa JOOl I J. P. QUIRK (1 2 l), Department of Soil Science and Plant Nutrition, School of Agriculture, The University of Western Australia, Nedhnd, WesternAustralia 6009, Australia G. S. P. RITCHIE (47), Department of Soil Science and Plant Nutrition, School of Agriculture, The University of WesternAustralia, Nedlands, Western Azlsh-alia 6009, Australia G. E. VARVEL (l), Soil/Water Conservation Research Unit, United States Department of Agriculture,Agricultural Research Service, University of Nebraska, Lincoln, Nebraska 68583

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Preface Volume 53 contains four excellent reviews that cover a broad spectrum of important advances and topics in the plant and soil sciences. Sustainable agriculture is one of the most discussed issues and venues for research in agronomy at the present time. The first chapter comprehensively reviews the history of crop rotations and future directions in this important area. Topics that are covered include twentieth century rotations, agronomic impacts of crop rotations, effects of rotations on soil quality, economics of crop rotations, and policy impacts. The second chapter provides a thorough discussion on how dissolution and precipitation affect soluble aluminum in acid soils. The author reviews factors that affect dissolution and precipitation, ways to model soluble aluminum including thermodynamic and kinetic approaches, and the effects of aluminum on aspects of acid soils. Water quality and conservation are of paramount importance in protecting and preserving our environment and are among the most active areas of research in agronomy. The role that nutrient management has on optimal water use efficiency and conservation is the topic of the third chapter. Discussions on conserving water supplies via optimization of water use efficiency and preservation of water quality through nutrient management are thoroughly covered. The fourth chapter is a definitive treatise on how interparticle forces affect soil physical behavior which, of course, has immense effects on plant growth and yield. Topics that are discussed include interparticle forces, soil water relations, swelling of clays, surface area and pore size, water stability of soil aggregates, and sodic soils. I thank the authors for their comprehensive and timely reviews.

DONALD L. SPARKS

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CROP R~XITONS FOR THE 21sr C

~

D. L. Karlen,' G. E. Varve1,z D. G. Bullock,3 and R. M. Cruse4 'National Soil Tilth Laboratory United States Department of Agriculture Agricultural Research Service Ames, Iowa 50011 *Soil/Water Conservation Research Unit United States Department of Agriculture Agricultural Research Service University of Nebraska Lincoln, Nebraska 68583 3Department of Agronomy University of Illinois Urbana, Illinois 61801 4Deparunent of Agronomy Iowa State University Ames, Iowa 5001 1

I. Origin of Crop Rotations 11. 20th Century Crop Rotations A. Pre-World War I1 B. Post-World War I1 Developments C. 2 1st Century Outlook 111. Agronomic Impacts of Crop Rotation A. Crop Yield B. Water Use Efficiency C. Nutrient Use Efficiency D. Disease and Pest Interactions E. Allelopathy W. Soil Quality Effects A. Soil Structure B. Aggregation C. Bulk Density D. Water Infiltration and Retention E. Soil Erodibility F. Organic Matter V. Biological Diversity A. Effects on Wildlife B. Alternative Land Uses VI. Economics of Crop Rotation VII. Policy Impacts on Crop Rotations VIII. Summary and Conclusions References 1 Advance in A p n q , Vdume 53

Copyright Q 1994 by Academic Press, Inc. All rights of reproduction in any form reserved

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D. L. KARLEN ETAL.

I. ORIGIN OF CROP ROTATIONS The practice of crop rotation or sequentially growing a sequence of plant species on the same land (Yates, 1954) has been in existence for thousands of years. As noted by Parker (1915), crop rotation developed primarily from the experiences of mankind relative to soil productivity. MacRae and Mehuys ( 1 985) stated that it was practiced during the Han dynasty of China more than 3000 years ago. Early agriculturists experienced low yields that resulted from continuous cropping, and throughout history, crop rotation was found to be necessary to maintain productivity. However, it was seldom, if ever, understood why. Early writers noted that crop rotation was in use in ancient Greece and Rome. Pliny mentioned the use of four rotational schemes, including two for rich soils and one each for second- and third-quality soils (White, 1970a,b). For rich soils, the two rotations mentioned included three-crop sequences of barley (Hordeurn vulgare L.), millet (Panicurn rniliaceurn L.),and turnip (Brussica r a p L.) or wheat (Triticurn aestivurn L.), millet or turnip, and emmer (Triticurn dicoccon Schrank), and 4 months of fallow followed by spring beans (Varafaba L.) or no fallow with winter beans (Hiemalis faba L.). For second-quality soils the suggested 2-year rotation was wheat and beans or another legume and for thirdquality soils it was emmer and beans or legume followed by a fallow period. Pliny also recommended that when fallow is not an option, the field should be put down to lupins (Lupinus albus L.), vetch (Vicia sativa L.), or beans. These crops could then be incorporated as green manure in preparation for growing emmer. Columella recommended similar crop rotation practices to those described by Pliny (White, 1970a,b). Systems cited by these ancient writers generally described the typical legume and cereal crop rotation in which some form of legume [pea (Pisurn sativurn L.), bean, vetch, or lupin] was alternated with a cereal crop. The actual use of crop rotations by the Romans has been debated at length by both medieval and classical historians, some of which insist the practice of rotation was rare. According to White (1970b), these historians argued that rotation systems were widely recommended by Roman agronomists, but in practice were not used extensively by farmers of the time. However, it is interesting to note accounts mentioning crop rotation by other writers not considered agronomists, which included Virgil’s detailed account of crop rotations as alternatives to traditional fallowing in his poems. Whether crop rotations were in widespread use is not known, but the practice was used and its benefit was known. According to Brehaut (1933), in his translation of Cato’s De agricultura, other writers, including Cat0 the Censor, indicated the use of rotations was prevalent. Cat0 noted the beneficial effects of lupins, beans, and vetch and indicated the

CROP ROTATIONS FOR THE 2 1st CENTURY

3

likely use of this crop rotation was in a legume-cereal system. In Italy during the first century B . c . , Varro also noted the importance of a green manure crop, especially legumes, in cropping systems prevalent at that time. Despite the beneficial effects of crop rotation, the practice fell out of favor with the demise of Roman power throughout Europe (White, 1970b). Less use of crop rotation and a return to the old crop and fallow system appeared to occur with the return to a more rural civilization as the more urban civilization prevalent in the Roman Empire disappeared. Throughout the Middle Ages, little mention is made of crop rotation and as noted earlier, the prevalent practice was probably the crop-fallow system. One exception mentioned for this period was the practice of alternating 2 years of wheat and 5 years of grass in a system called ley farming (crop rotation). This sequence was used by the Monks of Couper around 1400 in Britain (Franklin, 1953). Crop rotation was probably used to some extent during this period, but it appears that a crop-fallow system, with the use of manure, was the general system in use. Crop rotations, as we now know them, are often traced back to the Norfolk rotation. This was popular in England about 1730 (Martin et al., 1976). The Norfolk rotation, which was widely used at the time, consisted of turnip, barley, clover, and wheat in a 4-year sequence. The Norfolk and many other similar rotation systems were in use throughout the 18th century, but little was actually known about the specific benefits of rotating crops. The prevailing thought was that each of the crops in the rotation obtained their nutrients from different zones or parts of the soil. This perception was used to explain why a sequence of different crops yielded better than a single crop grown year after year. Between 1730 and 1840 the practice of crop rotation and the use of artificial manure (lime and other soil minerals) to supplement animal manure had become almost universal in England (Parker, 1915). One early English agricultural writer, Arthur Young, was not necessarily a proponent of this system. Young was a great apostle of mixed farming. He lauded the value of legumes, the use of crop rotation, and the feeding of livestock on the farm and the return of the manure to the land. Young insisted grass land and grazing were of primary importance and management of arable land of secondary importance to English agriculture. However, he did emphasize the importance of crop rotation and animal husbandry to agriculture at the time (Parker, 1915). As would be expected because of the heavy influence of English and Scottish settlers, most early agriculture in the United States was based on English customs. Several letters between Thomas Jefferson and George Washington (Bureau of Agricultural Economics, 1937) support this statement and indicate that crop rotation was also the prevalent practice in the United States. Jefferson wrote in a letter addressed to President Washington in 1794 that he was going to have to use a milder course of cropping because of the ravages brought about by overseers

4

D. L. KARLEN E T A .

during his absence. His rotation was first year, wheat; second, corn (Zea mays L.), potatoes (Solanum tuberosum L.), or pea; third, rye (Secale cereale L.) or wheat, according to the circumstances; fourth and fifth, clover or buckwheat (Fagopyrum esculentum Moench); and sixth, something he described as folding or buckwheat if it had not been used in the fourth or fifth years. Another letter, dated 1798, indicated he was using a triennial rotation of 1 year of wheat and 2 years of clover in his stronger fields or 1 year of wheat and 2 years of pea in the weaker fields followed by a crop of Indian corn and potatoes between every other rotation. Jefferson commented in both cases that he felt these types of cropping systems, with the addition of some manure, would help his fields recover their pristine fertility at Monticello. In later years, after retiring from the presidency, Jefferson returned to Monticello and noted in a letter to C. W. Peale in 1811 that his rotations were mainly corn, wheat, and clover; corn, wheat, clover, and clover; or wheat, corn, wheat, clover, and clover. It was apparent that he knew well the benefit of rotation with legumes by the prevalence of clover in each of these systems. In some of his letters and papers, George Washington described a good crop rotation plan that he found in use on Long Island in 1790. It consisted of corn with manure, oats (Avena sativa L.) or flax (Linum usitatissimurn L.), wheat with 4 to 6 pounds of clover and 1 quart of timothy (Phleum pratense L.), and meadow or pasture. From 1800 to 1810 this same rotation with some slight modifications came into quite general use in Pennsylvania. However, in Virginia, a rotation similar to that of Jefferson’s was used by many farmers (Parker, 1915). Jefferson, Washington, and many other progressive farmers of the time used rotations and manure extensively in an attempt to regain productivity levels similar to those when the virgin soils of the United States were first broken out. In other parts of the world, it was apparent crop rotations and other systems similar to the Norfolk rotation were in extensive use by farmers during the 19th century. Despite their extensive use of rotations, agriculturists of the time, such as Baron Justis von Liebig (1 859), believed that although crop rotation improved the physical and chemical condition of the soil, all plants would eventually exhaust the soil. Liebig felt that unless soils were heavily manured, all fields would eventually lose their fertility, regardless of crop rotation. Hall (1905) presents an excellent summary of the prevailing thoughts and experiments concerning crop growth and production during the 19th century. It was during this time period that researchers discovered legumes had the ability to assimilate and utilize nitrogen from the atmosphere, which enlightened researchers regarding the benefit of growing crops in rotation with legumes. As described by Hall (1905), this discovery provided an explanation as to the benefit of existing crop rotation studies and led to new investigations on crop rotations during the 19th century at Rothamsted, England (the world’s first agricultural

CROP ROTATIONS FOR THE 2lst CENTURY

5

research station). These studies further identified that some of the nitrogen fixed by the legumes in a cropping system becomes available for succeeding crops and clearly identified at least part of the beneficial effects of crop rotations.

11. 20th CENTURY CROP ROTATIONS A. PRE-WORLD WAR 11 The discovery in the latter part of the 19th century that legumes could fix nitrogen from the atmosphere was a major reason rotations remained popular into the early part of the 20th century. Nitrogen was the major limiting nutrient for most crops and it could only be supplied by the addition of manure or by incorporating a legume of some type in the cropping system. Use of crop rotation during this period, similar to patterns established throughout history, was greatly dependent on the amount of new or virgin land available for crop production. If cheap and plentiful amounts of fertile land were available, crop rotations were not extensively used. Only as land became more expensive and less plentiful were crop rotations utilized more extensively. Johnson (1927) presented examples of rotation experiments conducted in several different areas of the United States during the early part of the 20th century. In Georgia, the suggested rotation was corn, cowpea (Vigna unguiculata L.), oat, and cotton. Cowpea was sown during the last cultivation. The corn was harvested for grain and the cowpea was worked into the soil. Oat was sown in late fall and harvested in late May or early June. Cowpea was sown again as a green manure crop to be incorporated the next spring just before planting cotton (Gossypiurn hirsuturn L.). This crop sequence increased cotton yields as much as 100% after the first series of the rotation and even greater increases in productivity were maintained in successive rotations. Rotation experiments at the University of Missouri that began in 1888 included a 6-year corn, oat, wheat, clover, timothy, and timothy rotation and a 3-year corn, wheat, and clover rotation. According to Johnson (1927), after 30 years, yields of corn were increased 60.4%, oat 3%, and wheat 32% in the 6-year rotation and 30.8% for corn and 40.8% for wheat in the 3-year rotation over the yields of the corresponding continuously cropped areas. Manure applications averaged 6.8 tons annually in both rotation and continuous cropping systems. Results from an Ohio experiment were similar (Johnson, 1927). The main difference between the Ohio and Missouri experiments was the use of fertilizers instead of manure. Yields of corn, oat, and wheat in rotation were increased 29.9, 30.8, and 42.5%, respectively, above yields of those crops in continuous culture. In Delaware, corn grain yields increased 156.9% in a rotation of corn

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D. L. KARLEN ET AL.

(including a cover crop of rye and vetch), soybean [(Clycine m u (L.) Merr.)], wheat, clover, and timothy as compared to continuous corn when no fertilizer or manure was used. With nitrogen, phosphorus, and potassium applications corn grain yields still increased 24.4% in rotation as compared to the continuous corn system (Johnson, 1927). Cover crops were widely recommended for many cropping systems in the early 20th century (Johnson, 1927). Among these systems was one including two crops of kale (Brassica oleracea var. acephala DC.), three of cabbage (Brassica oleracea var. capitata L.), three of potatoes, three of sweet potatoes [(Zpornoea batatas (L.) Lam.)], and one of German millet [(Setariaifulica (L.) P. Beauv.)]. These 12 crops were grown in rotation during the 9-year period from 1912 to 1921 both with and without cover crops at the Virginia Truck Experiment Station. Similar to the experiments just described, both rotations received the same amount of fertilizer, nitrogen, phosphorus, and potassium. Rotation again increased yields, but yields were increased to an even greater extent with the use of cover crops. Yield increases ranged from 12.5% for kale to 62.5% for millet with cover crops. In this experiment, rotation was considered a major factor contributing to disease control in truck crops and probably contributed greatly to the yield increases. Other popular rotations for truck farmers in Virginia at this time included a 3-year rotation of potatoes, corn, and rye grown during both the first and second year, and sweet potatoes and rye grown during the third year. In Norfolk County, Virginia, early potatoes were grown as a spring crop. This was followed by a cover crop of native grass, which was cut for hay, or legumes such as soybean or cowpea, which were incorporated in the fall. Cabbage was then planted in November and harvested the following April or May just before planting a corn crop. Sometimes, soybean was planted directly in the row with the corn and rye was sown between the rows at last cultivation as intercrops. When the corn was harvested, the cycle, starting with early potatoes, was repeated. This Norfolk County, Virginia, rotation consisted of two main crops (potatoes and cabbage), two catch crops, the hay, and corn crop (Johnson, 1927). With small modifications, this was similar to most of the truck crop rotations used throughout the eastern United States. In the same symposium, Lyon (1927) described the effects of legumes and grasses in different crop rotations. Most of the systems he described were similar to a corn, oat, wheat, and hay rotation, where the hay was usually either a legume or a grass such as timothy. He concluded that with few exceptions, experiments conducted at eight experiment stations in the humid regions of the United States generally showed legumes to be superior to grasses for increasing yields of the following crops. In the drier parts of the country, however, grasses were generally superior to legumes because they usually did not deplete soil moisture as extensively as legumes. Crop rotation was not a widely accepted practice in the United States corn belt during the early 20th century. The soils were extremely fertile and after the virgin

CROP ROTATIONS FOR THE 2 1st CENTURY

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sod was plowed, they sustained corn yields at sufficiently high levels for many years. However, even on these extremely fertile soils, crop rotation greatly increased yields at several locations compared to growing monoculture corn (Wiancko, 1927). Despite the superiority of rotated corn yields, none of the other crops in the rotation produced net returns anywhere close to that of corn. Therefore, farmers wanted to grow continuous corn even though its production had greatly reduced the fertility of many soils. Wiancko (1927) concluded that corn was the principle crop of the corn belt and that fact had to be recognized and considered in crop rotations proposed for general use in the region. Crop rotations in the southern and southeastern United States usually revolved around the staple crops of cotton, tobacco (Nicotiana tabacum L.), rice (Oryza sariva L.), and peanut (Arachis hypogaea L.). Parker (1915) discussed several rotation schemes used for these crops during the late 19th and early 20th centuries. He presented rotations for both livestock and mixed grain and livestock farms. They usually had corn, oat, wheat, clover, and meadow in various combinations and sequences, with the main emphasis being on feed for livestock. Rotations for tobacco were usually tobacco, wheat, and clover; tobacco, wheat, and cowpea; or tobacco, wheat, red clover (Trifolium pratense L.), meadow, and corn. Cowpea was generally included where the legumes were used as a green manure to maintain the soil humus supply. Cotton rotations were similar to those of tobacco in that emphasis was placed on one crop, while other crops in the system were selected for maintenance of soil humus levels and/or their potential as livestock feed. Crops in the cotton-based rotations included corn, wheat, oat, peanut, cowpea, and crimson clover (Trifolium incartum L.) in 2- and 3-year sequences with cotton. Rice was most often grown continuously, but progressive farmers of the time were becoming aware of crop rotation benefits, and if possible they used a rice, rice, rice, fallow, corn, and pea or bean (as green manure) rotation. Western regions of the United States also utilized crop rotations extensively during the early part of the 20th century. Crop rotations varied widely because of large growing season precipitation differences across the region. In more humid parts of this region, crop rotations were similar to those discussed for the corn belt states to the east. Drier areas of the Great Plains used cropping systems developed for the region with respect to water conservation. Parker (1915) presented several of the rotations used during this period for what he termed grain farming and mixed grain and livestock operations (Table I). In these rotations, grain was Durum wheat (Triticum durum Desf.), winter wheat, rye, emmer, awnless barley, or 60-day oat. The specific selection depended on local conditions. Green manure/fallow referred to growing crops such as Dakota vetch, Canadian field pea, sweet clover, common millet (Panicurn miliaceum L.), or Hungarian millet [Setaria italica (L.) P. Beauv.], which were plowed under in early summer, and then allowing the land to rest for the remainder of the season. The term cultivated crop referred to such crops as Indian corn, Kafir corn (Sor-

Table I Qpiieal3- to 7-YearCrop Rotations Used for Grain Farming and Mixed Grain and Livestock Operations in the Western United States during the Early 20th Centuryn

Farming system Grain only

Option

Year 1

1

Grain

2

Grain Grain c-P c-crop c-crop Grain c-crop

3

c-crop

Grain

4

c-crop Grain Grain

g-m-f s-clovere s-clover

2 3 4

5 Grain and livestock

1

5 6 After Parker (1915). Green manure/fallow. Cultivated crop. Bromegrass (Brornus srerilus 1.). Melilotus oficinalis Lam.

Year 2

g-m-f Grain Grain Grain Bromed Grain

Year 3

Year 4

g-m-fb Grain g-m-f g-m-f g-m-f Brome Pea or vetch hay g-m-f

Grain Grain

Grain

c-crop Grain

csrop c-crop

Grain Grain

c-crop Millet or sudan grass Grain

Year 5 c-cropc Grain g-m-f g-m-f Grain Pea or vetch hay -

Year 6 -

g-m-f -

Grain

g-m-f

Year I

CROP ROTATIONS FOR THE 21st CENTURY

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ghum bicolor L. Moench), durra (Sorghum bicolor L. Moench), and proso millet (Panicurn miliaceum L.), which again depended on local conditions and the particular needs of the individual farmer. In western areas, where irrigation was available, crop rotations differed greatly from those used for dryland situations. The emphasis in these cropping systems was usually on some form of high value cash crop, similar to those in the more humid areas of the East and South. The high value cash crops were mainly potatoes and sugar beet (Beta vulgaris L.), usually in some form of rotation with a legume and grain crop. The importance of crop rotation was evident even with irrigation (Powers and Lewis, 1930). Substantial increases in soil nitrogen and total carbon were found where irrigation and crop rotation or use of manure occurred. They reported striking differences in crop yield, water use efficiency (yield per acre inch applied), net profit per acre, and water cost per unit of dry matter. Powers and Lewis (1 930) suggested that settlers on newly irrigated arid land should utilize a crop rotation to improve the nitrogen and organic matter content of their soils and that such practices would help ensure economical use of water and establishment of profitable crop production under irrigation. Examples of irrigated crop rotations used in different parts of the West included a 5-year rotation in Utah that consisted of sugar beet, oat and pea for hay, sugar beet, oat, and a fifth field in alfalfa (Medicago sativa L.); and a 5-year rotation used in Colorado that consisted of oat, pea, potatoes or sugar beet, barley or wheat, and a fifth field in alfalfa (Parker, 1915). These and many other similar cropping systems were used throughout irrigated areas of the western United States. The emphasis in most of the rotations was on some form of staple crop and an adaptable soil renovating crop, such as the alfalfa in the rotations just described. Alfalfa was well adapted to irrigation, provided an excellent source of forage, and as a legume, it contributed greatly to rebuilding the fertility of the soil in these irrigated systems. As noted earlier, selection of the specific cropping system during the latter part of the 19th and early part of the 20th centuries was based on the individual needs of the farmer, selection of crops adapted to a particular region, and the climatic limitations of the area. The other basic requirements in a crop rotation during this time period, regardless of the region, were the need for some sort of legume to provide nitrogen for successive crops and different forms of green manure crops, which were used to maintain fertility of the soils.

B. POST-WORLD WARI1 DEVELOPMENTS Cropping systems before World War I1 changed little, but following this world event, crop rotations that included legumes were de-emphasized. Increased avail-

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D. L. KARLEN ETAL.

ability of nitrogen from industrial sources during the 1950s and early 1960s hastened this change throughout the United States (Tyner and Purcell, 1985). Plentiful and inexpensive nitrogen fertilizer following World War I1 devalued legume rotations except for farmers with livestock systems that required the legume as a feed source (Olson and Sander, 1988). Post-World War I1 research reports document a consensus that synthetic fertilizers and pesticides could forever replace crop rotation without loss of yield (Aldrich, 1964; Benson, 1985; Shrader et al., 1966). Melsted (1954) concluded that to achieve maximum production with minimum soil deterioration, an adequate supply of fertilizer nitrogen was essential. Increased availability of nitrogen fertilizers, herbicides for weed control, and pesticides for insect and disease control reduced the use of extended rotations (Rifkin, 1983; Crookston, 1984; MacRae and Mehuys, 1985). These changes have resulted in extensive monoculture corn throughout most of the corn belt, with generally increasing yields. The introduction of improved crop varieties was a major factor (Power and Follett, 1987), but mechanization (i.e., replacement of draft animals, which required feed and land devoted to its production, with tractors and combines) also contributed to the general decline among farmers and researchers in perceived need for, and therefore use of, crop rotations. Mechanization and adoption of short, 2-year corn and soybean rotations or continuous monocropping enabled farmers to benefit from the economy of scale by specializing their operations, improving marketing practices, and having to invest in fewer pieces of equipment (Bullock, 1992; Colvin et al., 1990; Power and Follett, 1987). Furthermore, many of the government production control and income stabilization programs limited rotation options and forced farmers to abandon extended crop rotations (Francis and Clegg, 1990).

c. 2 1ST CENTURY OUTLOOK Intensive monoculture cropping has increased throughout the United States since World War 11, and crop rotations have diminished. However, in many areas, crop rotation has steadfastly remained the major cropping system. Current consensus is that crop rotation increases yield and profit and allows for sustained production (Mitchell et al., 1991). In many areas, including several different types of crops remains the most economical and feasible method for crop production because it is one of the most effective disease and pest control systems. More recently, increased energy costs resulted in renewed interest in crop rotations as a source of nitrogen (Tyner and Purcell, 1985). However, interest in rotations as a source of nitrogen is present only when energy and fertilizer costs are high. Both are uncertain at this time. Post-World War I1 abandonment of extended crop rotations, in favor of short rotations and monocropping systems, has generally been profitable. However,

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11

the change has had negative consequences, especially if on- and off-site environmental consequences are considered. Many effects are site specific, but they include decreased soil organic matter content, degraded soil structure, increased soil erosion, increased sedimentation of reservoirs, increased need for external inputs, and increased surface and groundwater contamination. Long-term effects of not using crop rotations are not clear, but it is reasonable to question if the substitution of capital, energy, and synthetic chemicals is sustainable (Bullock, 1992). These questions are raised as we look toward the 21st century, because, as stated by Hauptli et al. (1990), “Modern agriculture is a very recent development, when considered in the context of evolution or even human history.” Crop rotation has not been abandoned in the United States. Approximately 20% of the corn is grown in continuous monoculture, but most of the remaining 80% is grown in a 2-year rotation with soybean or in short (2- or 3-year) rotations with alfalfa, cotton, dry beans, or other crops (Power and Follett, 1987). The primary crop rotation change involves use of pasture and green manure crops. Few are included in current crop rotations. Many of the rotation factors, processes, and mechanisms responsible for increased yield remain unknown. Increased nitrogen supply is sometimes responsible (Russelle et al., 1987), but improvements in soil water availability (Benson, 1985; Roder et al., 1989), soil nutrient availability (Bolton et al., 1976; Higgs et al., 1976; Peterson and Varvel, 1989a,b,c), soil structure (Barber, 1972; Dick and van Doren, 1985; Griffith et al., 1988), soil microbial activity (Cook, 1984; Williams and Schmitthenner, 1962), and weed control (Bhowmik and Doll, 1982; Slife, 1976); decreased insect pressure (Benson, 1985), nematode populations (Dabney et al., 1988), and disease incidence (Dick and van Doren, 1985; Edwards et al., 1988); and presence of phytotoxic compounds and/or growthpromoting substances originating from crop residues (Barber, 1972; Benson, 1985; Bhowmik and Doll, 1982; Welch, 1976; Yakle and Cruse, 1983, 1984) have also been identified as contributing factors. Currently, no amount of chemical fertilizer or pesticide can fully compensate for crop rotation effects, and analysis of these individual factors generally does not explain the entire yield response associated with crop rotation. Determining how the factors associated with crop rotations interact and contribute to the currently undefined “rotation effect” will apparently continue to provide a major research challenge.

111. AGRONOMIC IMPACTS OF CROP ROTATION A. CROPYIELD Increased yield may be one of the most practical justifications for reintroducing crop rotations (Wikner, 1990; Karlen et al., 1991). Several studies showing

D. L. KARLEN ET AL.

12

that corn, grown in a 2-year rotation with soybean, yields 5 to 20% more than monoculture corn have been published (Strickling, 1950; Welch, 1976; Kurtz et al., 1984; Voss and Shrader, 1984; Peterson and Varvel, 1989c; Crookston et al., 1991). Data from a 15-year study in Iowa (Table 11) show the typical response. Crookston et al. (1991) reported that annually rotated corn yielded 10% more than continuous corn, and that first-year corn, following 5 years of soybean, yielded 15% more than continuous corn. Based on these results, they suggested Minnesota farmers consider using longer crop rotations. However, yield response to rotations greater than 2 years may (Crookston et al., 1991) or may not (Lund et al., 1993) occur. Increased emphasis on crop residue management to reduce soil erosion may also encourage crop rotations because they can largely eliminate corn yield decrease observed between no-tillage and conventional tillage production practices (Karlen et al., 1991). This response is particularly evident on poorly drained soils (Dick et al., 1991). Furthermore, because many cropping systems have a small profit margin, a 5% yield increase for corn may result in a 50% profit increase (Crookston, 1984).

Table I1 Crop Rotation Effect on Corn Grain Yield

in Northeast Iowa Year

Continuous

Rotation

Mg ha-l 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992

8.0 9.5 9.5 9.9 7.5 5.3 5.5 7.6 10.2 8.1 5 .O 6.6 10.6 8.3 9.1

9.4 10.2 9.7 10.4 7.7 6.8 7.3 8.8 10.8 9.0 6.3 8.0

8.0

9.0

11.2 9.4 10.0

LSD(O.05) = 0.3

cv = 4.3%

15-year average LSD(O.05) = 0.2

CROP ROTATIONS FOR THE 2 1st CENTURY

13

Crop yield increases due to rotation are not limited to corn. Grain sorghum in rotation with soybean (Brawand and Hossner, 1976; Clegg, 1982; Gakale and Clegg, 1987; Peterson and Varvel, 1989b; Roder et al., 1988; Langdale et al., 1990) or corn (Robinson, 1966) showed increased yield compared to continuous grain sorghum. Soybean yield also increased when grown in a rotation with corn, grain sorghum, or simply following a fallow period (Crookston, 1984; Dabney er al., 1988; Peterson and Varvel, 1989a).

B. WATERUSEEFFICIENCY The need to develop more water-efficient crop management practices may be one of the strongest incentives for adopting crop rotations. Crops should be managed in a rotation sequence so that complementary root systems fully exploit available water and nutrients (Karlen and Sharpley, 1994). Sadler and Turner ( 1994) suggested “opportunistic cropping” as a means for increasing agricultural sustainability through water conservation or by increasing productivity from applied water. Opportunistic cropping is not crop rotation in the typical sense, but this management practice requires farmers to remain sufficiently flexible to adapt their farming practices to utilize rainfall and/or irrigation water as efficiently as possible. Therefore, opportunities to rotate crops spatially and temporally may become increasingly important. Roder er al. (1989) evaluated yield and soil water relationships for a sorghum and soybean cropping system. They found that crop rotation increased soybean yield, but that nitrogen fertilization did not. The soybean yield advantage from rotation decreased as the amount of spring rainfall increased. Increasing temporal and spatial diversity by using different crop rotations may mimic natural ecosystems more closely than current farming practices. This change may lead to increased agricultural sustainability (Karlen et al., 1992). One example is in semiarid areas where saline seeps began to develop about 30 years after cultivation began, and especially after about 10 years of an alternateyear, crop-fallow rotation (Ferguson er d.,1972). Formation of saline seeps gradually became a problem as production agriculture disrupted annual crop growth associated with native plant communities in semiarid regions. Ferguson and Bateridge (1982) found that 50 years of crop-fallow farming significantly reduced soluble salt content of some soils. Although this was beneficial from an edaphic perspective, they found that up to 90 Mg ha-1 of salt was moved toward the water table where it resulted in groundwater salinization and became a source of salts for saline seeps. Undoubtedly, some water moves below the root zone of native vegetation, but the quantity is not large. Native vegetation is diverse with varying growth habits and rooting depths. Therefore, most precipitation infiltrating the sod is transpired

14

D. L. KARLEN ETAL.

(Ferguson et al., 1972; Halvorson and Black, 1974). With cultivation during periods of above-normal annual precipitation, and with improved soil water storage and conservation during fallow, increased use of summer fallow enhances percolation of water below the root zone and thus contributes to formation of saline seeps (Halvorson and Reule, 1976). By using flexible crop rotations involving small grains, grasses, deep-rooted crops, and a minimum amount of summer fallow, soil water loss by deep percolation could be prevented and development of saline seeps could be alleviated (Halverson and Black, 1974).

C. NUTRIENT USEEFFICIENCY 1. Nitrogen

Increased use of crop rotations may be mandated to improve nutrient use efficiencies and reduce losses of nitrogen to surface and groundwater resources. Crop rotation per se is important, but the sequence with which crops are grown may be more important (Carter et al., 1991; Carter and Berg, 1991). Karlen and Sharpley ( 1994) reviewed several studies showing how crop sequence could influence nitrogen movement through the soil profile and ultimately into groundwater resources. Several studies showed that soybean and alfalfa, which do not require supplemental nitrogen inputs, can effectively use or “scavenge” residual nitrogen remaining in the soil from previous crops (Johnson et al., 1975; Mathers et al., 1975; Muir et al., 1976; Olson er al., 1970; Stewart et al., 1968). Alfalfa roots may grow to depths greater than 5.5 m in some soils, and research has shown that nitrate can be utilized by the crop from any depth where soil solution is extracted by plant roots. Mathers et al. ( I 975) reported that alfalfa removed nitrate from the soil profile at a depth of 1.8 m during the first year of establishment and to a depth of 3.6 m during the second and third years. Olson et al. (1970) found that crop rotation reduced soil solution nitrate concentrations at a depth of 1.2 to 1.5 m by 34 to 82% compared to continuous corn. They found that the decrease in solution nitrate was directly proportional to the number of years in oats, meadow, or alfalfa production, and attributed this to combined recovery of nitrate by shallow-rooted oat crops followed by deep-rooted alfalfa crops. Soybean can also effectively scavenge residual soil N (Johnson et al., 1975; Havlin et al., 1990; Karlen e t a l . , 1991), but in Wisconsin, soybeans were not as effective as alfalfa because of their more shallow rooting depth (Jackson et al., 1987). This finding was supported by Olson et al. (1970), who also concluded that recovery of subsoil nitrates by deep-rooted legumes such as alfalfa will probably be more effective on medium and heavy textured soils than on sands. One of the persistent nutrient management questions associated with crop

CROP ROTATIONS FOR THE 2 1s t CENTURY

1s

rotation is whether the nitrogen contribution from legume fixation is responsible for much, if not all, of the beneficial rotation effect. Bullock (1992) reviewed several studies focusing on the fertilizer replacement value as the method for assessing nitrogen contributions from legumes grown in rotation with nonlegume crops such as corn or grain sorghum. He reported that this method overestimated the nitrogen contribution by legumes and underestimated the rotation effect. For example, soybean is given a fertilizer replacement value of 25 to 40 kg ha-1 in many midwestern states. The actual nitrogen contribution by the soybean crop is often much less or even negative. In the midwestern United States, soybean in a 2-year corn and soybean rotation may acquire only 40% of its nitrogen from dinitrogen fixation, while the remaining 60% is taken up from the soil (Heichel, 1987). When the grain is harvested and removed, there is an estimated net loss of 84 kg N ha-1 due to the large nitrogen content of soybean grain: The nitrogen contribution from alfalfa in rotation with maize in the upper midwestern United States is also less than suggested by fertilizer replacement value methodology. Fertilizer recommendations for corn following alfalfa in most midwestern states credit the alfalfa crop with a nitrogen contribution of 100 to 125 kg N ha-' (Bruulsema and Christie, 1987; Fox and Piekielek, 1988) based on fertilizer replacement methodology. However, the actual contribution measured with '5N methodology was only 24 kg N ha-1 (Harris and Hesterman, 1990). Based on these studies, Bullock (1992) concluded that rotation with legumes does not provide as much nitrogen as fertilizer replacement methodology estimates and that much of the yield benefit which has been credited to nitrogen contribution is actually due to other factors. Jensen and Haahr (1990) also concluded that with winter cereals, the rotation effect of pea was probably more important than the residual nitrogen effect. For winter oilseed rape (Brussicu nupus L.), the residual nitrogen effect from pea was equivalent to 30 to 60 kg N ha-1 if applied following oats. Removal of the above-ground pea residues, which contained less than 1% nitrogen, had no effect on the residual nitrogen value.

2. Phosphorus, Potassium, and Other Nutrients There is very little direct evidence that crop rotation affects phosphorus relationships (Bullock, 1992). Karlen and Sharpley (1994) concurred, but suggested that appropriate selection and use of a crop with a higher affinity for phosphorus may reduce soil phosphorus stratification and increase phosphorus-use efficiency, particularly if the nonharvested portion of the crop is returned to the soil. They suggested that selection of crops which can more efficiently utilize residual soil inorganic and organic phosphorus may be economically viable for farmers and enhance the sustainability of soil phosphorus fertility. Vivekanandan and Fixen (1991) reported that corn sampled at the six-leaf

16

D. L. KARLEN ET AL.

growth stage had a higher phosphorus concentration when following soybean than when following corn. Similarly, Copeland and Crookston (1992) observed that corn in a 2-year rotation with soybean accumulated significantly more phosphorus than did corn in continuous monoculture. This suggested that corn yield increases associated with crop rotation may have been due to improved general plant nutrition. However, in the same study, total phosphorus content of soybean was not affected by crop rotation except for the very early vegetative stages. Similarly, there was no consistent increase in leaf phosphorus concentration when sorghum was grown in rotation with cotton (Brawand and Hossner, 1976). They concluded that although there was a rotation effect, it could not be attributed to improved phosphorus nutrition. Copeland and Crookston (1992) reported that K and total micronutrient content increased for corn in a 2-year rotation with soybean as compared to continuous monoculture. They proposed that general improvement in plant nutrition may have been due to an improvement in corn root function and that causal agents such as mycorrhizae may have played a role. Following a similar argument, the same research group (Copeland et al., 1993) reported that increased water use by first-year rotated corn or increased water use efficiency of rotated soybean, as compared to continuous monoculture, demonstrated that rotation increased root surface and/or root activity which in turn improved water relations and increased grain yield. Inclusion of legume cover crops into a crop rotation in the southeastern United States also resulted in a beneficial redistribution of potassium to the soil surface from deeper in the soil profile (Hargrove, 1986). Extractable calcium and magnesium levels do not appear to be affected by crop rotation. Increased availability of micronutrients including iron, copper, and zinc because of microbiologically enhanced chelation may also be a beneficial effect of crop rotation and cover crops (King, 1990).

D. DISFASE AND PESTINTERACTIONS Crop rotation is a fundamental tool of integrated pest management. Francis et al. (1986a) coined the term “biological structuring” to describe the use of crop

rotations, management alternatives, biological phenomena, environmental conditions, and interactions of these factors to manage crop pests such as disease, weeds, and insects. Crop rotation affects pest pressure in various ways, but in general the literature supports Francis and Clegg (1990) who stated that “the greater the differences between crops in a rotation sequence, the better cultural control of pests can be expected.” While crop rotation does reduce pest pressure it should be noted that even when pest pressure is minimal, the rotational effect still exists. This suggests that pest control is a contributor to the benefit of crop rotation, but is not responsible for the rotation effect itself. However, it should be

CROP ROTATIONS FOR THE 2 1st CENTURY

17

recognized that we are unaware of all pests which detrimentally affect crops and thus it can be hypothesized that much of the rotational effect is due to alleviation of unknown pests. Crop rotation is an effective tool against certain pests, and efficacy may contribute to the rotation effect, but rotation does not control all pests (Bullock, 1992). Pests which are controlled by crop rotation have the following characteristics (Flint and Roberts, 1988). First, the pest inoculum source must be from the field itself. Crop rotation does not control highly mobile pests since they have the ability to invade from adjacent fields or other areas. Pests which can be controlled by rotation include soil and root-dwelling nematodes, soilborne pathogens (if they do not produce airborne spores), and vegetatively propagated weeds such as nutsedge (Cyperus). Second, the host range of the pest needs to be fairly narrow or at least must not include plants which are reasonably common in a given area. Third, the pest must be incapable of surviving long periods without a living host. In other words, the pest populations must decrease substantially within a year or two of removing a living host plant. 1. Weeds

Weeds can reduce crop yields provided their densities reach a biological threshold. Most research indicates that biological thresholds are greater than zero (Aldrich, 1987), but there are scattered arguments in the literature that biological thresholds are zero (Cousens, 1985). A combination of crop rotation, smother crops, and mechanical cultivation were used to control weeds prior to the introduction of the synthetic herbicide 2,4-D [(2,4-dichlorophenoxy)acetic acid]. Crop rotation alone was not sufficient to control weeds; all three methods had to be used as an integrated program with a primary goal of preventing weed reproduction (Regnier and Janke, 1990). Crop rotation helps control weeds because they thrive and increase in crops which have similar growth requirements to their own. For example, grasses thrive in continuous corn, while broadleaf weeds thrive in continuous soybean. In Nebraska, rotating corn and grain sorghum with a broadleaf crop is an effective method of controlling shattercane (Sorghum bicolor L.) because it allows for the use of herbicides which are phytoactive on cereals (Francis and Clegg, 1990). Similarly, Dale and Chandler (1979) reported that a corn and cotton rotation enabled growers to control johnsongrass (Sorghum halepense L.) much better than a continuous corn rotation because grass-specific herbicides could be used during the cotton phase of the rotation. Crop rotation introduces conditions and practices that are not favorable for a specific weed species and thus growth and reproduction of that species are hampered. For example, Forcella and Lindstrom (1988) found 25 weed seeds m-2 in a continuous corn field, but only 4 weed seeds m-2 where corn was

18

D. L.KARLEN ET AL.

grown in rotation with soybean. Not all crops are equal in their competitiveness with weeds. Van Heemst (1985) ranked 25 crops for their ability to compete with weeds based on a mean reduction in yield. Wheat was considered the most competitive and given a rank of first. Soybean ranked fourth and corn seventh. Regnier and Janke (1990) suggested that factors such as rate and extent of canopy development, plant spacing, and life cycle all contributed to a crop’s competitiveness. It has also been noted that cultivars within a species also compete differently with weeds. Bullock (1992) cited several references suggesting this difference may be attributable to production of allopathic compounds, especially if small grains such as rye, wheat, oats, or barley are included in the rotation. The importance of crop rotation was diminished with the advent of synthetic herbicides. However, there is ample evidence confirming that crop rotation irnproves weed control even with synthetic herbicides (Bullock, 1992). For example, after 7 to 8 years of standard chemical and mechanical weed control from 1500 to 3000 weed seed m-2 were found with continuous corn, while in a cornsoybean rotation, the soil had from 200 to 700 weed seed m-2 (Forcella and Lindstrom, 1988). Withholding herbicides for 1 year reduced continuous corn yield by 10 to 27% but did not reduce corn yield in the 2-year corn and soybean rotation. Interest in using crop rotation to control weeds is gaining popularity, especially among those persons focusing on sustainable agriculture. For example, compared to growing continuous corn, growing corn in a 2-year soybean and corn rotation or a 3-year soybean, wheat, and corn rotation reduced giant foxtail (Seturiafuberi Herrm.) seed at the 0- to 2 . 5 , 2.5- to lo-, and 10- to 20-cm depths (Schreiber, 1992). Similarly, Ball ( 1992) reported that cropping sequences were the most dominant factor influencing species composition in weed seed banks. Temporal diversity achieved through crop rotation and spatial diversity achieved through intercropping can markedly reduce weed population density and biomass production (Liebman and Dyck, 1993). Among 26 comparisons between monoculture and rotation cropping systems, they found that emerged weed densities with rotations were lower in 21 studies, higher in 1 study, and equivalent in 5 studies. They concluded that the success of rotation systems for weed suppression appears to be based on the use of crop sequences that create varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage to provide an unstable or inhospitable environment that prevents the proliferation of a particular weed species. Liebman and Dyck (1993) concluded that the relative importance and most effective combinations of various weed control tactics have not been adequately evaluated and recommended, therefore three research thrusts should be addressed. These included (1) determining effects of crop rotation and intercropping on weed population dynamics including weed seed longevity, weed seedling emergence, weed

CROP ROTATIONS FOR THE 2 1st CENTURY

19

seed production and dormancy, agents of weed mortality, resource competition between cultivated crops and weeds, and allelopathic effects; (2) determining how to combine specific components of rotation and intercropping strategies that may be important for weed control; and (3) designing and testing new integrated approaches for weed control at the scale of complex farming systems.

2. Insects Insect pests which have specific or at least narrow host ranges and which are incapable of extended migration are particularly susceptible to crop rotation (Ware, 1980). An example of this is the control of northern corn rootworm (Diabrotica sp.) in the central United States. In a monoculture corn production system, rootworm reaches an economic threshold about 30% of the time, but in a 2-year corn and soybean rotation, the economic threshold is reached less than 1% of the time. However, increased use of a 2-year corn and soybean rotation throughout much of the northern corn belt has resulted in selection for corn rootworms with a 2-year rather than a 1-year diapause. Therefore, reports exist of economic damage for corn grown in a short, 2-year rotation (Ostlie, 1987). For some insect species, crop rotation is not an effective control practice. For example, Johnson et al. (1984) reported that black cutworms (Agrotis ipsilon) are more of a problem when corn is rotated with either soybean or wheat than when it is grown continuously. Apparently, black cutworm moths are less attracted to corn residue than to either soybean or wheat residues for oviposition (Busching and Turpin, 1976).

3. Diseases Crookston (1 984) suggested that decreased crop yields associated with monoculture cropping systems were caused by increases in some unknown soil pathogen. Although attractive and frequently used to justify crop rotation as a method for preventing fungal diseases (Curl, 1963), Crookston’s suggestion is not universally accepted (Roder et a f . , 1988). It is not necessarily clear to what extent disease prevention contributes to the rotation effect. Bullock (1992) found that monoculture wheat has problems with fungal diseases, in particular take-all (Gaeumannomyces graminis var. tritici), but the severity of fungal diseases in continuous wheat often decreases within 3 to 5 years. The reduction in severity is known as “take-all decline,” and as a natural control of the disease it is effective. The mechanisms responsible for take-all decline are not completely understood, but changes occur in the microflora and microfauna in soils where take-all fungus is established. Part of the take-all decline is due to a build up of competitive and predatory microorganisms which control the take-all fungus (Crookston, 1984). Curl (1963) suggested that in some cases the control mechanism may be a pest-

20

D. L. KARLEN ET AL.

predator type of relationship, while in others, the organisms are simply competing for limited resources. Crookston et al. (1991) postulated that a buildup of beneficial organisms which help to control detrimental organisms might explain why second-year yields for a continuous corn cropping system often show a greater decline compared to yields of rotated corn than that observed during the later years. Meese et al. (1991) reported that withholding corn for 1 year is sufficient to obtain the maximum rotation effect, but Crookston et al. (1991) reported that withholding corn for more than 1 year would increase the rotation effect slightly. Both reported that soybean requires more than 1 year of absence to negate deleterious soybean yield effects. The exact nature of the agent responsible for the deleterious effect of continuous soybean is not clear, but Whiting and Crookston (1993), working in the northern U.S. corn belt, have reported that plant diseases are not playing a major role. Thus, it is reasonable to conclude that the yield increase observed for soybean in a soybean and corn rotation is not necessarily due to a reduction in seventy or incidence of plant diseases. Time does not decrease the seventy of all diseases (Bullock, 1992). Studies by Stromberg (1986) showed that gray leaf spot in corn (Cercosport zeae-maydis) in the southeastern United States becomes a severe problem if corn is grown continuously using no-tillage production practices. However, a rotation in which corn is absent for at least 1 year prevents the disease from becoming an economic problem.

4. Nematodes The use of crop rotations to control Meloidogyne and Heteroderu glycines species of plant parasitic nematodes on tobacco and soybean crops in North Carolina was established during the 1950s and 1960s (Barker, 1991). This approach for control is currently increasing in importance once again because many chemical nematicides are no longer available (Flint and Roberts, 1988). Negative impacts of nematodes on crop production are decreased by crop rotation because changing plant species generally reduces population levels of most plant parasitic nematodes (Dabney e f ul., 1988; Ferris, 1967; Edwards et al., 1988). A reduction of nematode pressure may account for most of the rotation benefit for soybean in the southeastern United States since cyst nematodes (Heteroderu glycines Inchinohye) in soybean can generally be controlled by crop rotation (Dabney et al., 1988). Bailey et al. (1978) reported similar conclusions with regard to root knot nematodes. Sasser and Uzzell (1991) reported that soybean yields were improved most by increasing the number of years during which a nonhost crop was grown. In other nematode studies, a 1-year rotation with barley (Carter and Nieto, 1975), clean fallow (King and Hope, 1934), or planting a resistant processing tomato cultivar (Flint and Roberts, 1988) were effective in controlling the cotton root knot nematode (Meloidogyne incognita).

CROP ROTATIONS FOR THE 2 1st CENTURY

21

The economic impact of 3-year cotton and soybean rotations in soils with varying population densities of Hoplolairnus Columbus was estimated by Noe et al. (1991). They calculated maximum yield losses to be 20% for cotton and 42% for soybean. Maximum nematode population densities at harvest were estimated to be 182 per 100 cm-3 of soil for cotton and 149 per 100 ~ m for - soybean. ~ They projected net incomes to range from a loss of $17.74 ha-1 for a soybean, cotton, and soybean rotation to a profit of $46.80 ha-' for a cotton, soybean, and cotton sequence. A range of economic assumptions and management conditions are considered in this study. Crop rotation research was de-emphasized in the early 1960s as priorities shifted to the development of resistant cultivars and the evaluation of nematicides (Schmitt, 1991). Resistant soybean lines (Brim and Ross, 1965, 1966)performed well in cyst-infested fields and gave the maximum yield potential for the environment in which they were grown. As these and other nematode resistant cultivars became widely grown, often in monoculture, nematode races shifted and, consequently, the use of resistant cultivars as a management tool is now limited. Schmitt (1991) reported that following 2 or more years of a nonhost crop, nematode populations were at low or undetectable levels and that soybean yields were not affected. Those results suggested that a more prudent use of resistant cultivars grown in rotation with nonhost crops would increase their longevity in fields infested with cyst nematode races 1, 3, or 4.

E, ALLELOPATHY Allelopathy occurs when one plant species releases chemical compounds, either directly or indirectly through microbial decomposition of residues, that affect another plant species. Liebman and Dyck (1993) stated that including allelopathic plants in a crop rotation or as part of an intercropping system may provide a nonherbicide mechanism for weed control. They found few studies that focused on use of allelopathy in rotations, but management of allelopathic cover crops for weed control has been extensively investigated (Bullock, 1992). Results of those studies are directly applicable to crop rotations. Liebman and Dyck (1993) found that exudation of allelochemicals from living roots of barley and oats have been suggested, but most studies of allelopathy have been conducted with cover crops or dead crop residues associated with notillage production practices. Studies such as those by Yakle and Cruse (1983, 1984) are typical of those efforts to understand the effects of this complex process, especially for monoculture corn production. With respect to weed suppression, Putman er al. (1983) found that compared to unplanted control treatments, residues of several fall-planted cereal and grass cover crops significantly reduced growth and dry matter production by several weed species during the following summer. Rye, wheat, and barley, which survived the winter and were

22

D. L. KARLEN ET AL.

subsequently killed by nonselective herbicides, had greater a~l~lopathic effects and suppressed weeds much more than oats, grain sorghum, or sorghum-sudan grass (Sorghum urundinaceum [Desv.] Stapf var. sudunense [Stapf] Hitchc.), which were killed by winter conditions. Shading and cooling of the soil may have contributed to this control, but several other studies have shown suppressive effects that cannot be attributed to the physical presence of mulch. Identification of the mechanisms governing the differential effect of cover crop residues on weed and crop species provides a major challenge for persons studying and using crop rotations.

IV, SOIL QUALITY EFFECTS The need to reduce negative on- and off-site impacts of agricultural practices will probably provide one of the s~ongestincentives for reintr~ucingcrop rotations into farm management plans. Kay (1990) reached a similar conclusion in stating that a major goal for agricultural research will be to identify and promote cropping systems which sustain soil productivity and minimize deterioration of the environment. To assess the effects of soil and crop management practices such as crop rotation on both factors, several projects focus on the concept of soil quality as an assessment tool (Karlen el a / . , 1992; Doran and Parkin, 1994; Karlen and Doran, 1993; Karlen and Stott, 1994). Using different crop rotations may improve soil quality by more closely mimicking natural ecosystems than current farming systems (Karlen et al., 1992). This woutd occur because temporal and spatial diversity across the landscape would be increased. Furthermore, management strategies that maintain or add soil carbon have good potential for improving the quality of our soil resources. Critical factors being included in most soil quality assessments include measurements of soil structure, aggregation, bulk density, water infiltration, water retention, soil erosivity, and organic matter (Karlen and Stott, 1994).All of these factors are influenced by crop selection and rotation. Therefore, it is logical to examine the effects of crop rotations on the various soil quality indicators as we assess the need to re-emphasize rotations in 2 1st century farming systems.

A. SOILSTRUCTURE Kay (1990) stated that the characteristics of plant species being grown, the sequence of different species, and the frequency of harvest were ail aspects of cropping systems that affected soil structure by influencing the formation of biopores by plant roots and soil fauna. The network of biopores subsequently

CROP ROTATIONS FOR THE 2 1st CENTURY

23

determined the amount and distribution of organic materials throughout the soil. Kay (1990) also stated that the effectiveness of different crops for improving soil structure at different soil depths was related to the amount of water extracted and photosynthate deposited by the plant, as well as persistence of photosynthate carbon at different depths. Elkins (1985) demonstrated how different crops can affect soil quality by creating biopores in compacted soils at depths that were not economically tilled. He found that bahiagrass (Paspalum notatum Flugge ‘Pensacola’) roots could penetrate soil layers that impeded cotton roots and that including bahiagrass in a rotation increased the number of soil pores that were 1.0 mm or larger in diameter. The biopores enabled the cotton to obtain water and nutrients from a depth of at least 60 cm and were effective for 3 years after the grass had been incorporated by plowing. Plants are important factors influencing the amount of stress imposed on soil structure at or near the surface because rooting densities generally decrease exponentially with depth in well-structured soils (Hamblin, 1985; Dwyer et al., 1988). The total amount of photosynthate deposited below ground and the resistance of this material to complete mineralization also varies considerably between plant species (Kay, 1990). For example, Davenport and Thomas (1988) reported that the total amount of rhizodeposition by bromegrass (Bromus inermis L.) can be twice as high as the amount deposited by corn. Furthermore, carbon originating from bromegrass was more persistent in the soil than that from corn (Davenport et al., 1988). Russell (1973) stated that, in natural grasslands, over 2.5 Mg ha-’ dry matter can be added to the soil each year as roots, and total root systems may contain more than 12 Mg ha-1 of dry matter, compared to only 2 to 5 Mg ha-1 of aboveground material. Abandonment of multiyear rotations in favor of short rotations has generally resulted in a degradation of soil structure as measured by soil aggregate stability, bulk density, water infiltration rate, and soil erosion (Bullock, 1992). Much of the blame for this degradation is attributed to decreases in soil organic matter content, but Bruce et al. (1990) found the relationships to be complex and easily erased or modified by tillage. Langdale et al. (1990) reported that crop rotations did not affect soil physical properties on selected Ultisols, but those findings are not predominant in the literature. Demonstrating a direct linkage among crop rotation, soil structure, and crop yield is very difficult, even though it is generally accepted that improved soil structure is beneficial to crop production (Johnston et a l . , 1942; Page and Willard, 1947). Strickling (1950) and Morachan et al. (1972) found no correlations between physical improvement in silt loam soils and corn yield in the midwest United States and both concluded that physical conditions were not limiting to yield. Bullock ( 1 992) suggested that the differences in reports may be due to climate since significant correlations of soil structure with crop yield seem to be weather dependent (De Boodt et al., 1961).

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D. L. KARLEN ET AL.

B. AGGREGATION Crop rotations that include legumes and/or grasses are generally beneficial to aggregate stability and formation of favorable soil structure (Robinson et al., 1994; Kay, 1990). Using measurements of mean weight diameter, Van Bavel and Schaller (1950) and Wilson and Browning (1945) showed that soil aggregation with continuous corn was half of that found with a corn, oat, and meadow rotation. Bullock (1992) argued that it is convenient to suggest crop rotations beneficially affect soil aggregate formation and stabilization, but quantifying the relationships is not that simple. When rotations involve numerous years of a sod, pasture, or hay crops, improvements in soil structure do occur (Olmstead, 1947; Strickling, 1950; Adams and Dawson, 1964; Tisdall and Oades, 1982; Power, 1990), but short rotations often reduce soil aggregation. For example, a cornsoybean rotation results in greater yield for both corn and soybean, but because soybean returns less crop residue, the rotation often degrades soil aggregation faster than does well-fertilized continuous maize (Power, 1990). This finding was consistent with that of Olmstead (1947), who reported that short rotations and commercial fertilizers do not maintain soil aggregates. Raimbault and Vyn (1991) observed improved aggregate stability due to crop rotation in Ontario. They reported that compared to measurements under continuous corn, soil aggregate stability in most years was highest under continuous alfalfa or where a legume (either alfalfa or red clover) was grown in rotation with corn. They also found that first-year corn grown in rotation yielded 3.9% more than continuous corn grown using conventional tillage, and 7.9% more than continuous corn grown using minimum tillage practices. Hussain et al. (1988) also concluded that crop rotation increased soil aggregation over time based on geometric mean diameter (GMD) values. Their calculations for continuous corn, corn following soybean, soybean following corn, and an oat/red and yellow sweetclover mixture following corn showed average GMD values of 150, 2 I 1, 225, and 31 1 pm, respectively. Evaluations of farming systems that include crop rotation have also shown significant differences in soil aggregation. For example, Jordahl and Karlen (1993) compared alternative and conventional farming systems in central Iowa and found that combined effects of alternative practices (i.e., a 5-year crop rotation including oats and meadow, manure/municipal sludge application, and ridge-tillage) resulted in greater water stability of soil aggregates than the conventional practices (i.e., a 2-year corn-soybean rotation with reduced tillage). They attributed the increased aggregation to the longer crop rotation, which included the oat and meadow, but also to application of 45 Mg ha-1 of a mixture of animal manure and municipal sewage sludge during the first 3 years of each 5-year rotation. Similarly, Reganold (1988) found a more granular soil structure and a more friable consistence in soil managed without the use of commercial

CROP ROTATIONS FOR THE 2 1st CENTURY

25

fertilizers and only limited use of pesticides (organic) than in that managed with recommended rates of commercial fertilizers and pesticides (conventional) in the Palouse region of eastern Washington. Other studies related to soil structure and aggregation also show that soil amendments, including animal manures and municipal sewage sludge, can lead to increased water stability of aggregates, decreased susceptibility to crust formation, and an increased proportion of large pores (Kay, 1990).

C. BULKDENSITY Cropping systems which return the most residue to the soil usually result in the lowest soil bulk density. Therefore, continuous corn will frequently result in lower soil bulk densities than corn-soybean rotations, even though crop rotation generally results in greater grain yield (Bullock, 1992). In plots with a pigeon pea (Cajunus cujun L.) and corn rotation, Hulugalle and La1 (1986) found that bulk density was always lower than in well-fertilized continuous corn. Hageman and Shrader (1979) found that after 20 years, soil bulk density following continuous corn was slightly lower than after a 4-year corn, oats, meadow, and meadow rotation (1.13 vs 1.17 g cm-3, respectively). They also found that annual application of 134 kg N ha-1 increased soil organic matter concentrations (52.1 vs 53.3 g kg- 1) and decreased soil bulk density (1.10 vs 1.20 g ~ m - compared ~ ) to the 0 kg N ha-1 treatment. These differences were attributed to less machinery travel and greater organic matter production with the corn, oats, meadow, and meadow rotation. Hageman and Shrader (1979) concluded that as soil organic matter increases, soil bulk density decreases. Bullock (1992) suggested that several factors, including traffic patterns, tillage, or sampling technique, may complicate assessments of crop rotation effects on soil bulk density. He concluded that benefits obtained from short rotations, such as a 2-year corn and soybean sequence as compared to wellfertilized continuous corn, were probably not attributable to lower soil bulk density. However, reduced bulk densities may contribute to the rotation benefit obtained from sod, pasture, or hay crops. Hammel (1989) measured bulk density and soil impedance after 10 years of continuous management in a long-term tillage-crop rotation experiment on Palouse (fine-silty, mixed, mesic Ultic Haploxeroll) and Naff (fine-silty, mixed, mesic Ultic Argixeroll) silt loam soils. He concluded that crop rotation did not significantly influence either soil property. Logsdon et al. (1993) reported that bulk densities were sometimes lower and the volume of large pores was slightly higher in fields where a 5-year corn, soybean, corn, oats, and meadow rotation was being used compared to that for a 2-year corn and soybean rotation.

26

D. L. KARLEN ET AL.

D. WATERINFILTRATION AND RETENTION In the southeastern United States, soil organic matter content, water infiltration rate, and aggregate stability all increased as the proportion of sod in the rotation increased (Adams and Dawson, 1964). Wischmeier and Mannering (1965) also reported a positive correlation between water infiltration rate and soil organic matter content for several midwestern soils with organic matter concentrations from 1 to 14%. Allison (1973) attributed increased water infiltration to improved soil structure and higher soil organic matter content. Recent farming systems studies in Iowa support this conclusion, i.e., steady-state infiltration measurements were somewhat higher for longer rotations where soil organic matter concentrations were slightly higher than those for shorter rotations (Logsdon et al., 1993; Jordahl and Karlen, 1993). Bullock (1992) concluded that crop rotation did not benefit production by increasing water-holding capacity, even in situations such as long-term pastures which resulted in substantial increases in soil organic matter content. This conclusion is based on several studies. Among these are results from Jamison (1953) who stated that organic matter does have a large water-holding capacity and that most of the water held by organic matter is held at potentials far less than - 1.5 MPa (the water potential at which water is not sufficiently available for survival of most plants). A second reason is that increased soil aggregation results in decreased plant available water (Jamison, 1953; Hillel, 1980). Bullock (1992) stated that this occurred because a larger fraction of the water is held at potentials less than - I .5 MPa and because of an increase in macropore volume and a decrease in the micropore volume. He supported these arguments with field data from Johnston et al. (1942) who reported that during dry years, continuous corn may yield more than rotated corn. However, during years of adequate rainfall, rotated corn generally yielded more than continuous corn (Johnston et al., 1942; Sahs and Lesoing, 1985). Hudson (1994) used a critical review of literature on soil organic matter effects on plant available water capacity to argue against this position. He found that for sand, silt loam, and silty clay loam soils, the volume of water held at field capacity increased at a much faster rate than that held at the permanent wilting point. Hudson (1994) concluded that on a volumetric basis, soil organic matter is an important determinant of available water-holding capacity, thus indicating a reevaluation of crop rotational effects on plant available water might be warranted.

E. SOILERODIBILITY Soil erosion requires two processes: (1) detachment of soil particles, and (2) transportation of the soil material by erosive agents such as water or wind

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27

(Hussain et a l . , 1988). Soil detachment associated with water erosion can be initiated by raindrops or overland water flow during a rainfall event. Detachment by wind involves skipping or saltation of soil particles across the soil surface. Soil management practices such as crop residue placement, application of animal manure, or using crop rotation can have both direct and indirect effects on soil physical properties which subsequently affect the detachment process (Bullock, 1992). With regard to crop rotation, Hussain et al. ( 1 988) reported that the rate of splash detachment from continuous corn treatments was higher than from soil under crop rotations. They found splash detachment rates of 48 mg ~ m for - ~ continuous corn, 40 for corn following soybean, 39 for soybean following corn, and 31 for an oat/red and yellow sweetclover mixture following corn. Johnston et al. (1 942) evaluated continuous corn, a corn-oat-sweetclover rotation, and continuous bluegrass and reported that over a 9-year period, the continuous corn treatments lost 793 Mg ha-’ of soil, while the rotation and continuous bluegrass treatments lost 202 and 42 Mg ha-’, respectively. They noted that runoff was in the same general order as soil losses. Stewart et al. (1976) reported that soil losses from corn in rotation with meadow were 14 to 68% of the soil loss from continuous corn. Reganold (1988) found a 16-cm difference in topsoil depth between adjacent organic and conventional farms in the Palouse. This difference was attributed to significantly greater erosion on the conventional farm between 1948 and 1985. He attributed the difference in erosion rates to crop rotation since the organic farm included green manure crops within the rotation, while the conventional farm did not. Contrary to the benefit of rotations which include forages or other surface cover during the spring, 2-year corn and soybean rotations can result in greater soil erosion than continuous corn (Bullock, 1992). For example, over an 18-year period, soil loss from a 2-year corn and soybean rotation was 45% higher than that from continuous corn (van Doren et al., 1984). This often occurs because the amount of surface residue following soybean is very low (Stewart et a l . , 1976; Laflen and Moldenhauer, 1979; Papendick and Elliott, 1984). Alberts et al. (1985) reported that soybean production results in an annual soil loss 3.4 times greater than that seen with corn production but noted that differences in erosion were not simply a function of less biomass. They concluded that corn residue is better at preventing soil erosion than is soybean residue, even when they are present in similar amounts. Laflen and Moldenhauer (1979), in a 7-year study, found that average annual soil losses were about 40% greater when corn followed soybean than when corn followed corn. They concluded the difference was caused by a “soil effect” because major differences in soil loss occurred during the period 30 to 60 days after planting, a point at which canopy development and residue cover were almost identical.

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F. ORGANIC MATTER Soil organic matter is the soil quality indicator for which the most information relative to crop rotation exists, but it is also the indicator for which the most unanswered questions remain. Crop rotation affects soil organic matter in several ways. Factors affecting it include rotation length, losses caused by tillage operations, mineralization, and interaction with fertilization practices.

1. Rotation Length Crop rotations which involve long periods of sod, pasture, or hay crops generally increase soil organic matter content during periods of these crops. This increase is presumably a primary factor that beneficially affects subsequent crops and contributes to the rotation effect (Bullock, 1992). Hussain et al. (1988) reported increased soil organic matter content with a 2-year corn and soybean rotation, but such findings are the exception. Generally, this short rotation results in lower soil organic matter levels than continuous corn, even though it provides a rotation effect (Dick et al., 1986a,b). The primary cause for this response appears to be that soybean simply does not produce as much biomass as corn (Dick et al., 1986a,b). An exception to this general conclusion often occurs in the southeastern United States where nonirrigated corn growth is frequently reduced by drought stress, and full-season, nonirrigated, determinate soybean cultivars have been shown to produce more than 7 Mg ha-' of aerial biomass (Hunt and Matheny, 1993). Results from Havlin et al. (1990) demonstrated that including grain sorghum in a rotation, rather than growing continuous soybean, increased organic carbon and nitrogen in the soil. They concluded that increasing the quantity of residue returned to the soil through higher yields or through greater use of high residue crops in the rotation, combined with reduced tillage, could improve soil productivity. Juma er al. (1993) concluded that after 50 years of research on Gray Luvisolic soils at the Breton Plots in Alberta, Canada, soil organic matter content is about 20% higher where a 5-year rotation has been used than where a 2-year, wheat and fallow rotation was followed. Similarly, Unger (1968) found that when tillage treatments were kept constant, continuous cropping resulted in a significant increase in soil organic matter concentrations compared to a cropfallow system.

2. Tillage Losses Tillage, which inverts and mixes the soil, introduces large amounts of oxygen into the soil and stimulates aerobic microorganism consumption of organic matter as a food source (Doran and Smith, 1987). When virgin eastern Oregon soils

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29

were cultivated, some lost over 25% of their organic matter in the first 20 years, with 35 to 40% being lost in 60 years (Rasmussen er al., 1989). Tillage for weed control during the fallow period was the primary cause for the loss of soil organic matter. Ridley and Hedlin (1968) found that after 37 years, soils which had initial organic matter concentrations of nearly 10% had 7.2% organic matter if cropped every year, compared to 3.7% in those fallowed every other year. Soils fallowed after every two or three crops had intermediate soil organic matter concentrations. Use of no-till systems can reduce the rate of soil organic matter loss, but not completely stop it. Collins er al. (1992) reported that after 58 years, total soil and microbial biomass carbon and nitrogen were significantly greater in annualcropping treatments than for wheat-fallow rotations. They concluded that residue management (i.e., reduced tillage) significantly affected the level of microbial biomass carbon and that annual cropping significantly reduced declines in both soil organic matter and soil microbial biomass. Similarly, Havlin et al. (1990) found that compared to native grassland, a 12-year wheat and fallow rotation resulted in total soil organic matter concentrations that were 4, 14, and 16% lower with no-till, stubble mulch, and conventional tillage, respectively.

3. Mineralization Effects FrequentIy, crop rotation benefits derived from organic matter are attributed to the release of nitrogen through mineralization (Bullock, 1992). However, Doran and Smith (1987) concluded that relationships among soil organic matter content, management practices including crop rotations, and nitrogen availability were not always predictable, constant, or direct. It is generally accepted that soil organic matter affects many of the soil quality indicators influencing mineral availability. These effects include increased water infiltration (Wischmeier and Mannering, 1965; Adam et al., 1970; Allison, 1973; MacRae and Mehuys, 1985), improved aggregate formation and stability (Spurgeon and Grisson, 1965; Harris er al., 1966; Fahad er al., 1982; MacRae and Mehuys, 1985), lower bulk density (De Kimpe et al., 1982), higher water retention capacity (Jamison, 1953; Hudson, 1993), improved soil aeration, and reduced soil erosion (USDA, 1980; Bezdicek, 1984; Reganold, 1988). Commercial agriculture has altered both the quality and quantity of soil organic matter in many soils (Robinson er al., 1994). Often, these soils may have taken hundreds or even thousands of years to reach stable soil organic matter conditions (Rasmussen et al., 1989). Destruction of soil organic matter by short rotations does not continue unabated until the soil is devoid of organic matter, but rather the soil organic matter reaches an equilibrium level (Joeffe, 1955; Allison, 1973; MacRae and Mehuys, 1985). When alternative tillage or crop rotations are used, a new equilibrium point is established. For example, Larson et al. (1972)

30

D. L. KARLEN ET AL.

indicated that the addition of 5 Mg/ha of maize and alfalfa residue applied annually could maintain organic C at a level of 1.81%. However, this soil organic matter level is considerably lower than that found in its precultivation state. No-till and reduced tillage (Karlen et al., 1989, 1991) cropping systems have shown gradual increases in soil organic matter content when compared to more intensive tillage management practices. Similarly, Haas et al. (1957) reported that within the top 30 cm of medium-textured soils in the Great Plains, organic carbon concentrations ranged from 20 to 36% of that found in adjacent virgin grassland. They attributed the differences to cropping practices that included rigorous tillage and fallow periods. Different crop rotations seem to result in different soil organic matter equilibrium levels, but Miller and Larson (1990) predict that soil organic matter concentrations will never return to levels observed in their virgin state.

4. Fertilizer and Manure Interactions Juma et al. (1993) concluded that application of nitrogen, phosphorus, potassium, and sulfur fertilizer and animal manure to Gray Luvisolic soils increased soil organic matter by increasing crop yields. They also reported that application of manure increased soil organic matter even more than fertilizer. This presumably occurred because in addition to its nutrient value, the 9 Mg ha-1 of manure that was added each year represented an additional source of organic matter. The report by Juma et al. (1993) supports conclusions by Boyle et al. (1989) who suggested that returning carbon to the soil is “a necessary expense that insures a sustainable harvest.” Both support suggestions by Karlen et al. (1992) that crop rotation, cover crops, and conservation tillage as the practices most likely to improve soil quality as we enter the 21st century.

V. BIOLOGICAL DIVERSITY Crop rotations have more biological diversity than that which occurs with monocultures. This diversity can be divided into temporal and spatial components. Temporal diversity results from a sequence of crops being grown in a given field (i.e., crop change with time), and has been an important tool in breaking pest cycles, reducing soil erosion, and increasing yields. The second component, spatial diversity, results from greater numbers of crops being grown at a given time on the landscape. Managing spatial diversity for improved crop production is poorly understood and little used in modern agriculture. While

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examples of managing spatial diversity to increase crop yields and conserve resources exist, i.e., use of tree shelterbelts for grain yield increases and soil conservation (Burvill, 1950; Ferber, 1974; Kort, 1986), little research emphasis has been placed on this component of crop production. Strip intercropping is the practice of growing crops in a series of narrow, adjacent strips that uses spatial diversity to increase yields. A well-designed strip intercropping system uses complimentary growth habits of adjacent crops to reduce plant competition in the strip border positions (Cruse, 1990). Crops are rotated annually in this system, resulting in a production practice which effectively utilizes both temporal and spatial diversity to improve yields. Strip intercropping research has dominantly addressed corn and soybean, i.e., a two-crop strip system (Francis et al., 1986b). Corn yields benefit while soybean yields are normally reduced. Recent additions of a third crop strip containing small grain has resulted in strip border interactions different from, and more favorable than, those of the corn/soybean strip system (Cruse, 1990). Furthermore, improved soil conservation has been observed by cooperating farmers, but replicated research is yet to be done. Temporal life cycle differences between adjacent strips when three or more crops are included apparently reduces crop competition and increases yield. Growing only two crops with temporally similar life cycles (i.e., corn and soybean) appears to create competition for water, light, and nutrient resources in the border positions and can result in lower yields for one or both crops than if they were grown in larger blocks or fields. Crop spatial arrangements effectively add another dimension, or opportunity, to rotation management in developing environmentally sound and highly productive cropping systems.

A. EFFECTS ON WILDLIFE Quality and spatial diversity of landscape cover has repeatedly been shown to influence wildlife abundance on agricultural landscapes (Kendeigh, 1982; Vance, 1976). Studies conclusively illustrate, for example, the favorable impact of increased crop diversity in the midwest United States on the ring-neck pheasant (Phasianuscolchicus) population (Fanis et al., 1977; Taylor et al., 1978; Vance, 1976). This is particularly true when small grain and/or forages are rotated with row crop production. Most wildlife species that rely on agricultural habitat for survival sustain their populations much more effectively on landscapes with diverse rotations than on those with monocultures or continuous row crops (Moen, 1983). It is well recognized that crop rotations can be managed for increased crop production. It is less well recognized that rotations can also increase wildlife populations on the landscape.

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B. ALTERNATIVE LAND USES Agricultural crop products traditionally have been used for food and fiber. A new role of agricultural crops is that of energy production. Crops for bioenergy production are receiving increased emphasis for at least three reasons: (1) petroleum energy reserves are finite; (2) selected bioenergy products are relatively clean and less polluting than petroleum counterparts; and (3) farmers view the energy market as very large and with significant economic potential. Traditional energy plants such as trees used for firewood have not integrated well into conventional crop rotation schemes due to their semipermanency. However, crops such as corn and switchgrass (Panicurn virgatum L.) are agricultural crops manageable in conventional farm systems and may well serve the cropsfor-energy nitch (McClelland and Farrell, 1992). This type of plant material may integrate well with other agricultural crops in a rotation. Furthermore, perennial energy crops such as switchgrass may serve a secondary purpose-that of soil erosion control. A seemingly large potential exists for sod-forming biomass crops on conservation reserve program land when the contracts for these lands expire. These crops could be worked into a rotation with limited row crop production or could conceivably remain as “permanent” vegetative cover. Currently, crops grown for bioenergy have at least one significant technological drawback. When those such as corn are produced for energy products like ethanol, there is a negative energy balance because of the nitrogen fertilizer requirement (Pimentel, 1991; USDA, 1986). This occurs because approximately one-third of the energy required to produce corn in the United States is attributable to nitrogen fertilizer. Increasing the use of nitrogen-fixing legumes in crop rotations, and producing corn following them, could solve this dilemma. Studies have repeatedly shown that legumes in rotation can contribute sufficient nitrogen to meet the nitrogen demands of various succeeding crops (Francis and Clegg, 1990). Rotations coupled with technological advances in ethanol manufacture may be imperative in creating a positive energy balance in the crops-for-energy agricultural system.

VI. ECONOMICS OF CROP ROTATION Crop rotation will have an impact on the returns associated with alternative tillage systems, and responses will be different on different soils and in different regions. In northeastern Iowa, Chase and Duffy (1991) found that for continuous corn, returns to land, labor, and management were higher for moldboard plow and chisel plow than for no-tillage or ridge-tillage treatments. For a cornsoybean rotation, moldboard plow, chisel plow, and no-tillage performed equally

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well. In Quebec, Canada, Lavoie ef al. (1991) used linear computer models to evaluate net farm income as affected by crop rotation, farm size, tractor size, and weather conditions. They projected that continuous corn grain production would be most profitable for wet, normal, and dry weather conditions. Their empirical results tended to verify the profit maximization behavior of farmers since corn grain production in Quebec increased from 62,600 ha in 1976 to 260,000 ha in 1986. Their model predictions suggested that reduced tillage and crop rotation practices would reduce net farm income by 10 and 45%, respectively. Lavoie et al. (1991) concluded that the relative importance of cropping systems would likely change as more data on long-term and environmental benefits of crop rotations become available, especially if the information caused major changes in public policies regarding land use and crop production practices. Carter and Berg (1991) compared a traditional 7-year, furrow-irrigated rotation of (1) alfalfa, (2) alfalfa, (3) dry edible bean, (4) dry edible bean, (5) winter wheat, (6) silage corn, and (7) spring wheat/alfalfa with a rearrangement that placed silage corn and winter wheat in the third and fourth years instead of in the fifth and sixth years because of concern for inefficient use of residual soil nitrogen following the alfalfa crops. They found the alternate rotation reduced the number of tillage operations over the entire cropping sequence. This reduced soil erosion 47 to loo%, sustained crop yields, and increased net farm income by an average of more than $125 ha-' each year for a 5-year cropping sequence. Questions regarding optimum crop rotations for various regions are not new. With regard to the U.S. corn belt, Wiancko (1927) concluded that because of the favorable soil and climatic conditions, corn would be the principle crop for this region, and crop rotations proposed for general use in the region must recognize this characteristic. A 1991 survey, conducted in three Iowa counties where Iowa State University had conducted an integrated crop management project (Table III), revealed that a majority of the project cooperators, neighbors of those cooperators, and randomly sampled farmers from throughout those counties considered it important to include small grain or forage in a crop rotation. The economic challenge seems to focus on maintaining profitability, while producing crops with practices that prevent soil degradation and loss of nitrogen and other nutrients to surface and groundwater resources.

VII. POLICY IMPACTS ON CROP ROTATIONS American farm policies have traditionally dealt with food cost, commodity supply, and farmer income (Doering, 1992). Farm policy influences profitability of crop rotation through five processes: deficiency payments, acreage reduction, base-acreage levels, crop prices, and risk reduction (Young and Painter, 1990).

D. L. KARLEN ET AL.

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Table I11 Iowa Farmer AttitudesUwith Regard to the Statement “A Good Cropping System Should Include Rotations of Small Grains or Forage Along with Row Crops” Survey grouph ICM cooperators ICM neighbors Random sample

Agree (%)

Undecided (%)

Disagree (%)

56 74“ 78

13

32 17 14

10 8

* Information provided by Dr. Steve Padgitt, Iowa State University. Ames, Iowa, from a survey associated with participants and nonparticipants in Iowa State University’s Integrated Crop Management (ICM) research program. ICM cooperators included those persons in Caroll, Kossuth, and Sioux Counties who had participated in a Model Farms program with Iowa State University. Neighbors are persons living adjacent to a coopcrator. Random includes persons living in those counties. L‘ In these counties 14% of the cooperator group had small grains or forages in addition to row crops. Among neighbors and participants in the random sample, 50% had small grains or forages in addition to row crops.

Concerns about environmental impacts have been peripheral to date, but questions regarding agricultural policy impacts on practices such as crop rotations are being asked more frequently. Factors including production surpluses, rising commodity program costs, and environmental degradation are encouraging a reexamination of current programs (Moore, 1989). Participation in U.S. agricultural commodity programs has generally resulted in decisions to use more erosive crop rotations (Poe et al., 1991). This has tended to occur because under conditions of program participation, on-site and off-site erosion costs that can affect crop rotation decisions have been internalized. Therefore, these factors influence field-scale management decisions only when long time periods (>40 years) are considered. The implications of crop rotation on susceptibility of the Texas High Plains to wind erosion and groundwater depletion were evaluated by Lee et al. (1989). Their simulations indicated that farm program participation, coupled with base acreage restrictions, encouraged production of continuous cotton. They projected that average annual wind erosion under continuous cotton would be two to six times greater than with alternate cropping systems. However, compliance with base acreage restrictions prior to the 1985 Food Security Act limited adoption of multiyear or multicrop production systems. Changes in policy were viewed as supporting 2-year rotations such as cotton and wheat or 3-year rotations such as cotton, sorghum, and wheat, both of which provide substantial wind erosion control. Until recently, agricultural policy has reflected goals of increased farm and

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rural income, low-cost food, improved rural conditions, improved technical efficiency of farming operations, and natural resource conservation as part of the agricultural productivity base (Doering, 1992). At times, programs designed to enhance the various goals have conflicted. Furthermore, the market often fails to alert agricultural producers to the real costs associated with on-site and off-site environmental damage (Doering, 1992). Agricultural policies, however, provide only one part of a farmer’s decision framework. Other factors include the relative costs associated with alternatives such as tractors vs draft animals, fertilizers vs manure, and pesticides vs cultivation. Nonfarm policies that affect the economy, trade, industry structure, resources, and the environment can also have more impact on the way farmers manage their land than official agricultural policy (Doering, 1992). He also states that national decisions about health, safety, and environmental quality have had and will continue to have a great influence on the way farmers farm. From this perspective, Doering (1992) concludes that federal policies toward agriculture do not appear to provide an incentive or disincentive for less intensive and more environmentally benign agricultural practices and cropping systems. He suggests that new policy approaches should target specific management practices, cropping patterns, input use, or sensitive locations. This approach will require new policy mechanisms to deal specifically and equitably with environmental concerns and society’s changing values, while recognizing actual production decisions that farmers face daily. In a report compiled from 12 interviews with agricultural policy-making individuals in Washington, D.C., Moore (1989) found that crop rotations, in principle, were viewed as beneficial for American agriculture. However, unqualified support for a crop rotation policy was not expressed. The primary concerns were focused on how uncontrollable conditions, such as international market prices or drought, would impact successful implementation of a crop rotation policy. Two perspectives that emerged from the interviews were a desire to deter monocropping practices and concern for maintaining farm incomes (Moore, 1989). The first perspective focused on total resource efficiency for society as a whole and emphasized benefits to be gained by encouraging crop rotations. These benefits included improved soil and water quality, increased farm flexibility, reduced program costs, increased diversity, reduced dependence on nitrogen fertilizers, reduced chemical input costs, and reduced insect pests. The second perspective emphasized known benefits of crop rotation to individual producers, i.e., maintaining productivity at the microeconomic level. To be effective, crop production or land use subsidies would be needed to compensate farmers for using crop rotations (Moore, 1989). Information and education regarding site-specific crop rotation practices and impacts, profitability of rotated crops, market infrastructures for new crops, an integration of new livestock production practices, new equipment, and reduced exports are some needs iden-

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tified as being crucial for increased adoption of crop rotations as we enter the 2 1st century. Potential incentives to encourage crop rotation include monetary compensation, long-term program stability, provision of appropriate knowledge and skills, and development of market infrastructure for various new crops (Moore, 1989). Several disincentives for continuing monoculture include regulations, liabilities for on-site and off-site damages, and internalization of external costs. It was suggested to Moore ( 1 989) that crop rotations could be required under certain site-specific circumstances and that routine groundwater monitoring might be required. The projected impacts of crop rotation policies were that corn would be less widely distributed, especially in nontraditional corn-growing areas. Farm labor and management requirements would increase-perhaps increasing opportunities for rural employment. General environmental quality would improve, although changes could not be guaranteed. Improved rural aesthetics, increased requirements for educational and training programs, and some redistribution of income among companies as they develop uses and markets for alternative crops would be expected. There would also be increased demand for production consultants and a probable reduction in the volume of U.S. exports. However, international prices may rise and actually result in higher export earnings. Changing current agricultural policy to accommodate crop rotations would focus on the core of policy issues by raising questions regarding the ultimate goals for U.S. agriculture (Moore, 1989). When determined, costs and benefits of the alternatives, trade-offs, and impacts of all aspects must be resolved to establish a solid basis for policy consensus. When this is accomplished, the policy stage will be set for encouraging and facilitating adoption of crop rotations in farm management practices.

VIII. SUMMARY AND CONCLUSIONS Advantages and disadvantages of crop rotation have undoubtedly been debated for thousands of years, as documented by historians (White, 1970b) who have stated that rotation systems were widely recommended by Roman agronomists, but often not adopted by local farmers. One reason for farmer hesitancy to use crop rotation may be that agricultural scientists are still unable to explain the mysterious “rotation effect.” Macroeconomic and microeconomic considerations have and presumably will always influence land use decisions, such as adoption of crop rotation. For the U.S. corn belt, this was well documented by Wiancko (1927), but economic considerations must include a more complete accounting for both on-site and offsite impacts of our soil and crop management practices.

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Benefits of crop rotation for land and water resource protection and productivity have been identified, but processes and mechanisms responsible for those benefits need to be better understood. This is a critical area for basic and applied research. Public policies that influence land use decisions, such as crop rotation, need to be as flexible as possible to encourage adoption of practices that are economically viable, environmentally sustainable, and socially acceptable. Following this agenda will ensure that crop rotations have a major role in 21st century agriculture.

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Muir, J., Boyce, J. S., Seim, E. C., Mosher, P. N., Deibert, E. J., and Olson, R. A. 1976. Influence of crop management practices on nutrient movement below the root zone in Nebraska soils. J. Environ. Qual. 5 , 255-259. Noe, J. P., Sasser, J. N., and lmbriani, J. L. 1991. Maximizing the potential of cropping systems for nematode management. J. Nemarol. 23, 353-361. Olmstead, L. B. 1947. The effect of long-time cropping systems and tillage practices upon soil aggregation at Hays, Kansas. Soil Soc. Am. Proc. 11, 89-92. Olson, R. A., and Sander, D. A. 1988. Corn production. In “Corn and Corn Improvement” (C. F. Sprague and J. W. Dudley, eds.), Agronomy, No. 18, pp. 639-686. ASA-CSSA-SSSA. Madison, Wisconsin. Olson, R. J., Hensler, R. F., Attoe, 0. J., Witzel, S. A., and Peterson, L. A. 1970. Fertilizer nitrogen and crop rotation in relation to movement of nitrate nitrogen through soil profiles. Soil Sci. SOC.Am. Proc. 34, 448-452. Ostlie, K. R. 1987. Extended diapause-northern corn rootworm adapts to crop rotation. CropsSoils 39, 23-25. Page, J. B., and Willard, C. J. 1947. Cropping systems and soil properties. Soil Sci. SOC. Am. PFOC. 11, 81-88. Papendick, R. I., and Elliott, L. F. 1984. Tillage and cropping systems for erosion control and efficient nutrient utilization. In “Organic Farming: Current Technology and Its Role in a Sustainable Agriculture” (D. F. Bezdicek, ed.), Am. Soc. Agron. Spec. Publ. No. 46, pp. 69-81. ASA-CSSA-SSSA, Madison, Wisconsin. Parker, C. P. 1915. “Field Management and Crop Rotation.” Webb, S t . Paul, Minnesota. Peterson, T. A., and Varvel, C. E. 1989a. Crop yield as affected by rotation and nitrogen rate. I. Soybean. Agron. J. 81, 727-731. Peterson, T. A., and Varvel, G. E. 1989b. Crop yield as affected by rotation and nitrogen rate. 11. Grain sorghum. Agron. J. 81, 731-734. Peterson, T. A., and Varvel, G. E. 1989~.Crop yield as affected by rotation and nitrogen rate. 111. Corn. Agron. J. 81, 734-738. Pimentel, D. 1991. Ethanol fuels: Energy, security, economics, and the environment. J. Agric. Environ. Ethics 4, 1-13. Poe, G. L., Klemme, R. M., McComb, S. J., and Amb~sious,J. E. 1991. C o m m o d i ~programs and the internalizationof erosion costs: Do they affect crop rotation decisions?Rev. Agric. Econ. 13, 223-235. Power, J. F. 1990. Legumes and crop rotations. In “Sustainable Agriculture in Temperate Zones” (C. A. Francis, C. B. Flora, and L. D. King, eds.), pp. 178-204. Wiley, New York. Power, I. F., and Follett, R. F. 1987. Monoculture. Sci. Am. 256, 78-86. Powers, W. L., and Lewis, R. D. 1930. Nitrogen and organic matter as related to soil productivity, J. Am. SOC. Agron. 22, 825-832. Putman, A. R., DeFrank, J., and Barnes, J. P. 1983. Exploitation of allelopathy for weed control in annual and perennial cropping systems. J. Chem. Ecol. 9, 1001-1010. Raimbault, B. A., and Vyn, T. J. 1991. Crop rotation and tillage effects on corn growth and soil structural stability. Agron. J. 83, 979-985. Rasmussen, P. E., Collins, H. P., and Smiley, R. W. 1989. “Long-Term Management Effects on Soil Pr~uctivityand Crop Yield in Semi-And Regions of Eastern Oregon,” Stn Bull. No. 675. USDA-ARS and Oregon State Univ. Agric. Exp. Stn., Pendleton. Reganold, J. P. 1988. Comparison of soil properties as influenced by organic and conventional farming systems. Am. J. Alternative Agric. 3 , 144-145. Regnier, E. E., and Janke, R. R. 1990. Evolving strategies for managing weeds. In “Sustainable Agriculture Systems” (C. A. Edwards, ed,j, pp. 174-202. Soil Water Consent Soc., Ankeny, Iowa.

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Ridley, A. 0..and Hedlin, R. A. 1968. Soil organic matter and crop yields as influenced by frequency of summer fallowing. Can. J . Soil Sci. 48, 315-322. Rifkin, 5. 1983. “Algeny.” Viking, New York. Robinson, C. A., Cruse, R. M., and Kohler, K. A. 1994. Soil management. In “Sustainable Agricultural Systems” (I. L. Hatfield and D. L. Karlen, eds.), pp. 109-134. Lewis Publ., CRC Press, Boca Raton, Florida. Robinson, R. G. 1966. Sunflower-soybean and grain sorghum-soybean rotations versus monoculture. Agron. J. 58, 475-477. Roder, W., Mason, S . C., Clegg, M .D., and Kniep, K. R. 1988. Plant and microbial responses to sorghum-soybean cropping systems and fertility management. Soil Sci. SOC.Am. J . 52, 13371342. Roder, W., Mason, S . C., Clegg, M. D., and Kniep, K. R. 1989. Yield-soil-water relationships in sorghum-soybean cropping systems with different fertilizer regimes. Agron. J. 81, 470-475, Russell, E. W. 1973. “Soil Conditions and Plant Growth,” 10th Ed. Longman, New York. Russelle, M. P., Hesterman, 0. B., Sheaffer, C. C., and Heichel, G . H. 1987. Estimating nitrogen and rotation effects in legume-corn rotations. In “The Role of Legumes in Conservation Tillage Systems” (J. F. Power, ed.), pp. 41-42. Soil Conserv. SOC.Am., Ankeny, Iowa. Sadler, E. J., and Turner, N. C. 1994. Water relationships in a sustainable agricultural system. In “Sustainable Agricultural Systems” (J. L. Hatfield and D. L. Karlen, eds.), pp. 21-46. Lewis Publ., CRC Press, Boca Raton, Florida. Sahs, W. W., and Lesoing, G . 1985. Crop rotations and manure versus agricultural chemicals in dryland grain production. J. Soil Wafer Conserv. 40, 5 11-5 16. Sasser, J. N . , and Uzzell, G., Jr. 1991. Control of the soybean cyst nematode by crop rotation in combination with a nematicide. J. Nematol. 23, 344-347. Schmitt, D. P. 1991. Management of Heterodera glycines by cropping and cultural practices. J . Nematol. 23, 348-352. Schreiber, M. M. 1992. Influence of tillage, crop rotation, and weed management on giant foxtail (Serariafaberi) population dynamics and corn yield. Weed Sci. 40, 645-653. Shrader, W. D., Fuller, W., and Cady, F. B. 1966. Estimation of a common nitrogen response function for corn (Zea mays) in different crop rotations. Agron. J. 58, 397-401. Slife, F. W. 1976. Economics of herbicide use and cultivar tolerance to herbicides, Proc. Annu. Corn Sorghum Res. Conf., 3Ist, Chicago pp. 77-82. Spurgeon, W. I., and Grisson, P. H. 1965. Influence of cropping systems on soil properties and crop production. Miss. Agric. Exp. Sfn. Bull. No. 710. Stewart, B. A., Viets, F. G . , and Hutchinson. G. L. 1968. Agriculture’s effect on nitrate pollution of groundwater. J . Soil Water Conserv. 23, 13-15. Stewart, B. A., Woolhiser, D. A., Wischmeier, W. H., Caro, J. H., and Frere, M. H. 1976. “Control of Water Pollution From Cropland,” Vol. 2. U.S. Dep. Agric. and Environ. Rot. Agency, Washington, D .C . Strickling, E. 1950. The effect of soybeans on volume weight, and water stability of aggregates, soil organic matter content and crop yield. Soil Sci. SOC. Am. Proc. 15, 30-34. Stromberg, E. 1986. “Gray Leaf Spot Disease of Corn,” Publ. No. 450-072. Virginia Coop. Ext. Serv., Blacksburg. Taylor, M. W., Wolfe, C. W., and Baxter, W. L. 1978. Land-use change and ring-necked pheasants in Nebraska. Wildl. SOC. Bull. 6 , 226-230. Tisdall, J. M., and Oades, J. M. 1982. Organic matter and water- stable aggregates in soils. J . Soil Sci. 33, 141-163. Tyner, F. H., and Purcell, J. C. 1985. Forage production economics. In “Forages, the Science of Grassland Agriculture”(M. E. Heath, R. F. Barnes, and D. S. Metcalfe, eds.), 4th Ed., pp. 4350. Iowa State Univ. Press, Ames.

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Unger, P. W. 1968. Soil organic matter and nitrogen changes during 24 years of dryland wheat tillage and cropping practices. Soil Sci. SOC. Am. Proc. 32, 426-429. USDA 1980. “Report and Recommendations on Organic Farming.” U.S. Dep. Agric., Washington, D.C. USDA 1986, “Fuel Ethanol and Agriculture: An Economic Assessment,” Agric. Econ. Rep. No. 562., U.S. Dep. Agric., Off.Energy, Washington, D.C. van Bavel, C. H. M., and Schaller, F. W. 1950. Soil aggregation, organic matter, and yields in a long-time experiment as affected by crop management. Soil Sci. SOC. Am. Proc. 15, 399-408. Vance, D. R. 1976. Changes in land use and wildlife populations in southeastern Illinois. Wildl. SOC. Bull. 4, 1-15. van Doren, D. M., Jr., Moldenhauer, W. C., and Triplet, G. B., Jr. 1984. Influence of long term tillage and crop rotation on water erosion. Soil Sci. SOC.Am. J . 48, 636-640. van Heemst, H. D. J. 1985. The influence of weed competition on crop yield. Agric. Sysr. 18, 8193. Vivekanandan, M . , and Fixen, P. E. 1991. Cropping systems effects on mycrorrhizal colonization, early growth, and phosphorus uptake of corn. Soil Sci. SOC. Am. J. 55, 136-140. Voss, R. D., and Shrader, W. D. 1984. Rotation effects and legume sources of nitrogen for corn. In “Organic Farming: Current Technology and Its Role in a Sustainable Agriculture” (D. F. Bezdicek, ed.),Am. SOC. Agron. Spec. Publ. No. 46, pp. 61-81. ASA-CSSA-SSSA, Madison, Wisconsin. Ware, G. W. 1980. “Complete Guide to Pest Control, With and Without Chemicals.” Thomson, Fresno, California. Welch, L. F. 1976. The Morrow plots-hundred years of research. Ann. Agron. 27, 881-890. White, K. D. 1970a. Fallowing, crop rotation, and crop yields in Roman Times. Agric. His?. 44, 281-290. White, K. D. 1970b. “Roman Farming.” Cornell Univ. Press, Ithaca, New York. Whiting, K. R., and Crookston, R. K. 1993. Host-specific pathogens do not account for the cornsoybean rotation effect. Crop Sci. 33, 359-543. Wiancko, T. 1927. Crop rotation in relation to the agriculture of the corn belt. J. Am. Sor. Agron. 19, 545-555. Wikner, I. 1990. Crop management research and groundwater quality. Proc. Best Manage. Pract. Maintain Groundwater Qual. pp. 41-45. Iowa State Univ., Ames and Pioneer Hi-Bred Int., Inc., Johnston, Iowa. Williams, L. E., and Schmitthenner, A. F. 1962. Effect of crop rotation on soil fungus populations. Phytopathology 52, 241-247. Wilson, H. A . , and Browning, G. M 1945. Soil aggregation, yields, runoff, and erosion as affected by cropping systems. Soil Sci. Soc. A m . Proc. 10, 51-57. Wischmeier, W. H . , and Mannering, J. V. 1965. Effect of organic matter content of the soil on infiltration. J. Soil Water Consew. 20, 150-152. Yakle, G. A., and Cruse, R. M. 1983. Corn plant residue age and placement effects upon early corn growth. Can. J. Plant Sci. 63, 871-877. Yakle, G. A,, and Cruse, R. M. 1984. Effects of fresh and decomposing corn plant residue extracts on corn seedling development. Soil Sci. SOC. Am. J. 48, 1143-1146. Yates, F. 1954. The analysis of experiments containing different crops. Biomerrics 10, 324-346. Young, D. L . , and Painter, K. M. 1990. Farm program impacts on incentives for greenmanure rotations. Am. J. Alternative Agric. 5 , 99-105.

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ROLEOF DISSOLUTION AND PRECIPITATION OF MINERALS IN CONTROLLING SOLUBLE ALUMINUM IN ACIDICSOILS G. S. P. Ritchie Department of Soil Science and Plant Nutrition School of Agriculture The University of Western Australia Nedlands, Western Australia 6009, Australia

I. Introduction 11. A Framework for Understanding Mineral Dissolution and Precipitation in Soils 111. Factors Affecting Dissolution and Precipitation of Aluminum-Containing Minerals A. Solution Properties B. Solid Properties N. Modeling Soluble Aluminum A. Chemical Thermodynamic Approaches B. l n e t i c Approaches to Modeling V. Aluminum in Acidic Soils: Principles and Practicalities References

I. INTRODUCTION Acidic soils are a worldwide phenomenon that may be natural or anthropogenic in origin. Acidic precipitation and farm management practices that disrupt the carbon, nitrogen, and sulfur cycles have apparently resulted in contemporary acidification rates that are much higher than rates estimated to occur in their absence (Binkley et af., 1989; Robson, 1989). Agricultural production on acidic soils may be severely limited by a number of nutritional (e.g., nitrogen or molybdenum deficiencies) or toxicity (e.g., aluminum or manganese) problems 47 Advances in Aponomy, Volume 53

Copyright 0 1994 by Academic Press, Inc. All rights of reproduction in any form reserved.

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G. S . P. RITCHIE

(Robson, 1989). Aluminum (Al) toxicity, however, is considered to be the most common cause of decreased plant growth in acidic soils. The quantity of toxic A1 in acidic soils has apparently defied prediction by chemical principles because the dynamic and diverse nature of soils distinguishes reality from ideality. The ultimate aim of soil scientists is to be able to predict Al speciation (solid and solution) in time and space and then deduce the quantity of A1 that is toxic to plants. There are several different forms of A1 in soils (Adams, 1984; Ritchie, 1989; Sposito, 1989a) which can all contribute to the toxic quantity of A1 in solution either directly or indirectly. AI-containing minerals are the ultimate source of A1 in most soils whereas organically bound, exchangeable, interlayer, and soluble, complexed A1 are sinks for Al3+ released during mineral dissolution. The sinks provide AP+ to the soil solution in the short term and hence, separately or collectively, may be seen as controlling the amount of AI3+ in solution. In the long term, even though A1 may be derived from mineral c o m ~ u n d sthe , quantity released cannot necessarily be predicted from equilibrium thermodynamics because morphological characteristics may result in the surface-free energy of the mechanism of structural breakdown being greater than the standard free energy of the reaction. When this occurs, kinetic considerations become more important than therm~ynamicsin controlling solution quantities of AP+ (Morse and Casey, 1988). Lewis and Randall (1923) pointed out that “thermodynamicsshows us whether a certain reaction may proceed and what maximum yield may be obtained, but gives no information as to the time required.” Hence our deductions about the processes controlling the dissolution and precipi~ationof A1 will always be at the mercy of the time scale of our observations. The processes and mechanisms of dissolution and precipitation have been under consideration by soil scientists and mineralogists for many years. In the context of A1 solubility, an understanding of dissolution mechanisms and kinetics helps us see the limitations of trying to apply equilibrium thermodynamics to predicting activities in soil solutions and to decide on the most appropriate course of action for our needs. The quantity of A1 in the soil solution is dynamic in time and space and the measurements we make represent one moment in the time and space of a pathway. Soluble A1 due to mineral dissolution and precipitation is the net result of the balance between thermodynamic and kinetic considerations, as affected by surface morphology, the uptake and release of nutrients and toxic ions by plants, and as affected by the composition and flow of water through the volume of soil being studied. When a mineral dissolves, whether it is a grain of feldspar in a granitic rock or kaolinite in a soil that is rewetting at the beginning of the wet season, the sequence of events that follows cannot be predicted by equilibrium

MINE^ D~SSOL~ION/PRECIPITATION

49

thermodynamics alone. A process or sequence of events begins which can be described in terms of a pathway. The pathway is controlled by thermodynamics, kinetics, and surface morphology, which answer the questions: (1) what is it and where can it go? (thermodynamics), (2) how quickly will it get there? (kinetics), and (3) what does it look like? (surface morphology). For soil scientists and others working in the field, there is a fourth question: how do I know when it’s there? Many mechanisms have been put forward to describe dissolution but few have addressed all three scientific components in~uencingthe process. Early work assumed the pathway was simply controlled by equilibrium thermodynamics (Garrels and Christ, 1965; Lindsay, 1979) but the inability of the theories to describe bulk solution concentrations led workers to postulate on nonequilibrium thermodynamics or on the physical structure of the dissolving surface and how they could lead to deviations from theoretical predictions based on the assumption of equilibrium (Helgeson, 1968; Hemingway, 1982; Hochella, 1990). In addition, the role of kinetics was also recognized to be so important (Morse and Casey, 1988) in some cases that it overshadows predictions from thermodynamic considerations. All the theories and mechanisms that have been suggested to explain dissolution have one aspect in common: they cannot be proved unequivocally. Hypotheses that explain behavior in terms of surface morphology require experimental evidence on the molecular scale (Sposito, 1986). Until now most of the evidence has come from bulk solution measurements or spectroscopic analyses that are limited in their ability to distinguish between the surface and the interior of a mineral. However, recent advances in spectroscopic and microscopic techniques are providing methods that can study the hydrated surface layers of a dissolving grain (Hochella, 1990; Brown, 1990; Mogk, 1990). This review considers the role of mineral dissolution and precipitation in c o n ~ l l i n gsolution quantities of A1 and our attempts to predict the outcome of these processes. Its purpose is to broaden our perspective and thereby increase our ability to predict (Al3+) accurateiy by providing soil scientists with possibilities for looking at the problem from a different perspective by drawing on examples from related disciplines such as geochemistry. The dissolution and precipitation of Al-containing minerals are by no means the only mechanisms controlling AP+ in soil solutions (Ritchie, 1994). It is an area, however, that requires more clarity so that its contribution to the overall scheme of events can be appreciated more appropriately. The new perspectives may then enable us to predict more accurately the variation in solution composition with time and space of acidic or acidifying soils, before and after amelioration. Within this framework, the chemical paradigms that have been mistaken for principles and the paradigms of mineral and solution phases that exist in our soils in apparent defiance of chemical principles will be discussed.

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G. S. P. RITCHIE

11. A FRAMEWORK FOR UNDERSTANDING MINERAL DISSOLUTION AND PRECIPITATION IN SOILS In a closed system, the amount and composition of a mineral that dissolves or precipitates may be described in terms of chemical thermodynamics and kinetics as affected by the surface morphologies of the dissolving and precipitating species (Fig. 1). It is not possible to understand fully the processes and pathways of precipitation and dissolution without considering the interactions among thermodynamics, kinetics, and surface morphology. Chemical thermodynamics describes the pathway and predicts mineral and solution speciation from the standard free energy change of a chemical reaction (AGO,) and the composition of the soil solution and the minerals present. Such considerations may assume that equilibrium can be achieved [i.e., the free energy ( G ) of the system reaches a minimum]; that non- or quasi-equilibrium exists [i.e., metastable products (e.g., smectites, Al-substituted goethite, and hematite) persist on a time scale considered long for soils]; or that an irreversible reaction occurs (i.e., a rock component dissolves completely). Even though the driving force for precipitation or dissolution may be great from a thermodynamic standpoint (i.e., a lot of free energy, AG,can be lost), the thermodynamic potential for a mineral to form or dissolve [( 1) in Fig. I ] may be overshadowed by kinetic considerations. The rate of precipitation or dissolution

,

Equilibrium

Transport

Irreversible

-

- Precipitation \

Equilibrium Solution

Surface Morphology

Topography Figure 1

Surface area

Structure

The three components influencing dissolution and precipitation.

MINERAL DISSOLUTION/PRECIPITATION

51

may be very small because the driving force (i.e., change in energy) is small. Thermodynamics indicates which reactions are possible whereas kinetics stipulate the time required for transformations and hence can frequently mediate the pathway of a reaction [(2) in Fig. 11. Kinetic considerations include transport of ions in solution, reaction rates in solution, and rates of nucleation, crystal growth, and dissolution. The energy changes described by chemical thermodynamics and kinetics during dissolution and precipitation may be modified by the surface morphology of the mineral (i.e., composition, structure, topography, thickness, and surface area). The surface morphology is the physical manifestation of the processes and rates of dissolution and precipitation. The soluble components predicted by thermodynamics can influence all the aspects of surface morphology [(3) in Fig. 11. For example, nucleation and crystal growth could generate new species on a surface. Conversely, the processes of dissolution could modify the surface by producing leached layers or crystal ripening (Morse and Casey, 1988) could produce crystals of smaller surface area. In turn, surface morphology can affect the release or incorporation of solution components which change the free energy of solution and hence mineral reaction pathways may be altered [(4) in Fig. 11. Kinetic factors can affect surface morphology [e.g., incongruent dissolution creates “leached layers” at a surface; ( 5 ) in Fig. I] just as much as surface morphology will dictate the speed of dissolution and precipitation [(6)in Fig. 11.

111. FACTORS AFFECTING DISSOLUTION AND PRECIPITATION OF ALUMINUM-CONTAINING MINERALS The surface and bulk properties of a mineral and the intensive and extensive properties of a solid-solution system can affect dissolution and precipitation by affecting each of the three components in the framework of Fig. 1 (Table I). Many of these factors are interrelated and hence the following discussion assumes all factors are constant other than the one being considered.

A. SOLUTIONPROPERTIES The state of saturation of a solution plays a fundamental role in determining the reaction pathway and rate, and the surface mechanism controlling precipitation and dissolution (Van Straten et al., 1984; Nagy and Lasaga, 1992). The dissolution of a solid may be represented by the following type of reaction:

52

G. S. P. RITCHIE Table I System, Solution, and Solid-Phase Properties That Influence the Dissolution and Precipitation of Al Mineralsa System

I . Temperature and pressure

Solution

2. .Saturation 3. pH 4. co, 5. Activity of water 6. Cations

7. Inorganic anions 8. Organic ligands 9. Ionic strength 10. pH buffeting 1 1 . Polydispersity a

Solid 12. Bulk composition

13. Surface composition 14. Activity of solid I S . Surface structure 16. Surface transmissivity 17. Surface thickness 18. Particle size 19. Particle surface area 20. Particle surface tension 21. Precipitation of other minerals

The numbers are used to refer to this table in Tables 11 and VI.

The extent to which the reaction proceeds to the right-hand side of Eq. (1) depends on the solubility product constant, Ksp: Ksp = (A13+)(OH)3

(2)

where round brackets denote activities. The right-hand side of Eq. (2) is referred to as the ion activity product (IAP) and can be used to estimate the saturation of a solution with respect to a particular mineral by estimating the relative saturation (RS):

RS

=

IAPIK,,

(3)

If RS < 1, the solution is undersaturated; if RS is > 1, the solution is supersaturated. The logarithm of RS is sometimes referred to as the saturation index (SI). The extent of saturation affects the reaction pathway of both dissolution and precipitation. Taking the dissolution of microcline in rainwater as an example, and assuming the very simple case that thermodynamic, partial equilibrium is possible (Tsuzuki, 1967), Fig. 2a shows that the reaction pathway depends on the initial A1 saturation of the solution as the microcline begins to dissolve. As the microcline reacts with water it releases A1 and Si into solution (A) at a rate that is sufficiently low for saturation to be < 1 with respect to any Al-OH or Al-Si-OH mineral. When the solution becomes saturated with respect to gibbsite (B), A1 will precipitate from solution while microcline continues to release A1 and Si, Eventually the Si activity will be high enough for kaolinite to precipitate (C) which will lower the A1 activity below that controlled by gibbsite. Hence, gibbsite will start to dissolve and, even though microcline and kaolinite are both present, A1 activity in solution will be controlled by gibbsite. During this stage,

MINERAL DISSOLUTION/PRECIPITATION

53

b

a 0

0

-a -:>: U W

-5

-5

ln

0

0

- -5

-5

m

= > ln

.-ln

'0 d

m

L

m

0 -10

0

-10

L

&

m

m

- = L

c 0

-10

-10 t

-

m

m

0

0

-5 loglH,SiO,

0

I

-5 log IHLSi0'1

0

Figure 2 The variation in A1 solubility at pH 5 (a) and pH 4 (b) during the weathering of microcline. The lines represent the ion activity product predicted from the K, of minerals at equilibrium: G , gibbsite; K, kaolinite; S, amorphous silica. (After Tsuzuki, 1967.)

both gibbsite and microcline will be sources of A1 for the kaolinite that precipitates. When all the gibbsite has dissolved, microcline continues to react to form kaolinite and the A1 activity decreases whereas Si activity continues to increase until it is equivalent to that associated with amorphous silica at equilibrium. At this point, kaolinite and amorphous silica are in equilibrium (D). If the microcline dissolved more quickly in the initial reaction with rainwater, then the line AB would not be so steep and there would be less likelihood that gibbsite would form before kaolinite precipitated. This is the first example of how three mineral phases can be present but A1 in solution is controlled by the least thermodynamically stable mineral. Even so, this is a very simplistic picture of what is happening and does not address the irreversibility of some of the reactions that occur (e.g., the precipitation of quartz). The state of saturation also affects reaction kinetics. The rates of dissolution and precipitation slow down as equilibrium is approached. Hence, as water flows through a soil, the rate of dissolution in each successive volume of soil decreases because the flowing water contains an increasing amount of A1 and is therefore nearer to equilibrium. This hypothesis is only relevant if other factors (such as pH, soluble organic ligands) that affect dissolution rates do not vary significantly between successive volumes of soil. With respect to mechanisms acting in situations far from equilibrium (i.e., the magnitude of the driving force is large), the rate of dissolution is controlled by the soluble quantity of the mineral components and the presence of other ions that may inhibit or catalyze the dissolution process (Nagy and Lasaga, 1992). In the case of precipitation, the rate-

54

G. S. P. RITCHIE

controlling step may be diffusion to the surface because surface reactions could have become very rapid at high supersaturations (Zhang and Nancollas, 1990). Hence, the nucleation rates for all possible intermediary phases become very rapid and essentially similar. As the driving force for the reaction decreases, the total change in free energy for the reaction, AG, (this includes the Gibbs free energy change, AGO,) may also influence the rate of reaction and alter the ratecontrolling step. In addition, even if the variation in the rate of reaction with AG, has the same shape (e.g., linear) for both precipitation and dissolution in solutions near equilibrium, one cannot necessarily conclude that the same mechanism is controlling both reactions. In the case of gibbsite at pH 3 and 80°C, Nagy and Lasaga (1992) found that the variation in dissolution rate with AG, could be explained most easily by postulating that dissolution occurs at dislocation screw defects on basal surfaces at saturations near equilibrium. In solution far from equilibrium, however, the dissolution rate was much greater and was consistent with the formation of etch pits. It was also possible that the functional dependence of rate on AG, was due to changes in solution or surface speciation of A1 with the extent of solution saturation. pH affects dissolution and precipitation because it takes part in the reaction, it acts as a catalyst, or it changes the reaction pathway or surface morphology. Lowering the pH (as in an acidifying soil) can change the reaction pathway by changing the extent of saturation (Tsuzuki, 1967). Figure 2 indicates that as the pH falls from 5.0 to 4.0 the reaction pathway of dissolution of microcline changes from: microcline + gibbsite .--, kaolinite + kaolinite

+ amorphous silica

to microcline + kaolinite + kaolinite

+ amorphous silica

Specific adsorption of H+ and OH- can alter the surface charge of a mineral and hence decrease the rate of nucleation by lowering the interfacial tension (Van Straten et al., 1984). Stumm and co-workers (Stumm and Wieland, 1990, and references therein) consider adsorption to consist of several stages of which the detachment of an activated surface complex is the rate-limiting step and hence controls the dissolution rate (Fig. 3). They found that the rate of dissolution of metal oxides was proportional to the surface concentration of H+ ions raised to the power equivalent to the charge of the metal cation (Fig. 4). Understanding the effect of pH on the dissolution rate of Al from layer silicates is not as straightforward because of the presence of pH-independent sites. In general, it appears that Al dissolution from kaolinite, anorthite, and montmorillonite is independent of H+ concentration in the pH region -4-9 whereas at pH 12; as may occur temporarily in soil around a dissolving grain of lime) dehydrates AI(OH), linkages to A10,and changes the reaction pathway to favor the precipitation of fine-grained, poorly crystalline boehmite (referred to as pseudo-boehmite) rather than bayerite (Hemingway, 1982). As mixing of OH- with the soil increases with time, the localized ratio of OH and Al will decrease until dehydration is no longer favored. At this stage, the pseudo-boehmite will stop precipitating and an Al(OH), solid phase will form. The pseudo-boehmite will then dissolve in response to the removal of A1 from solution as Al(OH), precipitates. Exchange of H+ for A13+ in the surface layers of a dissolving mineral will change the surface morphology and temporarily affect the dissolution rate (Casey and Bunker, 1990). Ionic strength (I) affects dissolution and precipitation by changing the activity of soluble mineral components, the relative amounts of the species of each component and by changing the surface concentration of H/OH ions. Increasing ionic strength decreases the activity of Al3+ and hence more Al3+ is released by the mineral dissolving in an attempt to restore the original equilibrium. This is balanced partially by a simultaneous increase in the ratio of AP+ and monomeric hydroxy species. Such changes can affect the reaction rate and pathways and the surface morphology. Accordingly, Furrer el al. (1991) found that the dissolution rate of montmorillonite was approximately doubled when the ionic strength was raised from 0.1 to I M . The presence of cations and anions other than those forming the minerals under consideration can change the speciation of soluble mineral components and hence the reaction pathways. They can also affect reaction rates and surface morphology by being specifically adsorbed, incorporated as an impurity, coprecipitating, or by precipitating on a mineral surface. Inclusion (as defined in Sposito, 1989b) lowers the activity of the solid and produces a strain on the crystal structure, both of which decrease solubility (Sposito, 1984). Precipitation of a new phase on a mineral will change the surface area and tension and may block sites for nucleation or dissolution, or hinder crystal growth. The presence of anions can alter the reaction pathway and rate by inhibiting or promoting polymerization and precipitation, forming new compounds or solid solutions with the components of pre-existing minerals, and by retarding crystallization (Zawacki et al., 1986; Hemingway, 1982; Bertsch, 1989; Davis and Hem, 1989). For example, specific adsorption onto variable charge surfaces of anions that form bidentate mononuclear surface complexes (e.g., oxalate, salicylate, citrate) will enhance short term (< 50 hr) dissolution (Fig. 5 ) , whereas

I " O \

I

P

/ \

O\/

I/=\

O\

I I

P

I/=\ /" /=\

O\

57

58

G. S. P. RITCHIE

specific adsorption of ligands that form multinuclear surface complexes or block surface reactive groups retard short-term dissolution (Fig. 5 ) (Stumm and Wieland, 1990). The extent to which an organic ligand increases the short-term dissolution rate of a-Al,O, correlates with the ability of an anion (within a given structural class) to complex A13+ in solution (Furrer and Stumm, 1986). In contrast, the presence of organic ligands does not significantly enhance the longterm dissolution of corundum (Carroll-Webb and Walther, 1988). The results for layer silicates are also inconclusive. The long-term dissolution of anorthite increases in the presence of oxalate at pH 4.2-9 (Amhrein and Suarez, 1988) whereas organic ligands do not affect kaolinite dissolution (Carroll-Webb and Walther, 1988). The presence of ligands that form soluble complexes with A1 can prevent the formation or rapid polymerization of hydroxy-Al at pH 12), dehydration of AI(0H); to 0x0 linkages will occur and boehmite will become the favored precipitate again (Hemingway, 1982). If iron is present, thermodynamic considerations indicate that the simultaneous precipitation of goethite and gibbsite at Si activities less than that required for kaolinite precipitation can affect the reaction pathway by favoring the formation of Al-substituted goethite or hematite rather than pure A1 hydrous oxides (Tardy and Nahon, 1985). Field evidence suggests that this could be important in some acidic soils. Fitzpatrick and Schwertmann (1982) found that the crystallinity of kaolinite and the Al substitution of ferric hydrous oxides in lateritic profiles increased with depth and with decreasing pH. In contrast, equilibrium modeling indicates that A1 contents of goethite tend to decrease as aridity and the concentration of Si in the soil solution increase (Tardy, 1971) and that Al-substituted goethite is thermodynamically more metastable than gibbsite at low activities of A1 (Figs. 6a and 6c) (Tardy and Nahon, 1985). These predictions assume that ideal solid solutions can exist in soils and that they are in equilibrium with other A1 minerals, such as kaolinite and gibbsite. Solutions well-buffered with respect to pH increase the rate of precipitation of aluminum hydrous oxides (May et af., 1979) but do not affect the dissolution of feldspars (Wollast, 1967). For A1 hydrous oxides, the reaction rate decreases as the difference between initial and final pH values gets larger in poorly buffered solutions, even if the solution is initially supersaturated with respect to a solid phase.

MINERAL DISSOLUTION/PRECIPITATION

10.0

(luartz /

,Gibbsite

< 4 o1

-a I

8.0

-

-b

Quartz ' 1 6 i b b s i te

I I

I

6.0

I I

-

Kaol ini te

10.0

I I I

'

-C

4.0

I

8.0

(0.01 )

-

.

-5.5

I 1

I

Quartz

I

6.0

6.0

I

I

I

4.0

6,0

I I i

_I 0

m

10.0

59

Al-GOETHITE

1 AI-HEMATITE

1

1

-5.0

-4.5

-4.0

-3.5

L o g L 1i4s1041 Figure 6 Equilibrium solubility diagram for gibbsite, quartz, and kaolinite at activities of water (a,) of 1.0 (a) and 0.5 (b) and for goethite (c) and hematite (d) with substituted A1 varying from 0.001 to 0.1%. (After Tardy and Nahon, 1985, Am. J. Sci., reprinted by permission of American Journal of Science.)

Carbon dioxide may influence both the reaction pathway and rate. Increasing partial pressure of CO, (as may occur in the rhizosphere) decreases the dehydration of Al(0H); and favors the formation of gibbsite rather than boehmite (Hemingway, 1982). An increase in the level of dissolved C 0 2 may increase the pH buffering of the soil solution and affect the rate of reaction as discussed earlier. Raising the temperature (as may occur in dry, hot weather experienced in arid and mediterranean climates) increases the rate of reaction and influences the reaction pathway by increasing the likelihood of dehydration of Al(0H); to A10, and changing the relative values of AGO, of minerals that may form. For example, gibbsite converts to boehmite at T > 368 K (Hemingway, 1982). Polydispersity of a species in solution with respect to size or molecular weight can affect its dissolution (Parks, 1990). The smallest particles with the highest surface area tend to dissolve first but reprecipitate as more well-ordered, larger crystals. Hence a polydisperse system may take a lot longer to dissolve unless the rate of reprecipitation is much slower than the rate of dissolution. Lowering the activity of water (as a soil dries or as water enters a smaller pore

G. S. P. RITCHIE

60

size) affects the reaction pathway, equilibrium activities of mineral components, and the composition of solid phases (Tardy and Nahon, 1985). Assuming that equilibrium is achievable, decreasing the activity of water increases the activity of A P + in equilibrium with hydrous A1 oxides and decreases Si activity at which gibbsite and kaolinite are in equilibrium (Fig. 6). Lowering the activity of water favors the formation of diaspore and boehmite over gibbsite but this depends on the choice of the equilibrium constant (Fig. 7). Thermodynamic considerations indicate that the percentage of A1 that can substitute in goethite or hematite, in equilibrium with kaolinite and quartz, increases as water activity decreases (Tardy and Nahon, 1985). The influence of water activity on mineral solubility indicates that the formation of boehmite rather than gibbsite would be favored in dry soils, particularly with a large clay-sized fraction. Gibbsite would tend to precipitate in larger pores whereas boehmite would precipitate in smaller pores in which water activity would be lower. Alternatively, if Si was present, gibbsite precipitation would be more prevalent in large pores and channels whereas kaolinite would be more stable in the fine pores.

t

11 0

'

'

*

'

'

'

0.2 0.4 0.6 0.8 activity o f water

' 1.0

Figure 7 The relationship between log[A13+]/[H+]3 and the activity of water (a,,,) for corundum (log K , = 9.73), diaspore (log Ksp = 7.92 or 8.95), boehmite (log KIP = 8.13). and gibbsite (log K , = 8.04). Log K , values taken from Lindsay (1979) or Tardy and Nahon (1985).

MINERAL DISSOLUTION/PRECIPITATION

61

B. SOLIDPROPERTIES The aggregation and composition of the bulk mineral and its surface layers will affect the composition of the solution and hence the reaction pathway and rate and surface morphology. It is still not clear which minerals dissolve congruently or incongruently and whether dissolution and precipitation occur through surface-controlled reactions or the development of leached layers. In addition, it has yet to be established unequivocally whether dissolution and precipitation occur at specific sites or uniformly across the surface of a mineral. These uncertainties all affect the activity of the solid and the quantity of soluble components in equilibrium with it. Solubility of a mineral decreases when the solid activity is < I which may be due to inclusion or the mineral surface having concave interfaces rather than flat surfaces. Precipitation on to interlayers, lattice defects, convex interfaces, low crystallinity, and small grain size increase the activity of a solid above the ideal value of 1.0 (Tardy and Nahon, 1985; Sposito, 1981, 1989b; Schott, 1990). Solubility increases with surface area which can result from increasing disorder (amorphous versus crystalline) or more structural defects (Parks, 1990). Pits, fractures, ledges, comers, and edges are all structural defects that may contribute to dissolution to different extents depending on the relative rates and qualities dissolved (Schott el al., 1989) (Fig. 8). The relative contribution of each defect to the overall dissolution of a mineral depends on the degree of saturation. For example, as relative saturation increases from values far less than unity (i.e., highly undersaturated), the fewer the sites at which a pit may form and hence the smaller the contribution of this process to overall dissolution (Schott er al., 1989). As dissolution proceeds, however, a decrease in surface strain energy at structural defects may counterbalance the increase in surface area and hence the increase in dissolution rate due to a high density of defects may not be as great as expected (Schott, 1990). In supersaturated solutions, amorphous materials tend to precipitate more quickly because the rough surface provides more sites for nucleation than the smooth surfaces of crystalline phases. Crystalline materials have a higher activation energy barrier to be overcome for precipitation to occur and a higher surface tension (or free energy) which limits solubility and decreases the dissolution rate (Van Straten er al., 1984). Hence, it is possible to have highly supersaturated solutions of sparingly soluble minerals. A less structured, higher specific surface and spongy solid phase would be expected to nucleate and grow a precipitate more quickly than a well-structured, low surface area solid. The phrase “crystal ripening” was coined by Ostwald to describe the process by which small grains tend to dissolve to form fewer grains which are larger (Morse and Casey, 1988). This process tends to lead to a wider distribution in grain sizes as time progresses and thereby affects the rates of dissolution and

62

G. S. P. RITCHIE What Determines Measured Dissolution Rate With Parallel Processes? Fastest process is normally rate-determtning unless i t s contribution to total dissolved concentration is insignlflcant

Point Defects Dislocations Microfractures

Kinks Grain o r Twin Boundaries

Comers Edges. Ledges

Entire Face With All Defects

Figure 8 A schematic illustration of the parallel processes involved in crystal dissolution. The horizontal length of each arrow indicates the relative rate of each process (actual rates can differ by many orders of magnitude). The vertical thickness of each arrow represents the relative quantity of material dissolved and delivered to aqueous solution by that process. Thus, while point and linear defects react most rapidly. they deliver less dissolved material to solution than slower dissolution of faces and pits occurring at edges, ledges, and corners. (Reprinted from Geochim. Cosmochim. Acru. v. 53, Schott, J., Brantley, S., Crerar, D., Guy, C., Borcsik, M., and Williams, C., Dissolution kinetics of strained calcite, pp. 373-382, Copyright (l989), with kind permission from Pergamon Press, Ltd., Headington Hill Hall, Oxford OX3 OBW, UK.)

precipitation because of the dependence of solubility on grain size and because the rate of nucleation decreases with increasing surface tension. Particle size also affects solubility because thermodynamics predicts that the heat of dissolution varies with particle size in different ways for different minerals. For example, hematite is less soluble than goethite at equal or large grain sizes, but more soluble when it is smaller (Tardy and Nahon, 1985). Similarly, amorphous silica is less soluble than quartz until the grain size of quartz become 30 p l 4 measured in a 0.005 M KCI extract are toxic to wheat (Cam et al., 1991). A 1% error in slope (Table III), a 0.1 unit error in pH (Table VI), or a 2% error in the log Ksp value (Table V) can result in an erroneous prediction of toxicity in the pH range of 4.2-4.25 if one assumes gibbsite is controlling A1 solubility. A 1% error in the slope of the solubility line at pH 4.0-4.35 is equivalent to a 2.5-3% error in log(A13+)which is often assumed to be accurate enough for the purpose of predicting (A13+) in soils. However, a 2.5-3% variation in log(AP+) is equivalent to a 33-35% error in (AP+) which at pH 4.2 is sufficient to change (A13+) from a nontoxic to a toxic value for the yellow earth soils studied by Carr et al. (1991). The size of these errors increases as pH decreases. The inaccuracies in predicting (A13+)that arise from not identifying all the complexing ligands in solution depend on the pH, the equilibrium constant (log K") for Al reacting with the unknown ligand (L) and the activity of the unknown ligand relative to that of Al3+ (Figs. 9 and 10). The concentrations of A1 species in Figs. 9 and 10 were estimated using an equilibrium program, TITRATOR (Cabaniss, 1987), assuming a hypothetical case in which A1(OH)2+,AI(OH)2+, Al(OH),, ALL, and HL were the species formed in solution. Log K" values were taken from Lindsay (1979) except for ALL and HL (log K" = 3); ionic strength was set at zero and the total concentration of A1 was 30 phl. The ligand concentration was 30 phl unless the L:Al ratio varied between 1 and 3.3. At pH 4.5 (Fig. 9a), as the log K" for Al-L increases from 3.2 (a weakly complexing ligand) to 6.98 (a strongly complexing ligand), (A13+)decreases from 50 to