Advances in Agronomy, Volume 33

ADVANCES IN AGRONOMY VOLUME 33 CONTRIBUTORS TO THIS VOLUME R. W. ARNOLD F. H. BEINROTH R. J . BURESH F. B . CADY M...

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ADVANCES IN

AGRONOMY

VOLUME 33

CONTRIBUTORS TO THIS VOLUME R. W. ARNOLD F. H. BEINROTH R. J . BURESH F. B . CADY M. E. CASSELMAN MARLING . CLINE G . D. FARQUHAR

R. J . HAYNES ERNESTA. KIRKBY T. M. MCCALLA KONRADMENGEL W. H. PATRICK,JR. J . A . SILVA

H . T. STALKER G. UEHARA

P. W. UNGER P. D. WALTON R. WETSELAAR

ADVANCES IN

AGRONOMY Prepared in cooperation with the AMERICAN SOCIETY OF AGRONOMY

VOLUME 33 Edited by N. C . BRADY International Rice Research Institute Manila, Philippines

ADVISORY BOARD H. J . GORZ,CHAIRMAN R.B. GROSSMAN T. M. STARLING I . B. POWELL

J . W . BIGGAR

M. A. TABATABAI M. STELLY, EX

OFFICIO,

ASA Headquarters 1980

ACADEMIC PRESS A Subsidiary of Harcourr Brace Jovanovich, Publishers

New York London Toronto Sydney San Francisco

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ISBN 0-12-000733-9 PRINTED IN THE UNITED STATES OF AMERICA 80 81 82 83

9 8 7 6 5 4 3 2 1

CONTENTS CONTIUBUTORS TO VOLUME 33 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PREFACE .....................................................

ix xi

CONSERVATION TILLAGE SYSTEMS

P. W. Unger and T . M . McCalla I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1 . Historical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III . Tillage Equipment and Use ............................... IV . Crop Yields and Quality ................................. V . Environmental Consideration ............................. VI . Infiltration and Water Conservation ........................ VII . Weed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII . Insects and Plant Diseases ................................ IX . Soil Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X . Soil Structure and Other Physical Properties . . . . . . . . . . . . . . . . . XI . Chemical Effects and Microbial Activity .................... XI1. Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XI11. Summary and Conclusions ............................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 5 6 13 16 30 35 38 39 43 48 49 51 53

POTASSIUM IN CROP PRODUCTION

Konrad Mengel and Ernest A . Kirkby

I. I1. 111. IV . V.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Potassium Availability in the Soil .......................... Potassium in Physiology ................................. Potassium Application and Crop Growth .................... Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References ............................................

59 60 74 91 103 103

UTILIZATION OF WILD SPECIES FOR CROP IMPROVEMENT

H . T . Stalker I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Biosystematics ......................................... V

112 113

vi

CONTENTS

111. The Gap between Hybridization and Utilization . . . . . . . . . . . . . . IV . Approaches for Utilizing Wild Germplasm Resources . . . . . . . . . V . Examples of Species Used in Wild Species Hybridization Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI . Specific Uses of Wild Species for Crop Improvement . . . . . . . . . VII . Summary and Conclusions ............................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

118 119 126 135 140 141

NITROGEN FIXATION IN FLOODED SOIL SYSTEMS. A REVIEW

R . J . Buresh. M . E . Casselman. and W . H . Patrick. Jr . I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Nitrogen Fixation in the Water Column and on the Soil Surface 111. Nitrogen Fixation in the Aerobic Layer of Flooded Soil . . . . . . . IV . Nitrogen Fixation in the Anaerobic Layer of Flooded Soil . . . . . V . Nitrogen Fixation in the Root Zone of Nonnodulated Plants . . . . VI . Nitrogen Fixation on the Leaf and Stem Surface of Plants . . . . . VII . Environmental Factors Influencing Nitrogen Fixation in Flooded Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII . Comparison of Acetylene Reduction and 15N Methodology . . . . IX * Contribution of Fixed Nitrogen to the Nitrogen Requirements of Plants ............................................ X . Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

150 152 160 161 163 170 174 180 183 185 187

EXPERIENCE WITH SOIL TAXONOMY OF THE UNITED STATES

Marlin G . Cline I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1 . General Reactions to the System .......................... 111. Use of Soil Taxonomy Internationally ...................... IV . Problems for Users of Soil Taxonomy ...................... V . Taxonomic Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI . Impact of Soil Taxonomy Internationally .................... VII . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

193 195 197 202 207 213 222 223

vii

CONTENTS COMPETITIVE ASPECTS OF THE GRASS-LEGUME ASSOCIATION

R . J . Haynes

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Competition in the Pasture Community ..................... 111. Physiological Considerations .............................. IV . Morphological Considerations ............................. V . Competition for Environmental Factors ..................... VI . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

227 229 231 239 248 254 256

NITROGEN LOSSES FROM TOPS OF PLANTS

R . Wetselaar and G . D . Farquhar I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

263 264 111. Possible Pathways of Nitrogen Losses from Tops . . . . . . . . . . . . . 279 IV . Associated Methodology Problems ......................... 293 V . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

I1 . Record of Observed Decreases of Nitrogen Content of Plant Tops

AGROTECHNOLOGY TRANSFER IN THE TROPICS BASED ON SOIL TAXONOMY

F . H . Beinroth. G . Uehara. J . A . Silva. R . W . Arnold. and F . B . Cady

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

304 304 Soil Classification in Perspective .......................... 309 Agrotechnology Transference Research ..................... 316 Quantitative Verification of Transferability within a Soil Family . 323 Prerequisites for Worldwide Agrotechnology Transfer . . . . . . . . . 332 Conclusion: Implication for Agricultural Development . . . . . . . . . 336 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338

I1 . The Transfer of Agrotechnology ........................... 111.

IV . V. VI . VII .

THE PRODUCTION CHARACTERISTICS OF Bromus inerrnis LEYSS AND THEIR INHERITANCE

P . D . Walton

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. The Nature of the Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

341 343

viii

CONTENTS

III . Seed Production and Establishment ........................ IV . Forage Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V . Forage Yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI . Plant Breeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

347 350 353 363 367

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

371

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

R. W. ARNOLD (303), Department of Agronomy, New York State College of Agriculture and Life Sciences, Cornell University, lthaca, New York 14853 F. H. BEINROTH (303), Department of Agronomy and Soils, College of Agricultural Sciences, University of Puerto Rico, Mayaguez, Puerto Rico 00708 R. J . BURESH* (149), Laboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana State University, Baton Rouge, Louisiana 70803 F. B. CADY (303), Biometrics Unit, Department of Plant Breeding and Biometry, New York State College of Agriculture and Life Sciences, Cornell University, lthaca, New York 14853 M. E. CASSELMAN (149), Laboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana State University, Baton Rouge, Louisiana 70803 MARLIN G . CLINE ( 1 93), Department of Agronomy, Cornell University, lthaca, New York 14853 G. D. FARQUHAR (263), Department of Environmental Biology, Research School of Biological Sciences, Australian National University, P.O. Box 47.5, Canberra City, Australia 2601 R . J . HAYNES (227), Department of Soil Science, Lincoln College, Canterbury, New Zealand ERNEST A. KIRKBY (59), Department of Plant Sciences, The University, Leeds LS2 9JT, England T. M. MCCALLA ( l ) , Agricultural Research, Science and Education Administration, USDA, University of Nebraska, Lincoln, Nebraska 68583 KONRAD MENGEL (59), Institute of Plant Nutrition, Justus Liebig University, 0-6300 Giessen, Federal Republic of Germany W. H. PATRICK, JR. (149), Laboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana State University, Baton Rouge, Louisiana 70803 J . A. SILVA (303), Department of Agronomy and Soil Science, College of Tropical Agriculture and Human Resources, University of Hawaii, Honolulu, Hawaii 96822 H . T. STALKER ( 1 1 l), Department of Crop Science, North Carolina State University, Raleigh, North Carolina 27650 *Resent address: International Fertilizer Development Center, P.O. Box 2040, Muscle Shoals, Alabama 35660. iX

X

CONTRIBUTORS

G . UEHARA (303), Department of Agronomy and Soil Science, College of Tropical Agriculture and Human Resources, University of Hawaii, Honolulu, Hawaii 96822 P . W. UNGER ( l ) , Conservation and Production Laboratory, Agricultural Research, Science and Education Administration, USDA, Bushland, Texas 79012 P. D. WALTON (341), Department of Plant Science, The University of Alberta, Edmonton, Alberta T6G 2E3, Canada R. WETSELAAR (263), Division of Land Use Research, Commonwealth Scientific and Industrial Research Organization, P.O. Box 1666, Canberra City, Australia 2601

PREFACE The orderly classification of soils in the field is essential to effective utilization of much production-oriented soil and crop research. Unfortunately, however, there is all too little exchange of information among scientists concerned with soil classification. Many soil classification schemes are sufficiently complicated as to make them not easily understood by most agronomists. This has become increasingly evident with the development in recent years of comprehensive international soil classification schemes. Two of the articles in this volume relate to the classification of soils and to the usefulness to agronomists of classification schemes. Cline reviews agronomists’ experiences with the most comprehensive of the international classification systems, that developed under the leadership of the U.S. Department of Agriculture. The views presented are not only those of other pedologists, but also those of production-oriented scientists and others who have used the new comprehensive soil classification scheme. Beinroth and coauthors relate the findings of soil and crop scientists who have compared the performance of soils that have similar characteristics but are located in different parts of the world. This information is useful in ascertaining the value of soil survey in agrotechnology transfer. Nitrogen continues to be a prominent subject for agronomic research. The fixation of nitrogen in flooded systems is reviewed by Buresh, Casselman, and Patrick. They emphasize the uniqueness of flooded systems in relation both to redox potential and to nitrogen-fixing organisms. High nitrogen losses directly from plants as they mature may account for much of the low rate of utilization of applied nitrogen. Wetselaar and Farquharreview this subject in their contribution. Potassium in crop production has received prominent research attention in recent years, especially in relation to methods of predicting response to this important element. Mengel and Kirkby provide an excellent review of research on potassium availability in the soil and the function of potassium in the plant. The most significant recent change in soil and crop management in the United States has been in tillage and crop residue management. Over wide areas, tillage systems that keep most of the crop residues on or near the soil surface have replaced conventional systems that focused on the moldboard plow. A comprehensive review of the effects of some of these new tillage systems is presented in the article by Unger and McCalla. In recent years, scientists have become increasingly successful in implementing crosses between cultivated plants and wild species. These crosses provide considerable potential to incorporate into cultivated plants such desired characteristics as disease and insect pest resistance and tolerance of drought. Stalker reviews research on this subject. Research related to the production of forage crops is the focus of two of the xi

xii

PREFACE

manuscripts in this volume. Haynes reviews competitive aspects of grass-legume associations and illustrates how this competition accounts for the dominance of either grasses or legumes, depending on the ecological stresses. The characteristics of Bromus inermis Leyss that control its production and inheritance are the subjects of the review by Walton. The authors of the contributions presented in this volume are from six different countries. We express sincere appreciation to them for their efforts. N. C . BRADY

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VOLUME 33

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ADVANCES IN AGRONOMY, VOL. 33

CONSERVATION TlLLAGE SYSTEMS P. W. Unger2 and T. M. McCalla3 Agricultural Research, Science and EducationAdministration,U.S. Departmentof Agriculture

I. Introduction . . . . . . , . . . . . . . . . . . . . . . . . . ............................... A. Definition of Conservation Tillage Syst ............................... B. Development and Use of Practices in the United States .... C. Purpose of Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Historical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Tillage Equipment and Use ......................................... A. Machinery Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Seedbed Preparation and Crop Seeding . . . . IV. Crop Yields and Quality . . . . . . . . . . . . . . . . . . . A. Grain Yields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Plant Protein Content and Mineral Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Residue Yields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Root Growth V. Environmental Consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Control of Wind Erosion . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Control of Water Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Runoff Water Quality . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Infiltration and Water Conservation .......... A. Runoff and Infiltration.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Evaporation .................... VII. Weed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Problem Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Control with Tillage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Control with Herbicides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Control with Rotations ....................................... VIII. Insects and Plant Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Plant Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IX. Soil Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Residue Factors Involved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Biological Effects of Residue . .

2 2 2 5 5 6 6 10 13 13 14 15 16 16 17 21 29 30 30 33 35 35 36 37 38 38 38 39 39 39

40 42

'Contribution from Agricultural Research, Science and Education Administration, U.S. Department of Agriculture, in cooperation with the Texas and Nebraska Agricultural Experiment Stations. zSoil Scientist, Conservation and Production Research Laboratory, Bushland, Texas. 3Microbiologist, University of Nebraska, Lincoln, Nebraska. 1 ISBN 0-12-W733-9

2

P. W . UNGER AND T. M. McCALLA

X. Soil Structure and Other Physical Properties .................................. ........ A. Aggregation .......... B. Porosity and Density .................................................. C. Other Physical Properties . . ................. XI. Chemical Effects and Microbial XII. Economics . . . . . . . . . . . . . . XIII. Summary and Conclusions ................................................ A. Accomplishments ..... ..... B . Needs .............................................................. References . . . .................................................

43 44 46 46

51 51 52 53

1. INTRODUCTION A. DEFINITION OF CONSERVATION TILLAGE SYSTEMS

Conservation tillage systems, as used in this review, are systems of managing crop residue on the soil surface with minimum or no tillage. The systems are frequently referred to as stubble mulching, ecofallow, limited tillage, reduced tillage, minimum tillage, no-tillage, and direct drill. The goal of these systems of plant residue management is threefold: to leave enough plant residue on the soil surface at all times for water and wind erosion control, to reduce energy use, and to conserve soil and water. These systems are used throughout the United States and the world, and can be applied to all kinds of crop residue in many cropping systems. B. DEVELOPMENT A N D USE OF PRACTICES IN THE UNITEDSTATES

Stubble mulching was developed as a result of severe wind erosion in the Great Plains of the United States and Canada during the 1930s. Anchored surface residue kept the soil in place despite the erratic climate of the Great Plains. Crop residue on the soil surface was soon found to be equally effective for controlling water erosion. A surface mulch of plant residue protects the soil against the beating action of raindrops and keeps the surface of the soil open, thus increasing infiltration over that of a bare soil. When enough residue is present, more water is conserved with a mulch system than with the moldboard plow system. Since the 1930s and 1940s, many effective herbicides have come on the market, which reduced the need for tillage to control weeds. Even though the use of crop residue on the soil surface has much merit in controlling soil erosion and conserving water, use of residue by farmers depends in the final analysis upon the effects of surface residue on crop yields. Crop yields are often reduced where plant residue is maintained on the soil surface,

3


particularly on heavier textured soils in the more humid and northern areas of the United States. This apparently is due to factors such as ( 1 ) lack of proper equipment and knowledge of how to manage the residue with the equipment; (2) colder, wetter, and less aerated soil; (3) weed, insect, and disease problems; (4) lower nutrient availability, such as lower nitrate production; and ( 5 ) changes in the microbial status of the soil and the possible production of phytotoxic substances. In many areas, however, crop yields are often increased by plant residue left on the surface. In addition, residue use may keep a crop from being lost by wind erosion. Despite the lower yields and other problems sometimes encountered with the use of crop residue on the soil surface, these systems are useful to U.S. farmers. Demands for improved water quality of the nation’s streams and groundwater have also stimulated the use of crop residue on the soil surface. At present, over 28 million hectares (over 70 million acres) of land are cropped by minimum or no-tillage methods (Table I). Limited tillage, especially sod planting, is one method used in many parts of the United States. In some instances, the use of crop residue on the soil surface alone is not Table I 1978 /1979 No-Till Fanner Survey” No-tillageb State Alabama Alaska Arizona Arkansas California Colorado Connecticut Delaware Florida Georgia Hawaii Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts‘

Minimum tillage*

1978

I979

66,170 2,670 410 2,020 490 29,340 7,640 8 1,750 1,620 210,320 233,290 139,980 86,600 393,360 8 10

68,110 2,750 870

1.900

96,830 1,340

1978

50

2,140 490 32,120 1 1,530 108,050 2,830 215,300 259,000 146,090 95,100 470,250 810 2.060 112,490 1,380

83,930 1,850 120,190 109,210 390,570 832,980 770 123,030 48,260 8 17,480 1,093,080 2,040,470 698,250 3,001,360 3,862,810 800,650 236,540 158,570 4,330

1979 78,430 2,250 114,930 124.6 I0 410,700 866,370 1,210 133,310 54,230 997,570 1,097,940 2,246,050 696,070 3,338,730 4,091,460 849,980 238,570 182,790 4,130

Conventional tillageb 1978 1,473,130 3,840 425,740 2,715,170 1,603,680 1,571,230 19,790 55,940 490,810 806,560 113,560 1,333,470 6,433,210 3,155,870 5,665,720 4,634,970 410,280 1,458,720 71,310 226,840 18,490

1979 1,510,680 3,840 432,620 2,699,340 1,673,270 1,537,840 18,540 52,210 505,580 728,450 116,960 1,327,400 6,324,560 3,237,560 5,463,380 4,164,310 416,430 1,548,560 76,160 230,230 18,490 (continued)

4

P. W. UNGER AND T. M. McCALLA

Table I (continued) No-tillageb State Michigan Minnesota Mississippi Missouri Montana‘ Nebraska Nevada New Hampshire New Jersey New Mexico New York North Carolina North Dakota Ohio Oklahoma Oregon Pennsylvania Puerto Rico Rhode Island South Carolina South Dakota Tennessee Texas Utah Vermont Virginia Washington West Virginia Wisconsin Wyoming Totals

1978

1979

19,750 47,630 29,950 98,580 8,090 234,320 28,640 400 17,400 2,550 13,150 147,310 3,440 289,360 2,350 12,110 176,040 950 30 8,390 72,240 74,340 59,300 2,430 127,280 3,170 15,580 39,460 -

I 8,050 54,230 35,920 109,470 8,170 225,410 28.940 810 23,470 3,560 13,150 149,330 4,650 297,450 3,160 10,440 176,850 730 30 8,980 62,320 81,950 49,450 2,430 127,840 7,420 17,400 39,660 -

2,890,790 3,092,730

Minimum tillageb 1978

I979

428,980 1,848,640 131,530 1,474,960 263,050 2,613,920 38,360 2,140 8,090 78,310 263,050 255,970 1,297,860 40,470 235,610 127,770 69,130 110

728,250 1,398,220 220,960 607.600 79,930 1,010 157,710 234,690 1,210 359,570 1,210

437,070 1,893,970 148,120 1,445,890 264,470 2,577,090 39,290 2,140 16,190 90,450 265,070 258,800 1,421,690 40,470 239,580 129,830 62,160 I10 734,720 1,643,060 23 1,080 508,040 77,900 1,210 169,970 39 1,460 2,020 361,390 1,210

Conventional tillage* 1978 1,547,470 5,709,230 2,215,900 2,580,760 2,023,470 3,532,580 63,310 9.960 172,800 368,470 443,550 1,391,830 9,531,570 3,914,610 3,681.9 10 576,760 864,020 193,540 2,110 740,150 3,567,990 1,161,390 8.8 17,690 4 15,620 3 11,610 404,530 1,543,830 26,710 1,920,030 220,560

1979 1,541,080 5,679,890 2,199,620 3,831,550 2,058,880 3,146,900 63,770 10,560 157,430 360,180 441,520 1,375,800 9,409,360 3,906,520 3,678,070 566,070 897,610 173,080 2,110 753,540 3,268,310 1,197,090 7,868,000 428,170 31 1,210 402.470 1,382,810 22,260 1,943,100 273,570

27,392,940 28,983,820 90,962,280 89,374,030

“The source of information is the State Agronomists of the Soil Conservation Service. Values have been converted to hectares. Published with permission from No-Till Farmer, Inc. 61 I East Wells Street, Milwaukee, Wisconsin 53202. *Definitions of each of the tillage systems are as follows: No-tillage: Only the intermediate seed zone is prepared. Up to 25% of surface areacould be worked. Includes no-till, till-plant, chisel-plant, rotary strip tillage, and many other forms of conservation tillage and mulch tillage. Minimum tillage: Limited tillage, but the total field surface is still worked by tillage equipment. Conventional tillage: Where 100% of the topsoil is mixed or inverted by plowing, power tiller, or multiple diskings. ‘Estimates provided by Cooperative Extension Service Agronomists.


5

enough for good soil conservation. In such cases, use of residue in combination with other proven mechanical and conservation cropping practices, such as terracing and contouring, is necessary for more effective soil and water conservation. C. PURFOSEOF REVIEW

McCalla and Army (1961) reviewed the use of crop residue on the soil surface, which at that time was called stubble mulching. Because of research since then and because new equipment and herbicides are available, an inventory of the available information developed since then was needed, along with an appraisal of the merits and faults of managing crop residue on the soil surface for control of water and wind erosion, conservation of water, production of crops, and conservation of energy. Although the use of crop residue on the soil surface has been widely researched, information is still scarce on many aspects of the system’s influence on the physical, chemical, and biological soil environment; on insect, disease, and weed control; on water conservation; and on crop production. Not enough information is available for full implementation of the system across the United States for various soils, climatic conditions, and crops. Improvements are needed in equipment for seeding and applying herbicides. Crop varieties developed specifically for residue management systems are badly needed. Also, the economics of systems involving crop residue need to be fully evaluated. This review is, in general, limited to the salient points that have been researched since 1961. We have not attempted to summarize all the literature pertaining to the use of crop residue on the soil surface. Since 1961, numerous papers have been published, and a number of symposia and reviews have been made concerning the use of crop residue on the surface. Some examples of the latter are American Society of Agronomy (1978), Soil Conservation Society of America ( 1973, 1977, 1979), Great Plains Agricultural Council ( 1962, 1968, 1976), Ohio State University (1972), Plant Protection Limited (1973, 1975), and USDA (1977). Only pertinent and illustrative data are used to describe the areas of application and merits and faults of crop residue on the soil surface. Where essential technical information is lacking, we have called the deficiency to the reader’s attention.

II. HISTORICAL The early work was reviewed by McCalla and Army (1961) and Jacks et al. (1953, and also in a number of the symposia and reviews cited earlier.

6

P. W. UNGER AND T.M. McCALLA

Mulches, to some degree, have been used in agriculture since man first began to cultivate crops. Orchards and garden crops were probably the first to be mulched, and mulches usually were several centimeters thick. The Chinese, many hundreds of years ago, used stone mulches in their agriculture. Also, most primitive agriculture allowed residue to remain on the surface because the simple tools, in most cases, did not cover the plant residue. Crop residue is commonly used as mulches; but paper, plastics, glass wool, cloth, metal foil, sugarcane trash, manures, leaves, peat, litter, stones, and dust mulches have also been used. Natural mulches are snow and volcanic dust. Snow mulch is valuable in protecting a crop such as wheat against winterkill. Snow also is a major source of water for crop production in many cold regions. Dr. F. L. Duley and Professor J. C. Russel conducted the first intensive research in the United States on the use of a mulch for crops. The work was started at Lincoln, Nebraska, in 1937 by the Nebraska Agricultural Experiment Station in cooperation with the Research Division of the Soil Conservation Service, U.S. Department of Agriculture. Since that time, these and many other researchers have studied the management of crop residue on the soil surface. This research has resulted in the development of the crop residue management systems that are now known as conservation tillage systems.

Ill. TILLAGE EQUIPMENT AND USE A . MACHINERY REQUIREMENTS

Regardless of cropping system, a complement of machinery is needed to prepare satisfactorily a seedbed, plant seeds, and control weeds and volunteer crop plants. Because conservation tillage systems rely heavily on surface residue for erosion control and water conservation, it is imperative that the machinery be capable of operating satisfactorily when large amounts of residue are on the soil surface and that most residue be kept on the surface. Tillage systems developed within the last half century are capable of retaining most crop residue on the soil surface. These systems are the stubble mulch system, developed in the late 1930s and early 1940s, and the reduced- or notillage systems, which are essentially still under development. 1 . Stubble Mulch Tillage

Stubble mulch farming is a year-round system of managing plant residue. Stubble mulch tillage is performed with implements that undercut residue, loosen soil, and kill weeds. Because the soil is tilled as often as necessary to control weeds during the period between crops, the stubble mulch system is a tillage-

7


intensive system that requires frequent operations to control weeds. The system was developed primarily for wheat (Triticum aestivum L.) and other small grain crops, but is also adaptable to such crops as sorghum (Sorghum sp.). Good management of a stubble mulch farming system begins with harvest. To minimize tillage problems, crop residue should be spread uniformly by the combine. In the Great Plains, sweeps or blades are generally operated at the 12- to 15-cm depth during the first operation after harvest and shallower during subsequent operations. Weed control generally is best when the soil is dry at the time of tillage. In the dry-farming areas of the Pacific Northwest and at more humid locations where straw production by small grain is usually higher than in the Great Plains, the first operation is similar to that in the Great Plains, but the second operation may be deeper than the first to avoid serious plugging of the equipment by the residue. When unusually large amounts of residue are present, a disk-type implement may be used for the first operation to incorporate some of the residue with soil. This hastens decomposition, but still keeps enough residue on the surface for erosion control. Other implements that may be used in heavy residue are stubble pulverizers or busters (Jacks et al., 1955) and skewtreaders or spike-tooth harrows in conjunction with one-way disk plows (Papendick and Miller, 1977). Tillage implements that maintain surface residue are (1) s w e e p 6 0 cm or wider; (2) rodweeders with semichisels or small sweeps; (3) straight-blade machines; (4) chisel plows; ( 5 ) one-way plows (which generally should be used only when large amounts of residue are present); and (6) rodweeders. The amount of residue remaining on the surface after one operation with several tillage machines is shown in Table 11. The power needed for stubble mulch tillage depends on such factors as the type and size of machine; the depth and speed of operation; and the texture, water content, and slope of soil. Promersberger and Pratt (1958) showed that Table I1 Effect of Tillage Machines on Surface Residue Remaining after Each Operationa Tillage machine Subsurface cultivators Wide-blade cultivator and rodweeder Mixing-type cultivators Heavy-duty cultivator and other type machines Mixing and inverting disk machines One-way flexible disk harrow, one-way disk, tandem disk, offset disk Inverting machines Moldboard and inclined disk plow a From

Anderson ( 1968).

Residue maintained (%)

90 15

50

10

8


Table I11 Measured Average Diesel Fuel Consumption for Specific Field Operations on Pullman Clay h m , Bushland, Texas"

Operation Dryland Sweep Sweep Surface-irrigated Moldboard plow Heavy tandem disk Heavy offset disk Lister bedder Disk bedder Rolling cultivator Chisel, 38-cm spacing Chisel, 50-cm spacing Chisel, 100-cm spacing Sweep-rodweed (bed-furrow cultivation) Seeding Grain drill, 25-cm spacing

Tillage depth (cm)

Diesel fuel (liter)

8

6.1 8.4

13

20-25 8-13 8-13

15-20 15-20 15-20

28.1 9.4 11.7 6.5 8.4 5.1 14.0-16.8 12.2 7.5 7.9 3.7

" From Allen er al. (1977). moldboard plowing and field cultivating (with sweeps) a clay soil at a depth of 13 to 18 cm required 21.4 to 23.2 kW hours/ha (1 1.6 to 12.6 hp hourdacre) and 3.3 to 10 kW hours/ha, respectively. Operating a Noble blade 15 to 27 cm deep required 10.7 to 13.3 kW hours/ha. Allen et al. (1977) reported fuel consumption values for performing various field operations, including some that are used in stubble mulch systems, on a clay loam soil (Table 111). Subsurface tillage generally required less power or fuel than disk or chisel tillage, and substantially less than moldboard plowing. Detailed descriptions of implements used in stubble mulch farming systems were given by Fenster (1960), FA0 (1971), and Jacks et al. (1955). Many types and sizes of equipment are available for doing a satisfactory job of stubble mulching. The essential part of any stubble mulching system is to maintain enough residue on the surface to control erosion adequately from harvest to harvest.

2 . Reduced- and No-Tillage Systems One of the major reasons for tilling a soil is to control weeds. Hence, if weeds are controlled by herbicides, the need for tillage is reduced. The development of


9

effective herbicides in recent years has permitted the development of reduced- and no-tillage cropping systems. As with stubble mulch tillage, a major goal of these systems is to maintain crop residue on the surface for soil and water conservation. In some cases, however, the land is moldboard plowed, but the number of secondary operations is greatly reduced. The following are reduced- and no-tillage systems that have been evaluated in research trials and are currently used by some farmers. Additional information pertaining to these systems is given by Fisher and Lane (1973), Lewis (1973), Griffith et al. (1977), Amemiya (19771,Reicosky eral. (1977),Allen etal. (1980), andungerand Wiese(1979). a . Fall Plow, Field Cultivate. In this system, the moldboard plow is used for primary tillage, but secondary tillage is reduced to one shallow cultivation with sweeps at planting. A disk or rotary tiller may be substituted for the field cultivator to produce a finer, firmer seedbed, but it also leaves the soil more erodible. This system is widely used on the dark, nearly level, medium- and fine-textured clay loam soils of the east central Corn Belt. b. Spring Plow, Wheel-Track Plant. This system uses strip seedbed preparation on soil that was initially plowed 12 to 24 hours before planting. By planting soon after plowing, the soil water content is such that wheels break the clods and firm the seedbed. The planted rows may be in the tractor or planter wheel tracks. This system affords greater protection against erosion than fall plowing because crop residue is maintained on the surface until planting. c . Fall Chisel, Field Cultivate. This system is similar to the fall plow, field cultivate system, except that chiseling 20 to 25 cm deep replaces moldboard plowing. Chiseling retains more surface residue than moldboard plowing and, therefore, more effectively controls erosion. d . Disk and Plant. Tillage in this system is performed with standard tandem disks operated 8 to 10 cm deep, heavy disks operated 15 to 20 cm deep, or a combination of the two. The system usually includes one fall disking and one or more diskings before planting in spring. To conserve surface residue, disking should be delayed as long as feasible, and tandem disks rather than heavy disks should be used, because the heavy disks penetrate deeper and incorporate more residue than do tandem disks. e . Till-Plant. In this system, tillage and planting are done in one operation. Normally, tillage is with wide sweeps operated 5 to 8 cm deep on the ridgetop, which moves old stalks and root clumps into the area between rows and provides a trash-free zone for planting. The ridges were made the previous year during cultivation or after harvest with rolling or disk-hiller cultivators, large disk cultivators, or disk bedders (after harvest). Ridges may be re-formed annually in cases where heavy disks are used to cut residue and level old ridges, or they may be permanent, in which case the only tillage needed is for reshaping the ridges in the fall or spring with a rolling or disk-type cultivator. On soils in the southeast United States with compacted subsurface layers, two

10


machines have been developed to loosen the layers and plant seeds directly over the loosened zone. The first is the “ripper-hipper,” which subsoils the intended plant row and forms a ridge over the slit with hillers or bedders. Planting can be done in the same operation. The second machine is the subsoiler-planter, which has subsoilers to loosen the compacted layer, treading wheels to firm the loose soil in the slits, and flexible unit planters. Colters can be used with both machines to cut the surface residue. f . Strip Tillage. In a strip tillage system, only a narrow band of soil is tilled. Rotary tillers can be adapted for strip tillage by removing some of the knives. The tillage zone may be 20 cm wide and 5 to 10 cm deep. A standard planter can be attached to the tiller, resulting in a one-pass operation, because stalks can be chopped by the tiller. Another form of strip tillage is the opening of a slot for seed placement in previously untilled ground. The system, referred to as no-tillage, zero tillage, or slot planting, is used to plant in residue of previous crops or in chemically killed sod. Tillage usually is done with nonpowered, fluted colters running ahead of planters that have disk openers. Narrow chisels, angled disks, or straight or slightly rippled colters can also be used to open the soil for seed placement. A press wheel or seed packer wheel is needed for good seed-soil contact after planting. For proper operation of no-tillage planters, crop residue should be uniformly distributed on the soil surface and corn (Zea mays L.) or similar residue should be chopped before planting. The equipment and practices just described can be used in various combinations. Some other possibilities are combinations of stubble mulch tillage and herbicides (Phillips, 1969) and of conventional, reduced, and no-tillage (Allen et al., 1980; Unger and Wiese, 1979). Choice of system must consider the equipment available, soil and climatic conditions, size and type of farming operation, and the producer’s managerial ability and personal preferences (Giffith et a l . , 1977).

B . SEEDBED PREPARATION A N D CROPSEEDING

Regardless of crop or region, a firm, moist seedbed is desirable for rapid seed germination and seedling emergence. In regions with adequate precipitation, the seedbed normally is moist enough at planting for rapid seed germination and seedling emergence and for good plant growth. Under such conditions, continuous cropping is possible. In areas of limited precipitation, leaving land idle for a season to store enough water for a crop may be necessary. This practice is called summer fallowing. During fallow, the land must be kept free of weeds, and enough residue must be kept on the surface for erosion control.


11

I . Small Grains Stubble mulch tillage, either in a summer fallow or continuous cropping system, is widely used for weed control in the Great Plains and the Pacific Northwest where small grains are grown. With stubble mulch tillage, soil may need packing or smoothing to obtain a favorable seedbed. When packing is desirable, it can be done with a treader, which is similar to a rotary hoe with the tongues reversed so that the soil is packed rather than loosened, or by shallow operation of a sweep-rodweeder. The treaders also break heavy stubble and surface crusts, control small weeds, and have a tendency to break surface clods. Because clods are important for controlling wind erosion, treaders must be used with caution where small amounts of residue are on the surface. Successively shallower operation of rodweeders after the initial plowing causes the soil to be firm near the surface and, therefore, no further packing is required. In areas with favorable precipitation, soil packing generally is not necessary because of settling due to precipitation. A major requirement of seeding equipment is trouble-free operation under all surface residue conditions. Small grains can be successfully seeded with shovel-, hoe-, or disk-opener drills. Shovel- and hoe-opener drills work well for placing seeds in moist soil that is overlain by dry surface soil, and they also form ridges that help control wind erosion. These drills generally perform well when large amounts of residue are present because they have high clearance and staggered shanks that support the openers and seed spouts. Drills with disk openers ridge the soil less than those with shovel or hoe openers. They are, therefore, less satisfactory for planting through dry surface soil and for controlling erosion. The disk openers also tend to destroy surface clods, which further decreases their effectiveness for controlling wind erosion. Drills with wide-spaced (25 to 35 cm) disk openers perform satisfactorily in high residue situations and have been used for no-tillage seeding of wheat in wheat (Allen et al., 1976; Unger, 1978a) or corn (Musick et al., 1977) residue, and wheat or barley (Hordeum vulgare L.) in grain sorghum [Sorghum bicolor L. (Moench)] residue (Musick et a l . , 1972, 1977). Because soil penetration with disk openers may be limited under high residue conditions, irrigation or timely precipitation may be required for satisfactory germination and seedling emergence. Small grains can also be seeded with no-tillage drills (Papendick and Miller, 1977) or with one-way disk plows, moldboard plows, or similar implements with attached seeding equipment and press wheels (FAO, 1971; Stonebridge and Fletcher, 1973). Where plows are used, enough residue or clods should remain on the surface for effective wind erosion control. 2 . Row Crops The seedbed requirements for row crops, such as corn, soybeans (Glycine max L.), sorghum, and cotton (Gossypium hirsutum L.), are similar to those for small

12

P. W . UNGER A N D T. M. McCALLA

grains. Because the row spacing for row crops is usually wider than it is for small grains, a wider clean zone can be used for seeding row crops. Seeding can be done with planters equipped with furrow openers or with listers or similar implements operated at a shallow depth. The furrows should be deep enough to produce a clean, moist seedbed, but not deep enough to cover the residue between the rows. Various types of equipment for no-tillage planting of row crops have been developed in recent years. A major requirement of these planters is adequate penetration of the soil for satisfactory seed placement. A colter may be needed ahead of the planter to cut surface residue, but both unit planters with double-disk openers (Unger and Wiese, 1979) and grain drills with some of the seed spouts blocked (Musick et a l . , 1977) have been used successfully without colters. As for small grains, timely precipitation or irrigation after planting assured uniform germination and seedling emergence in cases where large amounts of surface residue restricted the penetration of soil by the planting unit. Where irrigation is not practiced and large amounts of surface residue are present, straight, rippled, or fluted colters are necessary for satisfactory soil penetration for seed placement. Straight colters require less equipment weight for soil penetration than do fluted colters, but fluted colters loosen the soil more and, therefore, have been widely used on no-tillage planters (Griffith et a l . , 1977; Amemiya, 1977; Pitts, 1978). Besides soil penetration, another major requirement of no-tillage planters is a provision for adequate seed-soil contact, which normally is provided by ribbed press wheels or seed packer wheels (Griffith et al., 1977; Pitts, 1978). The planting unit described by Pitts (1978), which has a subsoiler for loosening compacted layers in the soil, also has a spike-toothed wheel to fill the slot made by the subsoiler, which prevents seeds from falling too deeply into the soil.

3 . Grasses and Legumes The small seeds of many grasses and legumes require precise planting for adequate stand establishment. Grasses and legumes have been successfully seeded in stubble mulch systems by scattering seeds from a seeder box mounted ahead of a treader, which covered some of the seed. A preferable method is to use a drill, equipped with double-disk furrow openers and depth bands, that is capable of handling rough seeds and drilling through stubble. This method keeps the soil protected against erosion and maintains more favorable soil water conditions for germination and emergence. The no-tillage method has been successfully used for planting grasses and legumes at numerous locations (Bennett, 1977; Olsen et a l . , 1978; Papendick and Miller, 1977; Reicosky et a l . , 1977). In this system, tilling and seeding are done in one operation in chemically killed sod or residue from previous crops, thus affording excellent protection against erosion.


13

IV. CROP YIELDS AND QUALITY A. GRAINYIELDS

Results from early studies showed that wheat grain yields generally were higher with stubble mulch than with clean tillage in arid and semiarid regions, but lower in subhumid and humid regions (Zingg and Whitfield, 1957). Similar results have been reported for subsequent studies (Johnson and Davis, 1972; Smika, 1976a; Papendick and Miller, 1977). The higher yields resulted from increased water conservation with stubble mulch tillage. Possible reasons for the lower yields in more humid regions with stubble mulch tillage include greater nitrogen immobilization, fertility imbalances, difficulty with stand establishment, reduced seedling vigor, greater weed infestations, and release of phytotoxic decomposition products (Kimber, 1966; McCalla and Norstadt, 1974; Papendick and Miller, 1977). Much research concerning reduced- and no-tillage crop production systems has been conducted in recent years. While better erosion control and lower production costs have caused much of the interest in these systems, producers are not likely to adopt reduced- and no-tillage systems where there is a risk of lower yields, even though production costs are lower (Griffith et al., 1977). As a rule, grain yields were little affected by tillage practices under conditions of adequate soil water, favorable precipitation, and good drainage, provided other factors such as soil fertility, weed control, and plant populations were equal (Bennett, 1977; Reicosky et al., 1977; Amemiya, 1977; Unger et al., 1977; Griffith et al., 1977; Elliott et al., 1977; Rowel1 et al., 1977; La1 et al., 1978). Under conditions of limited soil water and limited precipitation or irrigation, crop yields were equal and often significantly higher with reduced- and no-tillage systems than with conventional tillage (Amemiya, 1977; Fenster, 1977; Unger et al., 1977; Unger and Wiese, 1979; Anderson, 1976; Wicks and Nordquist, 1976; Phillips et al., 1976; Wicks and Smika, 1973; Phillips, 1969). The higher yields with the reduced- and no-tillage systems generally were attributed to increased soil water contents resulting from increased infiltration, decreased runoff, and possibly decreased evaporation. Although workable reduced- and no-tillage systems have been developed for many crops at numerous locations, unfavorable results have been obtained in some cases. In general, yields often are lower with reduced- and no-tillage than with conventional tillage on poorly drained soils (Griffith et al., 1977), with continuous cropping when volunteer crop plants cause excessive plant populations (Allen et al., 1975), and where extremely large amounts of residue are present, as in the Pacific Northwest, where weed control, planting, soil fertility, disease, insect, and rodent problems have been encountered (Papendick and Miller, 1977).

14


B . PLANTPROTEINCONTENTA N D MINERALCOMFQSITION

Plant residue contains inorganic nutrients that are potentially available to subsequent crops as the residue decomposes. The rate of decomposition, however, is strongly influenced by tillage methods. Methods that retain residue on the surface result in slow decomposition and, therefore, slow release of nutrients to crops. Residue incorporation with soil generally increases the decomposition rate, but the nutrients may be initially immobilized, then subsequently released in forms available to plants as decomposition progresses. The rates of decomposition and application of fertilizers affect the amounts of nutrients available to plants and, therefore, the mineral composition of plants. Zingg and Whitfield (1 957) summarized the protein contents of wheat grain from seven locations in the Great Plains. At six of the locations, average protein contents for 52 years of data were 13.5 and 14.1% with stubble mulch and clean tillage, respectively. Similar effects of tillage were found by Bennett et al. (1954) in Utah, and Unger (unpublished data) in Texas. Even lower protein contents of grain were obtained for no-tillage wheat. The percentages were 18.4, 18.0, and 17.4 with disk, sweep, and no-tillage, respectively, for dryland wheat and 16.4, 16.1, and 15.9 with the respective tillages for imgated wheat (Unger, unpublished data). The same protein trends were found for sorghum grain in an irrigated wheat-dryland grain sorghum cropping system. Average values were 14.7, 13.9, and 13.5% with disk, sweep, and no-tillage, respectively (Unger and Wiese, 1979). Jacks et al. (1955) reported that the effect of mulches on mineral composition of plants depended on the type and amount of mulch, stage of decomposition of the mulch, soil and climatic conditions, and kind of plant. One of the responses to heavy mulching was a marked increase in the K content of soil, which resulted in increased K contents of plant tissues. Jacks et al. (1955) also showed that Ca, Mg, Mn, and P contents of plant tissues generally were higher on mulched than on bare soil. In contrast, Estes (1972) showed that Ca, Mg, Zn, Mo, B, and A1 concentrations in corn leaf tissue were significantly decreased and K concentrations were significantly increased under no-tillage as compared with conventional tillage at five levels of lime application. Concentrations of P, Fe, and Mn were not affected (Table IV). Changes in element concentration in tissues apparently were related to soil pH and method of lime application. Also, reduced Ca and Mg uptake possibly resulted from greater K uptake (Estes, 1972). Griffith et al. (1977) reported that no-tillage decreased plant uptake of Cu, Zn, B, and Mn, possibly because of reduced root systems, which caused less contact with soil, and because of cooler and wetter soils, which decreased the availability of the micronutrients early in the season. They also reported that plant K was deficient with shallow or no-tillage only under unusually wet conditions on poorly drained soils. Similar results were reported by Bower et al. (1944) and

15

CONSERVATION TlLLAGE SYSTEMS

Table IV Effect of Tillage Method on Nutrient Composition of Corn Ear Leaves' Tillage method Nutrient Phosphorous (%) Potassium (%) Calcium (S) Magnesium (%) Zinc (ppm) Iron (ppm) Molybdenum (ppm) Manganese (ppm) Boron (ppm) Aluminum (ppm) a

No-tillage

0.34 1.66 0.11

0.50 26.0 154.0 2.0 77.0 9.I 56.0

Conventional

0.34 1.53 0.85 0.59 29.0 161.0 3.2 84.0 10.2 60.0

% Change relative to conventional

0.0

+8.56 -9.4b - 15.2b - 10.3b -4.3 -37.5b -8.3 - 10.86 -6.7b

From Estes (1972). Denotes significant difference between tillage methods (P = 0.05).

Lawton and Browning (1948) with stubble mulching in Iowa. Reduced soil aeration was suggested as a possible cause for reduced K absorption. The generally higher soil and plant K concentration with surface residue apparently results from leaching of readily water-soluble K compounds from the residue. C. RESIDUEYIELDS

Zingg and Whitfield (1957) summarized the early data pertaining to straw production by wheat at six locations in the western United States. Wheat grown in rotation with corn, oats (Avena sativa L.), and sweet clover (Melilotus sp.) produced less straw with stubble mulch than with clean tillage. Similar trends occurred for continuous wheat, but straw production was higher with stubble mulch tillage than with plowing in wheat-fallow systems at some locations. The average data for 101 crop-year comparisons showed a 3.9% decrease in straw production with stubble mulch tillage as compared with plowing. On a strawgrain ratio basis, the averages were 1.87 for stubble mulching and 1.98 for plowing. The straw yields and straw-grain ratios varied with varieties, soil fertility, and growing conditions. At similar grain yields, some semidwarf wheat varieties yielded less and others yielded more residue than taller varieties (Bauer and Zubriski, 1978). Relatively few residue yield data as influenced by tillage methods have been reported in recent years, but for studies at Bushland, Texas (Johnson and Davis, 1972; Unger, 1977), wheat straw-grain ratios averaged 2.43 and 2.15 with clean

16


and stubble mulch tillage, respectively, for 24 crop-year comparisons. For dryland grain sorghum, the ratios were 1.32, 1.15, and 1.18 with disk, sweep, and no-tillage, respectively (Unger and Wiese, 1979). Limited data for inigated continuous grain sorghum showed opposite trends. The values were 1.70 and 2.42 with clean and no-tillage, respectively (Allen et af., 1975). The high residue yields relative to grain yields with no-tillage resulted from volunteer sorghum, which caused high forage yields, but plant populations that were too high for favorable grain yields. The ratio with clean tillage was also higher than those normally expected (1.0 to 1.2) for irrigated grain sorghum (Eck and Taylor, 1969). D. ROOTGROWTH

Jacks et al. (1955), from their review of the literature, concluded that plant roots tended to accumulate at or near the soil surface when large amounts of mulch were present. Although data concerning the influence of stubble mulch tillage on root development are not available, recent studies showed higher root concentrations near the surface with no-tillage than with clean tillage (Ellis et al., 1977; Griffith et af., 1977). Besides the rooting pattern, weight and size of roots are also affected by type of tillage. No-tillage and strip rotary tillage resulted in lower, and chisel plowing and wheel-track planting resulted in higher corn root weights than conventional tillage. Also, corn roots were larger in diameter with no-tillage, which caused them to have less absorbing surface for water and nutrient uptake per unit weight of roots. However, when the residue resulted in adequate water near the soil surface, there was little correlation between root weight differences due to tillage method and grain yields (Griffith et al., 1977). Apparently, the improved soil water conditions resulting from surface residue with no-tillage compensated for the decreased root weight and penetration depth that were noted.

V. ENVIRONMENTAL CONSIDERATION Much emphasis has been placed on control of air and water pollution in recent years. Because wind and water erosion are major contributors to air and water pollution, respectively, much effort has been directed toward developing improved erosion control techniques. The value of surface residue for erosion control has long been recognized. Recent results show that erosion control can be improved by maintaining crop residue on soil surfaces with reduced- and notillage cropping practices.


17

A. CONTROLOF WINDEROSION

The urgent need to control soil erosion by wind effectively in the late 1930s and early 1940s stimulated the development of the stubble mulch farming system. Because of the success of stubble mulch farming in the drier part of the Great Plains and the continued threat of wind erosion in many years, the stubble mulch system has become the basic tillage method in many dryland farming areas.

I. Factors That Influence Wind Erosion Soil erosion by wind can be a problem wherever the required conditions of soil, vegetation, and climate prevail. These conditions are (1) a soil that is loose, dry, and reasonably finely divided; (2) a smooth soil surface on which vegetative cover is absent or sparse; (3) a large enough field; and (4) wind that is strong enough to move soil (Skidmore and Siddoway, 1978). Sandy soils are extremely susceptible to wind erosion because of little or no coherence between particles, rapid drying after wetting, and small particle sizes. Other soils, however, are also subject to wind erosion when they are dry and loose, and when the soil particles have been finely divided by tillage, raindrop impact, or freezing and thawing. Particles greater than 0.84 mm in diameter are generally considered nonerodible by wind. The smoothness of soils and the amount of residue on the surface are strongly influenced by tillage methods. Operations that leave a rough, cloddy surface or that keep residue on the surface can help to minimize wind erosion. Examples are moldboard, lister, or chisel plowing, which leave the surface rough, and stubble mulch or no-tillage, which keep residue on the surface. The potential for wind erosion is increased by surface smoothing and residue-destroying operations, such as disking, harrowing, cultivating, and land planing. When used for secondary tillage, these operations can also reduce surface roughness initially formed by primary tillage such as moldboard plowing. The impact of raindrops, soil freezing and thawing, and erosion itself can further reduce surface roughness. Field width parallel to the wind direction has a major influence on soil erodibility, which increases as field width increases. Hence, width in the direction of prevailing winds should be kept as narrow as possible. Wind erosion occurs on some highly erodible soils that are only a few meters wide (Skidmore and Siddoway, 1978). With adequate surface residues, the effect of field width is minimized. Soil movement begins at relatively low wind speeds (Chepil and Woodruff, 1963) and progressively increases as wind speed and turbulence increase. Therefore, to minimize wind erosion, the wind speed at the soil-air interface must be reduced to the threshold value below which no significant wind erosion will

18


occur (Skidmore and Siddoway, 1978). The influence of wind on soil erosion is extremely complex and includes the processes of soil particle movement, transport, sorting, abrasion, avalanching, and deposition (Woodruff and Siddoway, 1973). Detailed discussions of the mechanics of soil erosion have been established by Bagnold (1943), Chepil and Woodruff (1957), and Zingg et al. (1965).

2 . Wind Erosion Equation A generalized equation expressing the relative quantity of wind erosion from a field was first published by Chepil (1959). As new data have become available, the equation has been modified and is now generally given as

E =f ( f C K L V )

(1)

where E is the potential annual quantity of erosion per unit area and is a function, f, of I, soil erodibility; C, local wind erosion climatic factor; K , soil surface roughness; L , equivalent width of field (maximum unsheltered distance across the field along the prevailing wind erosion direction); and V, equivalent quantity of vegetative cover (Chepil and Woodruff, 1963). The mathematical relationships among the components of the equation are complicated. The relationships, however, have been computed and developed into tables or plotted on graphs, and are useful for estimating annual soil losses by wind erosion and for determining alternate land treatments for wind erosion control. A guide containing this information for the Great Plains states is available (Craig and Turelle, 1964). Tillage has a direct bearing on factors 1, K , and V through its effect on soil cloddiness, soil roughness, and equivalent quantity of vegetative cover. 3. General Principles of Wind Erosion Control

Values for each of the individual primary factors that influence wind erosion must be determined before the potential soil loss can be estimated. The I factor is determined from the percentage of soil particles smaller than 0.84 mm in diameter as determined by dry sieving or from reference tables of known average cloddiness of different soils during the wind erosion season. The C factor for a particular location is estimated from the wind erosion climatic map. Appropriate values for I and C are available in the wind erosion control guide (Craig and Turelle, 1964). Factors K, L, and V are based on field conditions. With I and C determined, appropriate charts and tables in the guide pertaining to K , L, and V are used to determine the erosion potential. The charts and tables can also be used in reverse to determine what conditions or practices are needed to reduce wind erosion to a desired level (Chepil and Woodruff, 1963).


19

4 . Residue and Tillage Effects on Wind Erosion

Surface residue affects wind erosion primarily through influences related to factor V of the wind erosion equation. The effect of tillage is related mainly to factors I and K of the equation. a . Function of Residue. The principal function of surface residue is to decrease the force of wind on the soil itself. Zingg (1954) showed that different amounts, kinds, and arrangements of anchored crop residue in the field eliminated from 5 to 99% of the wind action on the soil surface. The forces that move erodible particles are reduced when the force of the wind is transferred to the residue. Wind erosion is eliminated when the forces at the soil-air interface are reduced to a threshold value below which erosion will not occur. Wind erosion is influenced by the kind, amount, texture, height, and orientation of surface residue. The value of small grain residue for controlling erosion is well known. The wind erosion control guide uses this kind of residue as the basic type and gives equivalents for other types of residue (Craig and Turelle, 1964). On an equal-weight basis, small grain residue is more effective than that of sorghum or corn, which in turn is more effective than that of cotton or soybeans (Woodruff and Siddoway, 1973). The differences result from the different densities of the materials. As residue amounts increase, erosion decreases; therefore, as much residue as possible, up to the limit beyond which no further reduction occurs, should be maintained on the surface for effective erosion control. The amounts required vary with location and soil type. For highly erosive soils, residue production may not be adequate for effective erosion control, especially with such crops as cotton or soybeans, and possibly with corn, sorghum, and even small grains. The texture, or fineness, of residue also influences erosion. For equal amounts, fine residue gives more protection than coarse residue when it is equally distributed and anchored in the soil. Harvesting, tillage, and other crop production operations may remove, flatten, shorten, shred, or redistribute surface residue and, therefore, influence its effectiveness for controlling erosion. In general, standing residue is about twice as effective as flattened residue for controlling erosion. Also, tall residue is more effective than short residue. Sorghum stubble about 30 cm tall is effective under most conditions if it is dense enough to cause wind to flow over it rather than through it. Leaves are important for increasing the density of stubble (McCalla and Army, 1961). Harvesting and tillage operations that retain large amounts of tall, erect residue on the surface are, therefore, most effective for controlling wind erosion. Stubble mulch tillage undercuts surface residue and maintains most of it on the surface. However, even this tillage method destroys surface residue when used repeatedly and when performed at relatively high speeds. The no-tillage system,

20

P. W. UNGER A N D T. M. McCALLA

with no soil disturbance other than that needed to plant the seeds, is especially effective for maintaining surface residue and, therefore, for controlling wind erosion. b. Function of Tillage. In the last section, the effectiveness of stubble mulch and no-tillage were discussed relative to maintaining residue on the surface. In some cases, not enough residue is produced for erosion control, even when all of it is kept on the surface. In other cases, weeds are controlled and seedbeds are prepared with tillage, which reduces or destroys surface residue and, therefore, leaves the soil susceptible to wind erosion. To control erosion under such conditions, soil should be kept in a rough, cloddy condition. Some surface roughness results from normal crop production operations. Roughness for controlling wind erosion is also obtained by tillage or planting operations that cover or replace some of the erodible soil particles with less erodible material, thus reducing wind drag on the remaining particles. There is, however, some evidence that increasing roughness increases wind turbulence and velocity fluctuations at the surface so that some of the benefits gained by roughening the surface are lost (Lyles et al., 1971). The most effective soil roughness height is 5 to 13 cm (Armbrust et al., 1964; Woodruff and Lyles, 1967). Tillage operations that minimize soil pulverization and smoothing are effective for maintaining surface roughness. Special planters, such as the deep furrow and hoe drills for planting small grains in surface residues, produce roughness in the 5- to 13-cm height range and, therefore, are especially effective in providing erosion-resistant surfaces (Woodruff and Siddoway, 1973). Reduced- and no-tillage systems, as compared with conventional tillage, provide greater protection against wind erosion because the surface can be kept rougher due to fewer tillage operations and more residue on the surface. Other factors being equal, soils at locations where the value for the climatic factor is high require more nonerodible clods (>0.84 mm) to control wind erosion effectively than such soils at locations where the climatic factor is low. The degree of cloddiness produced by tillage depends on soil texture, soil water content at tillage, tillage tool, and speed of operation (Woodruff and Siddoway, 1973). Sandy soils have low cohesiveness and, therefore, contain relatively few clods that resist wind erosion. However, if the surface soil contains more than about 8% clay, a cloddy surface that resists wind erosion can be produced by cultivation of sandy soils (Harper and Brensing, 1950). Plowing, cultivating, or planting sandy soils while they are moist also leaves the surface more resistant to wind erosion than when these operations are performed while the soils are dry. For soils with higher clay contents, enough cloddiness can generally be obtained when the operations are performed within a wide range of soil water contents. However, least cloddiness (>0.84 mm) of a silty clay loam soil in Kansas was obtained at an intermediate soil water content (about 20%). Cloddiness increased at lower and higher water contents with moldboard, sweep, and one-way tillage


21

(Lyles and Woodruff, 1962). Emergency chiseling of high-clay soils effectively reduces wind erosion by increasing surface cloddiness, even when these soils are quite dry. In stubble mulch studies, cloddiness generally was greatest with 5-cm-wide chisels and 80-cm-wide sweeps, followed in order by the one-way disk, rodweeder with shovels, and 2.75-m-wide sweeps. In a wheat-fallow rotation in Montana, the nonerodible fraction (>0.84 mm) of a sandy loam soil also increased as the amount of residue on the surface increased (Black, 1973). Similar results were obtained for a silt loam soil in Idaho (Woodruff and Siddoway, 1973) and a clay loam soil in Texas (Unger, unpublished data). For no-tillage, Unger et al. (1979) reported fewer large dry aggregates (B0.84 mm) with no-tillage than with disk or sweep tillage on Pullman clay loam (Torrertic Paleustoll) at Bushland. The soil, however, was protected against wind erosion by surface residue. B . CONTROL OF WATEREROSION

Water erosion is the dominant problem on about 72 million of the 172 million hectares of cropland in the 48 contiguous states of the United States (Hayes and Kimberlin, 1978). With clean tillage systems, water erosion may occur at any time on most soils, but the potential is generally greatest while the surface is bare after plowing, during seedbed preparation, and at seedling establishment. Conservation systems, which involve surface residue, are especially effective for controlling water erosion (Wischmeier, 1973; Hayes and Kimberlin, 1978). Soil erosion by water is a process of particle detachment and transport that requires energy. Both rainfall and flowing water (runoff) have detachment potential, but transport is mainly by runoff. At upslope positions, the energy is supplied mainly by rainfall and the slope gradient. On bare soil, most of the kinetic energy of raindrops is dissipated at the surface where the impacting drops detach soil particles. Splash action and shallow sheet flow transport many of the detached particles to runoff concentrations. Drop impact also disperses soil aggregates, reduces surface roughness, and promotes surface sealing and crusting, thereby increasing runoff (Wischmeier, 1973). As runoff increases, rill and finally gully erosion may occur. Gully erosion is the most obvious type, but sheet and rill erosion are responsible for the major part of water erosion on cropland (Hayes and Kimberlin, 1978). I . Factors That Influence Water Erosion

For effective water erosion control, seeding and tillage practices should decrease raindrop impact on the soil, increase water infiltration, decrease runoff velocity, and decrease soil detachability (Wischmeier, 1973). These factors are

22


influenced by intensity and duration of rainfall; steepness and length of soil slope; texture, organic matter content, roughness, and ridging of soil; amount, type, and distribution of surface residue; and type of erosion control practice (e.g., contouring, strip cropping, terracing). The factors influencing erosion have been studied extensively and reviews and guidelines pertaining to erosion control have been published by Hayes and Kimberlin ( 1978), Kimberlin ( 1976), Stewart et al. (1975), Wischmeier (1973), and Wischmeier and Smith (1978). The guidelines generally involve use of the universal soil loss equation, which helps to establish relationships between the amount of erosion and the factors influencing erosion. 2 . The Universal Soil Loss Equation (USLE) The universal soil loss equation is A

= RKLSCP

(2)

where A is computed soil loss per hectare; R , rainfall factor based on the number of erosion-index units in a normal year’s rainfall at a specific location; K, soil erodibility factor; L, length of slope factor; S, slope gradient factor; C, crop management factor; and P, erosion control practice factor. All factors are unitless, except A and K . Units for A are metric tondhectare (or tondacre) per year and those for K are metric todhectare (or todacre) per erosion index unit (Hayes and Kimberlin, 1978). Values for the factors of the equation are available for many conditions at numerous locations (Stewart et al., 1975).

3 . General Principles of Water Erosion Control To determine potential erosion at a specific location, a value for each factor of the USLE must be determined. The R factor is based on rainfall records at a location and, therefore, is fixed. The K , L, and S factors at a given location are also based on prevailing conditions and, therefore, are not subject to change without major soil alterations. Changing the value of the C factor, however, offers a major potential for reducing erosion. Crop management practices involved include tillage, rotations, and residue management practices. When potential erosion at a given location cannot be reduced to acceptable levels by crop management, then engineering-type erosion control practices, which affect factor P, must be used. In the remainder of this section, we discuss mainly practices that affect the C factor of the USLE. 4 . Residue and Tillage Effects on Water Erosion

The influence of crops, rotations, and management on factor C of the USLE is illustrated in Table V. In general, the value of C decreases as increasing amounts

23


Table V Generalized Values of the Crop Management Factor, C, in the 37 States East of the Rocky Mountains" C value Line no.

Crop, rotation, and management*

Base value: continuous fallow, tilled up and down slope

Productivity levelr HigL Moderate

.oo

1.oo

0.54

0.62 0.59 0.52 0.49 0.48 0.44 0.35 0.30 0.24 0.28 0.26 0.23 0.24 0.20 0.17

1

Corn C, RdR, fall TP, conv (1) C, RdR, spring TP, conv ( I ) C, RdL, fall TP, conv (1) C, RdR, wc seeding, spring TP, conv ( I ) C, RdL, standing, spring TP, conv (1) C, fall shred stalks, spring TP, conv (1) C (silage)-W(RDL, fall TP) (2) C, RdL, fall chisel, spring disk, 40-30% rc ( I ) C (silage), W wc seeding, no-till pl in c-k W (1) C (RDL)-W(RDL, spring TP) (2) C, fall shred stalks, chisel pl, 40-30% rc (1) C-C-C-W-M, RdL, TP for C, disk for W (5) C, RdL, strip till row zones, 55-408 rc (1) C-C-C-W-M-M, RdL, TP for C, disk for W (6) C-C-W-M, RdL, TP for C, disk for W (4) C, fall shred, no-till pl, 70-50% rc (1) C-C-W-M-M, RdL. TP for C, disk for W (5) C-C-C-W-M, RdL, no-till pl 2d & 3rd C (5) C-C-W-M, RdL, no-till pl 2d C (4) C, no-till pl in c-k wheat, 90-708 rc ( I ) C-C-C-W-M-M, no-till pl2d & 3rd C (6) C-W-M, RdL, TP for C, disk for W (3) C-C-W-M-M, RdL, no-till pl 2d C (5) C-W-M-M, RdL, TP for C, disk for W (4) C-W-M-M-M, RdL, TP for C, disk for W (5) C, no-till pl in c-k sod, 95-80% rc (1)

0.50 0.42 0.40 0.38 0.35 0.31 0.24 0.20 0.20 0.19 0.17 0.16 0.14 0.12 0.11 0.087 0.076 0.068 0.062 0.061 0.055 0.051 0.039 0.032 0.017

Cottond 27 28

Cot, conv (Western Plains) (1) Cot, conv (South) (1)

0.42 0.34

0.49 0.40

Meadow 29 30 31

Grass & legume mix Alfalfa, lespedeza, or Sericia Sweet clover

0.004 0.020 0.025

0.01

1

2 3 4 5 6 7 8 9 10 11

12 13 14 15

16 17 18 19 20 21 22 23 24 25 26

0.18

0.14 0.13 0.11 0.14 0.11 0.095 0.094 0.074 0.061 0.053

(continued)

24 Table V

P. W. UNGER AND T. M. McCALLA (continued)

C value

Line no.

Crop, rotation, and managementb

Sorghum, Grain (Western Plains)d 32 RdL, spring TP, conv (1) 33 No-till pl in shredded 70-50% rc

Productivity levelC High Moderate

0.43

0.11

0.53 0.18

B. RdL, spring TP, conv (1) C-B, TP annually, conv (2) B, no-till pl C-B, no-till pl. fall shred C stalks (2)

0.48 0.43 0.22 0.18

0.54 0.51 0.28 0.22

W-F, fall TP after W (2) W-F. stubble mulch, 560 kg rc (2) W-F, stubble mulch, 1120 kg rc (2) Spring W, RdL, Sept TP, conv (N & S Dak)( I ) Winter W, RdL, Aug TP, conv (Kans) (1) Spring W, stubble mulch, 840 kg rc (1) Spring W, stubble mulch, 1400 kg rc (1) Winter W, stubble mulch, 840 kg rc (1) Winter W, stubble mulch, 1400 kg rc (1) W-M, conv (2) W-M-M, conv (3) W-M-M-M, conv (4)

0.38 0.32

Soybeansd 34 35 36 37

Wheat 38 39 40 41 42 43 44 45 46 47 48 49

0.21 0.23 0.19 0.15 0.12

0.11 0.10 0.05 0.026

0.021

From Stewart et al. (1975). This table is for illustrative purposes only and is not a complete list of cropping systems or potential practices. Values of C differ with rainfall pattern and planting dates. These generalized values show approximately the relative erosion-reducing effectiveness of various crop systems, but locationally derived C values should be used for conservation planning at the field level. Tables of local values are available from the Soil Conservation Service. Abbreviations used are defined as follows: B-soybeans; C--corn; c-k--chemically killed; convconventional; cot--cotton; F-fallow; M-grass-and-legume hay; pl-plant; W-wheat; wc-winter cover; kg rc-kilograms of crop residue per hectare remaining on surface after new crop seeding; % rc-percentage of soil surface covered by residue mulch after new crop seeding; 70-50% rc70% cover for C values in first column, 50% for second column; RdR-residues (corn stover, straw, etc.) removed or burned; RdL-all residues left on field (on surface or incorporated); TP-turn plowed (upper 13 or more cm of soil inverted, covering residues). bNumbers in parentheses indicate number of years in the rotation cycle. No. (1) designates a continuous one-crop system. High level is exemplified by long-term yield averages greater than 4700 kg corn or 6.7 metric tons grass-and-legume hay; or cotton management that regularly provides good stands and growth. dGrain sorghum, soybeans, or cotton may be substituted for corn in lines 12. 14, 15, 17-19, 21-25 to estimate C values for sod-based rotations.


25

of residue are maintained on the soil surface for increasing amounts of time during the crop production cycle. a . Function of Residue. The amount of crop residue available and how it is managed influences the erosion control effectiveness of tillage systems. Surface residue dissipates raindrop energy, thus decreasing soil detachment, surface sealing, crusting, and, in turn, runoff. Soil detachment and subsequent erosion are further decreased by surface residue because the residue decreases the shear stress exerted on the soil by runoff (Wischmeier, 1973). The residue serves as dams that slow the flow rate of water across the surface. 1. Residue rate. For maximum effectiveness, the soil surface should be completely covered with residue. Light-weight, hollow-stemmed materials, such as small grain residue, provide greater coverage per unit of weight than materials such as corn or sorghum stubble. Greater than 95% coverage is provided by about 5.6 metric tons/ha of small grain straw or about 8.2 metric tons/ha of chopped cornstalks (Wischmeier, 1973). The effect of wheat straw mulch rate and soil slope on water and soil losses during simulated rainfall is given in Table VI. Water and soil losses decreased as mulch rates increased. Soil slopes had little or no effect on water losses, but soil losses increased as the percent slope increased (Lattanzi et al., 1974). Similar results, reported by Wischmeier (1973), indicated that even the low rates of mulch kept infiltration rates relatively high. Runoff velocities with 1 . 1 and 2.2 metric tons/ha of wheat straw were about 50 and 33% of those with no mulch. The decreased runoff velocities apparently greatly lowered soil losses as mulch rates increased. 2 . Residue type. On Miami silt loam (Typic Hapludalf) with 5% slope in Indiana, soybean and wheat residue left on plots at harvest were equally effective in controlling erosion when compared on a dry-weight basis in late autumn. In contrast, 4.5 metric tons/ha of corn stover was only half as effective as an equal weight of wheat straw. At higher mulch rates, the differences among the materials decreased (Wischmeier, 1973). 3. Mulch distribution. Wischmeier (1973) also measured the influence of residue distribution on the silt loam soil in Indiana. Mulch in rows across the slope, with two-thirds of the area bare in alternate strips, was as effective in controlling erosion as uniform distribution of the same amount of residue over the entire area. Much of the soil detached from bare strips was deposited in the mulched strips, which showed that strip tillage can control erosion if the residue strips are on the contour. b. Function of Tillage. Conservation tillage systems, such as no-tillage and till planting, retain all or most crop residue on the surface. They are especially effective in controlling erosion by water. Numerous studies concerning the effect of these systems on erosion have been conducted, but we will show only two

Table Vi Water Loss by Runoff and Soil Loss in Runoff during Initial, Wet, and Very Wet Runs"

Rainapplied

Initial run, 60 minutes

Wet runr 30 minutes

Very wet runC 30 minutes

Mulch rate (metric todha)

0 0.5 2 8 0 0.5

2 8 0 0.5 L

8

Soil loss (g/m*)

Water loss (kg/mz)

S = 6%

S = 12%

S = 20%

S =2%

49.3 50.9 43.5 2.5 28.6 29.9 27.7 2.5 28.4 30.8 28.9 4.0

52.7 52.4 43.7 4.3 28.9 29.8 28.1 5.0 29.3 30.8 28.1 6.4

53.0

49.2 52.4

950 600 240 7 410 260 90 2

1230 750 260 10 550 370 130

400

560 380 120 5

51.1 44.0 4.1 28.2 29.5 27.2 9.3 29.1 30.1 28.2 12.5 ~~~

44.1

5.7 27.9 30.0 27.3 7.1 28.1 30.6 29.3 9.1

260 80 1 ~~~

Rain intensity was 6.4 c d h o u r . From Lattanzi er al. (1974). b S = slope. 'Wet run made 1 day after initial run; very wet run made 15 minutes after wet run. a

S =6%

Sb = 2%

5

S = 12%

S =20%

1870 970 3 10 6 770 480 150 3 720 480 150 3

2140 1250 490 1

L

820 520 210 8 810 540 220 5

27


examples, which show the tremendous potential of these systems for controlling erosion. Harrold and Edwards (1972) measured rainfall, runoff, and sediment yield on three watersheds for a storm near Coshocton, Ohio, having an expected recurrence frequency of over 100 years. More than 12.7 cm of rain fell in 7 hours. Corn was grown on all watersheds. Rainfall was identical and the slopes were similar for clean-tilled watersheds with sloping or contour rows (Table VII), but runoff and sediment yield from the contoured watershed were only 52 and 14%, respectively, of that from the sloping-row watershed. No-tillage (corn planted in sod) with contour rows resulted in 57 and 0.1% runoff and sediment yield, respectively, of that from the sloping-row watershed, even though the slope was much greater on the no-tillage watershed. Onstad ( 1972) in South Dakota, measured runoff and soil losses from plots on Egan and Wentworth silty clay loams (Udic Haplustolls) having about 6% slopes. Although average runoff, based on either the actual amount or percentage of rainfall, was lower than at locations with more rainfall, the results showed the value of surface residue and improved tillage practices in decreasing runoff and soil losses (Table VIII). Surface conditions other than residue also influence the effectiveness of tillage practices in controlling erosion. These include the amount of residue incorporated, surface roughness, surface ridging, and the portion of surface disturbed (Wischmeier, 1973). Residue mixed with the surface soil by chiseling or disking is less effective than residue on the surface, but incorporation is better than removal because the incorporated residue tends to increase infiltration and decrease runoff and, hence, erosion. Wischmeier and Smith (1965) showed 40% less runoff for conservation tillage corn systems where the residue was incorporated by plowing than where it was removed at harvest. Soil loss was reduced about 12% for each 2.2 metric tons/ha (1 todacre) of corn residue Table VII Runoff and Sediment Yield from Corn Watersheds at Coshocton, Ohio, during a Severe Rainstorm on 5 July 1%9"

Tillage

(a)

Rainfall (cm)

Runoff (cm)

Sediment yield (kdha)

Plowed, clean-tilled sloping rows Plowed, clean-tilled contour rows No-tillage, contour rows

6.6

14.0

11.2

50,700

5.8

14.0

5.8

7,200

20.7

12.9

6.4

70

Slope

(I

From H m l d and Edwards (1972).

28


Table VIII Average Rainfall, Runoff, and Soil Loss on Tillage Plots at Madison, South Dakota" Average values (1965 to 1970)

Tillage practice

Rainfall (cm)

Fallow Conventional Mulch Till-plant (with slope) Till-plant (on contour)

42.0 42.0 42.0 42.0 42.0

Runoff (cm)

Soil loss (todha)

5.0 2.9 2.4 2.1

17.5 6.0 3.7 3.5 0.9

ab b c c 1.0 d

ab b c c d

From Onstad (1972). bValues within a column followed by the same letter are not significantly different (Duncan Multiple Range Test, 5% level).

incorporated (Wischmeier and Smith, 1978); therefore, the value of incorporated residue as compared with removed residue for erosion control is obvious. On soils where the amount of surface residue is limited, tillage-induced roughness and cloddiness can increase infiltration, reduce runoff velocity, and thereby reduce the potential for soil loss. Because high-intensity rains early in the season rapidly decrease the roughness and cloddiness of bare soils, the resulting soil crust must be broken by cultivation to enhance subsequent infiltration (Wischmeier, 1973). As shown in Table VII, contour ridges or furrows greatly reduce runoff and soil losses as compared with ridges with the slope. Similar decreases are possible with graded furrows (Richardson et al., 1969). A major factor involved is runoff velocity. On sloping rows, the rapidly flowing water readily transports detached soil particles down the slope, whereas on contour or graded rows, the runoff velocity is much lower. Runoff velocity and erosion can be decreased, even on sloping rows, when residue is maintained in the furrows (Kramer and Meyer, 1969; Meyer and Mannering, 1963; Mannering and Meyer, 1961; Taylor et al., 1964). Any tillage operation, even stubble mulch tillage which undercuts the surface, disturbs the surface and reduces the amount of surface covered by residue. Because soil detachability is inversely related to surface cover, it presumably increases with tillage, thus increasing the potential for soil loss. Whereas soil loosening by tillage increases infiltration as compared with no-tillage (Wischmeier, 1973), erosion can be kept to a minimum if contour plowing is used in a strip tillage system. This allows the strips of undisturbed residue to trap soil detached from the tilled zone.

29


C. RUNOFFWATERQUALITY

The present concern for clean water demands that pollutants be kept from entering surface and ground waters. Sediment, which is the end product of soil erosion, is by volume the largest single pollutant of surface waters. It is also the principal carrier of some chemical pollutants (Stewart et al., 1975). Hence, decreases in erosion by water also decrease the pollution of surface waters by sediment and some chemicals. Water erosion control was discussed in Section V, B. The same principles that apply to controlling erosion also apply to controlling pollution. However, all soil lost from a given location does not necessarily enter downstream water because much of it may be deposited before it enters the streams. Stewart et al. (1975) roughly estimated sediment delivery ratios for drainage areas of different sizes (Table IX), but they recognized that soil texture, relief, type of erosion, sediment transport system, and areas of deposition within the watershed would all influence the amount of sediment delivered to the downstream waters. The inability to predict accurately the transport of pollutants from fields to downstream bodies of water is recognized as one of the greatest problems in recommending specific control practices for a given site (Frere et al., 1977). Accurate prediction or even measurement of pollutant transport from fields is not possible; therefore, we will present only qualitative data on runoff water quality. The transport of sediment depends primarily on the volume and velocity of water flow. When the velocity is reduced, the transport capacity is also reduced. Any sediment in excess of the reduced capacity settles out. Because larger and heavier particles settle out first, the remaining sediment has a higher percentage of fine particles. The finer material has a higher capacity per unit of sediment to absorb such chemicals as phosphorus and pesticides. Also, light-weight organic Table IX Influence of Drainage Area on Sediment Delivery Ration ~~

~~

Drainage area Square kilometers I .3 2.6 13.0 26.0 130.0 260.0 518.0

Square miles 0.5 1 .o

5.0 10.0

50.0 100.0 200.0

From Stewart et al. (1975).

Sediment delivery ratio 0.33 0.30 0.22 0.18 0.12 0.10 0.08

30


materials tend to be associated with the fine particles. The transported sediment that reaches downstream bodies of water, therefore, contains more clays, organic matter, nutrients, and pesticides than the original field soil (Frere, 1976; Frere et al., 1977).

VI. INFILTRATION AND WATER CONSERVATION A major dryland crop production goal in subhumid and semiarid regions is to store enough water in the soil between crops so that the subsequent crop will not be too severely stressed for water and, therefore, will produce a favorable yield. Even in more humid regions, storage of additional water in soil is beneficial for alleviating the adverse effects of short-term droughts. To conserve water, the water that would normally be lost by runoff must infiltrate into the soil and it must then be protected against loss by evaporation or use by weeds. Weed control is discussed in Section VII. At some locations, drainage of excess water is an important production practice, but drainage will not be discussed in this report. A. RUNOFFAND INFILTRATION

The processes of runoff and infiltration are closely related in that water that infiltrates a soil is prevented from leaving it as runoff. However, runoff reduction or even prevention does not necessarily mean that the water will infiltrate into the soil, because the water may be lost by evaporation. Some results pertaining to runoff were included in the discussion of water erosion control (Section V, B) (Tables VI, VII, VIII). The large decreases in runoff with conservation tillage result from surface residue, which dissipates the energy of falling raindrops, decreases dispersion and surface sealing, and increases infiltration. Surface mulches decreased runoff under such widely different conditions as found in the high rainfall areas of the tropics (Barnett et al., 1972; Rockwood and Lal, 1974) and the southeastern United States (Batchelder and Jones, 1972), and the low rainfall area of the U.S. Great Plains (Onstad, 1972). Some effects of residue on runoff were shown in Section V, B. Hence, only limited additional data will be used to illustrate the effects of residue on runoff and infiltration. In the tropics (Nigeria), Rockwood and La1 (1974) measured much lower runoff and soil losses from no-tillage areas than from bare-fallow and plowed areas. In Puerto Rico, Barnett et al. (1972) applied artificial rainfall and measured runoff from three soils with different slopes (Table X). Cropping treatment

31


Table X Cropping Effects on Rainfall Runoff fkom Three Soils in Puerto R i d Rainfall' (cm) Cropping treatmentb Fallow

Tobacco Conventional tillage

Mulch tillage

Grass strips

Pangola grass Full sod Tops removed

" From Barnett ef

Runoff (cm)

Slope Soil

(%)

1

2

3

1

2

3

Humatas clay Juncos silty clay Pandura sandy loam

35 32 26

6.4 6.5 6.6

13.0 12.4 13.0

19.4 18.9 19.7

0.0 4.4 0.5

7.2 13.0 5.8

7.2 17.4 6.3

Humatas clay Juncos silty clay Pandura sandy loam Humatas clay Juncos silty clay Pandura sandy loam Humatas clay Juncos silty clay Pandura sandy loam

38 33 28 38 34 26 37 32 29

6.0 6.6 7.2 6.0 6.2 6.5 6.4 6.2 6.8

11.9 12.6 12.9 11.9 12.3 13.1 13.0 12.6 12.5

18.0 19.2 20.1 18.0 18.6 19.6 19.4 18.8 19.3

1.4 4.0 0.2 1.4 3.6 0.0 1.1 3.8 0.1

8.2 11.6 4.8 9.9 11.5 2.9 10.6 11.0 3.2

15.6 5.1 11.2 15.0 2.9 11.7 14.8 3.2

Humatas clay Humatas clay

39 46

6.0 6.0

12.7 11.9

18.7 17.9

0.1 1.7

0.7 9.1

0.7 10.8

9.6

al. (1972).

Conventional tillage was plowing, smoothing, and planting on contour; mulch tillage was planting in sod; grass strips were one row wide between three clean-tilled rows; and Pangola grass conditions were as shown. Rainfall was applied at 6.35 cm/hour for 60 minutes (Storm 1); 12.7 c d h o u r for 60 minutes (Storm 2) beginning 10 minutes after Storm 1. Results for Storm 3 are the sum for Storms I and 2.

did not greatly affect runoff on Humatas clay (Typic Tropohumult), apparently because water conductivity through this fine-textured soil was lower than the potential infiltration rate. On Pandura sandy loam (Typic Eutropept), mulch tillage and grass strips greatly reduced runoff. According to Barnett et al. (1972), the high runoff from Juncos silty clay (Vertic Eutropept) is misleading because the rainfall entered the soil, but returned as interflow at the lower end of the plots. All soils were highly aggregated and remained so throughout all storms. The imposed rainstorms had occurrence frequencies of 2 years (Storm l), 50 to 100 years (Storm 2), and 150 to 500 years (Storm 3). Batchelder and Jones (1972) measured rainfall and irrigation-water runoff from topsoil, exposed subsoil, and mulched, exposed, subsoil plots on Groseclose clay loam (Typic Hapludult) in Virginia from 1966 to 1968 (Table XI). Except in 1966, runoff always was lowest from the mulched subsoil. In 1966, irrigation runoff was higher from mulched than from unmulched subsoil. This indicated that the mulched subsoil plots were excessively irrigated. In subsequent

32

P. W. UNGER AND T. M. McCALLA Table XI Soil Management Effect on Runoff from Topsoil and Exposed Subsoil"

Topsoilb Dates

Factor

1,

I0

9 May-20 Oct. 1966

Rainfall Rainfall runoff Irrigation Irrigation runoff Rainfall Rainfall runoff Imgation Irrigation runoff Rainfall Rainfall runoff Irrigation Irrigation runoff

48.1 11.8 34.1

48.7

24 May-3 1 Oct. 1961

10 May-25 Oct. 1968

5.5

31.5 9.5 19.8 5.9 49.9 12.4 17.1

3.1

12.6 0 0 31.5 11.5

0 0 49.9 16.6 0 0

Subsoilb 1,

48.7 11.3 34.1 2.2 31.5 1.9 11.5 2.2 49.9 10.9 14.2 2.0

I0

48.7 15.8 0 0 31.5 9.1 0 0 49.9 13.7 0 0

Mulched subsoilb 1,

I,

48.7 10.4 34.1 3.4 31.5 1.9 12.5 0.3 49.9 1.2 6.2 0.1

48.1 2.7 0 0 31.5 0.3 0 0 49.9 0.5 0 0

From Batchelder and Jones (1912). Values given in centimeters. bI, and I, refer to imgated and nonirrigated plots, respectively.

years when irrigation amounts were adjusted, the mulch effectively controlled runoff. Mulches greatly decreased runoff in several seedbed preparation studies by Hays (1961) and Taylor et al. (1964) in Wisconsin. Wheel-track planting or mulch planting (field cultivator) for corn after hay decreased runoff about 50% as compared with normal seedbed preparation. A stover mulch in corn after corn decreased runoff to less than it was from corn in a rotation on soils at La Crosse and Madison. Runoff from small grain was lower with field cultivation than with fall plowing. Even less runoff occurred with mulching and field cultivation (Table XII). In all studies, yields were not greatly affected by seedbed preparation techniques, but runoff control greatly reduced soil loss. Under furrow-irrigatedconditions on Pullman clay loam at Bushland, Allen et al. (1975) evaluated rainfall and irrigation water runoff from clean- and notillage areas cropped continusouly to grain sorghum. For four irrigations in 1971 totaling 36.4 cm, runoff totaled 9.7 and 4.1 cm from clean- and no-tillage areas, respectively. For the fifth irrigation, when 6.3 cm of rain fell immediately after application of 8.2 cm of water, runoff from the respective areas totaled 5.8 and 4.6 cm. Less runoff from the no-tillage areas was attributed to slower water advance due to residue in the furrows, which allowed deeper penetration of the water than in clean-tillage furrows. The additional water caused greater plant growth, but yields were not increased because uncontrolled volunteer sorghum plants raised plant populations to levels above the optimum for grain production.

33


Table XI1 Tillage Effects on Runoff and Crop Yields" Soil, period, crop seedbed preparation Fayette silt loam, 15% slope, 1955-1959 average (corn) Seedbed-Corn after hay-Normal Seedbed-Corn after hay-Wheel-track Seedbed-Corn after hay-Mulch Fayette silt loam, Lacrosse, 1954- 1959 average (Corn) Corn in corn-grain-hay rotation Corn after corn and corn stover mulch Miami silt loam, Madison, 1955-1959 average (Corn) Corn in corn-grain-hay rotation Corn after corn and corn stover mulch Fayette silt loam, 15% slope, 1957-1959 average (Small grain) Fall plowed Field cultivated Corn stover mulched plus field cultivated

Runoff (cm) .

Yield (kg/ha)

2.1 0.9 1.1

61 10 5440 5950

1.5 0.3

7530 7880

2.0 0.2

4930 4770

9.0 7.2 4.8

5720 5240 5540

"From Hays (1961).

B. EVAFQRATION

In many areas, evaporation accounts for the major loss of water from agricultural soils. In the Great Plains, for example, about 60% of the 50 cm of average annual precipitation is lost directly from soil by evaporation (Bertrand, 1966). Water evaporates from the soil surfaces after precipitation, but before it enters the soil; as the surface dries; and from within the soil, especially before plant canopies completely cover the surface. As plant canopies develop, evaporation decreases and transpiration increases. Soil water evaporation occurs in three stages (Lemon, 1956). In the first stage, evaporation is rapid and steady, and depends on the net effects of water transmission to the surface and the aboveground conditions, such as wind speed, temperature, relative humidity, and radiant energy. During the second stage, evaporation decreases rapidly as the soil water supply decreases. Soil factors control the rate of water movement to the surface, and aboveground factors have little influence. Evaporation during the third stage is extremely slow and is controlled by adsorptive forces at the liquid-solid interface. According to Lemon (1956), the greatest potential for decreasing soil water evaporation lies within the first two stages. Potential methods include (1) decreasing turbulent transfer of water vapor to the atmosphere; (2) decreasing capillary continuity; and (3) decreasing capillary flow and water-holding capacity of surface soil layers. Crop residue has been studied as a potential mulch for decreasing evaporation. Mulches greatly affect first-stage evaporation (Bond and Willis, 1969; Unger,

34


1976; Unger and Parker, 1976), but the long-term effect of mulches on evaporation is difficult to establish because of their interacting influences on water infiltration, distribution in soil, and subsequent evaporation. Consequently, higher soil water contents resulting from surface mulches may be due to lower evaporation, but water infiltration and distribution in soil may be involved also, especially in field studies where the researcher has little control over soil wetting by precipitation. In field studies in Colorado, Montana, and Nebraska, precipitation storage as soil water during fallow was 16% with no residue and 34% with 11 metric tons/ha of wheat straw on the surface (Greb ef af., 1967). When Unger (1978a) placed wheat straw on Pullman clay loam at Bushland, precipitation storage during fallow ranged from 23% with no mulch to 46% with 12 metric tons of mulch/ha. Dryland grain sorghum grown after fallow yielded 1780 and 3990 kg/ha with the 0 and 12 metric todha mulch treatments, respectively. Unger and Wiese (1979) used no-, sweep, and disk tillage for residue management and weed control from wheat harvest until sorghum planting in an irrigated wheatdryland grain sorghum cropping system. Precipitation storage, sorghum grain yields, and water-use efficiency were highest with no-tillage and lowest with disk tillage (Table XIII). Even though factors besides evaporation control undoubtedly were involved, these studies showed that residue from high-residue producing crops, when maintained on soil as a mulch, can greatly increase precipitation storage and crop yields in areas where water for crop production is limited. The thicker the mulch, the more effectively it decreases evaporation (Bond and Willis, 1969; Hanks and Woodruff, 1958; Unger and Parker, 1976). Because material density greatly influences the thickness obtained with a given weight of material, low-density materials, such as wheat straw, more effectively decrease evaporation than sorghum stubble or cotton stalks, which are more dense. For Table XI11 Effect of Tillage Method on Average Precipitation Storage, Sorghum Grain Yield, and Water-Use Emciency for the Sorghum Crop in an Irrigated Wheat-Dryland Grain Sorghum Cropping Systema ~~~~~~

Tillage method Factor

No-tiliage

Sweep

Disk

Precipitationb storage (%) Grain yield (kg/ha) Water-use efficiency (kdha-cm)

35 3140 89

23 2500 77

15 1930 66

From Unger and Wiese (1979). Precipitation averaged 34.8 cm during fallow and 26.4 cm during the growing season.


35

similar evaporation decreases, about two and four times as much sorghum stubble and cotton stalks, respectively, were needed as compared with wheat straw on a weight basis (Unger and Parker, 1976). Crop residue mulches effectively decrease evaporation during the first stage (Bond and Willis, 1969; Unger, 1976; Unger and Parker, 1976). However, for maximum water conservation over long periods, either enough water must be added to penetrate deeply into the soil profile or large amounts of residue must be present (Bond and Willis, 1971; Gardner and Gardner, 1969; Unger, 1976). Because the water content near the surface of mulched soil often is higher than that of bare soil, especially soon after water additions, mulches are very useful for improving seedling establishment (Army et al., 1961; Bertrand, 1966; Smika, 1976b).

VII. WEED CONTROL Weeds compete with crops for water, nutrients, and light; therefore, effective weed control is essential if crops are to produce maximum yields under the prevailing environmental conditions. The technology of weed control uses cultural methods and herbicides, either singly or in various combinations, to prevent weed seedling establishment and to eliminate those seedlings or plants that have survived the initial control measures. Best weed control is obtained when differences in the biological characteristics of crops and competing weeds are exploited (Wiese and Staniforth, 1973). A. PROBLEM AREAS

The life history of weedy plants has a major effect on methods used to control them in a given crop production system. Weeds may be annuals, biennials, or perennials. Annual weeds can be either summer or winter annuals. Summer annuals, whose seeds germinate during warm weather, often have growth habits similar to summer crops. Troublesome summer annuals include barnyard grass [Echinochloa crusgalfi (L.) Beauv.], crabgrass [Digitaria sanguinalis (L.) Scop.], sandbur (Cenchrus), pigweed (Amaranthus sp.), and green foxtail (Setaria viridis L . ) . Volunteer crop plants of summer and winter annuals may cause problems in succeeding crops when the crops are grown continuously. Winter annual weeds have life cycles similar to fall-planted small grains. Especially troublesome in winter wheat fields are cheatgrass (Bromus secalinus L.), hairy chess (Bromus cornmutatus Schrad.), downy brome (Bromus tectorum

36


L.), tansy mustard [Descurainia pinnata (Walt.) Britt.], and henbit (Larniurn urnplexicaule L.). Biennial weeds live more than 1 year but less than 2. Some common biennials are wild carrot (Daucus carota L.), burdock (Arctiurn sp.), common mullein (Verbascum thapsus L.), and bull thistle [Cirsiurn vulgare (Sair) Tenore], but they are not considered to be especially troublesome (Wiese and Staniforth, 1973). Perennial weeds may spread by seeds, rhizomes, bulbs, tubers, or stolons, and once established, they compete vigorously with most annual crops and may be difficult to control. Weed species differ in different sections of the country, but some of the most troublesome perennial weeds in the United States are Johnson grass [Sorghum halepense (L.) Pers.], quack grass [Agropyron repens (L.) Beauv.], nutsedge (Cypersus sp.), field bindweed (Convolvulus arvensis L.), leafy spurge (Euphorbia esula L.), perennial sow thistle (Sonchus arvensis L.), Bermuda grass [Cynodon dactylon (L.) Pers.], Canada thistle [ Cirsium arvense (L.) Scop.], horse nettle (Solanurn carolinense L.), silverleaf nightshade (Solanum elaeagnifolium Cav.), Russian knapweed (Centaurea repens L.), and woollyleaf bursage (Franseria tomentosa Gray) (Wiese and Staniforth, 1973). B . CONTROL WITH TILLAGE

Tillage helps control weeds by ( 1 ) killing emerging seedlings; (2) burying weed seeds and delaying growth of perennial weeds; (3) leaving a rough surface to hinder weed seed germination; (4) providing enough loose soil at the surface to permit effective cultivation; ( 5 ) leaving a clean uniform surface for efficient action of herbicides; and (6) incorporating herbicides when necessary (Richey et al., 1977). Different tillage methods affect soils differently; therefore, they also affect the degree of weed control obtained with their use. Some weeds can be controlled with clean tillage methods involving moldboard plowing or disking (one-way, tandem, offset), but these tillage methods either bury or mix crop residue with soil, thus increasing the potential for erosion and decreasing the potential for water conservation. Alternate tillage systems that retain varying amounts of residue on the surface include chisel, sweep or blade, till-plant, and no-tillage. Normally, cultivation is required for growing season weed control unless herbicides are used, as they are in the no-tillage system. Chisels are widely used to loosen soils while retaining surface residue, but weed control is difficult with chisels because only narrow bands in soil are disturbed. Better weed control is generally obtained with stubble mulch tillage (sweeps, blades, etc.) than with chisels because the entire surface is undercut, thereby severing the deep roots of weeds. Stubble mulch tillage, however, is less effective than plowing because seeds are not buried and the soil and weeds are not inverted as with plowing. With stubble mulch tillage, soil often remains in contact with roots, and plants may continue to grow if rain occurs soon after


37

tillage or if the soil is wet at tillage. Small weeds may be especially hard to kill with stubble mulch tillage. Skewtreaders and rodweeders can improve weed control in a stubble mulch system. The till-plant system provides essentially a clean seedbed, but only partially covers weeds and residue (Williams and Wicks, 1978). Weeds in the seedrow can be controlled with herbicides or by a rolling cultivator, which can operate in thc interrows without clogging (Richey et al., 1977). The till-plant system, even with some bare surfaces, can effectively control erosion when tillage is done on the contour (see Section V, B). With the no-tillage system, soil disturbance or loosening is limited to that required to place seeds in soil, and weeds are controlled with herbicides. Also, the system leaves the weed seeds on the surface in a poor environment for germination. When herbicides fail, tillage may be necessary to save a crop. However, effective mechanical cultivation may be difficult in an established no-tillage crop because the soil may be too firm (Richey et al., 1977). C . CONTROL WITH HERBICIDES

Herbicides are relied upon to control some weeds in many cropping systems involving tillage and to control all weeds in no-tillage systems. The mode of action of herbicides and the type of plants to be controlled largely determine which herbicides can be used in a particular cropping system. Herbicides must be compatible with present and future crops to avoid crop damage. Some herbicides must be incorporated with soil and, therefore, tillage is required. Disks, rotary tillers, and rolling cultivators are generally satisfactory for incorporating herbicides. Plows, even though they invert the soil, are less effective because they mix herbicides with soil only slightly. Chisels and sweep plows also cause little mixing of herbicides with soil. Tillage methods also strongly influence the effectiveness of surface-applied herbicides because they affect the amount of residue that remains on the soil surface. For maximum effectiveness, surface-applied herbicides should be uniformly placed on the entire soil surface. With some tillage methods, residue may intercept the herbicides, leaving some areas of soil untreated. In Indiana, corn plant residue covering 85% of the soil surface intercepted 30% of the applied atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-~-triazine]. Many areas under the residue remained untreated (Richey et al., 1977). In Texas, however, Unger et al. ( I97 I ) and Unger and Wiese (1979) completely controlled volunteer wheat with surface-applied atrazine when the atrazine was applied to areas having up to 1 1 metric tons/ha of wheat straw on the surface. Contact herbicides also controlled volunteer wheat under high residue conditions (Unger, 1977, unpublished data). Use of herbicides in no-tillage systems often has resulted in a shift in weed

38

P. W.UNGER AND T. M . McCALLA

species. In Indiana, herbicide-resistant weeds, such as milkweed (Asclepias syriaca L.), fall panicum (Panicum dichotomij7orurn Michx.), and briars, increased with continuous no-tillage (Richey et al., 1977). In Kansas, the weed population in a wheat-sorghum-fallow system shifted from broadleaf species, which were susceptible to atrazine, to sandbur, which was resistant. As a consequence, yields decreased unless sandbur was controlled with tillage. The herbicide-tillage combination resulted in yields of 3700 kg/ha as compared with 2300 kg/ha with sweep tillage alone (Phillips, 1969). D. CONTROL WITH ROTATIONS

Some weed species in some crops are difficult to control with tillage or herbicides because of the similar biological characteristics of the weeds and crops, and the vigorous growth habits of some perennial weeds. Species most difficult to control vary, depending on locations (Southern Weed Sci. SOC., 1979), but some examples are cheat, wild oats (Avena fatua L.), and downy brome in winter wheat; barnyard grass, foxtail (Setaria sp.), sandbur, and fall panicum in corn or grain sorghum; cocklebur (Xanthium sp.) and velvetleaf (Abutilon theophrasti Medic.) in soybeans; cocklebur, field bindweed, prickly sida (Sisa spinosa L.), spurred anoda [Anoda cristata (L.) Schlecht.], sicklepod (Cassia obtusifolia L.), and silverleaf nightshade in cotton; and Johnson grass, Bermuda grass, and nutsedge in all crops. Because problem weeds may be difficult to control with tillage or herbicides in continuous cropping systems, a crop rotation may be the most effective and economical control method available. Fields with summer annual weed problems can be rotated to winter grain crops. Then the problem weed can be controlled with tillage or herbicides during the period between crops. Conversely, fields with troublesome winter annual weeds can be rotated to spring- or summerplanted crops. Crop rotation also makes it possible to select crops that are most competitive against perennial weeds (Wiese and Staniforth, 1973). A crop rotation, or even skipping a crop, and using intensive weed control measures during the noncropped period may be necessary to reduce or eliminate heavy infestations of troublesome perennial weeds.

VIII. INSECTS AND PLANT DISEASES A. INSECTS

In recent years, considerable attention has been given to insect problems with no-tillage systems (Gregory and Musick, 1976). Many of the insect problems are not affected by tillage system, but others may be greater with no-tillage. Accord-


39

ing to Phillips and Young (1973), insects causing slightly greater problems with no-tillage included sod webworms, cutworms, armyworms, and root aphids (various species of each). Phillips and Young (1973) also reported that slugs (Deroceras laeve Muller) caused greater problems with no-tillage. Other reports, however, have shown different results for some insects. For example, Phillips and Young (1973) showed that wireworm (Melanotus cribulosus LeConte, and others) problems were similar for the two systems, but Musick and Beasley (1978) showed that wireworms caused more damage with no-tillage. The reported differences possibly resulted from differences in location, previous crops, and overall management of the systems. B . PLANTDISEASES

Considerable work has been done in recent years concerning the influence of crop residue on plant disease, as indicated by Cook et al. (1978). As for insects, disease problems generally were similar for conventional and no-tillage systems (Phillips and Young, 1973). Exceptions included anthracnose and yellow leaf blight for corn; and bacterial blight, bacterial pustule, wildfire, anthracnose, and sclerotial blight for soybean for which the disease incidence was greater with no-tillage. Other exceptions included “take-all” for small grain and Pytophtora, Rhizoctonia, Fusarium root rot, and stem rot for soybean for which the disease incidence was lower with no-tillage. According to Boosalis (1979), conservation tillage may affect plant disease by providing a habitat for plant pathogens; serving as foci for the dissemination of inoculum; affecting the growth and multiplication of pathogens associated with residue; altering the physical and chemical components of the soil; creating an environment hostile or beneficial to the pathogen; through residue decomposition, producing compounds that adversely or beneficially affect the pathogen; using pesticides that may affect the pathogen; and in altering the physical and chemical environment of the soil surface with residue that may affect the growth and configuration of roots to make them more resistant or vulnerable to soilborne diseases. An example of an indirect beneficial influence of crop residue was the reduction of stalk rot of sorghum in a reduced tillage system in Nebraska (Boosalis and Cook, 1973).

IX. SOIL TEMPERATURE A. EFFECTOF SURFACE RESIDUE

Plant residue placed on soil can significantly influence soil temperature. The primary mechanism of this effect is the change in radiant energy balance (Van

40


Doren and Allmaras, 1978), but an insulating effect may be involved also, because mulched soil normally is warmer than bare soil during cold weather, even during daylight hours (Unger, 1978b). The radiation balance is influenced by heating of air and soil, evaporating of soil water, and reflecting of incoming radiation by surface residue (Van Doren and Allmaras, 1978). As reflectance increases, soil temperature generally decreases. The insulating effect increases as residue mulch thickness increases (Unger, 1978b). B. RESIDUEFACTORS INVOLVED

Residue characteristics involved in the reflectance of incoming radiation include residue age, color, geometry (whether standing or matted on the surface), distribution, and amount. The general condition of the plant residue must be considered because plant aging, which causes yellowing and bleaching of the plants, increases reflectance in the visible wavelengths, but decreases it in the near infrared wavelengths. Further aging and decomposing of residue (after crop harvest) usually produces gray and darker shades, which reduce reflectance over the visible range (Van Doren and Allmaras, 1978). Because reflectance is highest with bright colored residue, the temperature difference between mulched and bare soil is greatest with bright-colored residue, then decreases as the residue ages. Gausman et al. (1975) compared the reflectance from bare soil with that from standing or surface-matted sugar cane (Saccharum oficinarum L.) after the cane was frozen and had bleached and turned yellow. Reflectance was greater from matted residue than from bare soil, but less from standing sugar cane residue than from bare soil or matted residue at all measured wavelengths, apparently because shadows occurred in the standing stubble. Gausman et al. (1975), using dried and bleached avocado (Persea americana L.) leaves, showed also that maximum reflectance is reached asymptotically as the thickness of target increases. For avocado, two leaves (leaf area index of 2) gave near maximum reflectance. For randomly placed residue, such as wheat straw or corn stover after harvest or initial subsurface tillage, complete surface coverage would be needed to obtain maximum reflectance. Under field situations, residue cover often is incomplete because of natural distribution or incorporation by tillage; therefore, some reflectance value less than maximum would be expected for a given residue condition (Van Doren and Allmaras, 1978). The greater reflectance from surface residue than from bare soil causes less soil heating and potentially less evaporation of soil water. However, Hanks et al. (1961), under conditions of their research, found no direct relationship between net radiation and evaporation. Factors within the soil, especially after the surface dried, apparently were more important than incoming radiation in determining water losses by evaporation during the fallow season.


41

40

P 35 I

W

30 I-

4

a 25

a W I-

20 I

1

22 23 24 25 DATES-AUQUST 1973

FIG. 1. Temperatures (10-cm depth) of Pullman clay loam at Bushland, Texas, during a hot period (21 through 25 August 1973) as affected by wheat straw surface mulch rates (metric tons/ha, numbers at lines). Air temperature is also given (from Unger. 1978b).

Radiation reflectance approaches a maximum as surface coverage by residue approaches 100% (Van Doren and Allmaras, 1978). Thus, residue in excess of those required to obtain 100% coverage should have no effect on soil temperature if radiation reflectance alone is involved. However, residue mulches at rates greater than those required for complete surface coverage do affect soil tempera-

10

P I

5

W

a 3

I - 0 4

a W

a

I -5 W

I-

a

;-lo

-I 5

FIG. 2. Temperature (10-cm depth) of Pullman clay loam at Bushland, Texas, during a cold period ( I through 7 January 1974) as affected by wheat straw surface mulch rates (metric tons/ha. numbers at lines). Air temperature is also given (from Unger, 1978b).

42


21

y

2c

W

a

2 I1 4

a

W

g

IC

W

k

c Y

C 0

1

2

-

0.084 X

+ 0.104

4 MULCH RATE

!*

' 0.943

8

- tonrlha

.

COLD PERIOD

12

FIG.3. Relationships between mean soil temperature (Y)and wheat straw mulch rates (X metric tons/ha) for seasons of fallow (spring, fall, winter, summer), and a 5-day hot, a 7-day cold, and a 5-day near-sorghum-planting period at Bushland, Texas, on Pullman clay loam (from Unger, 1978b).

tures (McCalla and Duley, 1946; Unger, 1978b), apparently because the mulches have an insulating effect. Effects of wheat straw mulches at rates from 0 to 12 metric tons/ha on temperature of Pullman clay loam at Bushland during a hot and cold period are shown in Figs. 1 and 2, respectively. The 4-metric todha rate covered almost 100% of the surface (Unger, 1978b). Relationships between mean soil temperature and mulch rates during different seasons and periods are shown in Fig. 3. The mulch rate effect on temperature was greatest for the hot period and least during winter. The trends (Figs. 1, 2, and 3), in general, were similar to those reported for residue-mulched soils by Allmaras et al. (1973) and Van Doren and Allmaras (1978).

c.

BIOLOGICAL

EFFECTS OF RESIDUE

In cooler parts of the United States, such as in the Corn Belt or the northern Great Plains, cool soil temperatures under a mulch in the spring of the year may adversely affect seed germination or plant growth. Favorable soil temperatures for germination and seedling emergence may occur up tn 7 days later in notillage than in conventional tillage seedbeds in the northern United States, but no temperature-induced delay of planting is expected at southern U.S. locations (Unger and Stewart, 1976).


43

Early planting of warm-season crops favors higher yields when the frost-free period is limited or when high temperatures, droughts, or insect and disease problems at later growth stages adversely affect yields. Because surface residue with stubble mulch and conservation tillage decreases soil temperature, considerable research has been conducted on how soil temperature affects plant growth. In Iowa, chopped corn stalks applied at a rate of 0 to 9 metric tons/ha lowered soil temperatures at a 10-cm depth by an average of 0.4'C/ton of mulch during May and June (Burrows and Larson, 1962). Straw mulches caused similar temperature decreases at other northern U.S. locations (Allmaras et al., 1964; Willis et al. 1957; Van Wijk et al., 1959), which decreased early corn growth. In South Carolina, where soil temperatures were considerably higher than those in Iowa, Ohio, and Minnesota, a mulch did not appreciably affect corn growth rate (Van Wijk et al., 1959). In central Texas, a straw mulch decreased soil temperature, but had little or no effect on early grain sorghum growth as compared with that on bare soil (Adams, 1962, 1965, 1967, 1970). At a much higher elevation in northwest Texas, increasing the straw mulch rate delayed the time that soil reached a favorable temperature for sorghum germination and growth. However, the temperature was near optimum before normal sorghum planting dates for the region, and thus the mulches affected the time of sorghum emergence only slightly. The sorghum on plots with large amounts of mulch grew more slowly, but yielded more than that on plots with little or no mulch. Yields were higher because more water had been stored in the plots with large amounts of mulch on the surface during the fallow period that preceded the sorghum crop (Unger, 1978b). Because most biological, chemical, and physical reactions depend on temperature, mulch-induced temperature differences in soil undoubtedly affect factors other than seed germination, seedling emergence, and plant growth. Interactions of soil temperature and soil water content resulting from a surface mulch greatly altered the distribution of corn roots in Minnesota (Allmaras and Nelson, 1971). Also, there is evidence that root zone temperature influences the uptake of water and nutrients and the distribution of products of photosynthesis within the plant. However, the final effect of these processes on plant growth and yield is not fully understood (Nielsen, 1974).

X. SOIL STRUCTURE AND OTHER PHYSICAL PROPERTIES

Water conservation and water and wind erosion control are major goals of conservation tillage systems. To achieve these goals, the conditions at the soil surface and within the profile must allow water to enter the soil readily and still keep the soil resistant to erosion. Soil physical factors that influence water infiltration and soil erodibility include soil aggregation, porosity, and density.

44


A . AGGREGATION

Soil aggregation refers to the cementing or binding together of several soil particles into secondary units. Water-stable aggregates, which do not disperse, are of special importance for high water infiltration, good soil structure, and good plant growth. Large stable aggregates at the soil surface are important also for controlling erosion by wind and water. The binding substances for natural soil aggregates have mineral or organic origins. Soils of humid tropic and subtropic regions have generally high infiltration rates, even on steep slopes (Table X). Many particles in tropic soils are the size of sand grains and consist mainly of altered minerals cemented by iron (Donahue et al., 1977). Because of the stable particles, the infiltration rate into tropic and subtropic soils often is similar to that for deep sands, and the rate remains high with prolonged rainfall. Organic substances that contribute to soil aggregation are derived from plant materials, either after alteration by soil animals, bacteria, and fungi, or directly from the plants. Earthworms beneficially affect soil structure by increasing infiltration (Hopp and Slater, 1961). While feeding on organic materials and burrowing in soils, earthworms secrete gelatinous substances that coat and stabilize soil aggregates. Water-stable aggregates are formed also with water-insoluble gummy substances secreted by bacteria, fungi, and actinomycetes (Donahue et al., 1977). Earthworm activity and intensive tillage are highly incompatible. Hence, there are few earthworms in most cultivated soils. Bacteria and other microorganisms, however, feed on decaying plant roots and other plant parts returned to the soil by tillage. Besides enhancing aggregation and thus water infiltration, soil microorganisms influence soil productivity through their effect on plant nutrients. For maximum earthworm activity, no-tillage is desirable. Where the soil is tilled, enough crop residue should be kept on or returned to the soil to provide an abundant food source for soil organisms, thus providing the potential for increased soil aggregation and water infiltration. The direct influence of plants on soil aggregation is manifested through exudates from roots, leaves, and stems, and leachates from weathering and decaying plant materials, which bind soil particles together; plant canopies and surface residue, which protect surface aggregates against breakdown due to raindrop impact, abrasion by wind-blown soil, and dispersion in flowing water; and root action in soil, which promotes the formation of aggregates. If aggregates formed through these processes are subsequently maintained on the surface, water infiltration will be higher than it is in intensively cultivated, poorly aggregated soils (Donahue et al., 1977). Crop rotations involving grasses and legumes have long been known to increase soil aggregation and maintain organic matter contents at higher levels than


45

do continuous row crops (Johnston et al., 1943; Mazurak et al., 1955; Van Bavel and Schaller, 1951; Wilson and Browning, 1946). On Marshall silt loam (Typic Hapludoll) in Iowa, aggregates were largest with continuous bluegrass (Poa pratensis) and successively smaller after red clover (Trifolium prateme), oats, and corn in a 10-year rotation, and after continuous corn. The clover maintained a loose, granular structure, whereas continuous corn resulted in a cloddy soil that was difficult to manage. With continuous corn, organic matter content decreased from 3.39% in 1931 to 2.86% in 1942. Organic matter contents with the rotation and with continuous bluegrass were similar. Less runoff and soil erosion were associated with the larger aggregates and higher organic matter contents. With limited water, yields of rotation and continuous corn were similar, but with adequate water, yields were higher with rotation corn (Johnston er al., 1943). Similar results were reported by Van Bavel and Schaller (195 1) and Wilson and Browning (1946). When row crops replaced sod crops, aggregation and infiltration decreased and soil losses generally increased (Adams, 1974; Jensen and Sletten, 1965; Mazurak and Ramig, 1963; Van Bavel and Schaller, 1951). The residual effect on aggregation increased with the age of the sod before plowing. Replacing grain crops with grasses increased aggregation and water infiltration, which generally improved with the age of the sod (Mazurak and Conard, 1959; Mazurak and Ramig, 1962; Mazurak et al., 1960). Cool-season grasses, as a group, more favorably affected aggregation and water infiltration than warm-season grasses (Mazurak and Conard, 1959). About 4 years in sod was needed before substantial increases in water infiltration were measured (Mazurak et al., 1960). In addition to retaining water on soils, thus providing more time for infiltration, crop residue and growing crops protect soil surfaces from dispersion due to raindrop impact and flowing water. The protection from flowing water may be important when the soil is irrigated or when precipitation causes runoff. The protection against dispersion maintains the favorable surface structure, which decreases surface sealing and, therefore, permits more rapid infiltration than would occur through a dispersed, sealed surface layer. Surface protection against dispersion is largely a function of the amount of coverage that the surface materials provide. Because the residue density and diameter differ among different crops, equal weights of different residue provide different amounts of surface coverage and, thus, differently influence infiltration (Van Doren and Allmaras, 1978). For example, wheat straw is about three times more effective than German millet (Setaria italica) straw and seven times more effective than grain sorghum stalk per unit weight of material for preserving infiltration. The protection provided by growing crops, like that of residue, is related to the extent of surface coverage. The importance of large (>0.84 mm), stable, dry aggregates or clods in the control of wind erosion was discussed in Section V, A.

46


B . POROSITY AND DENSITY

Soil porosity and bulk density are inversely related; therefore, any practice that affects one also affects the other. The factor generally determined is bulk density. Except in unusual soils, the bulk density of the tillage layer was normally lower in plowed soil than in unplowed soil, such as areas in grass or soil horizons not recently plowed (Unger, 1970, 1972). However, the method of plowing had little effect on soil density when relatively small amounts of crop residue were involved (Blevins et ul., 1977; Johnson, 1950; Unger, 1969; McCalla, 1959). With increasing amounts of residue, soil bulk density normally decreases (Black, 1973; Juo and Lal, 1977; Koshi and Fryrear, 1973). Bulk densities in the 0- to 15-cm layer of minimum-tillage soil in Nigeria were 1.38, 1.37, 1.49, 1.46, and 1.59 g/cm3 for bush fallow (bush regrowth), Guinea grass (Panicum maximum), pigeon pea (Cujunus cujun Millsp.), corn with residue returned, and corn with residue removed, respectively (Juo and Lal, 1977). Except for the last treatment, all residue was returned to the plots as a surface mulch each year. The low density in the fallow plots (bush fallow, Guinea grass, and pigeon pea) was attributed to high biological activity, which resulted in a porous surface horizon. Greater density in the corn plot without residue than with residue resulted from compaction of the surface layer (Juo and Lal, 1977), apparently because of no protection by surface residue against raindrop impact and soil dispersion. Under lower rainfall conditions, compaction of bare soil due to raindrop impact seems to be slight, because a surface mulch mainly decreased compaction due to tractor traffic (Koshi and Fryrear, 1973). C. OTHERPHYSICAL PROPERTIES

In addition to the previously mentioned physical properties, others affected by tillage include soil texture, crusting, hydraulic conductivity, and water storage capacity. Tillage-induced texture changes result primarily from the mode of action and depth of tillage. Plows that do not invert soil have little effect on soil texture. Soil-inverting plows can cause major changes in texture at the surface, especially when the texture changes rapidly with depth and plowing is deeper than previous plowing. Erosion on bare soil plots and earthworm activity in residue-covered plots also contribute to texture changes. In Nigeria, the amount of gravel increased by 5 to 7% and silt and clay decreased by 4 to 6% in corn plots without residue as compared with that in residue-covered plots (Juo and Lal, 1977). The fine materials apparently were eroded from the plots by runoff water. Wind erosion causes similar changes in texture. A sandy soil initially deep-plowed to bring clods to the surface to aid in wind erosion control increased the clay content


47

from 4 to 14%. Within 5 years, the clay content was at about 4% again because wind erosion during that period had removed or buried most of the clay initially brought to the surface (Chepil et al., 1962). Tillage methods that leave bare soil in the planted row may cause severe crusting and seedling emergence problems if heavy rainfall occurs before emergence. Crusting at other times may decrease infiltration and reduce plant growth. Any tillage method that keeps residue on the surface and, thereby, protects the soil against dispersion by raindrop impact and ponded or flowing water decreases crusting. Surface residue resulting from stubble mulch and especially no-tillage practices greatly reduce soil crusting (Johnson, 1950; Juo and Lal, 1977; Lal, 1976 Unger, unpublished data). When a crust has formed, it must often be broken to obtain satisfactory plant populations. Breaking the crust also may enhance subsequent infiltration of water. The saturated hydraulic conductivity of soil increases as soil porosity increases and density decreases. Conductivity was 46 and 65% lower in corn plots with and without surface residue, respectively, than in fallowed plots with residue (bush fallow, Guinea grass, pigeon pea). Juo and La1 (1977) reported that the amount of surface mulch, the type of vegetative cover, and the differences in rooting pattern probably influenced the saturated hydraulic conductivity of the soil. When Koshi and Fryrear ( 1 973) placed a cotton bur mulch on Acuff loam (Aridic Paleustoll) at rates from 1 1.2 or 22.4 metric tons/ha, hydraulic conductivity in crop rows was eight times greater with mulch than it was in bare soil. The higher infiltration rate and hydraulic conductivity that result from tillage practices keeping residue on the surface allow soil profiles to be more readily refilled with water. Consequently, soil water content often is higher in reducedor no-tillage cropping systems. Although higher water content may be detrimental to plant growth and yields on some soils at humid locations (Boone et al., 1976; Van Doren and Triplett, 1969), the additional water generally improves crop growth and yields, especially during short-term droughts and at subhumid and semiarid locations. Besides allowing soil profiles to be refilled more readily with water, reducedor no-tillage systems may also change the amount of plant-available water held in a soil. The water-holding capacities (difference between retention at -0.1 and - 15 bar matric potentials) were 13.6, 11.2, 13.7, 14.9, and 9.1% by weight for soil from bush fallow, Guinea grass, pigeon pea, corn with residue, and corn without residue plots, respectively (Juo and Lal, 1977). La1 (1976), who found higher water-holding capacities on no-tillage plots than on plowed plot%, attributed the differences to changes in organic matter content and texture in the surface horizon of plowed plots. The type of change was not specified, but apparently involved decreases in organic matter and fine soil particles in plowed plots because, when these remained constant, the available water-holding capacities of coarse-textured core and sieved soil samples were similar. As clay

48

P. W.UNGER AND T. M. McCALLA

content increased, the water-holding capacity, based on - 1/3 and - 15 bar matric potentials, increased slightly (Unger, 1975).

XI. CHEMICAL EFFECTS AND MICROBIAL ACTIVITY Some chemical and microbial effects of leaving crop residues on the surface are shown in Table XIV. Doran (1980) provided some additional information. Soil beneath the plant residue cover is generally cooler, wetter, and less aerated than where residue is plowed under. Doran, in his study of surface soil from long-term tillage plots at seven locations around the United States, found that the surface layer (0 to 7.5 cm) of most reduced-tillage soils had higher microbial populations, higher phosphatase and dehydrogenase enzyme activity, and higher levels of total nitrogen and potentially mineralizable nitrogen than conventionally tilled soils. Aerobic microorganism counts increased 10 to 80%, and anaerobic bacteria, including denitrifiers, increased 60 to 300% in the surface of reducedtillage plots compared to conventional tillage. At the depth adjacent to the plow layer (7.5 to 15 cm), populations of aerobic organisms (especially nitrifiers) with Table XIV Effect of Stubble Mulching on Some Chemical and Biological Properties of the Soil as Compared with Plowing" Type of determination ChemicalSoil Ammonia loss Nitrites Nitrates Nitrification rate Organic matter PH HCI-soluble phosphorus Biological Crop yields Bacteria Actinomycetes Fungi Earthworms Nematodes Denitrifiers Azotobacter Legume bacteria

Stubble mulch compared to plowing

Slightly higher with legume residues No difference, and low amount present About 5 to 10% less No difference

1

May be slightly higher in surface 2.5 cm of soil; in 2.5- to 15.2cm depth, no difference

Variable-may

be higher in dry years and lower in wet years

Greater number of organisms in surface 2.5 cm with residues on surface

May be higher number in surface layer of soil

1

From McCalla (1958).

No difference in numbers or effect on nodulation


49

plowing were significantly higher than those with no-tillage; however, populations of facultative anaerobes and denitrifiers were higher with no-tillage. This indicates that the biological environment of no-tillage soils is less oxidative than that for conventional tillage. Under such conditions, organic matter and total nitrogen would tend to increase. The significance of higher microbial populations in surface soil and more denitrifiers has not yet been identified, but these results indicated that soils with residue on the surface may need more nitrogen than those with plowed surfaces. However, corn used nitrogen more efficiently in no-tillage systems than in conventional systems (Moschler and Martens, 1975; Moschler et al., 1972) and wheat gave variable results with respect to nitrogen fertilizer and tillage system. More nitrogen was needed with no-tillage than with conventional tillage on heavy clay soils (Davies and Cannell, 1975). Phosphorus tends to increase in the surface soil of mulched soil, and is available to plants (Fink and Wesley, 1974). However, only fragmentary evidence is available in regard to transformation and availability of other nutrients needed by the crop. Phytotoxic substances that occur in plant residue or are produced by microorganisms may, in some instances, inhibit plant growth and may be related to reduced yield (McCalla and Norstadt, 1974; Elliott et al., 1978).

XII. ECONOMICS To be economically advantageous over an existing system, a new crop production system must be either less expensive or more efficient or both. A new production system is less expensive if it requires less labor, fuel, and equipment. A system is more efficient if it increases the quantity or improves the quality of products to be sold or used in relation to the production inputs. Because of rapidly changing prices, assigning dollar values to various cropping systems has little meaning. We will, therefore, discuss factors that affect expenses and income, and give only limited data regarding the economics of different systems. Labor and equipment (tractor, plows, fuel, etc.) expenses for crop production can be reduced by eliminating field operations, by reducing the number of time-intensive operations, or by using larger equipment. A lower labor and equipment requirement is a major advantage of the reduced- and no-tillage cropping systems because three, and sometimes more, operations can be eliminated (Allen et al., 1977; Hough, 1979). The labor and equipment savings are offset to some degree by higher expenses for herbicides. The high cost of herbicides was a major deterrent to adoption of early no-tillage systems. However, weed control with herbicides rather than with tillage is now more economical in some cropping systems (Unger and Wiese, 1979; Wiese et al., 1979). With the same size tractor, the labor and equipment requirement per unit area is greatly influenced by the type of tillage operation. As tillage depth and inten-

50

P. W.UNGER AND T. M. McCALLA

sity increase, the time required to perform the operation increases. With factors such as soil type and water content unchanged, the time required to perform different operations is related to the amount of fuel expended. Allen et al. (1977) reported fuel consumption values for performing different operations on Pullman clay loam at Bushland (Table 111). Some of the differences were related to depth of tillage. However, moldboard plowing required the most fuel and was followed in decreasing order by chiseling (narrow spacing), disking, and sweep plowing. The values would be different for other soils, plowing depths, and soil water contents, but for all conditions, eliminating the fuel-intensive operations reduces the labor and equipment requirement for tillage. The differences among systems would be minimized where two or more operations with disk or sweep plows are needed to obtain the weed control provided by one moldboard plowing. A further labor savings is possible by using larger equipment. However, larger equipment may require a more skilled operator and is also more costly. Thus, when considering the purchase of larger equipment, all advantages (labor savings, timeliness of operations, etc .) must be weighed against possible disadvantages (higher tractor and equipment costs, need for higher-skilled labor, alternate use of unused labor). When production expenses remain unchanged, then crop values must be increased to obtain higher returns from a new crop production system. Because of higher yields, stubble mulch tillage was more economical than one-way tillage for wheat production at Bushland, even though fuel use was the same (Allen and Fryrear, 1979). When production expenses are decreased and yields are inTable XV British National Average Expenses for Establishing a Cereal Crop into Stubble Based on the Cost of Owning and Operating New Equipment" Number of operations or expense items Operation or expense Ploughing Disking Herbicide Herbicide application Harrowing Seeding Tine cultivation

Conventional tillage I 2 1 pint 1

1 1

Totals From ICI-Plant

Minimum tillage

1 pint 1 1 1 3

Expenses (British pounddacre) Notillage

2 pints 1 1 1

Conventional tillage 6.75 5.20 1.87 1S O 1.60 3.30

20.22

Protection (1976).

Minimum tillage

NOtillage

-

-

1.87 1.50 1.60 3.30 8.50

3.74 1.50 1.60 3.80

16.77

10.64

51


Table XVI Cost of Tillage and Herbicides for Various Cropping Sequences with Surface Irrigation on the Southern High Plains" Operations and total expenses Cropping sequence Wheat to sorghum, double-cropped Wheat to wheat

Sorghum to sorghum

Wheat-sorghum-fallow

a

Clean tillage

Limited tillage

Disk, disk, bed, apply atrazine ( I .8 kg/ha) $52/ha Disk, disk, bed, cultivate $44/ha Disk, disk, chisel, bed, cultivate $49/ha Disk, disk, disk, bed, cultivate, cultivate $44/ha

Apply atrazine (1.8 kg/ha) $17/ha Disk-bed, cultivate $26/ha Shred, split beds, cultivate $30/ha Apply atrazine (3.4 kg/ha) and 2.4-D ( 1 . 1 kg/ha) $30/ha

From Wiese er a / . (1979).

creased, remain constant, or even slightly decreased, the reduced-tillage systems are more economical than tillage-intensive systems. In irrigated areas, additional benefits from reduced-tillage systems are derived from greater water conservation, which results in higher yields with reduced expenses for energy, equipment, and labor for pumping irrigation water (Allen and Fryrear, 1979; Section IV, A). Based on equipment and herbicide expenses, production expenses for some crops are about twice as high with conventional tillage as with limited- and no-tillage (Tables XV and XVI). In other systems, the expenses may not differ much for different production systems (Unger and Wiese, 1979). However, if yields are increased or if the reduced-tillage system permits better utilization of all farm resources, economic returns to the crop production enterprise are increased by using the reduced-tillage system (Brown and White, 1973; Unger and Wiese, 1979).

XIII. SUMMARY AND CONCLUSIONS

Much has been accomplished since 1961 in regard to the use of crop residue on the surface in conservation tillage systems. The rapid technological advances in the use of herbicides have done much to reduce the need for tillage. The direct-

52

P. W.UNGER AND T. M . McCALLA

drill system with no subsequent cultivation is well established in the United States, Europe, Asia (particularly in Japan), and Africa under certain conditions. Use of direct drilling or limited tillage controls water and wind erosion, conserves from 5 to 15 cm additional water from rain-fed agriculture, allows more timely planting of crops, and improves the quality of surface water. The amount of energy required for tillage is reduced by direct drilling and no-tillage, but additional energy may be needed for fertilizers and herbicides. The net energy balance may still be slightly in favor of direct drilling. In the United States, use of direct drilling or limited tillage will be necessary to reduce soil erosion losses to levels that meet the requirements of Section 208 of Public Law 92-500, as amended by the Congress in 1972. The wide interest in and adaptation of this practice in this country and throughout the world has been an important achievement.

B. NEEDS While great advances have been made in the use of direct drill or limited tillage, there are still considerable gaps in our knowledge of how to use the system most effectively. For optimizing crop yield and quality in conservation tillage systems, an in-depth understanding of the influence of tillage and residue management systems on the physical, chemical, and biological components of the plant-soil environment is essential. However, little effort has been made to obtain this information through an integrated approach. In most past research involving plant residue systems, the main factor determined was crop yield. Only minor emphasis was placed on other biological considerations. We know that tillage alone can have a dramatic effect on the biological equilibria and microbial population in the soil. The reductions in crop yield that have occurred with some conservation tillage systems have shown the need for a complete understanding of the biological changes in the soil environment as a result of the physical changes imposed by the system (Doran and McCalla, 1977). To determine the biological effects of minimum tillage and crop residue management, multidisciplinary research is needed to study the microbiology, biochemistry, chemistry, and physics of the plant-soil environment, as well as the physiology and disease and insect vulnerability of the crop itself. Because the soil environment with residue systems is generally cooler, wetter, and less aerated than with the moldboard plow system, microbial activity is slower and also results in different transformations of nutrients. Not until we more clearly understand the physical, chemical, and biological changes brought about by the use of crop residue on the surface in conservation tillage systems can we more effectively use the additional stored water and conserve the organic matter and nutrients in the soil. This information will help us to apply more wisely fertilizer and


53

other treatments to maximize yield, improve the quality of the food produced, and minimize some present harmful effects on crop yields. It will also enable us to reduce energy input, control water and wind erosion, improve crop quality by managing nutrient composition of the crop, and curtail the physiological disorders that result from insects and diseases. Only through a complete understanding of the physical, chemical, and biological effects of conservation tillage systems on the soil and plant environment can predictions be made of how the information obtained in one area can be applied in other areas. A basic need is the development of a model, with appropriate baseline parameters (lowest category or variable in an equation or model), that is applicable to interregional as well as interdisciplinary research. How is a model developed that will use both physical and biological data collected in different regions? One answer to this question may be in the choice of dependent variables for which data are available in the literature and which are most frequently used in agricultural research endeavors. One such universal “dependent variable” is crop yield. Of course, within research disciplines, equations will be developed that are defined by the parameters important to that particular area of study. However, baseline parameters should be chosen so that data measurements can be applied and used by other disciplines. Examples of a few such parameters would be soil temperature, soil air content, and soil water content. These parameters are affected by the physical properties of the soil and have a direct influence on the chemical and biological components of the soil-plant environment.

REFERENCES Adams, J. E. 1962. Agron. J. 54, 257-261. Adams, J . E. 1965. Agron. J. 57, 471-474. Adams, J . E. 1967. Agron. J. 59, 595-599. Adams, 1. E. 1970. Agron. J . 62, 785-790. Adams. J . E. 1974. Agron. J. 66, 229-304. Allen, R. R., and Fryrear, D . W. 1980. In “Conservation Tillage in Texas” (B. L.Harris and A . E. Coburn, eds.), pp. 31-45. Tex. Agr. Ext. Service B-1290. Allen, R. R., Musick, J . T . , and Wiese, A. F. 1975. Tex. Agr. Exp. Sin. PR-3332 C, pp. 66-78. Allen, R. R., Musick, 1. T . , and Wiese, A. F. 1976. Trans. Am. SOC.Agr. Eng. 19,234-236.241. Allen. R. R., Stewart, B. A., and Unger, P. W. 1977. J. Soil Wafer Conserv. 32, 84-87. Allen, R. R., Musick, J. T.. and Dusek, D . A. 1980. Trans. Am. SOC. Agr. Eng. 23, 346-350. Allmaras, R. R., and Nelson, W. W. 1971. Soil Sci. SOC.Am. Proc. 35, 974-980. Allmaras, R. R., Burrows, W. C., and Larson, W. E. 1964. SoilSci. SOC.Am. Proc. 28,271-275. Allmaras, R. R., Black, A. L., and Rickman. R. W. 1973. I n “Conservation Tillage: The Proceedings of a National Conference,” pp. 62-86. Soil Conserv. SOC. Am., Ankeny, Iowa. Amemiya, M. 1977. J. Soil Wafer Conserv. 32, 29-36. American Society of Agronomy. 1978. Prw. Symp., Crop Residue Management Systems, Houston, Texas. Spec. Pub. No. 31.

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ADVANCES IN AGRONOMY. VOL. 33

POTASSIUM IN CROP PRODUCTION Konrad Mengel and Ernest A. Kirkby Institute of Plant Nutrition, Justus Liebig University, Giessen, Federal Republic of Germany and Department of Plant Sciences, The University, Leeds, England

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. . . . . . . . . . . . . . . . . . . . . . . . . .. . . , . . .. . . . . . . . . A. Soil Potassium Fractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Factors and Processes of Potassium Availability . . . . . . . . . . . . . . . . . . . . . . . , . . , . C. Assessment of K Availability in Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

60 60

D. Plant Root Soil Interactions

71 74 74 81 83 85

11. Potassium Availability in the Soil

111. Potassium in Physiology

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

64 70

A. Potassium Transport across Biological Membranes and Cation Competition B. Cell Turgor and Water Economy of Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Energy Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Long-Distance Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Enzyme Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 IV. Potassium Application and Crop Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 A. Crop Response and Potassium Application . . . . . . . . . . . . . . . . . . . . , . . , . . . . , . . . 91 B. Effect of Potassium on Yield Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 C. Secondary Effects of Potassium on Crop Yield.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 103 V. Conclusions 103 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I. INTRODUCTION Potassium was first recognized as an essential element for plant growth following the work of the Englishman Home in 1762 from experiments in which he grew barley in pots of soil and used plant analysis as a means of investigating uptake. Later researchers such as Th. de Saussure and Carl Sprengel recognized that potash was present in plant ash obtained from a large number of different plant species. In reviewing the analytical data of the period, Liebig (1 841) proposed that K was in some way involved in plant metabolism. The experience of farmers around Giessen, the German university town in which Liebig worked, had indicated the beneficial influence of manuring crops with plant ash. Liebig recognized that potash was the essential growth factor in the ash. Furthermore, Liebig was aware +

59 Copyright @ 1980 by Academic Rcss. Inc. All rights of Rpaoducrion in any form reserved. ISBN 0-12600733-9

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KONRAD MENGEL AND ERNEST A. KIRKBY

that the clay fraction of the soil provided a source of K+ for plant growth. In his book, “Die organische Chemie in ihrer Anwendung auf Agrikultur und Physiologie” he wrote, “There must be a component in clay which has an influence on plant life and which directly participates in plant development. This component is the ever-present potash or sodium. The paramount importance of clay minerals in binding or releasing K+ recognized by Liebig has been confirmed by much subsequent research work. The same is also true of Liebig’s suggestion that K+ was involved in plant metabolism. Potassium is now known to be required by plants in large quantities, and potassium fertilizer application has had a considerable impact on crop production, particularly under conditions where there has been a shift from extensive to intensive agricultural practice (Amon, 1969). In this article, three main aspects of K+ in crop production are reviewed, namely, K availability in the soil, the function of K+ in the plant, and potash fertilizer application. The soil is considered as a source of K+ to plant roots. Pedological and mineralogical problems relating to soil K+ have been reviewed elsewhere by Rich (1968, 1972) and by Schroeder (1976). These aspects are considered here only insofar as they are of direct importance to the availability of K+ in the soil medium and hence to crop growth. The use of K+ in practical crop production is also emphasized in the discussions on the physiological role of K+ in the plant and in fertilizer application. ”

II. POTASSIUM AVAILABILITY IN THE SOIL A. SOILPOTASSIUM FRACTIONS

The potassium status of a soil may be assessed on its content of K+-bearing minerals, since the amount of these minerals present in a soil gives some indication of the potential source of K+ to plants. However, in terms of the ability of the soil to supply K+ to plant roots, the quantity of K+-bearing minerals plays only an indirect role. More important in determining the K+ supply to plants are the soil K+ fractions. These fractions, which have been established experimentally using different extraction techniques, are soil solution K+, K+ adsorbed to clay minerals or humus, and K+ present in minerals. The total quantities of K+ in these three fractions differ considerably between soils. However, in mineral soils in which K+ is present in average amounts, the soil solution K+ makes up about 1 to 3% of the exchangeable K+, which in turn represents only a small fraction-at most a few percent-of the total K+ (Scheffer et al., 1960). Potassium in soil solution tends to equilibrate with K+ in the adsorbed fraction so that these two soil K+ fractions are closely interdependent. The equilibrium between solution and adsorbed K+ is controlled to a large


61

extent by the degree of K+ selectivity of the adsorption sites in the exchangeable fraction. Adsorption sites of organic matter and of kaolinitic clay minerals are low in K+ selectivity. Potassium adsorbed on these sites is thus in equilibrium with a relatively high concentration of solution K+ (Ehlers et al., 1968). On the other hand, the 2: 1 clay minerals possess adsorption sites that are much higher in K+ selectivity and that bind K+ very strongly. This is especially true for illitic clay minerals (Ehlers et al., 1967). As shown in Fig. 1 , three types of adsorption sites may be distinguished: planar sites (p-position) with a low K+ selectivity, edge sites (e-position) with a medium K+ selectivity, and inner sites (i-position) with a high K+ selectivity. These highly selective inner sites are of particular interest, since the adsorbed K+ can be considered an integral part of the clay mineral. This is true of the micas, in which the “interlayer K+ ” bridges the adjacent layers by electrostatic bonds. Interlayer K+ is not easily displaced by other cation species and particularly not so by larger cation species such as Ca2+ and Mg2+. For this reason, this particular K+ fraction is termed “nonexchangeable K+.” All three K+ fractions, solution K+, exchangeable K+, and nonexchangeable K+, are interrelated and all play a part in supplying plants with K+. The interrelationshipsbetween the K fractions may be illustrated by considering what happens when a K+ salt such as KCl is added to the soil. At first the salt dissolves and the K+ concentration of the soil solution increases rapidly. Potassium is then removed from the solution by the adsorption sites, the rate at which this occurs depending on the particular equilibrium conditions of the system. This removal of K+ from soil solution is accompanied by an equivalent increase in the soil solution concentration of other cations. The application of K+ to a soil may saturate all three fractions with K+. However, the time required for K+ equilibrium to be reached under field conditions may be as much as several weeks. Saturation of i-positions by K+ is a particularly long-term process since the diffusion of K+ in the interlayer zone is p - position i position

I

I

-

K not exchangeable to IargeCations

*+

I +

‘e-

position

Hydroxy-Al(or Fe) Islands

FIG. 1. Model of an expandable 2: 1 clay mineral with interlayer K+,wedge zones, and p-. e-, and i-positions.

62


slow (diffusion coefficient lo-’’ to cm-2 sec-l). In examining this fixation process on a K+-fixing soil (44% clay), Karbachsch (1978) found that even after a very high fertilizer application rate of 1000 kg Wha, the K+ concentration of the soil solution was only on the order of 1 mM K+. As the experiment progressed, this concentration steadily declined until after a period of 90 days it fell to a value of 0.3 mM, which was only about 50% higher than the K concentration of the soil solution prior to K+ fertilizer application. Similar results have been reported by Amberger et al. (1974) who found that the process of K+ adsorption to the i-position of clay minerals extended over a period of about 6 months. From these results it is clear that soils in which the K fixation capacity is high may more or less immobilize fertilizer K+. The agricultural implications of this are considered in the final section of this article. All i-positions, even those occupied by cations other than K+, are involved in the K fixation process. Potassium fixation takes place by means of K+ adsorption to these K+-specific binding sites of the interlayer zone, and in this process cation exchange occurs. As shown in Fig. 1, the replacement of interlayer K+ by larger cation species (Ca2+,MgZ+)expands the lattice and wedge zones are formed. The reverse process occurs when these larger cation species are replaced from interlayer sites by K+ or NH:. The contraction in the mineral is accompanied by a decrease in cation exchange capacity. This is the process by which K+-depleted 2:l clay minerals fix K+. The K+ fixation capacity of soils differs widely and depends much on the type of soil clay minerals present in the soil and on their degree of K depletion. According to investigations of Arifin and Tan (1973) the proportion of wedge zones decreased for the various minerals in the following sequence: micas > illite > vermiculite > montmorillonite. These authors found K+ fixation capacities ranging from 0.3 to 0.6 me K/g clay. The data refer to K+ fixation under dry conditions. The so-called “wet K+ fixation capacity” is a lower value, due to the fact that under wet conditions only micas, illites, and vermiculites fix K+, whereas under dry conditions smectites are also able to fix K+, because of a shrinkage of the mineral. The behavior of clay minerals in relation to K fixation or release on drying, however, is by no means clear-cut (Ahmad and Davies, 1971). Potassium depletion of K+-fixing soil minerals may be of anthropogenic origin or may have occurred during soil development. Potassium fixation is often found on alluvial soils associated with high amounts of organic matter, as is the case for numerous K+-fixing sites in Bavaria in the Federal Republic of Germany. The role of organic matter in K fixation in these soils is not clear, but it is believed that the mineral constituents may have lost substantial amounts of K+ during the period of soil material transportation by water (Niederbudde, 1967), thereby making the soils prone to K+ fixation. Continuous cropping without K+ application may also induce the buildup of a


63

K+ fixation potential in the soil. An example of this has been reported by Nielsen (1970). In a long-term field experiment he observed the highest K fixation capacity on a soil which had received no K+ fertilizer for 70 years. Potassium fixation is also dependent on soil acidity, being generally low or absent on more acid soils. Under such conditions more soluble forms of A1 such as AI(OH)?j, Al(OH)2+are available which compete with K+ and are selectively bound to the i-position of the clay minerals (Nemeth and Grimme, 1972). In the long term, low pH conditions also favor the formation of Al-chlorites, a type of mineral which does not fix K+ (Laves, 1978). Under certain circumstances interlayer K+ may contribute to a considerable extent in supplying K+ for plant uptake (Niederbudde et al., 1969; Tabatabai and Hanway, 1969). Mengel and Wiechens (1979) found that under optimum conditions the nonexchangeable K+ fraction of a soil rich in K-bearing minerals (illite, vermiculite) could completely meet the K+ demands of ryegrass. This pot experiment also showed that the proportion of K+ absorbed from the nonexchangeable K+ fraction increased the more the exchangeable K+ fraction was depleted. Below a level of 300 ppm exchangeable K+, most of the K+ absorbed by the ryegrass originated from the nonexchangeable K+ source. Potassium quantities present in this nonexchangeable fraction may be considerable. The availability of this K+, however, decreases as K+ is released. Drews (1978) found that only a very small fraction-at most a few percent-of the interlayer K+ was available to Lolium perenne. In this permanent cropping experiment the rate of K + released from the nonexchangeable soil K + fraction finally became so small that the plants suffered severely from K+ deficiency. These observations of Drews (1978) as well as the experiments of Wiechens ( 1 975) and pot and field experiments of v. Boguslawski and Lach ( 1 97 1) clearly demonstrate that the huge pool of interlayer K can be tapped by plants to only a very limited extent. One may suppose that only interlayer K+ located in the marginal zones of clay minerals is available to plant roots in adequate amounts. The above view is supported by the work of Newman (1969). In studies on the release of K+ by micas he has reported that the K+ concentration in equilibrium with the interlayer K+ decreased as the interlayer K+ is depleted. The process of interlayer K+ release is not yet completely understood. According to v. Reichenbach (1972) it is an exchange process associated with diffusion in which K+ adsorbed to i-positions of the interlayer zone is replaced by other cation species. If the replacing species is a large one (Na+, Mg2+, Ca2+), then K+ exchange results in an expansion of the clay mineral and the formation of wedge zones (see Fig. 1). The resulting widening gap between the two layers of the mineral favors the diffusion of the replaced K+ out of the mineral. A demonstration of this type of behavior has been reported recently by Jackson and During (1979) for New Zealand topsoils of widely different clay mineralogy. Pretreatment of the soils with Ca2+ (as acetate) resulted in an expansion of the +

64


clay mineral and an increase in K+ desorption. Potassium desorption is also often associated with the oxidation of F2+ to Fe3+ (Farmer and Wilson, 1970; v. Reichenbach, 1972). Low pH and high moisture conditions are also beneficial to the release of interlayer K'. Net K+ release occurs if the rate of K+ release is higher than the rate of K+ fixation. Since the fixation rate depends directly on the K+ concentration in solution in contact with the clay mineral, a high net rate of release is only likely to occur if the K+ concentration in the solution is extremely low. Diluting the soil solution with distilled water should thus promote the release of interlayer K+. This has been demonstrated in a comparative leaching experiment by Drews (1978). The continuous leaching by water of a soil column containing 300 g soil over a period of 180 days resulted in a loss of 9.8 mg K/100 g soil. However, in the comparative parallel treatment, in which the water was replaced by 15-60 x M KCl no loss of K+ from the nonexchangeable fraction was observed. This experiment shows that net K+ release from clay minerals occurs only if the K+ concentration in the soil solution is extremely low. The question of whether plant roots have a direct influence on the release of interlayer K+ is discussed in the next section. B. FACTORS A N D PROCESSES OF POTASSIUM AVAILABILITY

The contact exchange process as postulated by Jenny and Overstreet (1938) was long held to be the most important means by which plant roots obtain and mobilize K+ adsorbed to clay minerals. This concept was criticized by Lagerwerff (1961), who concluded from his experimental data that the bulk of cations absorbed is taken from the solution and that the exchangeable cation fraction is only indirectly available by means of exchange with cations in solution. Further evidence against the predominant role of contact exchange in K+ uptake came from the findings of Barber et al. (1963). These workers evaluated the amount of K+ accessible to plant roots by interception, or in other words, the K+ in direct contact with the roots as they push their way through the soil. It was concluded that the amount of intercepted K+ was far too small to satisfy the needs of the plant. This conclusion is also consistent with the calculations of Mengel and Kirkby (1978) which show that even for a soil high in exchangeable K+ the amount of K+ in direct contact with plant roots can only satisfy a small fraction of the plant's K requirements. Experiments of Drew and Nye (1969) with Lolium perenne also revealed that only 6% of the total K+ demand was supplied by the soil volume of the root hair cylinder. Ninety-four percent of the K+ taken up therefore originated from beyond the limit of the root hair cylinder. It can thus be concluded that the bulk of K+ required by plants must be transported toward the roots.


65

The transport of K+ in the soil medium toward plant roots may take place by mass flow or diffusion. Differentiation between both processes is difficult, and only a rough calculation can be made. According to Barber et al. (1963) only about 10% of the total K+ requirement of crops is transported by mass flow, although the contribution can be somewhat greater when the amount of water transpired by the crop is increased. Generally, however, it is accepted that diffusion is the main process by which K+ is transported to plant roots. Both processes, K+ diffusion and K+ mass flow, have been incorporated into an equation that describes the K+ flux toward plant roots (Barber, 1962).

+ C ~ V+ u

J = Dl(d~l/dr) + Dp(d~2/dr)

(1)

where flux rate (total quantity of ions reaching the root per unit time per unit area of root surface) c, = K+ concentration in the soil solution cp = K+ concentration moving at the soil surfaces (adsorbed K+) c3 = K+ concentration in mass flow water v = velocity of water flowing through the soil toward the roots a = replenishment factor D, = diffusion coefficient of the ions in the soil solution D, = diffusion coefficient for the movement of ions at soil surfaces (exchange diffusion) J

=

This equation shows that quite a number of factors have an influence on the K+ flux rate in the soil medium. If one assumes that the mass flow component ( c 3 v ) is of minor importance and one also neglects the replenishment factor, then the flux rate can be seen to be controlled mainly by factors influencing diffusion. Two kinds of diffusion may be considered, diffusion in the soil solution and diffusion in the zone of adsorbed cations (exchange diffusion). Data for exchange diffusion are very rare in the literature. However, De Lopez-Gonzales and Jenny (1959) reported an exchange diffusion coefficient for Sr2+ of 1.5 x lo-* cm-2 sec-’. This is lower than the diffusion coefficient of S r p + in solution by a factor of lo3. It would therefore seem reasonable to assume that the “exchange diffusion” coefficient for K+ is also by some orders of magnitude lower than the K diffusion coefficient in solution, and that one may neglect the exchange diffusion component of the equation (Nye and Tinker, 1977). If the term for the exchange diffusion in Eq. (1) is dropped, then the equation for the K+ flux may be reduced as follows:

This is Fick’s diffusion law. Nye and Tinker (1977) argue that despite the heterogeneous nature of soil, it is legitimate to treat the soil as a quasi-

66


homogeneous body to which this law may be applied and that the diffusivity of such a system is described by the diffusion coefficient. They hold the view that this is valid so long as a representative sample of gas- and liquid-filled pores and adjacent adsorbed phases are included. In a more recent paper Nye (1979) has substituted the diffusion coefficient by a dispersion coefficient (see below). This alteration, however, does not affect the principle of the following deductions. According to Nye and Tinker (1977) the diffusivity may be described by the equation! D = D18fldC1ldC

(2)

where D1 = diffusion coefficient of K+ in free solution 8 = the fraction of the soil volume occupied by solution f , = impedance factor C, = concentration of K+ in soil solution C = concentration of K+ in the whole soil system From this equation it follows that the diffusivity of K+ in the soil media increases with 8, which in turn is closely related to soil moisture. The impedance factor also increases as soil moisture increases. This term represents the tortuous pathway along which K+ has to pass on its way through the soil medium to plant roots. It can readily be visualized that the tortuosity increases as the soil becomes drier. C1 is the K+ concentration in the soil solution, and C is the total K+ directly or indirectly involved in K+ transport. Generally the exchangeable K+ is used when measuring the term C. The ratio dC,ldC is of particular importance since it is the reciprocal of the buffer capacity.

vl)

b = dCIdC, = buffer capacity Substituting b for dCldC, the following equation is obtained: D

= D19fl/b

(3)

From this equation it is clear that the diffusivity of K+ decreases as the K buffer capacity increases. This close relationship between the K+ buffer capacity and the K+ diffusion coefficient has been shown experimentally by Vaidyanathan et al. (1968). The importance of soil moisture for the diffusion of K+ or related cation species has been demonstrated by several authors. Graham-Bryce (1963) found a diffusion coefficient for Rb+ of 1 x lo-' cm+ sec-I at a soil water content of 23%. The coefficient decreased to 5 x lo-* ern+ sec-' when the soil moisture was reduced to 10%.Patrick and Reddy ( 1977) measured a diffusion coefficient of 2.5 x 10-6cm-2sec-l for NH: in paddy soils. In comparison with these values,


67

the diffusion coefficient for K+ in pure water is about 1.5 X cm-' sec-'. As it seems likely that the diffusion coefficients for NH:, Rb+ , and K+ should not differ greatly, the coefficients cited above for NH: and Rb+ should also be more or less in the same order of magnitude as those of K+ . It can thus be seen that soil moisture is of crucial importance in K+ availability. From the equations cited above the K+ flux in the soil medium (J) can be described by the following equation: J =

-(*)

(2)

(4)

In this equation exchange diffusion and mass flow are neglected. The rate of K+ diffusion is controlled by the K concentration gradient ( d C , / d r )as well as by diffusivity . If the assumption is made that the rate of K+ absorption by the root is the same as that diffusing to the root surface, then the K+ concentration at the root surface remains constant. This, however, is an exceptional case, and generally the rate of K+ absorption by roots is higher than the rate of K+ transported toward the root surface. For this reason K+ depletion zones develop around the root, and the K+ concentration at the root surface thus declines. The degree of K+ depletion at the root surface can be expressed by the C,/Ci ratio, where Ci represents the initial K+ concentration before uptake begins and C, the K+ concentration at the root surface. Experimental evidence of Rb+ depletion around plant roots has been reported by Barber (1962) and Farr et al. (1969) using autographs. The K+ concentration at the root surface is of crucial importance in relation to K+ uptake, according to the following equation (Drew et al., 1969):

F = 21raaC,

(5)

where F = Flux rate across the root surface (mole cmP sec-') a = radius of the root a = root absorbing power C, = concentration of K+ in solution at the root surface

In this case a single root or single root segment is considered as a cylinder, so that the term 27ra represents the root surface of I-cm root length. The term a is the root absorbing power. A high power means that a high proportion of K+ impinging on the root surface is absorbed and vice versa. The root absorbing power depends much on root metabolism and is thus not a constant term but changes depending on metabolic conditions and plant species involved. From Eq. ( 5 ) it can be derived that the K+ flux across the root surface is related to the K+ concentration at the root surface (C,) in a linear way. This is not

68

KONRAD MENGEL AND ERNEST A . KIRKBY

completely correct, as the K+ uptake rate in relation to C , is rather described by a Michaelis-Menten type of curve, as shown by Barber (1979). However, in cases in which diffusion is the limiting process in transporting K+ to the root surface, the K+ concentration at the root surface is low ( d o p M ) , and in this low concentration range the relationship between K+ uptake and K+ concentration is more or less linear. If the time factor is integrated, K+ uptake can be described by the following equation:

M, = 21ratuC,t

(6)

c,

The term represents the average K+ concentration at the root surface during the uptake period t . The decrease of C , during this time is related to the replenishment of solution K+. The higher the K+ replenishment, the higher is the mean K+ concentration at the root surface. This K+ replenishment is controlled by the K+ buffer capacity (b) of the soil, which can be expressed as the ratio of K+ quantity (exchangeable K+) over K+ intensity (K+ concentration of the soil solution) (Mengel, 1974). Figure 2 from the data of Grimme et al. (197 1) shows the K+ buffer curve of a sandy soil and a loamy soil. From these curves it can be derived that if the same amount of K+ is absorbed by plants (quantity factor) from both soils, the K+ concentration in the soil solution of the sandy soil is depressed to a much higher extent than the K+ in the soil solution of the loamy soil. The steepness of the curve is a direct measure of the K+ buffer capacity. This K+ buffer capacity provides also some information about the K+-supplying power of a soil according

(c,)

me K+ in solution ( intensity )

FIG.2. K+ quantityhntensity relationship of a sandy and a clay soil. The steepness of the curves represents the K+ buffer capacity (after data of Grimme er al., 1971).

69


to the following equation (Nye, 1979):

U, = bdC,/dt

(6a)

where U, = K uptake r a t e h i t soil volume. It is evident from Eq. (6a) that the K + concentration at the root surface and the K+ buffer capacity are most important factors controlling K+ uptake of plants. The K+ concentration at the root surface is difficult to measure. It is related to the average K+ soil solution concentration according to an equation established by Baldwin et al. (1973): +

a a av X c, = C,/(l + InD*Bfl 1.65a -

)

(7)

where v x

D*

= flux of water through the root surface = radius of the depletion zone =

dispersion coefficient of the solute in the soil solution (cm' sec-')

This equation takes into account the mass flow (v).The term D* stands for the diffusion coefficient. According to Nye (1979) the dispersion coefficient is more appropriate than the molecular diffusion coefficient. Under normal plant water consumption D* is unlikely to exceed the molecular diffusion coefficient by more than a factor of 2. Major factors controlling C , are the K+ concentration in the bulk soil solution ( C , )and the root absorbing power (a).the former increasing and the latter decreasing the K+ concentration at the root surface. Also the extension of the depletion zone has an influence on C,; the larger the extension, the higher is the K+ concentration at the root surface. The theory of K+ flux toward plant roots as outlined above has been tested with young maize plants by Claassen and Barber (1976), using the following equation for the calculation of plant K+ uptake:

where I, I,,, K, C, E

= = = =

=

K+ uptake rate (influx) influx rate at infinite K+ concentration Michaelis-Menten constant

K+ concentration at the root surface K+ efflux

C , was computed from an equation established by Nye and Maniott (1969). From the plot CJC, versus the radial distance from the root, the K+ concentration

70


at the root surface (C,) was derived. Thus it was possible to calculate the K+ uptake according to soil and root parameters and to test this calculation by actual K+ plant uptake. Although four different soils with varying K+ levels were included in this experiment a fairly good correlation (Rz= 0.87) was obtained between the predicted and experimental K+ uptake. Calculated K+ uptake was overestimated by about 50% possibly because competition occurred between roots for soil K+. The good agreement between the predicted Kf uptake and the actual K+ uptake of plants proves that the theory of K+ flux and diffusion in soils is based on sound assumptions. The mathematical model used by Claassen and Barber (1976) takes into account the following factors: effective average diffusion coefficient, initial K+ concentration in the soil solution, and the buffer capacity. Hence these factors are the most important parameters controlling K+ availability in soils. C. ASSESSMENT OF K AVAILABILITY I N SOIL

Potassium concentration in soil solution, K+ buffer capacity, and the soil diffusivity should be considered in estimating K+ availability for practical purposes. Under practical farming conditions the soil diffusivity is difficult to assess in advance since it depends much on soil moisture. Potassium concentration in the soil solution and K+ buffer capacity until now have rarely been used in estimating soil K+ availability. In most cases exchangeable K+ is still regarded as a satisfactory measure of the K+ availability status of soils. This fraction, however; comprises both solution K+ and Kf adsorbed by varying strengths to adsorption sites (p-, e-, and even i-positions). Soils with the same values for exchangeable K+ may thus differ considerably in K+ concentrations in soil solution (Nemeth et al., 1970), because more selectively bound K+ is equilibrated with a relatively low K+ concentration and vice versa. If this specifically bound K+ is taken into account, exchangeable K+ may also be a good indicator of the K+ availability status. Rezk and Amer (1969) thus found a significant correlation between K+ uptake by plants and the “corrected” exchangeable K+ of the soil. This “correction” was obtained by dividing the exchangeable K+ through the Gapon coefficient. By this procedure numerical values are obtained that are closely related to the K+ concentration of the soil solution. Poor correlations between plant response and exchangeable K+ have been obtained especially in investigations where soils of different clay contents and degrees of K+ saturation have been used. Jankovic and Nemeth (1974) even found a negative correlation between the exchangeable soil K+ and sunflower seed yields harvested from five different sites. The same yields, however, were positively correlated with the K+ concentration of the soil solution. A close relationship between K+ concentration of the soil solution and the grain yield of


71

wheat, grown under field conditions, has also been reported by Nemeth and Harrach (1974). Similarly in studying the K+ availability of 21 different soils in a pot experiment with oats, v. Braunschweig and Mengel (1971) found a highly significant correlation between the K+ concentration of the soil solution and grain yield. More recently During and Duganzich (1979) have also reported that K+ uptake by white clover was best reflected by the K+ concentration of the soil solution. Exchangeable K+ alone correlated very poorly with uptake except in soils of very low K status. Recent experiments of Wanasuria et al. (1980) have shown that the K+ of paddy soils extracted by electroultrafiltration (EUF) was positively correlated with the grain yield, whereas no significant correlation with the exchangeable K+ was obtained. EUF-extractable K+ does reflect the K+ concentration of the soil solution (Nemeth, 1979). Although the K+ buffer capacity is of paramount importance for K+ availability, little quantitative data are available concerning its influence on K+ supply to crops. Nemeth (1975) in investigating three soils with different K+ buffer capacities found a close negative relationship between the K+ buffer capacity and the decrease in grass yield of four consecutive cuts harvested during the experimental period. Barrow (1966) reported that the correlation between the K+ uptake of clover and the content of exchangeable K+ was improved if in addition to the exchangeable K+, the K+ buffer capacity was also taken into account. Recent experimental results of Busch (1980) obtained in pot experiments with a number of soils differing widely in texture have shown that 50-80% of the variability in K+ uptake could be explained by the K+ buffer capacity and the K+ concentration of the soil solution. Only under extreme K+-deficiency conditions, was K+ uptake much controlled by other, still unknown factors. D. PLANTROOTSOILINTERACTIONS

The quantity of K+ absorbed by crops is also related to root growth, extension, and metabolism. Although root interception contributes only to a minor extent to the total K+ requirement of a crop, root extension and root density in the soil are of importance for the quantity of K+ accessible to plant roots. The extension of the K+ exploitation zone around a plant root represents the soil volume that can be "mined" for K+. Since K+ is mainly transported by diffusion toward plant roots, the bulk of K+ absorbed by plants originates from these zones around roots. It is easy to deduce from this relationship that a dense rooting system can exploit a larger soil volume for K+ than can a poor one. The rooting density ( L , ) can be defined as total root length per unit soil volume. The rooting density has an impact on the extension of the K+ exploitation zone around the root (Nye, 1979) according to the following relationship: x = 1 l(7rL.")~

72


where x = radius of exploitation. Thus in dense root systems, the K+ depletion zones around roots are less extended and often an overlapping of the exploitation (depletion) zone occurs. A further factor of importance is the soil volume accessible to plant roots. Pot experiments of Newman and Andrews (1973) have shown, for example, that if only a small soil volume is available, the amount of K+ absorbed by plants is also reduced. When the soil volume was restricted, dense rooting systems were observed and K+ uptake was inadequate. It seems likely that this resulted from root competition for K+ between overlapping depletion zones. Root extension and root mass are both of particular importance if available K+ in the soil is low (Chloupek, 1972). On the other hand, as has been shown by Maertens (1971) using young maize plants, only a small portion of a root system may suffice to ensure ample K+ uptake, provided the K+ availability is high. Thus, in general, the K+ absorption potential of a root segment by far exceeds the rate at which K+ is actually absorbed. In order to assess K+ uptake rates, K+ uptake should be calculated per unit root segment (e.g., cm root length). Such experiments and calculations have been canied out by Mengel and Barber (1974). These workers observed K+ uptake rates per unit length of plant root in the early weeks of plant development. Similar results have also been reported by Adepetu and Akapa (1977), who studied nutrient uptake of five cowpea cultivars. The K+ uptake rate per m root length of 5-day-old plants was four times higher than the K+ uptake rate of 30-day-old plants. These results clearly indicate that especially at an early growth stage, high K + availability is required and that the susceptibility to K deficiency is particularly marked during this period. Barber (1979) reported that maximum K+ uptake rates may differ considerably. Thus for young maize roots maximum K+ uptake rates of 2 pmole and for young soybean roots of 0.4 pmole K + cm-I sec were found (cm refers to root length). This author stresses the fact that the K+ content of the tops rather than the K+ content of the roots has a decisive influence on the maximum K+ uptake rate. Thus in 17-day-old maize plants maximum K+ uptake rates ranged between 1.3 and 4.0 pmole cm-' sec - I according to the K + content of the tops; rates being low in tops with high K+ contents and vice versa. The K+ absorption rate of the root is highly dependent on the root metabolism and particularly on respiration and thus on the carbohydrate content of the root (Mengel, 1967). Generally, younger plants have higher root carbohydrate contents than older plants, and even young root tips of older plants are less capable of absorbing K+ than root tips of younger plants (Vincent et al., 1979). Plant species and even cultivars of the same species may differ in their capability of exploiting soil K+. These differences can be explained in terms of rooting pattern and root metabolism, although the whole complex of K+ uptake by field crops growing in soil is still only poorly understood. Halevy (1977) compared +


73

two cotton cultivars differing in their capability of exploiting soil K+. The cultivar with the higher uptake potential for soil K+ was found to maintain vigorous root growth to a later growth stage than the cultivar with the poor K+ exploitation capability. This result suggests that root growth at a later stage in plant development in the higher K+ uptake potential cultivar was the cause of the difference in K+ uptake. A spectacular difference in the K+-exploiting capability exists between grasses and legumes. The legumes are inferior to grasses, and when grown together grasses successfully compete with legumes for soil K+. If abundant K+ is not available, the legumes suffer from K+ deficiency, whereas the grasses still grow vigorously and compete strongly for K+ (Blaser and Brady, 1950). This effect may be explained in part by differences in the rooting patterns of the two plant groups; although this explanation is not completely satisfactory. In this context an experiment of Malquori et al. (1975) is of particular interest, in which wheat and lucerne were grown in nutrient solutions. In one treatment where the K+ source of the nutrient solution was biotite, wheat was able to “extract” K+, whereas lucerne was not. This shows that of the two plant species, interlayer K+ was only accessible to wheat. Similar results have been obtained by Steffens and Mengel (1979), who grew ryegrass and red clover on a soil low in exchangeable K+. It was found that ryegrass was more capable of feeding from the nonexchangeable soil K+ than was clover. One may speculate as to the mechanism by which grasses are more able to utilize this interlayer K+ . In this context the results of Baligar and Barber ( 1978a) are of particular interest. These workers observed that after the addition of Rb+ to a number of different soils the K/Rb ratio of corn plants grown on the soils was more similar to the K/Rb ratio of the exchangeable soil fraction than to the K/Rb ratio of the soil solution. In an analogous experiment with onions the K/Rb ratio in the plants was found to be between the K/Rb ratio of the exchangeable fraction and the K/Rb ratio of the soil solution. Baligar and Barber (1978b) discuss their results in terms of exchange diffusion as proposed by Jenny (1966). Tinker (1978) in commenting on Baligar and Barber’s results suggests that the effect might be related to H+ excretion by roots displacing K+ and Rb+ from adsorbing sites around the root. This question needs to be investigated further. Exchange diffusion from the interlayer K+ of clay minerals to the surface of plant roots does not seem a likely mechanism for K+ release. In neither the experiment of Malquori et al. (1975) mentioned earlier nor the work of Ristori (1973, in which clay mineral was added to a nutrient solution, was evidence provided that close contact between clay mineral and root is essential for exploiting interlayer K+ . It is possible that an extremely low K+ concentration in the soil solution achieved by a high rate of K+ uptake is associated with a net release of interlayer K+ . Research in this direction merits further attention. The results of Drews (1978) are encouraging in this line of approach for he

74

KONRAD MENGEL AND ERNEST A . KIRKBY

found that plants grown under energy stress were less capable of exploiting interlayer K+ than control plants. The control plants also depressed the K+ concentration of the soil solution to a greater extent, and this may have resulted in a higher K+ release by clay minerals. According to Barber (1979) plant roots can deplete the K+ concentration of the nutrient solution to as low a level as 2 pM K+. Clay minerals are also capable of absorbing K+ from plants. In the experiment already mentioned by Malquori et al. (1975) in which biotite was added to a nutrient solution supplying wheat, these authors observed an expansion of biotite to a 14-h; peak indicative of K+ release. The maximum of the peak was obtained at the flowering stage. However, in the period following flowering the 14-h; peak declined, and the authors suggest that K+ released by plant roots after flowering was again fixed by interlayer adsorption sites. This observation is consistent with experiments of Kurdi and Babcock (1970), who found that at low K+ concentrapA4 K+)K was released by the roots and fixed by a tion in the root medium (4 bentonite suspension. The question of whether the excretion of H+ by plant roots can mobilize soil K+ is not yet clear. Newman (1969) found that at a low pH (3.5 to 4.0) the release rate of Kt from biotite was about twice as high as from more neutral pH. If a plant root depresses the pH of the rhizosphere due to H+ excretion, the release of interlayer K+ may therefore be promoted. Net Ht excretion of roots occurs when plants are fed with NHi-N (DeJaegere and Neirinckx, 1978) or in the case of legumes, when they are exclusively dependent on N fixation as an N source (Israel and Jackson, 1978). As yet, however, it is still uncertain whether this process of H+ release can mobilize interlayer K+ in quantities of importance in plant nutrition.

Ill. POTASSIUM IN PHYSIOLOGY A . POTASSIUM TRANSPORT ACROSS BIOLOGICAL MEMBRANES AND

CATION

COMPETITION

Physiology may be considered as a sequence of biochemical and biophysical reactions in living systems. For some of these reactions there is a direct or indirect association with K+, and it is for this reason that the entry of K+ into living systems merits attention. Of all cation species K+ is known to traverse biological membranes most rapidly. It has been shown by numerous authors that K uptake by plant cells is closely associated with metabolism, and especially with root respiration. Whether K+ is actively absorbed as defined by transport against an electrochemical potential is not completely clear. The application of the Nernst equation to studying electrochemical equilibria of ions in roots in bathing solutions has established this for other nutrient ions.


75

All anions are transported and accumulated against an electrochemical gradient (Bowling et al., 1966; Higinbotham et d., 1967), whereas the cations Ca 2+ and Mg2+invariably move into roots down an electrochemical gradient. The use of the Nernst equation for K+ transport studies provides evidence of both active accumulation (Pitman and Saddler, 1967; Bowling and Ansari, 1971) and passive equilibrium (Higinbotham et al., 1967; Pallaghy and Scott, 1969). The results of Etherton ( 1963) also indicate that the K+ concentration in the external solution can determine the form of uptake. At low external K+, the internal content was higher than that predicted by the electrochemical gradient indicating active uptake, whereas at high external concentrations the internal K+ was less than predicted and an outpump was suggested. To some extent the apparent discrepancy in the above results reflects the small differences obtained between observed internal concentrations of K+ and those predicted by the Nernst equations. Bowling (1976) has drawn attention to the possibility that since K+ is so mobile in plant tissues, passive diffusion may mask the activity of a K+ pump. The use of the Nernst equation may therefore be an inappropriate test for active K+ transport in higher plants. In reviewing the literature Higinbotham (1973) suggests that although there is good evidence for active K+ influx by algae a clear-cut case for higher plants has still to be made. Recent experiments of Cheeseman and Hanson (1979) with corn roots have shown that K+ can be taken up against an electrochemical gradient. The authors assume that this active K+ uptake is brought about by an ATPase which is inhibited by higher K+ concentrations and thus works only at concentrations

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