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i SOIL AND WATER QUALITY An Agenda for Agriculture Committee on Long-Range Soil and Water Conservation Board on Agriculture National Research Council NATIONAL ACADEMY PRESS Washington, D.C. 1993
ii NATIONAL ACADEMY PRESS 2101 Constitution Avenue Washington, D.C. 20418 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competencies and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sci- ences, the National Academy of Engineering, and the Institute of Medicine. This report has been prepared with funds provided by the U.S. Department of Agriculture, Soil Conservation Service, under agreement number 68-3A75-9-56; the U.S. Environmental Protection Agency, Office of Policy, Planning, and Evaluation under agreement number C X 818573-01-1; and The Joyce Foundation. Dissemination was supported in part by Pioneer Hi-Bred International, Inc., The Joyce Foundation, The W. K. Kellogg Foundation, the U.S. Department of Agriculture, Soil Conservation Service, and the Environmental Protection Agency, Office of Policy, Planning and Evaluation. Library of Congress Cataloging-in-Publication Data Soil and water quality: an agenda for agriculture/Committee on Long-Range Soil and Water Conservation, Board on Agriculture, National Research Council. p. cm. Includes bibliographical references and index. ISBN 0-309-04933-4 1. Soil managementâUnited States. 2. SoilsâUnited StatesâQuality. 3. Water quality managementâUnited States. 4. Sediment controlâUnited States. 5. Agricultural ecology âUnited States. I. National Research Council (U.S.). Committee on Long-Range Soil and Water Conservation. S599.A1S62 1993 333.76'0973âdc20 93-35470 CIP Â© 1993 by the National Academy of Sciences. All rights reserved. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the organizations or agencies that provided support for this project. Printed in the United States of America
iii Committee on Long-Range Soil and Water Conservation SANDRA S. BATIE, Chair, Michigan State University* J. WENDELL GILLIAM, North Carolina State University PETER M. GROFFMAN, Institute of Ecosystem Studies, Millbrook, New York GEORGE R. HALLBERG, Iowa Department of Natural Resources NEIL D. HAMILTON, Drake University Law School WILLIAM E. LARSON, University of Minnesota (Retired) LINDA K. LEE, University of Connecticut PETER J. NOWAK, University of Wisconsin KENNETH G. RENARD, Agricultural Research Service, U.S. Department of Agriculture, Tucson, Arizona RICHARD E. ROMINGER, A. H. Rominger and Sons, Winters, California+ B. A. STEWART, Agricultural Research Service, U.S. Department of Agriculture KENNETH K. TANJI, University of California JAN VAN SCHILFGAARDE, Agricultural Research Service, U.S. Department of Agriculture R. J. WAGENET, Cornell University DOUGLAS L. YOUNG, Washington State University Staff CRAIG COX, Project Director JOSEPH GAGNIER, Project Associate JANET OVERTON, Editor CRISTELLYN BANKS, Senior Secretary and Project Assistant * Virginia Polytechnic Institute & State University until September 1993. + Sworn in as Deputy Secretary, U.S. Department of Agriculture, May 12, 1993.
v Board on Agriculture THEODORE L. HULLAR, Chairman, University of California, Davis PHILIP H. ABELSON, American Association for the Advancement of Science, Washington, D.C. JOHN M. ANTLE, Montana State University DALE E. BAUMAN, Cornell University WILLIAM B. DELAUDER, Delaware State University SUSAN K. HARLANDER, Land O'Lakes, Inc., Minneapolis, Minnesota PAUL W. JOHNSON, Natural Resources Consultant, Decorah, Iowa T. KENT KIRK, U.S. Department of Agriculture, Forest Service, Madison, Wisconsin JAMES R. MOSELEY, Jim Moseley Farms, Inc., Clarks Hill, Indiana, and Purdue University DONALD R. NIELSEN, University of California, Davis NORMAN R. SCOTT, Cornell University GEORGE E. SEIDEL, JR., Colorado State University PATRICIA B. SWAN, Iowa State University JOHN R. WELSER, The Upjohn Company, Kalamazoo, Michigan FREDERIC WINTHROP, JR., The Trustees of Reservations,Beverly, Massachusetts SUSAN OFFUTT, Executive Director JAMES E. TAVARES, Associate Executive Director CARLA CARLSON, Director of Communications JANET OVERTON, Editor
vi The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Wm. A. Wulf is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academyâs purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. Wm. A. Wulf are chair and vice chair, respectively, of the National Research Council. www.national-academies.org
PREFACE vii PREFACE The list of environmental problems on the agricultural agenda has grown in the past 15 years. The long-standing concerns about soil erosion and sedimentation have been supplemented with new concerns about soil compaction, salinization, and loss of soil organic matter. The transfer of nitrates, phosphorus, pesticides, and salts from farming systems to surface water and groundwater has also become more important. Efforts to address the larger complex of environmental problems has been hampered by concerns about trade-offs. For example, best-management practices designed to reduce soil loss are now scrutinized for their role in increasing the leaching of nitrates and pesticides to groundwater. Other trade- offs arise between efforts to improve agriculture's environmental performance and efforts to reduce costs of production and maintain U.S. agriculture's share of world markets. In 1989 the Board on Agriculture of the National Research Council was asked to convene a committee to assess the science, technical tools, and policies needed to protect soil and water quality while providing for the production of food and fiber from U.S. croplands. More specifically, the committee was asked to â¢ investigate the threats to soil resources and recommend criteria to guide soil management; â¢ analyze fate and transport of agricultural chemicals to identify changes in farming systems required to improve water quality; â¢ identify remedial approaches that minimize trade-offs between
PREFACE viii improving soil or water quality, surface water or groundwater quality, or between different pollutants; and â¢ recommend policy and program options to improve long-term conservation of soil and water quality. The committee focused primarily on water quality, rather than water quantity, problems and on croplands rather than on forestlands or rangelands. The committee presents its work in two parts. Part One contains Chapters 1 through 4 and presents the committee's synthesis of the technical, economic and policy issues relating to soil and water quality. In Part Two, Chapters 5 through 12 describe in greater detail the scientific and technical knowledge on which the chapters in Part One are based. During its deliberative process, the committee first analyzed the physical, chemical, and biological processes that determine farming systems' impact on soil and water quality. The committee analyzed the effects of farming practices on soil, the role of soil in mediating the effect of farming systems on water quality, as well as the processes leading to the loss of nitrogen, phosphorus, pesticides, sediment, salts, and trace elements from farming systems. The committee studied the special problems posed by managing animal wastes and examined the important influence of the landscape in shaping the effects of farming systems on soil and water quality. The results of these analyses are presented in Chapters 5 through 12 of this report. The Appendix describes the methods used by the committee to estimate national, regional, and state nutrient budgets. Chapter 1 reviews the status of soil and water quality and discusses how and why emphasis has changed over the years from simply soil erosion and sedimentation to include soil degradation and water pollution. The presence of nutrients, pesticides, salts, and trace elements in crops, soil, and drinking water has created new problems that require new solutions. The search for solutions includes recognizing the importance of state and local policies as well as the needs and characteristics of the agricultural sector in efforts to improve soil and water quality. Based on the understanding gained by analyzing the processes that govern the interaction of farming systems and the environment, the committee identified promising opportunities for managing those processes in ways that protect soil and water quality and are profitable for the producer. The committee's analysis identified four major objectives for the management of soil and water resources: â¢ conserve and enhance soil quality as a fundamental first step to environmental improvement; â¢ increase nutrient, pesticide, and irrigation use efficiencies in farming systems;
PREFACE ix â¢ increase the resistance of farming systems to erosion and runoff; and â¢ make greater use of field and landscape buffer zones. These objectives and the technologies available to implement them in agricultural production are presented in Chapter 2. The task then became to develop strategies to implement those objectives and to identify the changes in concepts, technologies, and policies that might be needed. The farming system concept was central to the development of this report, and the need for a farming system approach at the farm enterprise, regional, and national levels underlies all of the recommendations that the committee developed. The advantages of using a farming systems approach to direct and target soil and water quality programs are presented in Chapter 3. Ultimately, to achieve long-term improvements in soil and water quality, the behaviors of some producers must be changed. A constant challenge in preparing this report was the attempt to link the social and economic factors that determine producer behavior with the physical, chemical, and biological factors that determine the effects of that behavior on soil and water quality. The committee used the understanding gained from studying these links to recommend a combination of policy and program reforms that will be needed to achieve long-term improvements in soil and water quality. The policy and program reforms recommended by the committee are discussed in Chapter 4. The debate over national policy to protect soil and water quality has intensified during the course of the committee's deliberation. The 1990 Food, Agriculture, Conservation and Trade Act and the 1990 Coastal Zone Act Reauthorization Amendments created new programs and new authorities that can be used to implement many of the committee's recommendations. Reauthorization of the Clean Water Act and the prospect of a new farm bill in 1995 provide more opportunities to move ahead with an agenda to protect soil and water quality. A great deal of progress could be madeâeven in the absence of new legislationâby integrating the multitude of federal, state, and local programs that are already addressing pieces of the soil and water quality problem. The opportunities to make current programs more effective are great and, in many cases, the authorities needed are already provided by legislation. It is the committee's hope that this report will help provide a framework to facilitate the integration of existing and new programs. SANDRA S. BATIE, Chair Committee on Long-Range Soil and Water Conservation Policy
ACKNOWLEDGMENTS xi Acknowledgments A report of this magnitude represents the combined efforts of many individuals from a variety of backgrounds. The committee thanks all those who contributed their ideas and experiences in technical and policy areas. During the course of its deliberations, the committee sought advice and special assistance. Among those who gave generously of their time were Raymond R. Allmaras, University of Minnesota; Ramon Aragues, Agricultural Research Service, Zaragosa, Spain; Peter E. Avers, U.S. Department of Agriculture, Forest Service at Washington, D.C.; Russell R. Bruce, U.S. Department of Agriculture, Agricultural Research Service at Watkinsville, Georgia; H. H. Cheng, University of Minnesota; C. V. Cole, U.S. Department of Agriculture, Agricultural Research Service at Colorado State University; Cornelia Butler Flora, Virginia Polytechnic Institute and State University; George R. Foster, U.S. Department of Agriculture, Agricultural Research Service at Oxford, Mississippi; Robert Grossman, U.S. Department of Agriculture, Soil Conservation Service at Lincoln, Nebraska; Benjamin F. Hajek, Auburn University; Roger Hanson, North Carolina State University; Fawzi Karajeh, University of California at Davis; Jean A. Molina, University of Minnesota; Gary B. Muckel, U.S. Department of Agriculture, Soil Conservation Service at Lincoln, Nebraska; Mathias J. Romkens, U.S. Department of Agriculture, Agricultural Research Service at Oxford, Mississippi; C. Ford Runge, University of Minnesota; David L. Schertz, U.S. Department of Agriculture, Soil Conservation Service at Washington, D.C.; Steven J. Taff, University of Minnesota; and Ward B. Voorhees, U.S.
ACKNOWLEDGMENTS xii Department of Agriculture, Agricultural Research Service at Morris, Minnesota. The committee is particularly grateful to Christopher D. Koss, President of the J. N. ''Ding" Darling Foundation, Key Biscayne, Florida, for his generous assistance in providing the four Ding Darling cartoons that help illustrate this report. The committee also acknowledges the special efforts of Amy Gorena, who served as a senior project assistant during the early stages of the study; Michael Hayes, who provided editorial expertise during development of the manuscript; and Rolla Chuang, who assisted as a student intern sponsored by the Midwest Universities Consortium for International Activities, Inc.
CONTENTS xiii Contents EXECUTIVE SUMMARY 1 Basic Concepts 1 The Agenda 4 Implementing the Agenda 8 Influencing Producers' Decisions 13 PART ONE 1. SOIL AND WATER QUALITY: NEW 21 PROBLEMS, NEW SOLUTIONS Soil and Water Quality Problems 21 Search for Solutions 30 Time to Move Ahead 34 2. OPPORTUNITIES TO IMPROVE 35 SOIL AND WATER QUALITY Conserving and Enhancing Soil Quality 38 Increasing Input Use Efficiencies 55 Increasing Resistance to Erosion and 95 Runoff Creating Field and Landscape Buffer 103 Zones 3. A SYSTEMS APPROACH TO SOIL 107 AND WATER QUALITY MAN- AGEMENT Linkages Among Objectives 107 Linkages Among Programs 108
CONTENTS xiv Advantages of the Farming Systems Approach 110 Farming System as Unit of Analysis and Management 113 Targeting Problem Areas and Farms 127 Implementing a Systems Approach 137 4. POLICIES TO PROTECT SOIL AND WATER QUAL- 145 ITY Environmental and Agricultural Policy 146 Factors Affecting Producers' Decisions 160 Continuum of Policies 162 PART TWO INTRODUCTION:SOIL, WATER, AND FARMING 187 SYSTEMS 5. MONITORING AND MANAGING SOIL QUALITY 189 Defining Soil Quality 190 Importance of Soil Quality 191 Importance of Monitoring Changes in Soil Quality 204 Extent of Degradation of U.S. Soils 218 6. NITROGEN IN THE SOIL-CROP SYSTEM 237 The Nitrogen Cycle 237 Nitrogen Mass Balance 240 Opportunities to Reduce Nitrogen Losses 266 7. PHOSPHORUS IN THE SOIL-CROP SYSTEM 283 The Problem of Phosphorus Delivery to Surface Waters 283 Sources of Phosphorus 284 Phosphorus in the Soil-Crop System 289 Transport Processes 299 Possible Management Methods for Phosphorus Loss 302 Reduction 8. FATE AND TRANSPORT OF PESTICIDES 313 Fate and Transport Processes 314 Reduction of Pesticide Pollution 329 Assessments of the Knowledge Base 333 Proper Use of Pesticides 334 9. FATE AND TRANSPORT OF SEDIMENTS 337 Effects of Erosion and Sedimentation 337 Sedimentation Processes 338
CONTENTS xv Sediment Estimation and Prediction 342 Technologies Treatment Technology 351 10. SALTS AND TRACE ELEMENTS 361 Overview of Salinity and Drainage Prob- 363 lems Sources and Effects of Salinity 369 Sources and Effects of Trace Elements 377 Alternative Management Options 387 11. MANURE AND NUTRIENT MAN- 399 AGEMENT Resource Utilization or Waste Disposal 399 Improving Manure Management 403 12. A LANDSCAPE APPROACH TO 417 AGRICULTURAL NONPOINT SOURCE POLLUTION Nonpoint Source Pollutant Attenuation 418 Mechanisms Process-Place Interactions 425 Implementing a Landscape Approach 426 APPENDIX 429 REFERENCES 449 GLOSSARY 489 ABOUT THE AUTHORS 495 INDEX 499
TABLES AND FIGURES xvi Tables and Figures TABLES 1-1 U.S. Department of Agriculture and U.S. Environmental 23 Protection Agency Soil and Water Quality Programs 1-2 Cropland and Pastureland Soils Affected by Saline or Sodic 26 Conditions 1-3 New Initiatives in the 1990 Food, Agriculture, Conservation 28 and Trade Act 2-1 Regional and National Estimates of Nitrogen Inputs, Out- 61 puts, and Balance on Croplands 2-2 Nitrogen Budgets for Four Farms (A, B, C, and D) in South- 62 eastern Minnesota 2-3 Crops Receiving Fertilizer Nitrogen Before, During, and 67 After Seeding 2-4 Regional and National Estimates of Phosphorus Inputs, Out- 72 puts, and Balances on Croplands 2-5 Percentage of Soil Tests Reporting High to Very High Lev- 74 els of Soil Phosphorus 2-6 Proportion of Cropland Soils Tested for Nutrient Levels, 76 Major Field Crops, 1989 2-7 Use of Integrated Pest Management for 12 Major Crops in 85 the United States, 1986 2-8 Highly Erodible, Not Highly Erodible, and Nondesignated 100 Lands on which Conservation or Conventional Tillage Systems Are Used for Various Crops, 1990
TABLES AND FIGURES xvii 3-1 Application of Farming System Approach at Different Geo- 114 graphic Scales 3-2 Ranking of Information Sources by Surveyed Farmers 126 3-3 Expenditures for Soil and Water Quality Programs as a Per- 143 centage of Expenditures on Pesticides, Synthetic Fertiliz- ers, and Commodity Programs 4-1 Constraints to Adopting New Technologies and Program 147 Responses to Nonadoption 5-1 Reference and Measured Values of Minimum Data Set for a 202 Hypothetical Typic Hapludoll from North-Central United States 5-2 Indicators of Change in Soil Quality and Their Relationship 208 to Components of Soil Quality 5-3 Some Pedotransfer Functions 212 5-4 Organic Carbon Additions Necessary to Maintain Soil 225 Organic Carbon at Present Levels at Several Locations 5-5 Amounts of Organic Carbon Needed Annually in Residue to 226 Maintain Soil Organic Carbon on Lands with Different Slopes and Erosion Levels 5-6 Extent of Salinity and Associated Problems by Land Use in 231 California 5-7 Salinity and Drainage Problems by Major Irrigated Areas 232 6-1 Nitrogen (N) Inputs, Outputs, and Balances in the United 241 States under the Low, Medium, and High Scenarios 6-2 Nitrogen Accumulation and Nitrogen Replacement Value 243 Estimated for Alfalfa and Soybeans 6-3 State and National Nitrogen Inputs and Outputs (metric tons) 244 6-4 State and National Nitrogen Contributions to Total Inputs 250 and Outputs 6-5 Nitrogen and Phosphorus Fertilizer Use: Top Ten States 256 6-6 Estimated Nitrogen Balance for Crop Production in the 262 United States, 1977 6-7 Potential Reductions in Nitrogen Fertilizer Applied to Corn 268 7-1 Phosphorus Inputs and Outputs in the United States, 1987 291 7-2 State and National Phosphorus Inputs and Outputs (metric 294 tons) 7-3 State and National Phosphorus Inputs and Outputs as Per- 296 centage of Total Mass of Phosphorus Inputs 7-4 Soils Testing Very Low to Medium or High to Very High 305 for Soil-P (percent) 8-1 Partition Coefficients and Half-Lives of Pesticides Used in 318 Florida
TABLES AND FIGURES xviii 9-1 Conservation Tillage Systems in the United States 353 9-2 Surface Soil Cover, Soil erosion, and Runoff from Different 354 Wheat Tillage Systems 9-3 Runoff and Soil Loss from Watersheds under Convention- 354 ally and Conservation Tilled Systems 9-4 Cropland Area under Various Forms of Conservation 355 Tillage, 1985 10-1 Concentration of Trace Elements Commonly Observed in 385 Forage Crops 10-2 Recommended Maximum Concentration of 15 Trace Ele- 386 ments in Irrigation Waters for Long-Term Protection of Plants and Animals 10-3 Total Removal by Crops of Cadmium and Zinc from Sludge- 389 Treated Greenfield Sandy Loam Soils, 1976â1981 11-1 Manure and Its Associated Nutrient Content 401 11-2 Economic Value of Nitrogen, Phosphorus, and Potassium in 401 Manures 11-3 Quantity of Livestock or Poultry Manure Needed to Supply 405 100 kg of Nitrogen over the Cropping Year with Repeated Applications of Manure A-1 Factors Used to Estimate Total Nitrogen and Phosphorus 435 Voided in Manures A-2 Nitrogen Voided in Recoverable Manures 436 A-3 Phosphorus Voided in Recoverable Manures 437 A-4 Estimates of Nitrogen Fixation by Legumes 438 A-5 Estimated Rates of Nitrogen Accumulation and Nitrogen 440 Replacement Value for Alfalfa and Soybeans in Low-, Medium-, and High-Fixation Scenarios A-6 Factors Used to Estimate Nitrogen and Phosphorus in Crop 442 Residues A-7 Nitrogen and Phosphorus Content of Harvested Crops 443 A-8 Inputs and Outputs of Nitrogen and Phosphorus on Crop- 445 lands in the United States, 1987 FIGURES 1-1 Percentage of land eroding by sheet and rill erosion at 25 greater than the soil loss tolerance level 1-2 Farm production regions used in this report 27 1-3 Sources and types of nonpoint source pollution in affected 29 U.S. rivers and lakes 1-4 Interactions of factors that influence producer's decisions 31
TABLES AND FIGURES xix 2-1 Changes in soil quality affect water quality 46 2-2 Nutrient cycle and pathways in agroecosystems 56 2-3 Pesticide pathways in agroecosystems 56 2-4 Irrigation pathways of water in agroecosystems 57 2-5 Economic return from insurance nitrogen (N) and deficit N 92 applications 2-6 Distribution of erosion events over 38 years on a field in 102 Missouri 3-1 Proportion of national nitrogen and phosphorus inputs and 111 balances contributed by each farm production region 3-2 Conceptual diagram of three-dimensional targeting 129 3-3 Use of a geographic information system to target and direct 136 soil and water quality programs 3-4 Conservation expenditures by the U.S. Department of Agri- 140 culture (USDA) and related state and local programs, 1983 to 1990 4-1 History of land set-aside programs in the United States as 173 cropland area reductions by type of program (1933â 1991) and net farm income (1945â1990) 4-2 States with water quality laws that affect agriculture 178 5-1 Processes of soil degradation 218 5-2 Interactions of factors that cause soil degradation 219 5-3 U.S. pH soil test summary as percentage of soils testing 6.0 235 or less in 1989 6-1 The nitrogen cycle 238 6-2 Amount of fertilizer-N and manure-N applied in relation to 265 annual average nitrate concentration in groundwater in Big Spring Basin, Iowa 6-3 Yield response of corn to nitrogen applied to three soils 276 6-4 Yield response of corn to fertilizer for three crop rotations 277 6-5 Nitrogen recovery related to fertilization rate 278 7-1 The phosphorus cycle 290 7-2 Relationship between broadcast phosphorus (PB) and 298 extractable soil phosphorus (Ps) 7-3 Economic returns on investments of annual applications of 306 phosphorus (P) fertilizers 7-4 Decrease of soil-P over time, measured as Mehlich 1- 308 extractable phosphorus, on Portsmouth soil during the residual phase 8-1 Interactions and loss pathways of organic chemicals (OCs) 315 in soils 8-2 Pesticide transport and transformation in the soil-plant envi- 316 ronment and the vadose zone 8-3 Mass balance of a hypothetical aerial foliar-spray applica- 324 tion of an insecticide
TABLES AND FIGURES xx 9-1 Crop residue levels on planted acreage by region in 1992 356 10-1 Typical salt accumulation patterns in surface soils for vari- 373 ous methods of water application 10-2 Detrimental effects of salinity on plant growth 374 10-3 Relative salt tolerance of agricultural crops 376 10-4 Possible abiotic and biotic processes affecting the reactivi- 379 ties and mobilities of trace elements 10-5 Total selenium concentrations in the top 30.5 cm (12 382 inches) of soil (A) and in shallow groundwater from 1984 to 1989 (B) in the San Joaquin Valley 10-6 Heavy metal contents in Greenfield sandy loam treated 384 with composted sludge from 1976 to 1981 10-7 Concentrations of selenium in tissues of various edible crops 388 11-1 Schematic of livestock-crop system showing gap in tradi- 403 tional manure recycling system because of use of rela- tively inexpensive fertilizers 11-2 Ratio of amount of manure produced to amount of crop- 408 land available for manure application 11-3 Average amount of manure nitrogen produced by animals 409 per unit area in relation to animal spacing 12-1 Conceptual diagram of a landscape showing potential for 419 grass vegetative filter strips and riparian buffer zones to intercept nonpoint source pollutants transported by sur- face water runoff and groundwater flow 12-2 Conceptual diagram comparing (A) cropland enrolled by 428 field in the Conservation Reserve Program (CRP) with (B) the same area of land set aside in riparian buffer zones
SOIL AND WATER QUALITY xxi SOIL AND WATER QUALITY
SOIL AND WATER QUALITY xxii