Advances in Agronomy, Volume 52

Advisory Board Martin Alexander Eugene J. Kamprath Cornell University North Carolina State University Kenneth J. Fr...

Author: Donald L. Sparks

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Advisory Board Martin Alexander

Eugene J. Kamprath

Cornell University

North Carolina State University

Kenneth J. Frey

Larry P. Wilding

Iowa State University

Texas A& M University

Prepared in cooperation with the

American Society of Agronomy Monographs Committee M. A. Tabatabai, Chairman S. H. Anderson D. M. Kral P. S. Baenziger S. E. Lingle W. T. Frankenberger, Jr. R. J. Luxmoore

G. A. Peterson S. R. Yates

S I N

T

52

Edited by

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

ACADEMIC PRESS A Division of Harcourt Brace & Company San Diego New York Boston London Sydney Tokyo Toronto

This book is printed on acid-free paper. @

Copyright 0 1994 by ACADEMIC PRESS, INC. All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Academic Press, Inc. 525 B Street, Suite 1900. San Diego, California 92101-4495 United Kingdom Edition published by

Academic Press Limited 2&28 Oval Road, London NWI 7DX

International Standard Serial Number: 0065-2 I I3 International Standard Book Number: 0- 12-000752-5

PRINTED IN THE UNITED STATES OF AMERICA 9 4 9 5 9 6 9 1 9 8 9 9

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9 8 1 6 5 4 3 2 1

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

vii ix

POULTRY WASTEMANAGEMENT: AGRICULTURAL AND ENVIRONMENTAL ISSUES J . T. Sims and D . C. Wolf

I . Poultry Waste Management: Contemporary Issues

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

11. Poultry Wastes: Production and Characteristics ................... I11. Nitrogen Management for Poultry Wastes ........................

n! Phosphorous Management for Poultry Wastes ....................

2 13 23 35

V. Trace Elements. Antibiotics. Pesticides. and Microorganisms in Poultry Wastes

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

VI. Poultry Waste Management Programs ............................ V I I. Conclusions ..................................................... References ......................................................

51 59 71 72

RAINWATERUTILIZATION EFFICIENCY IN RA~U-FED LOWLAND RICE Pradeep Kumar Sharma and Surjit K. De Datta I. Introduction .................................................... I1. Constraints .....................................................

I11. Potentials .......................................................

w.

Efficient Utilization of Rainwater ................................ V. Research Priorities .............................................. VI. Summary ....................................................... References ......................................................

85 87 91 92 112 112 113

WETLANDSOILSOF THEPRAIRIE POTHOLES

J . L . Richardson. James L . Arndt. and John Freeland I. Introduction .................................................... I1. Climate. Basic Hydrologic Concepts. and Wetland Classification . . 111. Geologic Factors ................................................ rv. Water Quality .................................................. V

121 124 138 141

vi

CONTENTS

V. Wetland Soil Properties ......................................... VI. Soil Sequences .................................................. VII. Soils on Prairie Pothole Edges ................................... VIII. Conclusions and Future Work ................................... References ......................................................

148 1SO 161 163 165

NEWDEVELOPMENTS AND PERSPECTIVES ON SOIL POTASSIUM QUANTITY/~NTENSITY RELATIONSHIPS

V. P. Evangelou. Jian Wang. and Ronald E . Phillips I. Introduction .................................................... I1. Electrochemical Considerations .................................. 111. Quantityhtensity ..............................................

IV. Basis of Molecular Interpretation of QuantityAntensity ........... V. Rapid Approaches for Quantity/Intensity Determinations ......... VI. Experimental Observations and Future Quantity/Intensity Applications .................................. References ......................................................

173 176 181 189 209

215 220

MORPHOLOGICAL AND PHYSIOLOGICAL TRAITS ASSOCIATED WITH WHEAT YIELD INCREASES

INMEDITERRANEAN ENVIRONMENTS Stephen P. Loss and K. H. M . Siddique I . Introduction .................................................... I1. Constraints in Mediterranean Environments ...................... 111. Biomass Production and Partitioning ............................. IV. Water Use ...................................................... V. Radiation Use ................................................... VI . High-Temperature Stress ........................................ VII . Use for Breeders ................................................ VIII . Concluding Comments .......................................... References ......................................................

229 232 236 251 258 261 262 265 266

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

277

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

JAMES L. ARNDT (12 I), Department of Soil Science, North Dakota State University, Fargo, North Dakota 581OS SURJIT K. DE DATTA (as), Office oflnternational Research and Development, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, and International Rice Research Institute, Manila, Philippines V. P. EVANGELOU (17 3 ) , Department of Agronomy, University o f Kentucky, Lexington, Kentucky 40546 JOHN FREELAND (12 l), Department of Soil Science, North Dakota State University, Fargo, North Dakota 58105 STEPHEN P. LOSS (229), Division of Plant Industries, Department of Agriculture, Western Australia, South Perth, Western Australia 6151, Australia RONALD E. PHILLIPS (17 3 ) , Department of Agronomy, University of Kentucky, Lexington, Kentucky 40546 J. L. RICHARDSON (121), Department of Soil Science, North Dakota State University, Fargo, North Dakota 58105 PRADEEP KUMAR SHARMA (as), Ubon Rice Research Center, Ubon Ratchathani 34000, Thailand, and International Rice Research Institute, Manila, Philippines K. H. M. SIDDIQUE (229), Division of Plant Industries, Department ofAgriculture, Western Australia, South Perth, Western Australia 6151, Australia J. T. SIMS (l), Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19717 JIAN WANG (17 3 ) , Department of Agronomy, University of Kentucky, Lexington, Kentucky 40546 D. C. WOLF (l), Department OfAgronomy, University ofArkansas, Fayetteville, Arkansas 72701

This Page Intentionally Left Blank

Preface Volume 52 includes a number of advances in the crop and soil sciences that should be of great interest to the readership. The first chapter is a comprehensive review of agricultural and environmental issues associated with poultry manure management, including discussions on production and characteristics of poultry wastes, nitrogen and phosphorous management of poultry wastes, trace elements, antibiotics, pesticides, and microorganisms in poultry waste, and poultry waste management programs. The second chapter discusses aspects of rainwater utilization efficiency in rain-fed lowland rice, including constraints, potentials, efficient utilization, and research priorities. The third chapter discusses wetland soils of the prairie potholes. Topics that are covered include climate, basic hydrologic concepts and wetland classification, geologic factors, water quality, wetland soil properties, soil sequences, and soils on the prairie pothole edges. The fourth chapter is a comprehensive review of the advances in soil quantity/ intensity ( Q / I )relationships, an index that has been widely employed over the years to assess nutrient availability in soils. Discussions on electrochemical considerations, quantitylintensity interpretations and applications, and rapid techniques for making Q/I measurements are included. The fifth chapter deals with morphological and physiological traits associated with wheat yield increases in Mediterranean environments and discusses constraints in these environments, biomass production and partitioning, water and radiation use, high-temperature stress, and use for plant breeders. I appreciate the fine contributions of the authors. DONALD L. SPARKS

ix

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POULTRY WASTEMANAGEMENT: AGRICULTURAL AND ENVIRONMENTAL ISSUES J, T. Simsl and D. C. Wolf* 'Department of Plant and Soil Sciences University of Delaware Newark, Delaware 19717 ZDepartment of Agronomy University of Arkansas Fayetteville, Arkansas 72701

I. Poultry Waste Management: Contemporary Issues A. Water Quality and Nutrient Management B. Pesticides, Antibiotics, and Heavy Metals in Poultry Wastes C. Dead Poultry Disposal 11. Poultry Wastes: Production and Characteristics A. Poultry Production Operations and Types of Waste B. Properties and Composition of Poultry Wastes C. Appropriate Use of Poultry Waste Analyses 111. Nitrogen Management for Poultry Wastes A. Forms in Poultry Wastes B. Nitrogen Transformations in Storage and Handling C. Nitrogen Losses Due to Drying Poultry Wastes D. Nitrogen Transformations in Soils E. Crop Response to Nitrogen in Poultry Wastes IV Phosphorous Management for Poultry Wastes A. Phosphorous Concentration and Form in Soils Amended with Poultry Wastes B. Phosphorous Retention and Movement in Soils Amended with Poultry Wastes V. Trace Elements, Antibiotics, Pesticides, and Microorganisms in Poultry Wastes A. Trace Elements B. Antibiotics, Coccidiostats, and Pesticides in Poultry Wastes C. Microbial Population of Poultry Wastes VI. Poultry Waste Management Programs A. Overview of Agricultural Management Plans for Poultry Wastes B. Nutrient Management Plans VII. Conclusions References 1

Advances in Agrnnmy, W u m e 12 Copyright Q 1994 by Academic Press, Inc. All rights of reproduction in any form reserved.

2

J. T. SLMS AND D. C. WOLF

I. POULTRY WASTE MANAGEMENT: CONTEMPORARY ISSUES The poultry industry is one of the largest and fastest growing livestock production systems in the world. Globally, almost 40 million metric tons of poultry meat and 600 billion eggs were produced in 1991 (Foreign Agricultural Service, 1992). The dominant producers of poultry meat and eggs are the United States, China, the former Soviet Union countries, Brazil, France, and Japan (Table I). On a worldwide basis, poultry meat and egg production is growing at an annual rate of approximately 5%. The economic impact of the poultry industry to global and national economies is significant and of increasing importance. For example, in 1991 the United States produced 6.1 billion broiler chickens, 285 million turkeys, and 68 billion eggs, with a total production value of $14.7 billion (Economic Research Service, 1992). In comparison, the total dollar value of poultry production in the United States in 1980 was $9 billion (National Agricultural Statistics Service, 1992). Much of the U.S. poultry production is for export purposes. In 1991 the United States exported approximately 623,000 metric tons of broiler and turkey meat and nearly 2 billion eggs. Major importers of U.S. poultry products were China (Hong Kong), Japan, Mexico, and Canada (Economic Research Service, 1992). The localized nature of poultry production also means that it can represent a large percentage of the agricultural economy in many states or regions. In Delaware, for example, the poultry industry accounts for nearly 70% of the total agricultural income in the state, with the value of processed and delivered broilers in 1991 equivalent to $1.2 billion (Delaware Department of Agriculture, 1992). Although economically successful, the poultry industry is currently faced with a number of highly complex and challenging environmental problems, many of which are related to its size and geographically concentrated nature. The development of management programs that meet the increasing demand for poultry products, while minimizing the environmental effects of poultry wastes on soils, crops, surface waters, and groundwaters, will be the focus of this article. Other environmentally related issues, such as air quality and odor control, disposal of dead or diseased poultry, food safety, and animal health and welfare, also confront the poultry industry. However, from an agricultural perspective, the role of poultry wastes in the contamination of groundwaters by nitrate nitrogen (NO,-N), the eutrophication of surface waters by nitrogen and phosphorus, and the fate of pesticides, heavy metals, and pathogens applied to soils in poultry wastes are the central environmental issues at the present time. This article will provide a brief overview of each of these issues, a description of the types and compositions of poultry wastes, and a review of recent research addressing the agricultural and environmental aspects of poultry waste manage-

3

POULTRY WASTE MANAGEMENT Table I Global Production of Poultry Meat and Eggs and Recent Growth in the Poultry Industry Poultry meat production (lo00 Mg RTC" equivalents) Country

North America Canada Mexico United States South America Argentina Brazil Venezuela Europe France Germany Italy The Netherlands Spain United Kingdom Eastern Europe Hungary Poland Romania Former Soviet Union (includes 12 countries) Africa and Middle East Egypt South Africa Saudi Arabia Turkey Asia and Oceania Australia China Japan South Korea Taiwan All other countries Total

Egg production (million pieces)

1988

1993b

1988

1993

656 592 9272

727 1040 12,157

5721 17,859 69,410

5630 21,110 70,200

370 1997 373

520 3195 34 1

3300 14,850 2700

4730 14,750 2400

1434 576 996 485 829 1056

1870 640 1056 565 864 1260

15,300 17,960 I 1,234 10,761 10,856 11,736

15,700 15,600 1 1.570 10,800 10,400 1 1,420

465 35 1 370 3107

350 350 3 10 2527

4695 8220 7650 82,204

4100 7500 7200 65,250

279 545 248 236

225 560 290 335

2840 3723 2765 6200

3000 4355 3040 8100

40 1 2744 1471 235 418 3187 32,693

455 5200 1370 350 510 3856 40,923

3238 139,100 40,137 7204 4400 34,129 538,192

3784 20,500 43 ,Ooo 8500 4800 33,177 595.1 16

"RTC, Ready to cook. 1993 values as forecast by USDA Foreign Agricultural Service.

4

J. T. SIMS AND D. C. WOLF

ment. We will conclude by describing current best management practices for the use of poultry wastes in agriculture and by offering alternative approaches that may reduce the environmental impacts of poultry wastes.

A. WATERQUALITY AND NUTRIENT MANAGEMENT Poultry wastes contain all essential plant nutrients (C, N, P, K, S , Ca, Mg, B, Cu, Fe, Mn, Mo, and Zn) and have been well-documented to be excellent fertilizers (Bouldin e? al., 1984; Edwards and Daniel, 1992; Hileman, 1967b; Pennsylvania State College, 1944; Perkins et al., 1964; Simpson, 1990; Sims, 1987; Sommers and Sutton, 1980; Stephenson et al., 1990; Wilkinson, 1979). However, improper management of poultry wastes has been shown to contribute to NO,-N pollution of groundwaters and eutrophication of surface waters (Edwards and Daniel, 1992; Liebhardt et al., 1979; Magette et al., 1989; Ritter and Chirnside, 1987; Weil ef al., 1990). Groundwater contamination by N03-N is an issue of global concern; the causes and related environmental effects of NO,-N pollution have been discussed in a number of comprehensive review articles [Greenwood, 1990; Keeney, 1982; Strebel et al., 1989; U.S. Department of Agriculture (USDA), 19911. In brief, the basis for much of this concern is the potential effects of NO,-N on the health of human infants and animals. Infants younger than 3 months of age that consume water contaminated with NO,-N are susceptible to methemoglobinemia, also referred to as “blue-baby syndrome.” Methemoglobinemia is not caused directly by NO; but occurs when NO; is reduced to nitrite (NO:) by bacteria found in the digestive tract of human infants and animals. Nitrite can then oxidize the iron in the hemoglobin molecule from Fez to Fe3+,forming methemoglobin, which cannot perform the essential oxygen transport functions of hemoglobin. This can result in a bluish coloration of the skin in infants, hence the origin of the term blue-baby syndrome. Methemoglobinemia is a much more serious problem for very young infants than for adults, because after the age of 3-6 months the acidity in the human stomach increases to a level adequate to suppress the activity of the bacteria that reduce NOT to N O ? . Although documented cases of methemoglobinemia are extremely rare, the U. S . Environmental Protection Agency has established a maximum contaminant level of 10 mg N03-N/ liter (45 mg NOJliter) to protect the safety of U.S. drinking water supplies [U.S. Environmental Protection Agency (USEPA), 19851. The European Economic Community (EEC) (1980) has established a similar standard of 1 1 mg N03-N/ liter (50 mg NOJliter). Animals can also be susceptible to methemoglobinemia, although the health advisory level for most livestock is much higher, approximately 40 mg NO,-N/liter (180 mg NOJiter). Eutrophication is defined as an increase in the nutrient status of natural waters +

POULTRY WASTE MANAGEMENT

5

that causes accelerated growth of algae or water plants, depletion of dissolved oxygen, increased turbidity, and a general degradation of water quality. The enrichment of lakes, ponds, bays, and estuaries by N and P from surface runoff or groundwater discharge is known to be a contributing factor to eutrophication. The levels of N required to induce eutrophication in fresh and estuarine waters are much lower than the values associated with drinking water contamination. Although estimates vary, and depend considerably on the N:P ratio in the water, concentrations of 0.5 to 1.O mg N/liter are commonly used as threshold values for eutrophication. Marine or estuarine environments, where salinity levels are greater, are more sensitive to eutrophication and thus have lower threshold levels of N (c0.6 mg N/liter) (USDA, 1991). The eutrophication threshold for most P-limited aquatic systems is even lower, ranging from 10 to 100 p g P/liter (Mason, 1991). Water bodies with naturally low P concentrations will, therefore, be highly sensitive to external inputs of P. Once eutrophic conditions are established, algal blooms and other ecologically damaging effects can occur, including low dissolved oxygen levels, excessive aquatic weed growth, increased sedimentation, and greater turbidity. Decreased oxygenation is the primary negative effect of eutrophication because low dissolved oxygen levels seriously limit the growth and diversity of aquatic biota and, under extreme conditions, cause fish kills. The increased biomass resulting from eutrophication causes the depletion of oxygen, especially during the microbial decomposition of plant and algal residues. Under the more turbid conditions common to eutrophic lakes, light penetration into lower depths of the water body is decreased, resulting in reduced growth of subsurface plants and benthic (bottom-living) organisms. In addition to ecological damage, eutrophication can increase the economic costs of maintaining surface waters for recreational and navigational purposes. Algal surface scums, foul odors, insect problems, impeded water flow and boating due to aquatic weeds, shallower lakes that must be dredged to remove sediment, and disappearance of desirable fish communities are among the most commonly reported undesirable effects of eutrophication. Strebel et al. (1989) cited three main causes of NO,-N pollution of groundwaters in Europe: (1) intensified plant production and increased use of N fertilizers, (2) intensified livestock production with high livestock densities that cause enormous production of animal wastes on an inadequate agricultural land base, and (3) conversion of large areas of permanent grassland to arable land. Eutrophication of surface waters by N and P reflects both the contribution of agricultural inputs that are primarily nonpoint in nature, such as soil erosion and runoff, and inputs from direct discharge of wastewaters from municipalities, industry, urban stormwater systems, and recreational developments (Mason, 1991 ). Atmospheric deposition of N, as both precipitation (“acid rain,” primarily as nitric acid, HNO,) and particulate matter, and fixation of atmospheric N by aquatic organisms also contribute to the total pool of N in surface waters. Am-

6

J. T. SIMS AND D. C. WOLF

monia gas that has volatilized from areas of concentrated animal production may also be deposited by precipitation in nearby surface waters. Groundwater and surface water contamination by N and P in poultry wastes is primarily an issue of nonpoint source pollution. The manures, litters, sludges, composts, and wastewaters originating from poultry production operations are normally used in large-scale land application programs and are rarely concentrated enough to be considered a point source of N or P. Some exceptions exist, such as manure storage areas, the direct discharge of wastewaters from poultry processing plants into streams or rivers, and the disposal of large quantities of dead poultry in landfills due to a major disease outbreak. Situations such as these are subject to regulation and long-term monitoring by environmental protection agencies and will not be discussed in this article. We will focus on nonpoint source pollution caused by poultry wastes used for the production of agricultural crops. The causes and management of N and P pollution from poultry wastes can be viewed at essentially three scales: field, farm, and regional. At the smallest scale, such as an agricultural field where poultry manure is used as a fertilizer, the overapplication or poorly timed application of manure can result in excess nutrients in the soil and/or enhanced losses of nutrients by physical processes such as leaching, erosion, runoff, or volatilization. At the farm scale, wherein literally hundreds of thousands of animal units can be produced annually on only a few hectares of land, the environmental issue is the availability of adequate cropland to use the nutrients generated in the production and processing operations. A similar scenario exists at a state or regional perspective; however, at this level management of poultry wastes must be integrated into a broader nutrient management program that considers all sources of nutrients, including commercial fertilizers, legumes, and municipal sludges, composts, and wastewaters. It is imperative to keep the issue of scale in mind when addressing nutrient management of poultry wastes. Management programs that identify proper application rates and techniques for individual fields are of little value if a farm or region has an enormous surplus of waste. Larger scale solutions must be developed that address surpluses at the farm and regional level. Poultry production is often highly localized within a state or region. In the United States, 90% of the 6.1 billion broiler chickens produced in 1991 were grown in 15 states; 55% of the eggs were produced in eight states (National Agricultural Statistics Service, 1992). This localization has often been due to favorable transportation, marketing, or climatic conditions. Unfortunately, many areas in the United States where the poultry industry is concentrated are unfavorable from the point of view of effective use of the wastes generated by the industry. Two examples of the nature of environmental problems that can arise when the poultry industry is concentrated in relatively small geographic area are the Delmarva (Delaware-Maryland-Virginia) peninsula and northwestern Arkansas.

POULTRY WASTE M ~ A ~ E M E N T

7

1. Nutrient Management and Water Quality: The Delmarva Peninsula In 1991 over 537 million broiler chickens were produced on the Defmarva ~ n i n s u l a an , area with about 800,000 ha of cropland (W. Satterfield, Delmarva Poultry Industry, Inc., personal communication). More than 220 million broilers were produced in Sussex County, Delaware, alone, generating an estimated 270,000 Mg of manure (wet weight basis). The annual economic value of the nutrients in this manure, using current estimates (Stephenson et a f . , 1990), would be approximately $8 to $10 million. Virtually all of this manure is used in land application programs for the approximately 120,000 ha of grain crops and vegetables grown in the county. Approximately 50% of the cropland is used for soybeans (Gtycine mux L.), which require no fertilizer or manure N. Current manure recommendations for corn (Zea mays L.), wheat ( ~ r j ~ jaes~ivum c u ~ L.), barley (Hordeurn vulgaris), and vegetables typically range from 4 to 8 Mg/ha (no manure is recommended for soybeans). Based on these estimates, the manure generated by the poultry industry could supply essentially all nutrients needed by all crops, if it were evenly distributed throughout this county. Unfortunately, the unfavorable economics of manure tr~sportationcurrently prevent movement of manure more than a few kilometers. Further complicating the nutrient management issue is the fact that fertilizer consumption (sales) in Delaware averaged 175 kg N/ha (soybeans excluded) and 16 kg P/ha (a11 crops) (Delaware Department of Agriculture, 1992). Beyond this, the rapidly urbanizing nature of Deiaware and many other northeastern states may mean that more cropland will be needed for land application of the municipal wastes and wastewaters generated, and thus less cropland will be available for poultry waste application. Finally, although location of the poultry industry on the Delmarva peninsula makes economic sense, because of the ready access to literally tens of millions of consumers in the eastern United States, from a water quality perspective the geographic location presents major problems. The peninsula is dominated by coarse, welldrained soils that overlie shallow water tables (often less than 5 m), in a temperate area with plentiful rainfull (- 125 cm/year). Groundwaters discharge into highly sensitive and important surface waters, including the Chesapeake Bay, the Delaware Bay, and Delaware’s Inland Bays (a national estuary). The relatively flat topography of the peninsula reduces erosion and runoff, but enhances infiltration and groundwater recharge. Groundwater NO,-N concentrations in many areas of this peninsula commonly exceed the 10 mg Nlfiter drinking water standard established by the U.S. EPA (Hamilton and Shedlock, 1992). Ritter and Chirnside (1987) surveyed more than 200 wells in southern Delaware, 70% of which were from individual homes. They reported that more than 34% of the wells tested in Sussex County had NO,-N concentrations in excess of 10 mg

J. T. SIMS AND D. C. WOLF

N/liter and cited intensive agricultural activity, particularly land application of poultry manure, as the cause. Concentration of the poultry industry in an area without adequate cropland can also result in the accumulation of soil P to excessive levels. Most land management programs for poultry wastes are based on N management to reduce the likelihood of groundwater contamination by NO,-N. The N : P ratio of poultry wastes, however, usually results in the addition of P beyond crop removal in harvested biomass, except in extremely P-deficient soils. For example, application of poultry manure at the rate normally recommended to meet the N requirements of corn (5 Mg/ha, dry weight basis), at yield goals typical to the Delmarva peninsula (7 Mg/ha), adds about 135 kg P/ha to the soil, relative to P removal of approximately 25 kg P/ha in harvested corn grain. The net effect of N-based manure management, therefore, is ever-increasing soil P levels. Recent soil test information summaries from the state of Delaware confirm this P buildup in manured soils. Soil test summaries from 1991 to 1992 for Sussex County, Delaware showed that 77% of soil samples from agricultural fields had high or exces0.025 N H,SO,); 28% had sive levels of soil test P (Mehlich 1, 0.05 N HCI soil test P values in excess of 140 mg P/kg, twice the level at which no fertilizer P would be recommended (K. L. Schilke-Gartley, University of Delaware, personal communication). Mozaffari and Sims (1994) measured soil test P in the surface horizons (0-20 cm) of 48 cultivated fields from Sussex County with a history of frequent manure use. The median value for soil test P was 128 mg P/kg; 9 of the 48 soils were rated as high in P (>35 mg P/kg) and 35 as excessive in P (>70 mg P/kg). Other surveys of soil test P in areas dominated by animalbased agriculture have shown similar trends. Baker (1986) sampled 70 agricultural fields in Lancaster County, Pennsylvania and found that the soil test P (Bray P1, 0.03 N NH,F 0.025 N HCI) levels averaged 131 mg P/kg (range = 36 to 41 1 mg P/kg), relative to a desired value of 50 mg P/kg. The fate and environmental impacts of P from poultry wastes are discussed in more detail in Section IV. Clearly, however, in areas where surface waters are sensitive to eutrophication, effective P management of poultry wastes is critical. This management must include an understanding not only of how manure P reacts with soils, but of the processes that can transport P from waste-amended soils to surface waters, such as erosion, runoff, artificial drainage, and, in certain excessively well-drained soils, leaching and groundwater discharge.

+

+

2. Nutrient Management and Water Quality: Arkansas In 1991, Arkansas ranked first in the United States in poultry production with over 980 million broilers, fourth in turkey production with 24 million turkeys, and sixth in egg production with 3.7 billion eggs (Arkansas Agricultural Statis-

9

POULTRY WASTE MANAGEMENT Table I1 Number of Poultry and Quantity of Poultry Waste Produced in Arkansas and Delaware during 1991

Source

Arkansas Broilers Turkeys Laying hens Delaware Broilers Laying hens

Waste Total Produced waste Number per bird produced (millions) (dry kg) (dry Gg)

Typical level (%)

Total produced (Gg)

N

P

K

N

P

K

36 12 6

13 8 1

19 14 4

980 24 16

0.9 18.6 12.7

882 446 203

4.1 2.8 3.0

1.5 1.7 3.3

2.2 3.2 2.2

220 0.7

0.9 12.7

198 9

4.1 3.0

1.5 3.3

2.2 2.2

8 0.3

3 0.3

4 0.2

tics Service, 1992). The value of commercial broiler, turkey, and egg production was approximately $1.37 billion, $186 million, and $286 million, respectively. The total farm value of poultry and eggs produced in 1991 in Arkansas was $1,851,925,000 (National Agricultural Statistics Service, 1992). In addition to the meat and eggs produced, the poultry industry in Arkansas, as in Delaware, generated substantial quantities of poultry waste (Table 11). Estimates for the amount of nutrients contained in the poultry waste would suggest that the value of waste material as fertilizer would be $28 million to $40 million in 1991 (J. T. Gilmour, unpublished data). Stephenson et al. (1990) and Smith and Wheeler (1979) have calculated the fertilizer value of broiler litter as $31.23/Mg and $32.67/Mg, respectively. The majority of the poultry waste is recycled as an organic amendment on pastureland in western Arkansas. In fact, the increase in broiler production in Arkansas has been paralleled by an increase in beef production largely due to the availability of an economical source of fertilizer in the form of poultry waste. Broiler litter has been used extensively on tall fescue (Festuca arundinacea Schreb.) and bermuda grass [Cynodon dactylon (L.) Pers.] pastures. The annual maximum broiler litter application rate for cool-season grasses recommended by the University of Arkansas Cooperative Extension Service is 9 Mg/ha, with no more than 5.6 Mg/ha in a single application. The USDA Soil Conservation Service recommendation is 6.7 Mg/ha per year with no more than 3.4 Mg/ha in a single application. Both recommendations are based on providing adequate N fertility for forage production, as is common in most state animal waste application programs (Wallingford el al., 1975). Arkansas, Delaware, and most other states do not currently consider P or heavy metals as limiting factors

10

J. T. SIMS AND D. C. WOLF

in land application of poultry waste. However, excessive P levels are increasingly being recognized as a limitation for poultry waste application to soils (see Section IV). Because the soils in the Ozark region tend to be shallow and are often over limestone aquifers that are used as sources of drinking water, increasing concern has been expressed regarding the role of poultry litter in NO3-N and fecal coliform contamination of groundwater (Daniel et al., 1992; Wolf, 1992; Wolf and Daniel, 1989). Edwards and Daniel (1992) recently presented an excellent review of the environmental impact of on-farm poultry waste disposal. Steele and McCalister (1991) reported that well water from a poultryproducing area averaged 2.83 mg NO,-N/liter compared to 1.73 mg NO,-N/liter for a forested control area in the Ozark region of northwestern Arkansas. The NO,-N levels in springs were also evaluated and ranged from 2.58 to 3.23 mg/ liter in the poultry-producing area, compared to 0.02 to 0.40 mglliter in the control area (Adamski and Steele, 1988). Scott et al. (1992) reported data from the sampling of 63 wells and 18 springs in a poultry-producing area of northwestern Arkansas and reported median NO,-N concentrations of 0.4 and 3.2 mg/ liter, respectively. However, 20 of the wells and 10 of the springs had median NO3-N levels of 5.6 and 5.9 mg/liter, respectively. These findings suggest that application of poultry litter to pasture land had adversely impacted groundwater quality as shown by NO,-N concentrations above the 3 mg/liter level in wells and springs. However, preliminary results from a recent survey of domestic well water samples in northwestern Arkansas suggest that less than 5% of the samples collected exceeded the 10 mg N/liter maximum concentration limit set by the U.S. Environmental Protection Agency (S. L. Chapman, personal communication, 1992). In addition to NO,-N contamination of groundwater, surface runoff can contaminate lakes and streams with P and result in eutrophication. Because land application rates for poultry waste are generally derived from plant requirements for N, excessive levels of P can be applied to and accumulate in the soil. The 1989 summary of soil test results for over 2000 soil samples collected from pastures in selected Arkansas counties showed that the addition of manure had resulted in large increases in available P and modest increases in extractable K in soils with a history of manure application (J. T. Gilmour, unpublished data). This summary showed soil test P (Mehlich 3, 0.2 N CH,COOH + 0.025 N NH,NO, + 0.015 N NH,F 0.013 N HNO, + 0.001 M EDTA) increased from a weighted mean of 59 mg P/kg for soils that had not been amended with manure to 106 mg P/kg in soils amended with manure. Fertilizer P is not recommended for forage production when soil test levels are >50 mg P/kg. Extractable K was also increased by manure addition from 142 mg K/kg in nonamended soils to 168 mg K/kg in soils amended with manure. No fertilizer K is recommended when soil test levels are >150 mg K/kg. Because P addition to lakes

+

POULTRY WASTE MANAGEMENT

I1

and streams can often be the critical nutrient to initiate the eutrophication process, concern regarding high P levels in manure-amended soils continues to grow (Decker, 1992). Erosion of surface soil with high P concentrations can represent a potentially serious environmental problem as does direct transport of soluble P or surface-applied poultry waste into water systems. Contamination of groundwater and surface water with pathogenic microorganisms is also an important environmental concern. Fecal coliform and Escherichia coli are generally used as indicators of pathogens in water sources. Runoff from areas where poultry waste has been applied can contaminate surface water with fecal microorganisms. In northwestern Arkansas, fecal coliform levels often exceed the 200 fecal coliforms/100 ml limit established for primary contact water, and poultry waste applied to pasture land may often be the primary source of fecal coliforms (Arkansas Department of Pollution Control and Ecology, 1992). Because nutrient and bacterial contamination of groundwater and surface water has had such an important impact on drinking and recreational water sources in Arkansas, there is little doubt that greater attention will be focused on management practices to protect water quality and recycle nutrients in poultry waste in the poultry-forage-beef production systems that dominate production agriculture in the state.

B. PESTICIDES, ANTIBIOTICS, AND HEAVY METALS INPOULTRY WASTES Nutrients are not the only constituents of poultry wastes that can have an environmental impact. Pesticides used to control insects in poultry houses and heavy metals, antibiotics, and coccidiostats used as feed additives for nutritional or disease-related purposes are also of concern. Limited research, however, has been conducted on the fate of these waste constituents following their application to agricultural soils. Pesticide degradation and mobility in soils are issues of great national interest. Most studies have evaluated the fate of pesticides directly applied to soils for the control of weeds, insects, or pathogens. One example of a pesticide used in poultry production is cyromazine, an s-triazine larvacide that is mixed with poultry feed and passed through the animal to control fly populations in broiler houses. Recent preliminary research has shown that heavy manure applications and intensive rainfall can cause cyromazine losses in runoff (Pote et al., 1994). Antibiotics and coccidiostats include compounds such as amprolium, salinomycin, streptomycin, tetracycline, and terramycin. Very little research has been conducted on the environmental fate of any of these chemicals after manure or litter containing them is applied to the soil.

12

J. T. SIMS AND D. C. WOLF

Heavy metals are often the land-limiting constituent in organic waste management programs for municipalities and industries. As an example, in Delaware, the length of time an agricultural field can receive municipal sewage sludge is ultimately based on total heavy metal inputs. Lifetime site loading rates currently used for Cd, Cu, Ni, Pb, and Zn applied to a soil with a cation exchange capacity between 0 and 5 cmol/kg are 5, 140, 140, 560, and 280 kg/ha. Heavy metal concentrations in poultry wastes can be similar to or even exceed those reported for domestic sewage treatment plants. Metals are normally added to the poultry diet as salts, such as CuSO,, NaSeO,, or as acids, such as 3-nitro-4-hydroxyphenylarsonic acid; they may also occur naturally in the grains used in the diet. The median values for As, Cd, Cr, Cu, Ni, Pb, and Zn reported for sewage sludge in the northeastern United States were 10, 15, 500, 800, 80, 500, and 1700 mg/kg, respectively (Baker, 1985). Malone et al. (1992) collected broiler litter samples from 60 poultry farms in Delaware and found that Cu and Zn values ranged from 289 to 920 and 315 to 680 mg/kg. Analyses of 275 manure samples submitted by farmers to the University of Maryland from 1985 to 1989 had average values of 168 and 223 mg/kg for Cu and Zn; maximum values were 527 and 620 mg/kg, respectively (Bandel, 1988). Kunkle et al. (1981) reported average As, Cd, Cu, Hg, Pb, and Se values after five flocks of broiler chickens were 35, 0.5, 319, 0.3, 3, and 0.3 mg/kg. The addition of heavy metals in poultry wastes to soils is not regulated at the present time, despite the similarity in heavy metal concentrations noted with wastes that are regulated. This suggests that research on,the fate of metals in soils amended with poultry wastes may be needed to determine if guidelines or regulations similar to those mandated for municipal and industrial wastes are necessary for poultry wastes.

C. DEADPOULTRY DISPOSAL Animal mortality, a common problem in the poultry industry, can result in significant waste disposal problems for farmers; these problems can be enormously greater if a major disease outbreak occurs. In 1991 more than 36 million chickens, excluding broilers, were lost due to mortality (National Agricultural Statistics Service, 1992). The number of broilers lost is more difficult to estimate given the large number of individual farmers involved in broiler production. However, based on the normal mortality estimates of 2-3% commonly used for broilers by the poultry industry, over 120 million broilers die and must be disposed of each year. Until recently, on-farm disposal has normally involved burying the dead poultry in large pits, with little if any consideration given to the potential for groundwater pollution as the carcasses decompose. Recent advances in composting and farm-based acid-rendering tanks have provided some alternatives for normal mortality, but are still inadequate to handle catastrophic losses involving tens of thousands of birds. Further, the possible transmission of dis-

POULTRY WASTE MANAGEMENT

13

h

m .-

fcn

400

'

+ Poultry Compost

Y

Y

aJ

Y

2 n

300

Figure 1 Effect of composting raw poultry manure on the rate and extent of N mineralization in an Evesboro loamy sand soil, relative to the typical pattern of N uptake by corn (Sims er a [ . , 1993).

ease organisms during the handling and land application of dead poultry composts is a major concern to the poultry industry. Initial research has shown that two-stage composting can destroy many pathogenic organisms, but the fear of increasing poultry mortality by the distribution of inadequately composted poultry wastes remains. Composting dead poultry with a carbon source (e.g., straw) and with poultry manure has been shown to decompose poultry carcasses successfully (Murphy and Handwerker, 1988; Palmer and Scarborough, 1989; Sims et al., 1993). The dead poultry compost, as with other composted wastes, is a stable material that releases N more slowly than does raw manure or broiler litter (mixture of poultry excreta and woodchips or sawdust). Composting of dead birds has the potential, therefore, to improve the agronomic and environmental efficiency of land application programs using poultry wastes by improving the synchrony of N release with crop N uptake (Fig. 1).

11. POULTRY WASTES: PRODUCTION AND CHARACTERISTICS As with all industries, there are many different types of waste materials generated during the production of poultry and eggs. Effective environmental man-

14

J. T. SIMS AND D. C. WOLF

agement of any poultry waste begins with an understanding of its composition and the physical, chemical, and microbiological reactions that control the fate of potential pollutants in the waste following land application. Simpson (1990) recently reviewed the topic of agricultural use of poultry wastes and identified the three most common poultry wastes as (1) poultry manure (urine and feces) or poultry litter (a mixture of manure and the woodchips used as a base in broiler houses), (2) dissolved air flotation (DAF) sludge originating from poultry processing plants, and (3) composts produced from hatchery wastes and dead birds. Wastewaters from poultry processing plants are also commonly applied to agricultural lands, but these operations are relatively small in magnitude relative to programs that involve land application of manures, litters, sludges, and composts. Wastewater irrigation also normally requires strict adherence to regulations established by state environmental agencies. Limited information is available on the nature and use of wastewaters, DAF sludges, and poultry composts. Consequently, our discussion will focus on the production and composition of poultry manure and litter, although some information on dead poultry composts will be provided because of the emerging importance of this issue.

A. POULTRY PRODUCTION OPERATIONS AND TYPES OF WASTE The major poultry production operations include broiler chickens, turkeys, and eggs (layer chickens). Broilers account for approximately 80% of the poultry meat produced in the United States and 72% of the production on a worldwide basis (Economic Research Service, 1992). Other types of poultry operations include breeders, used to produce eggs for broiler and layer operations; pullet replacement operations that produce chickens for layer and breeder operations; and miscellaneous poultry such as ducks, geese, and pigeons. The production facilities used for all poultry operations are similar and, for all practical purposes, today consist solely of total confinement housing. Some limited semiconfinement or free-range poultry operations exist, but from a poultry waste management perspective, the vast majority of manures, litters, sludges, and composts originate from broilers, layers, and turkeys produced in total confinement housing. Two types of confinement housing are commonly used for poultry operations: (1) caged pit systems and (2) floor/litter systems. A variety of confinement designs exist, but the houses illustrated in Fig. 2A are reasonably typical examples of these two systems. Caged pit systems are most commonly used for layer or pullet operations and consist of cages suspended above either a deep or shallow pit. Manure from the birds falls into a pit, where it is removed periodically by scraping or flushing. Caged pit manure contains no bedding material and is nor-

,

Solid manure spreader

A r Bird cages

v-

A

2

1

Solid manure /spreader

Figure 2 Typical (A) confinement systems and (B)storage structures for a poultry operation. Adapted from Soil Conservation Service (1992) and Sims er al. (1989).

16

J. T. SIMS AND D. C. WOLF

mally semisolid or liquid in nature, depending on the type of removal system used. Floor systems are used for broilers, turkeys, or pullets and are normally single-story houses with an earth or concrete floor covered with from 5 to 15 cm of a litter material such as sawdust, wood chips, or other carbonaceous substance. The litter acts to absorb moisture, which in turn reduces the incidence of disease and helps maintain poultry health. A partial cleaning of wet, crusted, or “caked” litter normally occurs after each flock is removed from the house. A complete cleanout and replacement of the litter is done less frequently, usually between 12 and 24 months after introduction of the original litter material. Once removed from the poultry house, manures and litters are often applied immediately; if not, they are stored in roofed structures, tarpaulin-covered stacks, windrowed piles, or, in the case of liquid manures, in lagoons or in concrete or steel storage tanks. Concern over the environmental impact of uncovered manure storage piles has resulted in government cost-sharing to provide roofed storage barns (Fig. 2B) that can maintain the manure or litter in a dry, easily handled state until the proper time for land application. Storage locations should be in well-drained areas and sufficiently removed from any surface water to avoid contamination by runoff. Liquid or semisolid manures normally originate from layer operations. Information on the design and construction of manure storage facilities is normally available from local or national soil conservation agencies or cooperative extension. From the perspective of efficient manure use in agriculture, the primary goals of these structures are to prevent pollution during storage (e.g., leaching, runoff) and to maintain the manure or litter in a form that allows for uniform application by manure spreaders or injection equipment. One alternative waste handling and storage technique that is receiving great interest is a “composter” that can be attached to an existing storage structure. The primary purpose of these composters is to dispose of dead poultry under conditions of normal mortality by composting the birds with straw and manure (Palmer and Scarborough, 1989). The dead poultry compost can then be combined with other manure and land-applied or handled separatedly if its physical properties or composition makes it more suitable for certain crops than others. Knowledge of the quantity of poultry manure or litter produced on a farm or within a given geographic area is essential for the design of an effective a waste management program. Although reasonably accurate estimates of the quantity of fresh manure produced by various poultry types are available, farm-scale or regional estimates are generally lacking. Overcash et al. (1983a) reported that the average daily fresh manure production for broilers was 87 kg/1000 kg live weight, and for laying hens was 73 kg/1000 kg live weight (18 and 25 kg/1000 kg live weightlday on a dry weight basis). Converting this to the quantity removed from a typical broiler house or caged pit operation, the values were 20 kg/1000 kg live weight/day for broilers and 11 kgl1000 kg live weight/day for laying hens in a deep pit operation. As noted by Malone (1992), however, a

POULTRY WASTE MANAGEMENT

17

number of production, handling, and storage factors affect the actual quantity of manure/litter generated for various poultry types. Among these are feed composition and feed efficiency, the type of bedding, the frequency of crust removal and total cleanout operations, the number of flocks in a house between replacement of the bedding material, the final live weight of the poultry, and management practices such as type of watering system, house ventilation system, and floor type (soil versus concrete). He cited estimates of litter production from the literature and personal communications that ranged from 0.7 to 2.0 dry Mg/ 1000 broilers and an average value of 1.O dry Mg/ 1000 broilers. A recent study on the quantity and quality of litter produced in Delaware, conducted by Malone et al. (1992), showed that the amount of broiler litter produced ranged from 1 .O to 1.1 wet Mg/ 1000 birddflock as a function of type of cleanout program used to remove the litter from the poultry house. It is clear that we can only estimate the amount and timing of manure or litter production. However, values such as those obtained by Malone et al. (1992) can be useful in farm and regional management of poultry wastes. As an example, consider a typical broiler operation on the Delmarva peninsula with five poultry houses, six flocks per year, and 200 ha of cropland devoted to corn (75 ha), wheat (25 ha), and the soybeans (100 ha). Broiler litter production from this operation would be approximately 650 wet Mg/year. If distributed uniformly and to nonleguminous crops, the application rate of 6 Mg/ha would provide most, if not all, of the nutrient requirements for this farm. Similar calculations can be made for different sized farms or for entire counties or regions to determine if an adequate land base is available to support an existing or expanding poultry industry.

B. PROPERTIES AND COMPOSITION OF POULTRY WASTES A large database is available documenting the physical and chemical properties of poultry manures and litters (Barrington, 1991; Bomke and Lavkulich, 1975; Kunkle et al., 1981; Midwest Planning Service, 1985; Overcash et al., 1983b; Smith, 1973). Very little information is available on the composition of processing wastes, wastewaters, and composts. As with other organic wastes, the moisture content, pH, soluble salt level, and elemental composition of poultry manures and litters have been shown to vary widely as a function of type of poultry, diet and dietary supplements, litter type, and handling and storage operations. A summary of several studies of manure and litter composition is provided in Table 111 to illustrate the magnitude of this variability. Several noteworthy points can be drawn from this table. First, the total N and P contents of poultry manures and litters are among the highest of all animal manures. Compare the values in Table 111 with typical reported values for total N in fresh beef,

Table 111 Summary of Several Studies Documenting the Elemental Composition of Poultry Manures and Litters" Content (rnglkg)

Content ( 8 ) Description of waste Fresh chicken manure' Mean Range Fresh turkey manure' Mean Range ~ o u ~ t rlitter' y Mean Range Broiler litter' Mean Range Broiler litter" Mean Range Broiler littere Mean Range Cape pit manuree Mean Range

NH 4

P

K

S

6.1 3.7-8.8

0.6 0.4- I . I

2.2 I 2-2.9

2.0 1.2-2.7

-

1 .o

-

-

5.2-14.9

0.6-1.3

4.8 2.9-6.1

0.8 0.5- I .2

I .5 0.5-2.4

2.4 1.3-3.2

-

4.0 2.7-6.4

3.5 1.4-6.8

0.9 0.5- I . I

1.6

1 .8

-

0.5-3.5

1 . 1 -2.7

-

3.1 1.3-7.4

4.0 2.3-6.0

-

I .6 0.6-3.9

2.3 0.7-5.2

0.2-0.8

2.3 0.8-6.1

3.9 1.2-7.7

1.1 0.1-2.0

1.9 0.7-3.6

2.4 0.8-4.9

0.1-1.5

2.4 0.7-8.3

4.3 0.3-10.3

1.1 ND-2.5

2. I 0.3-3.8

2.6 0.1-6.7

0.7 0. I- I .5

4.4 I .3-6.5

I .5 ND-2.9

19 0. I - 5 . 1

2.8 0.7-4.7

0.7 0.1-1.5

"All data reported on dry weight basis. 'Overcash ef u / . (1983b). 'Stephenson e1 a / . (1990). dMalone (1992). eV. A . Bandcl (personal communication. 19891.

-

0.5

0.7

Ca

Mg

B

cu

N

Mn

Zn

-

-

-

0.6 0.6-0.6

-

-

-

-

-

-

0.4

-

-

-

0.3-0.5

-

-

-

0.5 0.2 -0.9

473 25- 1003

348 125-667

106- 669

0.7 0.1-1.9

377 21-84s

355 88-772

34 1 64-777

2.3 0.3-12.5

I .o 0.1-2. I

25 I 2-798

309 55-717

338 23-798

10.1 0.2-26.7

I .4 ND- 1.5

I60 2- 1053

296 4- 1061

226 10-937

8.1

-

315

POULTRY WASTE MANAGEMENT

19

dairy, horse, and swine manure: 4.2, 3.5,2.4, and 5.2%; or values for total P in the same manure: 0.9,0.6,0.4, and 1.5% (Sommers and Sutton, 1980). Second, poultry litter values for N and P are usually lower than those for fresh manure, reflecting both the losses the occur following excretion of the waste and the dilution effect from combining manure with carbonaceous materials that are very low in N and P. Overcash et al. (1983b) reported that the N and P content of various bedding materials ranged from 0.2 to 0.8% and 0.1 to 0.2%, respectively. Malone et af. (1992) analyzed 14 samples of wood-based litter and found an average N and P content of 0.3 and 0.02%. Third, NH4-N is a significant nitrogenous component of poultry manures and fitters, as is uric acid (2.6% in fresh manure, 0.9% in litter) (Overcash et al., 198313). Uric acid metabolizes rapidly to N&-N in most soils. The net result of the high NH,-N and uric acid contents in poultry wastes is a large percentage of N that can be converted to NO,-N, often within a few weeks. As discussed in more detail in Section 111, this can increase the likelihood of NO; nitrogen leaching from poultry manureamended soils unless manure/litter is applied in a manner and at a time that closely matches crop N uptake patterns. Fourth, the use of poultry wastes as soil amendments for agricultural crops will provide appreciable quantities of all important plant nutrients. As an example, the application of 9 Mgiha of broiler litter (75% solids), a rate commonly used to meet the N requirement of agronomic crops, will provide approximately 270 kg Niha (70 kg NH,-Niha), 100 kg P/ha, 165 kg/ha of K and Ca, 45 kg/ha of S and Mg, and 2-5 kgfha of Mn, Cu, or Zn. Typical fertilizer recommendations for nonirrigated corn (yield goal of 7 Mglha) in the eastern United States, on soils with m ~ i u m soil tests for all nutrients, would be 125 kg N/ha, 30 kg P/ha, and 100 kg K/ha. Calcium and magnesium requirements are normally met by liming, whereas S, B , Mn, Cu, and Zn are only recommended for certain crops in specific situations known to cause deficiencies of these elements. As noted earlier, and as shown in this example, the application of poultry manure based on crop N requirements often provides more of other nutrients than is required by the crop (e.g., an excess of 70 kg Plha). The implications of long-term manure use on the economics of soil fertility management and potential environmental impacts of excessive soil nutrients are discussed in more detail in Sections IV and VI. Manure testing can also identify other properties, elements, or compunds that may have an impact on crop production or the environment. Phytotoxic effects of manures are relatively uncommon. However, if applied at excessive rates, the soluble salts, NH4-N, and alkaline nature of most poultry wastes can produce crop growth problems. Shortall and Liebhardt (1975) reported that broiler litter rates of 90 Mgiha or greater significantly reduced corn yields due to high soil salinity levels. Weil et al. (1979) also reported that excessive manure rates (>50 Mg/ha) reduced germination, emergence, and seedling growth of corn due to a combina~ionof high soluble salts, NH4-N and nitrite-N. Both of these stud-

20

J. T. SIMS AND D. C. WOLF

ies, however, found that the effects of excessive manure were transitory and were reduced by normal rainfall and leaching within 1 year. It should be noted, however, that these studies were conducted on well-drained soils in a humid region (mid-Atlantic United States) where climatic conditions would be conducive to rapid leaching of salts and nitrification of NH,-N. Poultry manure is normally an alkaline material, with pH values ranging from 7.5 to 8.5. Its effects on soil pH can be significant but somewhat contradictory. Sims (1986b) found that addition of three broiler litters (pH from 8.5 to 8.9) raised the pH of an Evesboro loamy sand soil (Typic Hapludults) from 6.5 to 7.5 immediately after application, but that the final soil pH after 20 weeks was about 5.5. The initially high pH could reduce micronutrient availability, particularly Mn and Zn; the final more acidic pH that resulted from the nitrification of added and mineralized NH,-N could cause phytotoxicity from excessive A1 and Mn in some soils. As mentioned earlier there is limited information available on the presence or concentration of heavy metals and pesticides in poultry wastes. New instrumentation available to many testing laboratories, such as inductively coupled plasma (ICP) spectrometers and gas chromatograph-mass spectrometers, is likely to make multielement and organic compound analyses of manures and litters more common in the near future, In addition to the results of Kunkle et al. (1981) mentioned earlier and the data shown in Table 111 for Cu and Zn, some recent data on the heavy metal content in broiler litter were obtained from ICP spectrometry analyses conducted by North Carolina State University (J. C. Barker, personal communication). The means (mg/kg), standard deviations, and number of samples analyzed were as follows: As (26, 19, 11); Cd (0.4, 0.3, 7); Cr (9, 0.7, 2); Cu (225, 95,458); Hg (0.2, 0.07, 3); Ni (7, 7, 4), Pb (6, 7, 4); Se (0.2,0.02, 3); and Zn (315, 105,460). All values are expressed on a wet weight basis and hence represent the actual concentration applied in the field. For reference purposes, the total solids contents of 534 broiler litter samples analyzed by North Carolina State averaged 78% (range of 58-97%, SD = 6%).These concentrations can also be compared to maximum metal concentrations recommended for sewage sludges applied to lands. Ritter (1987) summarized these for the mid-Atlantic region of the United States (in mg/kg, on a dry weight basis) as follows: Cd (25), Cr (lOOO), Cu (IOOO), Hg (lo), Ni (200), Pb (lOOO), and Zn (2500). No maximum concentration value was reported for As or Se.

C. APPROPRIATEUSEOF POULTRY WASTEANALYSES These studies leave little doubt that poultry manures and litters are valuable fertilizer materials, although the wide ranges in nutrient composition reported raise the question of the most effective use of poultry waste analyses. Certainly

21

POULTRY WASTE MANAGEMENT Table IV Statewide Nutrient Budge for Delaware, Illustrating the Magnitude of the Nutrient Management Problems of the Poultry Industry Nutrient generated or used (mg), statewide basis Source or use of nutrienta

Nitrogen

Nutrient source Poultry manure Fertilizer sales

7865 19,275

3495 2955

6990 15,500

Total Nutrient use by crop Corn (69,700 ha) Soybeans (80,600 ha) Wheat (24,300 ha) Barley (10,900 ha) Vegetables (32,400 ha)

27,140

6450

28,940

9760 0 2180 980 3640

940 1085 330 150 435

1560 1810 545 245 725

Total * Annual nutrient balance Statewide (Mg) Per hectare (kg) -

16,560

2940

4885

+ 10,580 + 48

+ 35 10 + 16

+ 24,055

Phosphorus

Potassium

+llO

“Values for source, use, and balance for N, P, and K based on information from the Delaware Department of Agriculture (1992) and Malone et al. (1992), and estimated nutrient requirements using recent soil test summaries for Delaware. bTotal area: 217.900 ha.

analyses of poultry manure or litter from well-defined production systems can help to establish the potential nutrient supply for a farm or region. This is of economic value because it can help farmers avoid the unnecessary purchase of commercial fertilizers. Research-based information on the content and availability of nutrients in poultry wastes is needed not only for crop management, however, but for the development of state or regional land use plans. An example of a larger scale application of data on waste properties is given in Table IV for poultry manure use in Delaware. The N, P, and K contents of over 200 manure samples produced under different management conditions were combined with actual values of the mass of manure generated to obtain estimates of manure N , P, and K production for the state (Malone et al., 1992). Combining these data with fertilizer sales and reasonable estimates of crop requirements for these nutrients shows the existence of a large surplus of N, P, and K, equivalent to approximately 48 kg N, 16 kg P, and 110 kg K for every hectare of cropland

22

J. T. SIMS AND D. C. WOLF

Site number Figure 3 The difference between total N actually applied, based on poultry manure samples collected during field application, and the amount estimated to be applied based on laboratory analyses of stockpiled manure samples. Results from a 17-site field experiment (Igo er al.. 1991).

in the state. Unfortunately, this is a common situation in areas where animalbased agriculture is concentrated on an inadequate land base (Power and Papendick, 1985; Power and Schepers, 1989). Clearly, a critical need exists for state and industry cooperation in the development of waste management plans and infrastructures that focus on the redistribution of excess manure to nutrientdeficient areas. Recent studies, however, question the use of analyses of stockpiled manure or litter to determine field level application rates. In one study, the N loading rates for broiler litter from 17 different on-farm storage areas, estimated from analysis of stockpiled litter samples, were compared to the actual loading rate based on analysis of samples collected during application to field corn (Igo et al., 1991). As shown in Fig. 3, when desired application rates were applied to large field plots using commercial manure spreaders, overapplication of 10-20 kg N/Mg of litter commonly occurred, as did underapplication of 5- 10 kg N/Mg. Therefore, the accurate application of a recommended litter rate for corn (-5 Mg/ha), based on analysis of the wastes, commonly resulted in the application of excess manure N approaching the total N requirement of the crop (- 100 kg N/ha). Clearly, an approach more comprehensive than N analysis and equipment calibration is needed to avoid over- or underapplication of N from organic wastes. Approaches to improve the efficiency of manure and litter use are described in Section VI.

23

POULTRY WASTE MANAGEMENT

111. NITROGEN MANAGEMENT FOR POULTRY WASTES Land application of animal waste is an important management practice to recycle nutrients, to improve or maintain soil fertility, and to improve soil biological and physical properties [Council for Agricultural Science and Technology (CAST), 19921. Historically, the most important nutrient considerations in developing poultry waste application recommendations have been the concentration and availability of N. Due to the common duct for urine and feces elimination in poultry, N levels of poultry waste are generally higher than those of other livestock wastes.

A. FORMSIN POULTRY WASTES The total N present in poultry waste can be separated into four forms (Fig. 4). Complex forms of organic N in poultry waste include constituents of feathers and undigested feed. Labile organic N is largely uric acid and urea. Uric acid in the fresh waste is rapidly hydrolyzed by the enzyme uricase to urea (Fig. 5). The urea is hydrolyzed by the enzyme urease to form ammoniacal-N. The NH4-N is the third form of N found in poultry waste. Nitrate, the fourth form, is generally absent in poultry waste unless the waste has been stored in an aerobic moist state. The concentration and distribution of these forms of N can vary with the particle size of various poultry waste components (solid or liquid excreta, woodDECOMPOSITION

Organic N

Complex Organic N

AMMON

.

[Ammonium

NITRIFICATION

1 Fixation

'

.

Runc,.

Ii LeaC'

Figure 4 Forms and fates of N in poultry wastes.

J. T. SIMS AND D. C. WOLF

24

Uric acid

Urea

Ammonia

Figure 5 Generalized reaction for the conversion of uric acid to ammonia.

chips, etc.). For instance, studies by Ndegwa et al. (1991) showed that the N concentration in the fine fraction of poultry litter ( 1 0 . 8 3 mm) was greater than in larger sized particles.

B. NITROGEN TRANSFORMATIONS IN STORAGE AND HANDLING The majority of N excreted in poultry manure is in the form of uric acid that can be rapidly converted to urea and NH,-N if temperature, pH, and moisture are adequate for microbial activity (Bachrach, 1957; Rouf and Lomprey, 1968; Siege1 et al., 1975). The hydrolysis reactions result in elevated pH levels that facilitate NH,-N volatilization (Reynolds and Wolf, 1987b). Losses of NH,-N from poultry wastes begin to occur immediately after excretion and can be influenced by conditions within the production house. For instance, Weaver and Meijerhof (1991) found that NH,-N losses from broiler litter became greater as relative humidity in the house increased. Nitrogen loss during storage and handling is determined by climatic conditions and the specific manure management system used. Estimates of N loss range from 10 to 80% of the N excreted (Midwest Planning Service, 1985; Soil Conservation Service, 1992). For poultry litter stored under roofed facilities, estimated losses during storage and handling are 30 to 45% of the total N content. For manure diluted by 250% and held in storage ponds or lagoons, the N loss may be 70 to 80% of the total N in the waste. Maximizing the nutrient value of poultry wastes, therefore, requires the use of management practices that will optimize N conservation during storage and handling (Barrington, 1991).

C . NITROGEN LOSSESDUETO DRYINGPOULTRY WASTES Drying poultry waste will enhance volatization if the conversion of uric acid and urea to NH,-N is complete. Oven drying fresh poultry manure from a laying

POULTRY WASTE MANAGEMENT

25

hen operation at 66°C resulted in a decrease in the total N level from 5.65 to 4.01% in the wet and dry manure, respectively (Gale et al., 1991). In the wet manure, 34% of the total N was in the NH4-N form and NO,-N levels were 5 1 mg/kg. When fresh poultry manure was air dried for 10 days, Giddens and Rao (1975) found that 47.6% of the total N was lost via NH,-N volatilization. Parker et al. (1959) reported that hen manure and broiler manure lost 17 and 12%, respectively, of their total N when dried for 10 hours at 78°C. In a study comparing methods of drying poultry litter, Wood and Hall (1991) reported that up to 15% of the total N was lost during drying. The P, K, Cu, Fe, and Zn levels were not influenced by drying. Nitrogen losses during drying influence not only the final N content of the manure or litter, but the accuracy of manure analyses used to determine proper field application rates. If laboratories conducting manure analyses dry samples at different temperatures prior to determination of total N, or do not dry them at all, they are certain to obtain different analytical results. Combined with this are possible changes in total N content that occur between the time of sample collection, analysis, and application due to NH3-N volatilization. This again illustrates the need to use manure analyses for N as guidelines, not as absolute values.

D. NITROGEN TRANSFORMATIONS IN SOILS As with most biological systems, the temperature, moisture, and pH of the system largely determine the biological transformations that occur in soil amended with poultry waste. When poultry waste is added to soil, mineralization of organic N and nitrification of NH4-Noccur rapidly under favorable conditions (Fig. 4). Incorporation of the waste into the soil will result in more rapid conversions than does surface application. Immobilization, NH,-N volatilization, and denitrification also occur in poultry waste-amended soils.

1. Ammonia Volatilization Ammonia volatilization during application of poultry waste can result in substantial N losses. Not only does the loss occur during application of the waste, but NH,-N volatilization continues when the poultry waste is allowed to remain on the soil surface. The volatilization process depends on conversion of uric acid to urea and then NH3-N. With adequate moisture and suitable temperature and pH, the process can be complete within 24 hours (Lacey ef al., 1981). The result is that as much as 50% of the total N in poultry waste is often in the NH,-N form (Reddy el al., 1980a). When poultry waste is surface applied, in excess of 50% of the total N in the

26

J. T. SLMS AND D. C. WOLF

waste material may be lost via volatilization. In laboratory and field studies, Wolf et al. (1988) found that as much as 37% of the total N in fresh manure from laying hens could be lost as NH3-N in 5 11 days when the waste was surface applied to a Bowie fine sandy loam and a Captina silt loam. Studies with 18 broiler litter samples by Schilke-Gartley and Sims (1993) showed that surface application of the litter resulted in NH,-N volatilization losses of from 4 to 3 1% of the total N within 12 days (average of 74% of added NH,-N). Immediate incorporation of the broiler litter reduced average NH,-N volatilization losses to 3% of total N, relative to 20% for surface application. Most of the NH,-N volatilization from these broiler litters occurred within 3 days, as illustrated in Fig. 6 . High temperatures, moist soil conditions, low hydrogen ion buffering capacity, high pH, and windy conditions can facilitate the gaseous loss of NH,-N (Adriano et al., 1974; Donovan and Logan, 1983; Muck and Richards, 1983; Reynolds and Wolf, 1987a). It is also possible that application of poultry waste to crop residue or forage vegetative cover would enhance NH,-N volatilization by preventing the poultry waste from coming in contact with the soil (Beyrouty et al., 1988; Donovan and Logan, 1983; Reynolds and Wolf, 1988). Contact with the soil allows the NH,-N to be retained on cation exchange sites. Incorporating the poultry waste immediately after application has been shown to reduce volatilization losses (Giddens and Rao, 1975). Precipitation or irrigation can also

90 -0

a

80

-0

70

.--N .-

' I0

I

a

A

A

A

A

k

. ......

A

60 50

I

-0

-0 -0

A

A

A

40

U

A 20

I

0

1

2

3

,

,

4

,

,

5

,

,

,

6

,

,

7

,

,

8

,

,

!

,

,

,

,

9 1 0 1 1 1 2

Time (days) Figure 6 Patterns of NH3-N loss from surface applications of poultry manure (PM) to a Hammonton loamy sand soil (Schilke-Gartley and Sims. 1993).

27

POULTRY WASTE MANAGEMENT

reduce gaseous losses by transporting the NH,-N into the soil where it can be retained (Lauer et al., 1976).

2. Mineralization-Nitrification-Immobilization Mineralization, the conversion of organic N to inorganic N, is critical to providing available forms of N to plants. Predicting the rate and amount of plant available N produced in poultry waste-amended soils is necessary for proper plant nutrition and to protect the quality of groundwater and surface water (Castellanos and Pratt, 1981a; Liebhardt et al., 1979; Pratt et al., 1973; Weil et al., 1990). Net mineralization of organic N is calculated using Eq. (1). % Net mineralization =

[(NH,-N

+ NO,-N),

- (NH4-N

+ NO,-N),k

-(NH,-N

+ NO,-N),,,,,]

[(TotalN)wa,Ie- (NH4-N -tNo3-N)wdqte]

x 100 (1)

where (NH,-N),, and (NO,-N),,, are the NH,-N and NO3-N concentrations (in mg/kg) in the soil treated with poultry waste, (NH,-N),, and (N03-N)ckare the NH,-N and NO,-N levels in the control (“check”) soils that did not receive poultry waste, and (Total N),,,,, , (NH,-N),,,,, , and (N03-N),,,,, are the total N, NH,-N, and NO,-N added to the poultry waste at time zero. Selected mineralization values are given in Table V for poultry manure, litter, and compost. Sims (1986b) added three different poultry manures to an Evesboro loamy sand in a laboratory study and showed that 30-60% of the organic N in two of the manures was mineralized under favorable moisture conditions in a 150-day incubation. He also showed that from 7 to 37% of the organic N was mineralized when the incubations were carried out at 0°C and that increasing the temperature to 40°C increased net mineralization. Nitrification, the sequential oxidation of NH t; to NO ;to NO; , was inhibited by moisture stress at 25°C which resulted in an accumulation of NH,-N. All incubations displayed N immobilization, conversion of inorganic N to organic N in microbial biomass, during the initial phase of the studies. Nitrogen mineralization in a clay and a sandy soil amended with ground and pelleted poultry manure was studied by Hadas et al. (1983). Their results indicated that mineralization was a two-stage process. At 25”C, from 34 to 42% of the total N in the poultry manure was mineralized in the initial rapid phase. The second phase was a slow-release process, and after 9 to 13 weeks, 42 to 50% of the total N had been mineralized and the authors suggested that two distinct substrates resulted in the two phases. Incubations were conducted at 14, 25, and 35°C and the results showed that nitrification was inhibited at 14”C, which resulted in an accumulation of NH4-N in the soil. Bitzer and Sims (1988),

Table V Selected Net Mineralization Percentages for Poultry Manure, Litter, and Compost

Poultry waste

Application rate (g wasteikg)

mg total N kg soil

Total N content

Temperature

mJ)

Soil

C"

Manure

6.5

320

4.93

Evesboro loamy sand

25

Manure

33.3

1130

3.4

Sandy

25

Manure

33.3

1130

3.4

Clay

25

Litter (mean of 20) Manure (mean of 2) Manure

3.4

I80

5.32

Kalmia sandy loam Norfolk sand

Composted manure Manure

130 270

4.59

-

270

1.7

45

1700

3.8

San Emigdio fine sand San Emigdio fine sand Bowie fine sandy loam

Incubation time (days)

Total N mineralized

(5'~)

Ref.

23

30 90 150 7 90 7 90 140

16 38 40 34 48 38 47 66

20-23

182

42

Chescheir et al. ( 1986)

23

7 70 7 70 14

39 48 18 29 37

Castellanos and Pratt ( 1 98 1 b)

23 25

Sims ( 1986b)

Hadas

ef al.

(1983)

Hadas er al. (1983) Bitzer and Sims (1988)

Castellanos and Pratt ( 1 98 1 b)

Gilrnour et al. (1987)

POULTRY WASTE MANAGEMENT

29

Manure sample

Figure 7 Timing of N availability when poultry manure is added to a soil. Initial N , represents NO,-N immediately after extraction; other bars represent net N mineralKC1-extractable NH,-N ized during 0- 14 and 14-140 days (Bitzer and Sims, 1988).

+

in a study with 20 poultry manures, also reported that mineralization of manure N occurred in two phases and that, when combined with the immediately available inorganic N in poultry manure, it could result in large accumulations of available N in the soil within 14 days (Fig. 7). Studies by Gale and Gilmour (1986) showed that poultry litter decomposition, as measured by CO, evolution, was a three-phase process. During the initial 7 days of incubation, the rapid phase of mineralization resulted in large increases in inorganic N levels in a Captina silt loam incubated at 25°C. There was a linear relationship between net C and net N mineralized during the rapid phase of decomposition. The second or intermediate phase lasted from 7 to 14 days and the slow phase that began at 14 days showed either no net mineralization or immobilization of N. Chescheir er al. (1986) also suggested that substantial immobilization occurred during the first 14 days in two soils amended with poultry manure. Because broiler and laying hen manure contains approximately 17, 13, and 4% by weight hemicellulose, cellulose, and lignin, respectively, it would appear likely that these materials would be important substrates for microbial utilization during the slow phase of decomposition (Smith, 1973). During the rapid, intermediate, and slow phases, the percentages of the litter C evolved as CO, were 25, 10, and 65%, respectively (Gale and Gilmour, 1986). First-order rate constants for the rapid phase followed the Arrhenius equation for 11, 18, and 25°C incubation temperatures. Nitrification is inhibited by lack of oxygen, low temperature, inadequate moisture, pH values < 5 or >8, and NH,-N toxicity (Alexander, 1977). Accumula-

30

J. T. SIMS AND D. C. WOLF

tion of toxic levels of N02-N has been reported in soils amended with poultry waste (MacMillan et a f . , 1972; Weil e t a l . , 1979).

3. Denitrification Denitrification is the conversion of NO: or NO 7 to N, or N,O by microbial activity. It can be an important, but difficult to quantify, mechanism for N loss in soils amended with poultry waste. Denitrification losses are greatest in poorly drained soils with high organic matter content and may be from 50 to 100% of the inorganic N in the soil (Soil Conservation Service, 1992). Denitrification losses in animal waste-amended soils have been related to soil texture with values of 35, 20, 10, and 0% for clay, clay loam, silt loam, and sand, respectively (Gilbertson and Norstadt, 1979). In a 7-year field study conducted by Cooper et al. (1984), poultry manure was incorporated in a Davidson clay loam to provide 25 or 49 Mg total N/ha during a 5-year period. They found that 51 to 58% of the total N applied could not be recovered 7 years after initiating the study and hypothesized that denitrification was the most likely mechanism to account for the N loss. The authors noted that the soil was waterlogged from April to July, which would have resulted in anaerobic conditions. They also suggested that the application of high rates of poultry manure could have resulted in sufficient levels of available C to stimulate microbial activity and denitrification. Reddy et af. (1980a) conducted laboratory studies to evaluate denitrification potential in a Norfolk sandy loam amended with 10 g poultry manure/kg soil and incubated at 22°C. Following 30-, 60-, 90-, or 120-day aerobic incubations, the soil was saturated with water and the disappearance of NO,-N evaluated. The results showed that the 30-day aerobic incubation followed by saturated conditions resulted in almost 75% of the NO,-N being lost after 24 hours under anaerobic conditions. The authors concluded that the 60-, 90-, and 120-day aerobic incubations resulted in depletion of available C and even though NO,-N was present and the soil was anaerobic, there was not sufficient C available for appreciable denitrification to occur. Meek et a f . (1974) found that annual applications of cattle manure resulted in less NO,-N leaching to a depth of 80 cm than did single manure applications. The authors suggested that annual manure applications provided higher levels of soluble organic C that could be used by bacteria carrying out denitrification. As was recently noted by Russelle (1992), the ability to predict the influence of management decisions on N losses in pasture and rangeland has been greatly hampered by the spatial and temporal variability of N cycling. Additional information on denitrification in manure-soil systems appears to be a critical need (Bouldin et al., 1984).

31

POULTRY WASTE MANAGEMENT

Mathematical models have been developed to predict how much poultry waste should be applied to supply crop needs (Mathers and Goss, 1979; Meisinger and Randall, 1991). To predict plant available N supplied by poultry waste, several researchers have presented decay coefficients or rate constants (Bitzer and Sims, 1988; Gale and Gilmour, 1986; Gilmour and Gale, 1986; Pratt et al., 1973; Sims, 1986b). These approaches are discussed in Section VI.

E. CROPRESPONSETO NITROGEN INPOULTRY WASTES 1. Forages Poultry waste is often used as an organic fertilizer in forage production systems. The addition of poultry waste to tall fescue, orchard grass (Dactylis gofmerata L.), and bermuda grass has been shown to increase dry matter production (Fig. 8). In many cases the amount of N applied was in excess of the amount recommended for forage production and could result in groundwater and surface water contamination. Excessive waste application rates can result in undesirable effects on the forage crop and the animals consuming the forage or hay. During a 7-year study in which over 18 Mg/ha-year of broiler litter was applied to tall fescue used in a grazing study, Stuedemann et al. (1975) noted problems with grass tetany and fat necrosis, and found NO,-N levels in

c

0'

0

100

200

300

400

500

600

700

800

N Waste Application (kg/ha) Figure 8 Dry matter production of various forages during the first year following amendment with poultry waste in nonirrigated field studies. Values were calculated from data taken from Hileman (1973), Huneycutt et a/. (1988). Quisenberry et al. (1981), and Vandepopuliere et al. (1975). 1, Fescue with manure; 2, bermuda grass with litter; 3, fescue with litter; 4, orchard grass with litter; 5 , fescue with manure.

32

J. T. SIMS AND D. C. WOLF

the forage of 3300 mg/kg. Based on findings from the research, the authors concluded that broiler litter should be applied to tall fescue at rates of 5 9 Mg/ ha-year. In a greenhouse study, Hileman (1971) amended three soils with 11-45 Mg/ha broiler litter and found that tall fescue would not germinate 2 weeks after the waste was added, nor would it germinate in two successive planting. He attributed the problem to high salt levels, especially K, and high NH,-N levels attributable to uric acid and urea hydrolysis. Nutrient imbalances in soils amended with poultry waste could also increase the potential for grass tetany problems (Wilkinson et al., 1971). Addition of poultry waste has also been shown to result in the disappearance of legumes in pastures due to the addition of readily available N that would provide a competitive advantage for the grasses (Huneycutt et al., 1988). These researchers also reported that application of broiler litter at a rate of 500 kg N/ha-year during the 5-year study resulted in approximately the same bermuda grass dry matter production as did 224 to 336 kg N/ha of NH,NO, fertilizer. Hileman (1973) stated that dry matter production of orchard grass was only slightly increased by applying more than 9 Mg broiler liter/ha in a 3-year field study. He also noted that after 3 years of litter application, pH and extractable Ca levels were decreased and available P and K levels were increased in the soil (Hileman, 1967a). Vandepopuliere et al. (1975) also noted a decrease in the soil Ca level with poultry waste application at one site. Soil physical properties such as decreased bulk density, increased water-holding capacity, and increased water-stable aggregation are also improved by poultry waste addition to soil (Hafez, 1974; Weil and Kroontje, 1979). 2. Corn Corn (Z. mays L.) response to poultry waste addition has been examined in numerous field studies (Fig. 9). Kalmia and Fallsington sandy loams were amended with three poultry manures and NH,NO, at rates designed to provide comparable amounts of plant available N (PAN). For manures, PAN = 80% x [NH,-N NO,-N] 60% X [organic N]) of 0,90, 180, and 270 kg/ha (Bitzer and Sims, 1988). The 2-year average irrigated corn yields for manures at the three PAN rates were 9.0, 11.O, and 12.3 Mg/ha, compared to 10.9, 12.4, and 12.4 Mg/ha for NH,NO, . Early season leaching losses of NO,-N were suspected of reducing corn yields with manure at one site relative to NH,NO, , wherein the majority of N was applied via sidedressing. This study illustrates one of the major concerns about use of poultry manure as an N source-the decreasing efficiency of N recovery at higher N rates that may subsequently result in NO,-N leaching. Consider the fact that increasing PAN from 180 to 270 kg N/ha in-

+

+

33

POULTRY WASTE MANAGEMENT

- Kalmla

* Fallslngton * Elkton

*Cecil

* Evesboro ~

0

1,000

~~~

2,000

3,000

Plant Available N (kg/ha) Figure 9 Influence of poultry waste additions on corn yields in five soils. Values were calculated from data taken from Bitzer and Sims (1988), Carreker er af. (1973), Shortall and Liebhardt (1975), and Sims (1987).

creased yields by only 1.3 Mg/ha in this study for poultry manure, and not at all for NH,NO,. It should also be noted that the difference in actual rate of poultry manure applied to provide these two N rates was relatively small: 4.2 and 6.3 Mg/ha. Rates greater than 6 Mg/ha are commonly applied in Delaware due to improperly calibrated application equipment or by farmers with inadequate land for manure use at lower, currently recommended manure application rates (4-5 Mg/ha). Similar results were obtained on an Evesboro loamy sand soil by Sims (1987), where poultry manure was compared with NH,NO, as an N source for conventional (CT) and no-tillage (NT) irrigated corn. In all cases, the highest corn yields were obtained with the highest rate of manure addition studied, but efficiency of N recovery by the crop decreased markedly as N rate increased. The 3-year average efficiencies for N recovery for poultry manure at rates of 84, 168, and 252 kg PAN/ha were 50, 37, and 36% (CT) and 31, 28, and 31% (NT). When poultry waste has been applied at excessive rates, corn yields have been reduced and high N03-N levels have been found in groundwater (Carreker et al., 1973; Shortall and Liebhardt, 1975; Wed et al., 1990). Studies demonstrating the impact of poultry manure on groundwaters were mentioned in Section I. The recent work of Weil et al. (1990) and ongoing studies by Sims et al. (1991) further illustrate the potential impact of poultry waste applications on NO; nitrogen concentrations in groundwaters (Fig. 10). The toxicity of high levels of poultry waste has been related to excessive soluble salts, especially K, NO,-N, and NH3-H (Liebhardt, 1976; MacMillan et al., 1972; Weil e l al., 1979). Toxicity symptoms exhibited were reduced germination, burned leaf tips and mar-

J. T. SIMS AND D. C. WOLF

lo

-I

25

--c- Poultry manure I. Urea . a. . Control (ON)

A

20

I

15

10

1989

1990

1991

...- ..... Figure 10 Groundwater NO,-N concentrations in the shallow water table of the Atlantic Coastal Plain of the United States. (A) Groundwater NO,-N concentrations in manured and nonmanured, irrigated, commercial corn fields (Weil et al., 1990). (B) Effect of applications of poultry manure and urea at 224 kg N/ha year on groundwater NO,-N levels, relative to an unfertilized treatment (J. T. Sims, unpublished data).

POULTRY WASTE MANAGEMENT

35

gins, and stunted root growth. Winter rainfall has been shown to leach the salts from the soil and eliminate accumulation of toxic materials in a coarse-textured soil in Delaware (Liebhardt, 1976).

IV.PHOSPHOROUS MANAGEMENT FOR POULTRY WASTES Phosphorus (P) is an essential plant nutrient, and proper management of soil, fertilizer, and manure P is vital for the successful production of agronomic crops. Phosphorus is not toxic to humans or animals. The sole environmental effect of P is its role in the eutrophication of surface waters (see Section I). From an agricultural perspective, P contributes to eutrophication by its movement into surface waters by the processes of erosion (sediment-bound P), runoff (soluble inorganic and organic P), or subsurface flow in artificial drainage and groundwater discharge. Reducing the impact of P from agricultural soils on eutrophication requires management programs that integrate the quantity/intensity relationships of soil P with these transport processes. Simply put, we seek both to minimize the accumulation of soil P to excessive levels and to reduce the transport of soluble or sediment-bound P to sensitive water bodies. Animal-based agriculture, such as the poultry-grain production systems common in many areas of the world, is perhaps the most difficult situation to manage with respect to P. The major difficulties associated with P management in agricultural soils that are routinely amended with poultry wastes were described in Sections I and 11. To reiterate the main point, in most areas where poultry manuresllitters are regularly applied to agricultural soils, the levels of total, available, and even soluble soil P can become excessive relative to crop needs. If these areas are near surface waters sensitive to eutrophication, and if transport processes exist that can redistribute soil P to these waters, the issue of environmental management programs for P must be addressed. These programs should be watershedbased and should review the current status of soil P and the overall nutrient budget for the area; the dominant crop rotations present and their P requirements; the susceptibility of the dominant soil series to erosion, runoff, and leaching of P; and the presence of drainage systems or groundwaters that can discharge P directly into surface water. They should be based on a sound understanding of the chemical, physical, and biological reactions undergone by P in soils, as affected by P source (manures, fertilizers), soil properties (pH, content of clays and Fe/Al oxides, organic matter, soil structure, etc.), and management practices (tillage, irrigation, P application techniques). The ultimate goal of environmentally based P management programs is to reduce surface water degradation while maintaining acceptable crop yields. This section provides a review of re-

36

J. T. SIMS AND D. C. WOLF

search on the fate of P from poultry manures in soils; management programs for P are discussed in more detail in Section VI.

A. PHOSPHOROUS CONCENTRATION AND FORM IN SOILS AMENDEDWITH POULTRY WASTES Phosphorus levels in soils amended with animal manures for many years are commonly well in excess of the critical values used by soil testing programs to identify soils that will respond to fertilizer P. Soil test summaries can provide a broad view of the impact of animal-based agriculture, or any other crop production system, on soil test P, as described in Section I. Because soil test results are normally accompanied by geographic locations and crop management information, the magnitude and distribution of soils that are excessive in P are becoming more apparent. A recent survey of four regional soil testing committees representing 34 states was conducted by Sims (1993) and found that the major environmental issue related to soil P was animal waste management, and poultry waste management in particular. Soil test extractants do not measure, however, the amount of total soil P, the distribution of soil P between organic and inorganic forms, or the biological availablility of soil P. Indeed it is fair to state that from the point of view of soil testing programs, the process of measurement and interpretation of P for environmental purposes is in its infancy. In fact, until fairly recently many soil testing laboratories did not measure the actual value of soil P once a defined “very high” value was reached. Actual P values were not determined because there was no need from a fertilizer recommendation standpoint and because of the time and expense of the additional dilutions and laboratory analyses that would have been required on many samples. Although researchers have measured actual values and studied the fate of P in heavily manured soils, it is only recently that new instrumentation (as with heavy metals and pesticides) has made routine measurement of actual soil test P values possible. As an example, the University of Delaware Soil Testing Program only began measuring and reporting actual soil test P values to farmers in 1991; prior to that the highest value reported was 100 mg Plkg (by the Mehlich 1 soil test, 0.05 N HCl 0.025 N H,SO,). Results of the first annual soil test summary after initiation of this procedure showed that, on a statewide basis, the percentages of samples from commercial cropland rated as low, medium, high, and excessive in P were 11, 21, 28, and 40%,respectively. In Sussex County, site of the poultry industry, 29% were rated as high and 48% as excessive. Of the samples from Sussex County in the excessive category, 63% ranged from 67 to 134 mg Plkg, 30% from 135 to 268 mg P/kg, and 7% exceeded 268 mg Plkg. Of the samples that indicated manure had been applied, 66% were in the excessive range. The critical value for soil test P in Delaware is 35 mg Plkg; beyond

+

POULTRY WASTE MANAGEMENT

37

this point no fertilizer or manure P is recommended, with the possible exception of a small amount of starter fertilizer placed in a band at planting. Clearly a large percentage of the cropland in the state and county can be considered nonresponsive to P. Further, the Delmarva peninsula is bordered on the east by the Delaware Bay and a national estuary, the Inland Bays of Delaware, and on the west by the Chesapeake Bay, water bodies that are highly sensitive to eutrophication. In this instance the information available from soil testing programs and the continued inputs of P in poultry manure/litter certainly suggest the need for an environmental P management program. Unfortunately, it is rare to find long-term studies that document the rate of accumulation of P in soils amended with any type of manure/litter or the fate of the added P. It is even more unusual to find studies that determine the amount of time required for normal crop uptake to reduce soil P to a “medium” or “low” soil test value. Sharpley et al. (1984) reported that the application of 67 Mg/ha/ year of beef feedlot manure for 8 years to a Pullman clay loam (Torretic Paleustolls) used for continuous grain sorghum production increased total P in the surface 30 cm from 353 mg/kg in an untreated check plot to 996 mg/kg. Available 0.025 N HCI) increased from 15 to 230 mg P/kg; P (Bray PI, 0.03 N NH,F critical values for Bray P (breakpoint between medium and high) typically range from 30 to 50 mg P/kg. Meek er al. (1982) found that application of feedlot manure to a Holtville soil (Typic Torrifluvents) used for a variety of crops (sorghum, lettuce, barley) for 3 years at 90 Mg/ha/year increased Olsen P (0.5 N NaHCO,, pH 8.5) from 9 mg/kg to 68 mg/kg. Olsen P values 5 years after cessation of manure applications and continued cropping were 65 mg P/kg. Critical values for Olsen P normally range from 20 to 30 mg P/kg. Similar long-term studies with poultry manure/litter are uncommon. Robertson and Wolford (1970) reported that the application of 26 Mg/ha/year (wet weight basis) of poultry (layer) manure for 5 years to a Breckenridge sandy loam resulted in yields equivalent to those due to commercial fertilizer and increased soil test P (Bray P1) from 50 mg P/kg in a check plot to 147 mg P/kg; 52 Mg/ha/year of manure increased soil test P to 189 mg P/kg. Mitchell etal. (1992) reported that soil test P (Mehlich 1) in the surface horizon of an Esto loamy sand soil that had received broiler litter for 20 years at about 7 Mg/ha/year was 180 mg P/kg, more than seven times the critical value used in Alabama. Soil test P in a nearby pasture area was less than 10 mg P/kg. Poultry manure applications at recommended rates can markedly increase soil test P levels even in the short term. Sims et al. ( 1991) conducted on-farm evaluations of best management practices for broiler litter on eight farms in southern Delaware. Applications of broiler litter at recommended rates for corn, wheat, and soybeans increased soil test P (Mehlich 1) levels by from 38 to 121 mg P/ kg in 2 years (Table VI), relative to an unamended control soil. The agricultural significance of soils with extremely high soil test P values

+

+

38

J. T. SIMS AND D. C. WOLF Table VI

Increase in Soil Test P in Two Years from Applications of Broiler Litter at Recommended Rates for Corn, Wheat, and Soybeans"

Crop rotation

Soil series

Irrigated cornlwheatlsoybeans Sorghumlsorghum Full-season soybeansldry land corn Full-season soybeanslsoghum Irrigated cornlwheatlsoybeans No-till soybeanslirrigated corn No-till soybeanslfull-season soybeans

Evesboro loamy sand Rumford loamy sand Kenansville loamy sand Kenansville loamy sand Evesboro loamy sand Evesboro loamy sand Evesboro loamy sand

P added in broiler Soil test P litter (mglkg) (kglhal rotation) Initial Final 550 234 283 337 514 373 337

148 144

217 200

153 99 150

227 218 241 202 246 185 174

"Adapted from Sims er al. (1991). Soil test extractant was Mehlich 1 (0.025 N H I S 0 4 + 0.05 N HCI). In Delaware, a soil test P value of 35 mglkg is considered high and no fertilizer P is recommended except for a low rate of banded starter fertilizer for certain crops.

relates primarily to the length of time required to deplete these soils back to responsive levels. Studies on the long-term P-supplying capacity of high-P soils are available, but are usually limited to only a few soil types or cropping systems. In general, however, these studies show that it can take years or even decades to reduce soil test P levels in high-P soils back to levels that could be characterized as medium. For example, McCollum (1991) reported the results of a 30-year field study conducted on a Portsmouth fine sandy loam (Typic Umbraquult) used for corn and soybean production. The soil had an initial soil test (Mehlich 1) level of -100 mg/kg; it required 16 years of continuous cropping to decrease soil test P to the critical level of 20 mg P/kg. As stated earlier, soils with equivalent or much greater values of soil test P than the Portsmouth soil are common on the Delmarva peninsula and other areas where poultry wastes are applied annually. In a slightly different study, McCallister et al. (1987) applied 0, 1 1, 22, or 33 kg P/ha to a Sharpsburg silty clay loam (Typic Argiudolls) and grew irrigated corn for 12 years. Despite excellent grain yields (12-year average of 9.9 Mg/ha), soil test P (Bray P1) levels in the check plots declined only slightly, from approximately 15 to 8 mg P/kg; addition of 33 kg P/ha/year increased soil test P to about 30 mg P/kg after 12 years. The authors attributed the long-term P-supplying capacity of the Sharpsburg soil to rapidly available pools of Fe- and Al-bound P in the surface horizon and to subsoil reserves of P. As mentioned above, soil testing results can identify an accumulation of P to

POULTRY WASTE MANAGEMENT

39

excessive levels in soils and perhaps provide a general indication of their longterm supplying capacity for P. Soil tests, however, provide little information on the forms of P present in soils. Sequential fractionation methods can partition soil P into differentially soluble pools that can then be correlated to soil properties for use in management programs, as illustrated by the study of McCallister et al. (1987). Several other examples illustrate the value of understanding the form of P in waste-amended soils. McCoy et al. (1986) fractionated P in a Sassafras sandy loam soil amended with 100 kg P/ha in sludge compost and found that approximately 82% of soil P was in the Al- or Fe-bound form, 3% was found as Ca-P, and 15% as residual, undefined forms of P. The poor plant availability of P in the compost was attributed to the sludge treatment process that used A1 and Fe to precipitate P as insoluble compounds. Sharpley et al. (1984) reported that the long-term effect of adding feedlot manure to a Pullman clay loam was to increase the relative amount of total inorganic P, not organic P. Application of 67 Mg/ha/year for 8 years increased total inorganic P from about 180 mg P/kg in a check plot to 900 mg P/kg; total organic P increased from approximately 200 to 425 mg P/kg. The authors stated that although feedlot waste is regarded as an organic soil amendment, most (78%) of the P in the waste was in the inorganic fraction, hence the greater increase in soil inorganic P with time. The increase in available P (Bray P) in this soil paralleled the increase in total inorganic P, suggesting that the inorganic forms of feedlot waste may be more important for plant P nutrition and/or that organic P is rapidly mineralized and converted to inorganic P. Other authors have reported similar increases in inorganic P when organic wastes are added to soils (Chang et al., 1983; Sims, 1992). From an environmental point of view, fractionation of soil and sediment P has been used to characterize the biological availability of P to algae, a primary factor in the eutrophication process. Sonzogni et al. (1982) suggested that bioavailable P is primarily a combination of dissolved inorganic P and nonapatite, inorganic, particulate P (P adsorbed by Fe and A1 oxides). They cited several fractionation schemes that could be used to characterize biologically available P in soils and sediments and stated that extraction of soils with 0.1 N NaOH at a wide solution: soil ratio (500: 1 to 1000: 1) for 17 hours could be used for a rapid estimation of biologically available P. Vaithiyanathan and Correll (1992) reported that the discharge patterns of P from an Atlantic Coastal Plain watershed into a nearby river were related to the forms of soil P present in the watershed. Over 75% of the total P in agricultural soils in this watershed was found as inorganic P; 98% of this P was occluded and nonoccluded Fe phosphates. They also reported that 94% of the P exported from the agricultural fields was found as particulate P; hence the biological availability of Fe-bound P in transported sediments is likely to have a major effect on eutrophication of downstream lakes. Previous work by Dorich et al. (1985) that related P sequentially extracted by NH,F (ALP), NaOH (Fe-P), and HCl (Ca-P) to algal availability of P under

40

J. T. SIMS AND D. C. WOLF

laboratory conditions provides supportive evidence for the importance of Febound P. Their studies showed that NaOH-extractable P was highly correlated with algal uptake of P during a 14-day laboratory incubation ( r = 0.95). Studies on the distribution of P among various inorganic and organic fractions in soils amended with poultry wastes are rare, however. The studies described here strongly suggest that information on the distribution and biological availability of P could be an important component of an environmental management program for P in agricultural areas dominated by the poultry industry.

B. PHOSPHOROUS RETENTION AND MOVEMENT INSOILS AMENDEDWITH POULTRY WASTES Accumulations of P to such high levels in the surface horizons of agriculture soils raise two other questions of environmental importance. First, what is the capacity of these soils to adsorb the additional P that may be added in manures, litters, and fertilizers? And second, what is the nature and magnitude of P loss from these soils by erosion, runoff, drainage, and leaching to groundwaters? Studies on these issues with poultry manure/litter are available, but uncommon. More research is available with other somewhat similar organic wastes (e.g., dairy manure, sewage sludges). Results from several studies will be reviewed and used to describe our current understanding of the mechanisms involved and the type of management practices needed to reduce P loss and transport to sensitive surface waters. Phosphorus is retained in soils by a number of different mechanisms, collectively referred to as “P fixation”; it can also be immobilized in an organic form if the C :P ratio of an added organic material is high, normally >300: 1. A number of excellent review articles are available on the fast and slow processes involved in the removal of P from solution (adsorption, precipitation) and the factors controlling its reversion to a soluble form (desorption, mineral dissolution) (Barrow, 1980; Fixen and Grove, 1991; Olsen and Khasawneh, 1980; Sample et al., 1980; Sanchez and Uehara, 1980; Sharpley and Halvorson, 1994). Sample et al. (1980) stated that the primary soil constituents involved in P retention were the hydrous oxides of Fe and Al, the alumino-silicate minerals, soil carbonates, and soil organic matter. Oxides of Fe and A1 are of greater importance in the more weathered soils of humid regions; in areas of low rainfall soil carbonates have a greater influence on P retention. Sample et al. (1980) also described the mechanisms involved and the techniques used to study P fixation, and stated that this process was a “continuum embodying precipitation, chemisorption, and adsorption, if the processes are viewed throughout the entire zone of soil influenced by a fertilizer application and through a time span encompass-

POULTRY WASTE MANAGEMENT

41

ing an entire growing season or longer.” Precipitation was defined as the formation of discrete, insoluble mineral forms of Al-P, Ca-P, or Fe-P. Adsorption was described according to the approach of Bache (1964) and Muljadi et al. (1966) as having distinct stages related to the energetics of the chemical reaction and the nature of the reactive site. Uehara and Gillman (1981) attributed differences in P adsorption among soils to variations in the specific surface area and reactivity of soil colloids and their capacity to occlude P. Fixen and Grove (199 1) characterized P bonding mechanisms on soil colloids as ligand exchange reactions in which phosphate replaces aquo and hydroxyl groups on oxide surfaces, forming monodentate, bidentate, or binuclear bonds of progressively decreasing reversibility. The authors also described the process of P desorption and the hysteresis (lack of complete reversibility) commonly observed following P fixation. Various mechanisms have been proposed to explain hysteresis. Among them are precipitation, occlusion within newly formed precipitates of Fe/AI hydrous oxides, and solid-state diffusion. Regardless of the process involved, it is clear that reversion of P to a less desorbable form increases with time after application. The long-term conversion of soluble P to forms that are much more slowly available has implications for plant P uptake and P desorption into runoff, drainage, or leaching waters. It also implies that the contribution of P to eutrophication is usually best controlled by reducing particulate transport, as P will primarily be found in soils in precipitated or adsorbed forms. The role of organic matter in the retention and release of P will be of particular importance in manured soils. Although the organic fraction (e.g., humus) is not thought to have a major capacity to adsorb P directly, metal-organic matter (OM) complexes (e.g., Al-OM, Ca-OM, Fe-OM) that form in soils amended with organic wastes can play a much greater role. Organic matter has been shown to have other effects on P fixation. For instance, organic acids were shown to compete for the same adsorption sites as phosphate anions and reduce the capacity of soil minerals to retain P (Nagarajah et af., 1970). Solubilized organic matter may also be redistributed to new sites in the soil where it can coat soil minerals and reduce their importance in P fixation. Sharpley and Halvorson (1994) reviewed P transport in agricultural runoff and emphasized the need for more research on the biological availability of soluble and particulate organic P in runoff from manured soils or soils with large amounts of crop residues. Subsurface transport of P in artificially drained soils can also be affected by organic matter. Anaerobic decomposition of organic matter can reduce Fe oxides, resulting in the release of adsorbed P into drainage waters (Mitsch and Gosselink, 1986; Ponnamperuma, 1972). Much of the research on the role of organic matter as a source of P and as a soil constituent that can affect P solubility and movement has been conducted in organic soils. Further research is needed on the mineralization, fixation, and desorption of P in soils amended with poultry manures and litters.

42

J. T. SIMS AND D. C. WOLF

1. Phosphorous Adsorption and Desorption Amending soils with manures, litters, or other organic wastes has been shown to affect the adsorption-desorption process for P. These processes are normally studied by the use of adsorption isotherms that relate the amount of P added to a soil to the concentration of P in solution after an equilibration period (usually 24 hours). Adsorption isotherms do not provide information on the mechanisms of P retention, and, when conducted with whole soils, only indirectly indicate the soil constituents involved in P retention. They do, however, provide reasonable estimates of the potential for a soil to retain additional P and are useful for comparing the effects of management practices (tillage, manuring) or soil properties (horizonation, texture, clay, AllFe oxides, etc.) on P adsorption. The Langmuir equation is the most frequently used approach to estimate the “adsorption maxima” for soils: = (1

kbC kC)

+

where Q is the amount of P adsorbed per unit weight of soil, C is the equilibrium concentration of P in solution, b is the maximum amount of P that can be adsorbed, and k is a constant presumed to represent the energy of bonding of P to the surface of the solid phase. Adsorption isotherms can also be used to determine the equilibrium concentration of P at the point of zero sorption (EPC,), as illustrated in Fig. 1 1. White and Beckett (1964) suggested that the EPC, value provides an indication of the potential of a soil or sediment to gain or lose P when placed in contact with natural waters. If EPC, values exceed ambient concentrations of P in a stream or lake (typically 0.01 to 0.1 mg Plliter) the soil or sediment would tend to desorb P into solution, increasing the potential for eutrophication. The adsorption and extractability of P in a Hayesville loam (Typic Hapludults) amended with anaerobically digested poultry manure were examined by Field et al. (1985). No significant effect of manure effluent on P adsorption maxima was found, even at extremely high effluent rates (1000 mg Nlkg soil). Soil test extractable P (Mehlich 1) was linearly related to the rate of P addition, but decreased by approximately 52% after a 90-day incubation, suggesting that a rapid process for the fixation of available P existed in this oxidic soil. Reddy et al. (1980b) investigated the effect of manure (beef, poultry, swine) loading rate on P adsorption and desorption in two soils, a Norfolk loamy sand (Typic Paleudults) and a Cecil sandy loam (Typic Hapludults). Poultry manure applied at a P loading rate of 178 mglkg increased soluble P from 0.2 to 9.8 mg/kg and Mehlich 1-extractable P from 49 to 214 mg/kg. Desorbable P measured by four 1-hour extractions was increased from 1.5 to 39.0 mglkg and the EPC, from 0.06 to 7.8 mg Plliter. The authors also measured changes in P adsorption

43

POULTRY WASTE MANAGEMENT

.-

0

2

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14

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Equilibrium P Concentration (mg/L) Figure 11 Example of a P adsorption isotherm for high-P soil, illustrating the EPC, concept (hypothetical data).

maxima with depth in the Norfolk soil following 5 years of application of swine lagoon effluent. The EPC, values at a depth of 0-15 cm were 0.01, 1.7, 4.1, and 22.0 for the Norfolk loamy sand and 0.01, 0.03, 0.16, and 0.88 mg P/liter for the Cecil sandy loam for annual P loading rates of 0, 81, 161, and 322 kg P/ ha. Phosphorus adsorption maxima, as estimated by the Langmuir equation or a single-point sorption isotherm (1000 mg/kg P), were also decreased by application of swine manure. The amounts of P adsorbed from the 1000 mg P/kg addition were, for the rates of 81, 161, and 322 kg P/ha, 58, 55, and 18 mg/kg in the Norfolk soil and 220, 190, and 76 mg P/kg in the Cecil soil. The authors attributed the decreased sorption at high manure rates to saturation of P sorption sites on A1 and Fe oxides by organic anions from manure mineralization. Singh and Jones ( 1976) had previously reported a similar phenomenon when poultry manure and other organic residues were added to a Mission silt loam (Typic Vitrandept). In that study P sorption at equilibrium concentrations of 0.1 and 1.0 mg P/liter were approximately 100 and 600 mg P/kg in the untreated soil and 50 and 275 mg P/kg in the poultry manure-amended soil. Desorption of P was also greater in soils amended with poultry manure; for equivalent adsorption values of 300 mg P/kg, 0.01 M CaCl, soluble P concentrations after 150 days of incubation were approximately 0.1 1 and 0.69 mg P/liter for the check and poultry manure-amended soils, respectively. As noted by Reddy et al. (1980a), incubation of poultry manure decreased the P sorption capacity of the Mission soil with time. Amounts of P sorbed at an equilibrium concentration of 1.O mg P/

44

J. T. SIMS AND D. C. WOLF

liter were approximately 525 mg/kg in the check and 300 mg P/kg in the poultry manure-amended soil after a 150-day incubation period. Mozaffari and Sims (1994) compared P adsorption maxima from the profiles of four soils that had received broiler litter and P fertilizers on a regular basis for years with the maxima from border areas separating these fields from drainage ditches (Fig. 12). Phosphorus sorption maxima estimated from the Langmuir equation ranged from 95 to 2564 mg/kg in cultivated soils and from 200 to 2000 mg/kg in field border areas. Two clear trends were observed for P sorption. First, P sorption was consistently greater in subsoils and was highly correlated with clay content ( r = 0.90). Second, when clay contents were similar, P sorption was usually greater in field border areas than in cultivated fields, particularly in the upper 40 cm, suggesting that previous cultural practices (fertilization, manuring, liming) may have reduced the capacity of cultivated areas to retain additional P, as has been seen in other studies (Barrow, 1974; Fox and Kamprath, 1970; Guertal et af., 1991; Reddy ef al., 1980b). Adsorption and desorption data have normally been used in the United States to provide general estimates of the suitability of a site for continued P applications. However, in some countries where animal-based agriculture has resulted in soils high enough in P to be a threat to groundwater and surface waters, more stringent approaches have been taken. In the Netherlands, where 43% of the grassland and 82% of the maizeland in areas with a manure surplus were esti-

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POULTRY WASTE MANAGEMENT

45

mated to be saturated with respect to P, regulations have established a critical limit for the concentration of orthophosphate in groundwaters at 0. I mglliter (Breeuwsma and Silva, 1992). Associated with this has been the definition of a "critical degree of P accumulation in soils," defined as 25% of the P saturation capacity of the soil, which is calculated as follows: DPS = Pa,,/ PSC, x 100 where DPS is the degree of phosphorus saturation (%); P,, is the oxalate extractable P content, surface area basis; PSC, is the phosphate sorption capacity, surface area basis. Phosphate sorption capacity is not determined directly from the Langmuir equation, but is calculated from an empirical equation developed by Schoumanns et al. (1987) that combines laboratory data and constants obtained from other adsorption equations (Freundlich and Elovich): PSC, = {[S, (1

+ a In t)(c/c,)"] + P,,}Td7.1

where S, is the phosphate adsorbed at a reference concentration and time (e.g., 50 mg P/liter for 24 hours), in mmol/kg; a is a dimensionless constant from the Elovich equation; t is the reaction time (days); c, is the reference concentration of phosphate (mg/liter); c is the phosphate concentration (mg P/liter) in solution at equilibrium; n is a dimensionless constant from the Freundlich equation; Po, is the oxalate-extractable P (mmol/kg); T is the thickness of soil layer considered (depth in cm); d is the soil bulk density (g/cm3); and 7.1 is the factor that converts mmol/kg to kg P,O,/ha. In essence, this equation uses data from two rather simple laboratory measurements (oxalate-extractable P and results of a singlepoint adsorption isotherm) to identify soils that have become sufficiently saturated with P to pose a threat to groundwaters.

2. Phosphorous Losses by Erosion, Runoff, and Leaching Phosphorus becomes an environmental problem only when it is transported to a surface water sensitive to eutrophication. Because of the affinity of P for soil colloids, the dominant process involved in P transport from most agricultural soils is erosion. The loss of soluble P in runoff, drainage waters, and groundwater discharge is normally an issue only in soils that have become excessive in P. The most common situation where significant losses of soluble P by processes other than erosion have been reported has been in soils amended with animal wastes. Management practices to control losses of sediment-bound and soluble P differ conceptually and practically and are discussed in Section VI. This section focuses on the processes involved in P transport to surface waters. Erosion can be defined as the transport of soil from a field in water or wind; runoff is water that runs off a soil surface instead of infiltrating. Studies have shown that smaller, lighter soil particles, such as clays and humus, are prefer-

46

J. T. SIMS AND D. C. WOLF

entially transported in erosion and runoff. These particles have also been shown to be enriched in P relative to the whole soil from which they were transported. Other studies have shown that, from an environmental perspective, the biological availability of soluble P and particulate P transported in erosion and runoff to aquatic organisms is perhaps of greater importance than the total P load to a water body. Sharpley et a / . (1992) stated that the biological availability of P is a “dynamic function of physical and chemical processes controlling both soluble P and biologically available particulate P.” They stated that key processes regulating soluble P transport include desorption-dissolution (release of P from vegetation and decaying organic residues); for particulate P, physical processes that regulate the size and nature of particles transported and their chemical reactivity for P were of more importance. A large body of literature is available on the loss of P from agricultural fields due to erosion and runoff. The management of transport processes for bioavailable P has been reviewed recently by Sharpley and Halvorson (1994). However, limited research as been conducted directly investigating P losses from manured fields. Three recent studies illustrate the nature of this problem. Mueller et al. (1984) examined the effect of tillage on P losses when dairy manure was applied to a Dresden silt loam soil (Mollic Hapludolls). Total P, dissolved molybdatereactive P, and algal available P (estimated by resin extraction of unfiltered runoff sample) were measured in runoff under three tillage systems (conventional, chisel plow, and no-till) with and without the application of dairy manure at the rate of 8 Mg/ha (dry weight basis). Results of this study, summarized in Table VII, illustrate some of the difficulties in controlling P losses from manures

Table VII Influence of Tillage and Dairy Manure Application on Runoff Losses of Total and Algal-Available P from a Dresden Silt Loam” Total P

Algal-available P

Treatment

Concentration (mglliter)

Loss (g/m2)

Concentration (mg/liter)

Loss (g/m2)

Conventional ( - DM) Conventional ( + DM) Chisel plow ( - DM) Chisel plow ( + DM) No tillage ( - DM) No tillage ( DM)

3.6a 3.4a 2.2b 1.7bc 0.7d 1.5c

237a 158ab 92bc I Od 71cd 133bc

0.73ab 0.76ab 0.49b 0.75ab 0.24~ 1.14a

52b 39bc 20cd 7e 24cd 98a

+

aFrom Mueller er al. (1984). Means within a column followed by the same letter are not significantly different at P = 0.01, bDM, Dairy manure.

POULTRY WASTE MANAGEMENT

47

applied to agricultural soils. First, in the absence of manure, reduced tillage operations were shown, as in many other studies, to decrease the concentration and total losses of total P and algal-availableP. The authors attributed this reduction to lower sediment losses with chisel plowing and no-tillage. Combining notillage and dairy manure, however, resulted in similar total P losses and greater algal-available P losses than did conventional tillage. Applying manure to the soil surface where it was directly exposed to rainfall and runoff more than offset the advantages of lower losses of sediment-bound P. Incorporating manure, however, decreased P losses, particularly for chisel plowing, probably because the manure enhanced infiltration and thus reduced sediment loss in runoff. Rapid incorporation of manure by chisel plowing was found to be the most effective practice to reduce total and soluble P losses from soils. Unfortunately, there are two major constraints to this approach to manure management. The first is the fact that due to time and labor constraints many farmers choose to apply manure during the fall and winter months when more time is available for spreading operations, but soil temperature or moisture may make tillage operations undesirable. Also, in areas where soils are highly susceptible to compaction and erosion, farmers often must apply manure when the soil is frozen to avoid equipment-related damage to soil physical properties. Second, as shown in Table VII, recommended or required soil conservation practices may reduce sediment loss, but increase P loss, presenting farmers with a difficult choice between conserving soil or nutrients. In areas dominated by pastureland, incorporation of manures or litters is usually not possible. As pastures are commonly located on steeply sloping land, this can greatly increase the likelihood of N and P loss from surface-applied manure. McLeod and Hegg (1984) evaluated the quality of surface runoff water from a fescue pasture (Cecil clay, Typic Hapludults, 3-5% slope) that received surface applications of organic wastes (dairy manure, poultry manure, sewage sludge) and commercial fertilizer. The percentages of total P added in manure that was lost in runoff were 2.4, 1.3, and 1.2% for poultry manure, dairy manure, and sewage sludge, respectively. Poultry manure also had the highest losses of total N, NH,-N, and total suspended solids, and the highest chemical oxygen demand in the runoff water. Most of the total P loss occurred within the first 7 days of application when runoff waters had extremely high P concentrations (- 12 mg total P/liter). Total P concentrations in the background runoff were 0.28 mg P/liter. This study and that of Mueller et al. (1984) illustrate the fact that even when sediment loss is minimal, as is common in patures and no-tillage situations, P losses from manured soils can be significant, particularly if intense rainfall events occur shortly after manure application. Some management approaches to resolve this problem are discussed in Section VI. Phosphorus can also be transported to surface waters in runoff from feedlots, barnyards, or manure storage areas. Best management practices for poultry ma-

48

J. T. SIMS AND D. C. WOLF

nure encourage stockpiling of manure and applying close to the time of planting. Little information is available on nutrient losses in runoff should stored manure be exposed to rainfall. Schellinger and Clausen (1992) evaluated the effectiveness of a vegetative filter strip (mixture of tall fescue, perennial ryegrass, and Kentucky bluegrass) in reducing the losses of N, P, solids, and bacteria in runoff from soil (Massena silt loam, Aeric Haplaquept) in a dairy barnyard. The 22.9-m strip was ineffective in reducing losses of total P and total dissolved P. Initial concentrations for these two variables were 20 and 18 mg/liter; after passing through the strip concentrations were 19 and 18 mg/liter, respectively. The poor performance of the strip in removing P, and other runoff constituents, was believed to be due to exceeding the hydraulic retention capacity of the strip. Careful attention to the hydraulic characteristics of filter strips for areas with extremely high nutrient loading capacity (e.g., manure storage areas) will be required to succeed in removing pollutants from runoff water. Leaching of P to groundwaters is rare because of the high adsorptive capacity for P of the clays and Fe/AI oxides that frequently accumulate in subsoils (Logan, 1991; Nelson and Logan, 1983; Ozanne et al., 1961; Sawhney, 1978). Organic forms of P may leach to greater depths than soluble inorganic P due to their less reactive nature with soil colloids (Hannapel et al., 1964; Rolston et al., 1975). The situations in which P leaching is most likely to occur involve well-drained, deep, sandy soils (Gerritse, 1989; Humphreys and Pritchett, 1971; Mattingly, 1970; Neller, 1946; Ozanne et al., 1961), particularly those that receive frequent applications of organic wastes and wastewaters. Unfortunately, the poultry industry is concentrated in many such areas, including the Atlantic Coastal Plain of the United States, the sand mountain region of northwestern Alabama, and in the Netherlands. Losses of P in artificial drainage systems are more common than losses in groundwater discharge, particularly in soils with high organic matter contents. However, studies on the leaching of and subsurface transport of P in soils amended with poultry wastes are virtually nonexistent. The most detailed research on P leaching has been conducted with wastewater irrigation systems and animal feedlots. Adriano et al. (1975) investigated P leaching in a Melita sand (Entic Haplorthods) used for wastewater irrigation from a food processing plant. Extractable (Bray 1) P was much higher in two spray areas than in a control plot or nearby corn field, averaging 334, 562, 55, and 77 mg/kg, respectively, at depths of 0-60 cm. The intensive irrigation at the sites (150 and 300 cm/year) increased average extractable P in the 1.5- to 6.6-m depth to 36 mg/kg, relative to 5 mglkg in the control soil. Soluble orthophosphate in subsurface discharge from the spray areas ranged from 0.54 to 1.54 mg P/liter at one site and 0.04 to 1.80 mg/liter at the other, relative to surface discharge standards (at that time) of 0.05 mg Plliter. Beek et al. (1977a,b) determined the distribution and form of P in a sandy soil from the Netherlands that had been irrigated with sewage waters for either 30 or 50 years. Total P levels in nonirrigated check soils were generally less than 100 mg P/kg at all depths (to 90 cm);

POULTRY WASTE MANAGEMENT

49

soils irrigated for 30 and 50 years, respectively, had total P values at 50 cm of approximately 350 and 650 mg/kg. Most of the P within the top 50 cm of soil was found as inorganic A1 and Fe phosphates. Studies by other authors have shown similar trends for P leaching from manured soils. Kuo and Baker (1982) reported that 20 years of dairy manure application increased total and NaHC0,extractable P (0.5 M , pH 8.5) with depth in a Briscot loam (Typic Fluvaquents) and a poorly drained Schalcar muck (Terric Medisprists). As in the study by Beek et al. (1977b), little difference in P was noted between manured and control soils at depths greater than 50 cm. For example, total P was approximately 1800 and 1200 mg/kg in the surface 15 cm of the manured Briscot and Schalcar soils, compared to 800 mg/kg in a nearby well-drained, unamended soil; total P in these soils at 50 cm ranged from 750 to 800 mg/kg. Fractionation of soil P indicated that under poorly drained conditions, P accumulated more as organic P and AUFe oxide P than as Ca-P (apatites). The more amorphous, and hence reactive, nature of Fe oxides under reducing conditions may have contributed to increased P retention by this solid phase. Campbell and Racz (1975) measured total P and 0.5 M NaHC0,-extractable P under a cattle feedlot on a Almasippi loamy fine sand (Gleyed Rego Black). Total P and extractable P were 882 and 76 mg/kg in the surface 15 cm, relative to 661 and 2 mg/kg in a nearby field; values for the same parameters at 60-90 cm were 458 and 17 mg/kg (feedlot) versus 509 and 0.2 mg/kg (field). Other studies with animal wastes and feedlots have shown similar trends for soil P distribution with depth (Meek et al., 1979; Sharpley et al., 1984). Mozaffari and Sims (1994) and J. T. Sims (unpublished data) determined the distribution of soil test P with depth at 34 locations comparing agricultural fields that routinely received broiler litter and fertilizer P, with field border areas, and nearby forests (Fig. 13). The results of these studies provide further evidence that accumulations of total or soil test extractable P at depths of more than about 50 cm will be rare, even in heavily fertilized, sandy soils. Subsurface transport of P in artificial drainage may be a more serious environmental concern than direct leaching of P to groundwaters, because drainage waters normally enter streams and rivers that interact with water bodies sensitive to eutrophication. Again, there is little information on the effects of poultry wastes on P concentrations in drainage waters. Studies with other manures, however, illustrate the nature of the problem. Hergert et al. (198 la,b) reported that dairy manure increased P losses in tile drainage from a Glossoboric Hapludalf because of the large amount of inorganic P added in the manure and the effects of organic P on sorption-desorption of inorganic P. Dissolved molybdate-reactive P increased from less than 50 pg/liter to more than 2200 pg/liter immediately following the application of 200 Mg/ha of dairy manure during spring high-rainfall conditions. The authors hypothesized that under conditions of high drainage flow inorganic phosphates (primarily Ca-P) were dissolved by the low ionic strength drainage waters resulting in high P concentrations in the waters; under conditions

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POULTRY WASTE MANAGEMENT

51

of slow-moving subsurface flow, the ionic strength was higher, enhancing precipitation and giving more time for readsorption of P than under rapid drainage conditions. Other studies have shown that under the anaerobic conditions common to poorly drained soils, decomposition of organic matter can reduce Fe oxides, increasing the release of soluble P into drainage waters. Soluble P released by the mineralization of soil organic matter will also contribute to P losses in drainage. Miller (1979) found marked increases in total and dissolved P concentrations in tile drainage waters from three organic soils in Ontario, relative to mineral soils. Cogger and Duxbury (1984) attributed P leaching losses from cultivated organic soils to mineralization of large quantities of organic P combined with low contents of A1 and Fe oxides capable of retaining solubilized P. Reducing unnecessary inputs of manures and fertilizers and managing the water table to reduce subsidence were techniques recommended by these authors to minimize P losses in tile drainage. These studies raise questions about the fate of manure P applied to artificially drained, organic soils that have high P mineralization potentials and low P adsorption capacities. Although localized in nature, soils such as these may be the site of concentrated animal production. For example, a significant portion of the poultry industry in Delaware is located on sandy soils with high levels of organic matter, which are only farmed because of artificial drainage; these soils continuously receive excessive P in broiler litter and have low P sorption capacities. Although, as shown in Fig. 13, P leaching does not occur to great depths in these soils, the water table rises to the surface of these soils frequently during the year. Excess water is drained from the fields by an extensive system of drainage ditches that ultimately discharge into a nearby national estuary (Delaware’s Inland Bays) that is highly sensitive to eutrophication. The effect of alternating reducing and oxidizing conditions on P loss in drainage water from these soils is a major environmental concern. Similar situations exist for the dairy industry in central and southern Florida.

V. TRACE ELEMENTS, ANTIBIOTICS, PESTICIDES, AND MICROORGANISMS IN POULTRY WASTES A. TRACE ELEMENTS Trace elements such as As, Co, Cu, Fe, Mn, Se, and Zn are often added to poultry feed to increase the rate of weight gain and feed efficiency, increase egg production, and prevent diseases (Tufft and Nockels, 1991). Drinking water may also contribute trace elements. Because little of the trace element is absorbed by the fowl, a substantial portion of the material is excreted in the waste. Feed spillage may also add trace elements to poultry waste. In addition to copper sulfate, As compounds such as arsanilic acid or sodium arsanilate [4-aminophenylarsonic acid] at 50 to 100 mg/kg feed, 3-nitro-4-hydroxyphenylarsonicacid at

52

J. T. SIMS AND D. C. WOLF

25 to 50 mg/kg feed, and 4-nitrophenylarsonic acid at 188 mg/kg feed have been added to rations (Bhattacharya and Taylor, 1975). In some situations, B concentrations in poultry waste can be elevated due to the use of boric acid for insect control in poultry houses. 1. Concentrations The levels of trace elements in poultry waste vary widely; representative values are summarized in Table VIII. The most definitive values are those provided by Webb and Fontenot (1975), Kunkle et al. (1981), and Morrison (1969) because they showed that the level of trace element in the waste is related to addition of trace elements to the diet of the birds. When Cu was included in the feed, the Cu concentration in the waste was five to six times higher than in waste from birds not receiving Cu in the feed (Johnson et al., 1985; Webb and Fontenot, 1975). Kunkle et al. (198 1) reported that the Cu level in broiler litter was linearly related to Cu added in the diet and was concentrated in the litter by 3.25 times. The addition of As to the diet resulted in a sevenfold increase in As in the litter (Morrison, 1969). Such information would certainly indicate that knowledge of the diet of the birds provides valuable information on the trace element content to be found in the waste material.

2. Impact and Fate Land application of poultry waste can provide trace elements such as Cu and Zn, required for crop production. There is some concern that long-term application of high rates of Cu could be toxic to crops grown on coarse-textured soils or crops grown on fine-textured soils subject to anaerobic conditions (Meek et al., 1975). Morrison (1969) found no evidence that As was taken up by plants where broiler litter containing high As levels had been applied for 20 years. Total As was <6 mg/kg in surface horizons of a Captina silt loam with or without a history of poultry manure addition (Sharpley el al., 1991). Application of poultry waste at levels that increase the soil available P levels could result in a P-induced Zn deficiency in soils with low levels of Zn or where Zn-sensitive plants are grown. Meek et al. (1975) reported that citrus trees fertilized with turkey manure developed Zn deficiency symptoms while adjacent trees not treated with manure remained healthy. The Zn deficiency could also be induced by addition of P fertilizer. Perhaps a more important process is the chelation of trace elements by organic compounds in the poultry waste. Chelation can increase trace element availability to plants (Prasad et al., 1984), resulting in possible transport of the chelates or complexes beyond the root zone in the soil. The fulvic acid fraction of poultry manure was characterized by Pandeya (1992) as having 70% of the total acidity

Table VIII Concentrationsof Total Trace Elements in Poultrv Waste Concentration (mg/kg, dry weight) Element As

4-Hydroxy-3-nitrophenylarsonicacid in broiler diet 4-Hydroxy-3-nitrophenylarsonicacid not in broiler diet B

Cd co

cu

Waste type Litter Litter Litter Litter Liner Litter Litter Litter Litter Litter Litter Liner Manure Manure Litter Manure Litter Litter Manure Litter Litter Litter Litter Litter

Mean

Range

40 76 nd 17 14 35 20 3 44 36 54 38 53 30 6 1

1-60 nd a 10-22 nd 0-77 3-60 12-30 3 32-56 nd 23-125 nd 48-58 26-33 nd nd 1-1 nd

1

2 1 127 326 77 32 319

1

nd 305 -346 58- 100 25 - 39 156-599

No. of samples 41 1

nd 55

24 8 11 1 2 164

106 55 4

2 1 1 2 1 12 164 2 4

2 8

Ref. Webb and Fontenot (1975) Westing et al. (1981) Ray (1978) El-Sabban et al. (1969) Messer er al. ( 1971) Kunkle et al. (1981) Morrison (1969) Morrison ( 1969) Hileman (1967b) Stuedemann er al. (1975) Stephenson et al. (1990) El-Sabban era!. (1969) Shortall and Liebhardt (1975) Weil et a/. (1979) Westing er a/. ( 1981 ) Bruhn er al. (1977) Hileman (1967b) Westing er al. (1981) Lowman and Knight (1970) Stuedemann e r a / . (1975) Wood and Hall (1991) Vandepopuliere er al. ( 1992) Hileman ( 1967b) Kunkle et al. (1981) ( ronrinues)

Table VIII-Continued Concentration (mg/kg, dry weight) Element

cu Copper sulfate used continuously in broiler diet No copper added to broiler diet Copper sulfate used continuously in broiler diet No copper added to broiler diet

Fe

Mn

waste type

Mean

Range

Litter Litter Litter Litter Litter Litter Litter Manure Manure Manure Manure Manure Litter Litter Litter Litter Litter Litter Manure Manure Manure Manure Manure Litter Litter

84 473 593 255

nd 25-1003 nd 132-329 37-99 415 -630 51-101 29-232 20-52 nd nd 28-48 529- 12,604 698-726 1016-2288

51

515 81 126 29 146 179 38 2.377 712 1,625 1

,ooo

1,023 601

630 1,349 717 2,300 1,216 228 406

Iooo-looo nd nd 450-950 790-2205 483-950 nd nd 175-280 363-451

No. of samples 55 106 1

46 35 24 24 20 12 1 1

2 I06 2 4 2 1 55 12 14 2 1

1 2 4

Ref. El-Sabban er al. (1969) Stephenson er al. (1990) Westing er al. (1981) Webb and Fontenot (1975) Webb and Fontenot (1975) Johnson et al. (1985) Johnson er al. (1985) Bitzer and Sirns (1988) Lowman and Knight ( I 970) Ammerman er al. (1981) Long er al. (1969) Weil er al. (1979) Stephenson et al. (1990) Wood and Hall (1991) Vandepopuliere er al. (1992) Hileman ( 1967b) Westing er al. (1981) El-Sabban er al. (1969) Lowman and Knight (1970) Bomke and Lavkulich (1975) Weil er al. (1979) Ammerman eral. (1981) Long et al. (1969) Hilernan (1967b) Vandepopuliere et al. (1992)

Mo

Se Zn YI YI

“nd. Not determined

Litter Litter Litter Litter Manure Manure Manure Manure Manure Manure Manure Litter Litter Litter Litter Litter Litter Litter Litter Litter Litter Litter Manure Manure Manure Manure Manure Manure Manure

321 228 37 1 348 318 47 1 349 334 245 378 259 4 9 8 1

299 272 218 125 267 496 315 523 406 43 I 325 34 1 298 388

nd nd nd 125-667 nd 239-610 276-408 nd 217- 330 320-408 235-283 2-5 nd nd nd 291 -308 nd 189-258 105-145 nd nd 106-669 320- 660 230-635 232 -5 30 nd nd 280-309 325-450

164

55 1 106 1

20 14 1

12 4 2 2 1 164 1

2 164

4 2 55 1

106 20 14 12 1

1 4 2

Stuedemann et al. (1975) El-Sabban ef al. (1969) Westing et al. (1981 ) Stephenson et al. (1990) Long et al. (1969) Bitzer and Sims (1988) Bomke and Lavkulich (1975) Ammerman et al. (1 98 1 ) Lowman and Knight (1970) Shortall and Liebhardt (1975) Weil et al. (1979) Hileman (1967b) Westing e t a / . (1981) Stuedemann er al. (1975) Westing ef al. (1981) Wood and Hall (1991) Stuedemann er al. ( 1975) Vandepopuliere ef al. (1992) Hileman (1967b) El-Sabban et al. (1969) Westing et al. (1981) Stephenson et al. (1990) Bitzer and Sims (1988) Bomke and Lavkulich (1975) Lowman and Knight (1970) Ammerman er al. (1 98 I ) Long er al. (1 969) Shortall and Liebhardt (1975) Weil et al. (1979)

56

J. T. SlMS AND D. C. WOLF

in the form of carboxy groups. Poultry litter humic acid material was shown to have 60% of the total acidity as phenolic hydroxyl groups (Prasad and Sinha, 1981). The acidic functional groups of organic fractions in poultry waste would be important in chelating or complexing trace elements in soils.

B. ANTIBIOTICS,COCCIDIOSTATS, AND PESTICIDES INPOULTRY WASTES 1. Sources Antibiotics that have reportedly been used in poultry production systems include bacitracin, bambermycin, chlortetracycline, dihydrostreptomycin, erythromycin, lincomycin, neomycin, oxytetracycline, penicillin, spectinomycin, streptomycin, tetracycline, and tylosin (Bhattacharya and Taylor, 1975). Several chemicals are used to control the internal protozoan parasites that cause coccidiosis. Some of the common coccidiostats in poultry diets are monensin, lasalocid, clopidol, halofuginone, and salinomycin (Minchinton et a!. , 1973; Stephenson et al., 1985). It is a common practice to treat broiler houses with disinfectants between flocks. It is possible that the disinfectants could be present in poultry waste and in the soil near the houses. In layer house operations, chemicals are often included in the poultry diet to aid in insect control. The chemical is passed through the fowl and prevents larvae development in the waste. Examples of some larvicides would be rabon, zoalene, unistat, nicarbazin, furazolidone, and nitrofurazone (Bhattacharya and Taylor, 1975) and cyromazine (Pote et al., 1992). Such materials are generally not included in the diets of broilers, but are restricted to caged-layer operations. Wills et al. (1990) reported that topical application of cyromazine and dimethoate to caged-layer manure had no detrimental effect on filfth fly predators. It is possible that herbicides or insecticides could be isolated in litter samples if the pesticides were present in the bedding material, but pesticides do not appear to be a common problem.

2. Concentrations The amount of chemical residue found in poultry waste is related to the amount, frequency, retention, and stability of the material. Webb and Fontenot (1975) evaluated broiler litter samples and found that the level of chlortetracycline was over 15 times greater in litter when the antibiotic was used continuously in the diet as compared to when it was included intermittently in the diet (Table IX).

Table IX Commonly Used Antibiotics, Coccidiostats, and Larvicides Concentration (mg/kg, dry weight)

Name Material Antibiotic

Coccidiostat

Larvicide

Chemical

Common

Mean 27.3 12.5 0.8

Amprolium Chlortetracycline Chlortetracycline Neomycin sulfate Nicarbazin Ox ytetracycline Penicillin Amprolium Zoalene

Arnprol Aureomycina Aureomycin Neomycin

-

193 (10

2-Chloro- 1-(2,4,5-trichlorophenyl) vinyl dimethyl phosphate

Rabon '

406

"Used continuously in broiler diet. intermittently in broiler diet. 'Diet contained 800 mg/kg rabon.

Terramycin Propen Amprol

0 81.2 10.9

12.5

Range 0.0-77.0 0.8-26.3 0.1-2.8 -

35.1 - 152.1 5.5-29. I 0-25

Not determined 186-580

No. of Samples

Ref

29 26 19 12 25 12 2 1 1

Webb and Fontenot (1975) Webband Fontenot (1975) Webb and Fontenot (1975) Webb and Fontenot (1975) Webb and Fontenot (1975) Webb and Fontenot (1975) Webb and Fontenot ( 1975) Ray (1978) Ray (1978)

I

Wasti et a/.(1970)

58

J. T. SIMS AND D. C. WOLF

3. Impact and Fate The consequences of land application of poultry waste containing antibiotics, coccidiostats, disinfectants, or pesticides have not been adequately evaluated. Nitrogen mineralization and corn growth were not influenced by chlortetracycline or oxytetracycline in beef cattle feces (Patten et al., 1980). However, Tietjen (1975) reported changes in biodegradation and crop response to antibioticcontaining manures. Aflatoxin formation has been reported in feedlot manure, but the importance of aflatoxin production in poultry waste needs to be assessed more completely (Fontenot and Webb, 1975). Growth deformation in vegetable crops in soils amended with poultry manure has been related to the presence of 4-amino-3,5-dichloro-6-methylpicolinic acid that resulted from metabolism of an impurity in the coccidiostat clopidol (Minchinton et al., 1973).

C. MICROBIAL POPULATION OF POULTRY WASTES 1. Types and Levels Poultry waste contains a large and diverse population of viruses, bacteria, fungi, and protozoa. Typical total microbial viable colony-forming units (CFU) counts of loxto 10' CFU/g dry waste have been reported (Halbrook et al., 195 I ; Johnson et al., 1985; Lovett et al., 1971; Nodar et al., 1990a,b). Fungi were found at levels of lo4 and lo5 CFU/g dry waste (Lien et a / ., 1992). Toxigenic fungi have been isolated from poultry litter (Lovett, 1972). In a study of the microbial population of seven poultry litters ranging in age from 1 to 36 weeks, Schefferle (1965a) reported total bacterial plate counts of 1.1 X 10"' to 1.5 X IO"/g fresh weight. The proportion of the total bacterial population capable of hydrolyzing uric acid ranged from 14 to 42% with a mean value of 24% (Schefferle, 1965b). The majority of the aerobic bacteria converted uric acid to urea, but some bacteria were capable of complete hydrolysis of uric acid to NH,-N. Giddens and Rao (1975) measured the total bacterial population in fresh poultry manure and in poultry litter after 3 days of incubation and reported levels of 9.7 and 58.6 x 10y/g dry weight, respectively. Fungal populations after 3 days for the manure and litter were I .O and 2.6 X 10s/gdry weight, respectively. Pathogenic microorganisms present in poultry waste represent a potentially serious health concern because of the diseases they could cause (Bhattacharya and Taylor, 1975; Fontenot and Ross, 1981; Fontenot and Webb, 1975; McCaskey and Anthony, 1979). The most frequently studied bacterial pathogens are Clostrzdium spp. and Salmonella spp. Alexander et al. (1 968) studied 44 samples of broiler, hen, and turkey waste and reported that 13 of the samples were negative for pathogenic bacteria, but Clostridium spp. were recovered from 60% of the samples. Kraft et al. (1969) studied fresh poultry manure from 91 houses and

POULTRY WASTE MANAGEMENT

59

reported that they isolated Salmonella spp. from 29% of the samples with levels of < I/g to > 3 x 104/gdry waste. Because of the difficulty and expense in conducting specific pathogen analyses, most studies have used bacterial indicators such as fecal coliforms or E . coli to assess potential fecal pathogen contamination of groundwater and surface water. Poultry produce approximately 45 g (dry weight) of fecal material/day, and the fresh waste contains approximately 1O6 coliforms/g dry waste (Geldreich e t a l . , 1962; Lien et al., 1992; Lovett et al., 1971). Giddens and Barnett (1980) evaluated total coliform levels in runoff from fallow and grassland amended with poultry manure and found levels as high as 3.8 x lo6/100 ml. Analysis of runoff samples from unamended tall fescue plots studied by Quisenberry et al. (198 1) had mean fecal coliform levels of 5.2 X 104/100 ml and exceeded the primary contact limit of 200/ 100 ml. Baxter-Potter and Gilliland (1988) noted that bacterial levels in agricultural runoff often exceed water quality standards regardless of management practices. The first runoff-producing rainfall event following waste application generally contains the highest pollutant levels (McLeod and Hegg, 1984). Grass buffer strips have been shown to reduce fecal coliform levels in manure-polluted runoff (Doyle et al., 1975). Survival of fecal indicators and pathogens generally decreases as (1) temperature increases, (2) the waste dries, and (3) the waste is exposed to sunlight (Menzies, 1977; Reddy et al., 1981). The presence of Cu in litter may also influence microbial population dynamics (Johnson et a f . , 1985).

VI.POULTRY WASTE MANAGEMENT PROGRAMS Management programs for poultry wastes must reflect both the potential value of the waste as a resource and a realistic appraisal of the negative effects waste constituents may have on the environment. The concentrated nature of the poultry industry commonly results in large quantities and varieties of wastes (litters, manures, dead bird composts, wastewaters, sludges) being produced in relatively small geographic areas. Transportation costs and the lack of a waste-processing and distribution infrastructure require that a comprehensive approach to poultry waste management be developed to take advantage of all beneficial end uses for the diverse waste products of this industry. The predominant resource value of most poultry wastes is as a source of plant nutrients for agronomic crop production. Other end uses, reviewed by Edwards and Daniel (1992), are (1) as a feed material for ruminants, (2) as a fuel source, either through direct burning or methane generation, and (3) as a component of composts or organic fertilizers for specialty crops. Major environmental impacts of poultry wastes, discussed earlier in this article, can be briefly summarized as ( 1 ) groundwater contamination by nitrate-N, (2) eutrophication of surface waters

60

J. T. SWIS AND D. C. WOLF

by N and P in runoff, (3) long-term fates of heavy metals and pesticides on soils, waters, and the food chain, and (4) pollution of drinking waters by pathogens such as E . coli and subsequent effects on human and animal health. Clearly, the environmental impact of greatest concern will be directly related to the use of the poultry waste. Avoiding degradation of groundwaters and surface waters by nutrients, pesticides, and pathogens is the most pressing issue associated with land application of poultry wastes. Toxicological effects of these waste constituents are of more concern when the wastes are processed and used as animal feeds. Sound waste management plans must reflect and prioritize the risks associated with each end use to maximize resource value and minimize environmental impacts. The focus of the management practices discussed here will be the use of poultry wastes as fertilizer materials for crop production. The literature on the advantages and disadvantages of refeeding poultry wastes to ruminants is voluminous and exceeds the scope of this article, as does the use of wastes as fuels. Readers are referred to several reviews of these topics (Fontenot and Ross, 1981 ; McCaskey and Anthony, 1979; Shuler, 1980; Smith and Wheeler, 1979).

A. OVERVIEW OF AGRICULTURAL MANAGEMENT PLANS FOR POULTRY WASTES The components of an effective waste management program for the agricultural use of organic wastes are illustrated in Figs. 14 and 15 and include (1) site selection, (2) production and collection, (3) storage, handling, and treatment, (4) transfer and application, and ( 5 ) utilization. Legal and regulatory requirements must also be considered in designing a plan. Although there is no single waste management plan that is appropriate for all locations, site-specific optimization of each of these components is essential to avoid wasting resources and pollution of nearby environments. The localized nature of the poultry industry in many areas also requires that regional waste management plans be developed using the same principles as farm-wide plans. Whatever the scale, comprehensive waste management plans assist in identifying potential problems in waste utilization and provide the basis for long-term plans for the most efficient use of these potentially valuable resources. Some key aspects of each component will be considered to illustrate the process involved in developing a waste management plan. 1. Site Analysis and Selection

Natural land features should be carefully considered when developing an agricultural waste management plan. As illustrated in Fig. 14, site analysis must

POULTRY WASTE MANAGEMENT

61

Fence

Existing contours

-95-

.&

Cntirdnews

Figure 14 Typical site analysis for an agricultural waste management system. Adapted from Soil Conservation Service (1992).

include appropriate locations for production, storage, and treatment facilities, as well as the suitability of soils on the site for land application of wastes. Proximity to streams, ponds, and drainageways and an understanding of groundwater hydrology are also vital components of site analyses, as are other potential environ-

62

J. T. SIMS AND D. C. WOLF

Figure 15 Waste management options for a poultry operation. Adapted from Soil Conservation Service (1992).

mental impacts such as odors, dusts, and noise. Flexibility is another important design consideration. A well-designed plan allows for future expansion of the operation or incorporation of conservation measures such as vegetated filter strips near streams.

POULTRY WASTE MANAGEMENT

63

2. Production, Handling, Storage, and Treatment A waste management plan should have as one of its highest priorities the minimization of waste generated. Any design feature that can reduce the volume of solid waste or wastewater will facilitate the ease and efficiency of operation of the plan. Examples include diverting clean runoff away from wastewater lagoons and avoiding spills of feed and other solid materials. Waste collection should reflect the patterns of waste production and be closely tied to storage capacity so that wastes can be stored in locations that are protected from rainfall and runoff and are maintained in a physical condition suitable for the appropriate application technique. Outside storage of broiler litters, for example, can result in a wet material that is difficult to handle and apply uniformly and can contaminate the storage site with salts and NO,-N, making it unsuitable for crops and a pollution threat for nearby waters. Treatment facilities, such as lagoons or composting facilities, should be properly constructed and have adequate capacity to handle normal and unexpectedly high volumes of waste; they should also monitor waste properties to determine the effectiveness of the treatment operation.

3. Transfer and Application Transportation of wastes to the site of ultimate use and application at the site must be considered carefully. The unfavorable economics of waste transportation often result in limited distribution of nutrients throughout a farm, causing the buildup of some nutrients (e.g., P) to excessive levels in fields short distances from the site of waste generation. Outdated or poorly maintained application equipment can restrict the rates of waste that can be applied or result in poor distribution during application. As an example, newer “spinner” type manure spreaders can uniformly apply much lower rates of broiler litter than older, flail type spreaders, allowing for more precise application of desired rates of N and P. Timing of application to maximize crop recovery is another critical factor, one that is closely related to production patterns and storage capacities. Application of poultry wastes during fall and winter, when crops have not been planted or are not actively growing, is normally discouraged. Unfortunately, during these months, farmers have more time available for transfer and application operations than during the spring when other operations (cultivation, planting, herbicide application, etc.) are necessary. In some areas it is often necessary to apply animal wastes when the soil is frozen to avoid compaction and erosion problems that can result from heavy equipment traveling over the normally wetter soils of spring. Poorly timed applications of wastes that result in excessive losses of nutrients in runoff or by leaching are one of the most difficult challenges to resolve. Computer modeling approaches to determine the most effective means to schedule poultry waste applications to soils are being developed to help resolve this problem (Edwards et al., 1992).

64

J. T. SIMS AND D. C. WOLF

4. Utilization Maximizing the resource value of poultry wastes often requires a combination of end uses. A common example of this is a small farm that produces more waste material than is needed to meet the nutrient requirements of the crops grown on the farm. In this situation, the poultry producer must identify other options to avoid potentially contaminating surface waters or groundwaters by overapplying nutrients to cropland. If options such as refeeding or incineration for energy generation are not available, distribution to nearby farms or industries that market and apply wastes may be necessary. Well-established infrastructures to redistribute poultry wastes to nutrient-deficient areas are uncommon, however, particularly on a regional scale. In Delaware and Arkansas, for example, the Cooperative Extension system, in cooperation with the poultry industry, has developed a local network for farmers that wish to obtain excess broiler litter from nearby poultry operations. However, at this time, a comprehensive plan to deal with the large excess of nutrients present in these states (Table IV) has not been developed. In most poultry operations, the first step in an effective waste management plan will be an assessment of the capacity of available cropland for nutrients in the wastes. Most nutrient management programs for animal wastes are similar to those developed for sewage sludges and are usually oriented toward identifying the appropriate application rate for a specific crop and field. However, unlike municipalities that often apply sludges to a number of different farms, most poultry growers have a fixed amount of cropland available to receive wastes. A comprehensive, farm-wide nutrient budget is essential to ensure that waste production does not exceed the capacity of the entire farm for nutrients. Once an efficient nutrient management plan has been designed, further steps can be taken to identify alternative end uses for excess wastes.

B. NUTRIENT MANAGEMENT PLANS Poultry wastes contain all essential plant nutrients, several nonessential heavy metals, natural and synthetic organic compounds, and a variety of pathogenic organisms; each of these waste constituents could conceivably limit the rate of poultry waste application to agricultural lands. Current approaches to land application of litters, manures, and wastewaters, however, are almost exclusively based on meeting crop nitrogen requirements, for several reasons. First, groundwater contamination with nitrate N in areas of intensely concentrated animal production is recognized as a serious and documented environmental problem. Second, although excessive P levels in soils that frequently receive poultry wastes are common and the role of soil P in eutrophication of sensitive surface

POULTRY WASTE MANAGEMENT

65

waters is well-known, basing the application rate of poultry wastes on crop P requirement creates serious logistical problems. The extremely low rates of manures and litters required, if any, to meet crop P needs result in large surpluses of these wastes, often without adequate alternative end uses. In some areas, however, as discussed in Section IV, concern about excessive soil P has resulted in its use as a land-limiting constituent for poultry wastes. Third, limiting annual or total application rates of poultry wastes based on the loading rate of nutrients other than N or P is usually unnecessary because they rarely affect crop production or the environment. And finally, the lack of clearly documented, significant, environmental impacts of heavy metals, pesticides, and pathogens has, at least to this point in time, made their use as land-limiting constituents for poultry waste application unjustified. Nutrient management programs normally consist of four steps: (1) identification of crop nutrient requirements at realistic yields, (2) the use of soil testing to estimate nutrients available for the crop from previous applications of fertilizers and wastes, (3) an assessment of the nutrients that will be provided when the waste is applied, and (4)efficient application techniques that provide the desired amount of waste at the proper time to maximize crop nutrient uptake. Examples of N- and P-based nutrient management plans will be given to illustrate these steps and the fundamental differences in these two approaches to nutrient management. Recent advances in soil and plant testing that can improve the efficiency of nutrient management of poultry wastes will also be discussed.

1. Nitrogen Management Plans for Poultry Wastes Current approaches to N management for poultry wastes normally base land application rates on the amount of predicted or potentially available N (PAN) needed to provide adequate N for a crop at a realistic yield goal. Waste management practices, based on local soil and climatic conditions, are then relied on to minimize the excess amount of N required, as a result of anticipated N losses (system inefficiency), to attain optimum yields. For most organic wastes the application rate needed to provide &heproper amount of available N is estimated by one of two approaches, the decay series or the fertilizer equivalence. The decay series approach has been widely adopted as a means to estimate both initial and residual availability of N in poultry wastes. A decay series is essentially a quantitative estimate of the amount of N that will be mineralized from an organic waste over a period of several years, and is usually based on laboratory N mineralization studies. Pratt er al. (1973) proposed a decay series of 0.90-0.100.05, for poultry manure, indicating that 90% of &heorganic N would mineralize in the first year, 10% of the remaining organic N in the second year, and 5% in the third year. Although use of a decay series is conceptually sound, it is obvious that many factors can affect the success of this approach, including heterogeneity

66

J. T. S M S AND D. C. WOLF

of wastes, annual variations in climate, and cropping system effects (e.g., tillage and irrigation), to name but a few. Further, as the decay series only estimates the amount of N that will become available, some technique to adjust (increase) the waste application rate to account for potential N losses by volatilization, denitrification, or leaching would be required. Multiyear field calibration studies are essential to verify a decay series. Sims (1987) evaluated a decay series of 0.60-0.20-0.10 for broiler litter, in combination with an adjustment for volatilization losses of NH,-N, in a 3-year field experiment with irrigated corn. Results showed that, although successful in producing comparable grain yields as fertilizer N, the efficiency of N recovery obtained was low enough to be of concern from an environmental viewpoint, averaging 36% for broiler litter and 56% for fertilizer N. The fertilizer equivalence approach determines N availability in organic wastes more empirically. Field studies comparing several rates of fertilizer N and organic wastes are used to determine the amount of total N in an organic waste needed to obtain yields or N uptake by a crop equivalent to that obtained with fertilizer N. Results are expressed as an equivalent rate (kg N/ha) or as a percentage of total N. A recent example of this approach was a 3-year field study with silage corn that reported, based on silage yield, that the fertilizer equivalence for dairy manure ranged from 73 to 122 kg N/ha or 27 to 47% of manure total N; based on N uptake the fertilizer equivalence was 26-60% (Jokela, 1992). Perhaps the most notable advances in recent years with regard to increasing N use efficiency from organic wastes have been in the area of soil and plant testing. An accurate soil test for N has been a long but elusive goal for soil scientists. The complex and dynamic nature of N cycling has made it difficult to use chemical extractants to estimate N availability in advance of planting. Similar problems have prevented the adoption of rapid chemical tests for available N in organic wastes (Castellanos and Pratt, 1981b; Chescheir et al., 1986). Residual tests for NO,-N have had a history of success in arid-zone soils, but not in humid regions (Hergert, 1987). In 1984 a significant breakthrough in soil N testing occurred that has shown the potential for markedly improving the efficiency of organic N sources for certain agronomic crops. The presidedress soil nitrate test (PSNT) was conceived and evaluated to address the problem of overfertilization of N in the northeastern United States, particularly in fields with histories of manure and legume use (Magdoff el al., 1984). The PSNT has four basic tenets, summarized as follows: ( 1 ) all fertilizer N for corn, except a small amount banded at planting, should be sidedressed when the crop is beginning its period of maximum N uptake; (2) soil and climatic conditions prior to sampling integrate the factors influencing the availability of N from the soil, from crop residues, and from previous applications of organic wastes; (3) a rapid sample turn-

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around (30 cm. The PSNT has since been evaluated in over 300 field studies in the northeastern (Magdoff et al., 1990) and midwestern United States (Blackmer et al., 1989) and has been repeatedly shown to be successful in identifying N-sufficient soils. Some of the logistical difficulties associated with the need for a rapid sample analysis have been overcome by the development of “quicktest” kits and electrodes that can be used in the field (Jemison and Fox, 1988). Even more encouraging are the results of a recent study with the leaf chlorophyll meter, which showed that this extremely rapid, in-field measurement of leaf “greenness” was as accurate as the PSNT in identifying N-sufficient sites (Piekielek and Fox, 1992). Another new approach to assessing N sufficiency for corn is the stalk nitrate test (Binford et al., 1990). This post-mortem test uses the concentration of NO, in the lower portion of the stalk at corn maturity to identify fields that receive excessive N from fertilizers or manures. The implications of these tests for organic waste use are straightforward, but not simple. For most farmers poultry manure or litter would be applied according to a decay series or fertilizer equivalence approach. A PSNT soil sample would be taken and, if necessary, additional fertilizer would be applied via sidedressing. However, studies from soils commonly amended with animal wastes have shown that often little or no sidedress N is required, even when manure was not applied in the current year (Fox et al., 1989; Meisinger et al., 1992). Roth and Fox (1990) found that the “economic optimum N (EON) rates” (N rate where economic return on fertilizer N investment is maximized) for 11 fields with and without long-term histories of manure use averaged 34 kg N/ha for manured fields and 128 kg N/ha for nonmanured sites. Alternatively, for maximum environmental efficiency, farmers could apply a suboptimum rate of waste that would be very unlikely to produce excessive soil N. A PSNT soil sample or leaf chlorophyll meter reading would be taken and the remainder of the crop’s N needs, if any, supplied via sidedressing fertilizer N at a time when crop N uptake efficiency is high. The greatest difficulty with the PSNT approach to organic waste use, apart from logistical problems associated with the rapid analytical turnaround, has been the presence of high percentages of soils that have been shown to need less, or no, manure/sludge than is generated by the farm or municipality. Simply put, these tests have shown that, particularly for animal-based agriculture, more N is often produced than is needed by the farming operation, given the land available and the economics of waste handling and application. This once again illustrates the need for organic waste management at a larger scale, state or regional in scope, oriented toward redistribution of waste N to nutrient deficient areas. Nitrogen use efficiency can be improved by other means as well, although efforts to control N losses under field conditions can be difficult and expensive

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and may increase one form of loss while reducing another. The use of conservation tillage practices can be expected to reduce erosion and runoff losses of N. Reducing water movement off a field, however, may increase infiltration and perhaps N03-N leaching and denitrification. Surface applications of wastes may also reduce soil-waste contact and accelerate waste drying, enhancing NH, volatilization but decreasing the rate of N mineralization. Other conservation practices that have the potential to reduce N losses include the use of winter cover crops to trap residual N from wastes, and controlled drainage systems or artificial wetlands to enhance denitrification in field border areas. It has been possible to increase N recovery from fertilizers by the use of improved application techniques, timing, and placement (banding, sidedressing, fertigation) or by developing more efficient or enhanced fertilized materials (slow-release N sources, chemical nitrification inhibitors). Logistical and economic constraints, however, have hindered the widespread development of improved handling and application techniques for animal wastes, although some progress has been made in waste processing, primarily in the areas of composting and pelletizing. As mentioned earlier, composting stabilizes the N in wastes, decreasing the likelihood of N losses via leaching or denitrification, whereas pelletizing provides a drier material with much greater flexibility in terms of nutrient content and application techniques. Composting converts raw waste to a more humuslike material, suitable for application at extremely high rates, but with limited N supplying capability. Pelletizing can convert organic wastes into enriched, fertilizer-like materials that have broader uses and fewer restrictions on transportation and handling. The use of nitrification inhibitors with raw wastes, or probably more effectively with pelletized materials, can increase the efficiency of N recovery as well. Sallade and Sims (1992) found that adding thiosulfate to a poultry manure-amended soil inhibited nitrification and thus decreased the likelihood of NO,-N losses by leaching or denitrification. Composting and pelletizing represent the type of large-scale improvement needed in the centralized processing and distribution of poultry wastes to create additional end-uses that increase the geographic distribution of poultry waste nutrients.

2. Phosphorous Management for Poultry Wastes Best management practices for poultry wastes that focus on controlling N losses will almost always result in continuous increases in soil P, as discussed in Section IV. In general, approaches to reduce nonpoint source pollution of P from agricultural soils have two major components. First, the transport processes by which P moves from an agricultural field to surficial waters must be controlled by conservation practices that minimize erosion and runoff. Minimum tillage

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operations, grassed waterways, and field border areas are commonly used to reduce particulate P losses, but are not always effective in controlling the loss of the more soluble, bioavailable forms of P, such as dissolved P and P in fine sediments (clays, fine silts). Second, in addition to controlling transport, processes to reduce the enrichment of soil particles and runoff waters by P must be developed. Practically speaking this involves the use of nutrient management programs that prevent soil test P levels from increasing beyond existing excessive values, while attempting to develop crop rotation strategies that can enhance the rate of depletion of P in these soils. The overall strategy for the environmental management of P for agricultural operations that routinely use animal manures should be a systems approach with the components discussed in the following sections. a. Develop a Farm-Wide Nutrient Budget The initial step in effective environmental management of P is the acknowledgement that a field, farm, or even region in a state may not possess adequate soil resources to use all the P generated by animals and municipalities, as clearly illustrated for Delaware in Table IV. It is therefore imperative to, on each scale, develop a quantitative P “budget” that clearly delineates the amount of P available for land application, the current P status of the soil, and crop removal under realistic yield conditions. The amount of P available can be estimated from manure production and P content, although some doubt exists as to the current accuracy of farmer estimates of quantity and timing of manure production. A detailed soil testing program should then be conducted that will quantify existing soil P levels and the rates of manure or fertilizer P required to adjust all soils on the farm to an optimum P level. Sims (1986a), based on crop removal studies under greenhouse conditions, estimated that from 4 to 15 years would be required for soil test levels of P to decline to less than a high value. Given that crop removal of P is relatively low for most grain crops, it is important to know if a field would be unsuitable for waste application, due to high P levels, for this many years. b. Allocate On-Farm Nutrients in Accordance with the Budget Once a nutrient budget is constructed, P-deficient fields can be identified and preferentially used as sites for land application of manures and litters. If an overall P excess exists, as is true for many animal-based operations, alternative methods for utilization of animal wastes must be conceived, designed, and implemented. It may even be possible to prepare long-term plans that anticipate, based on actual soil test values, when certain fields or areas on a farm will decline to P levels that require organic waste application to ensure adequate crop production.

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c. Minimize the Use of Unnecessary “Off-Farm” Nutrients An accurate farm nutrient budget can clearly identify the quantity of fertilizer or “off-farm’’ waste P required to achieve and maintain an optimum level of soil P. In general, for an animal-based operation, the long-term benefits of building up soil P to high levels with fertilizer P or P from other sources (e.g., sludges or composts) are debatable. Once soils have reached a desirable maximum level of soil test P they may no longer be suitable for manure application, forcing farmers to develop prematurely alternative end uses for on-farm wastes. Conversely, crop production on soils testing low or medium in P may result in reduced yields in the near term if P fertilizers are not used. It is also possible that the use of small quantities of P in “starter” fertilizers may produce increases in crop yields even in soils that have high soil test levels of P. d. Implement and Evaluate Appropriate Conservation Measures In addition to maintaining soil P at an optimum, but not excessive, level, farmers should implement the conservation measures necessary to prevent P loss by erosion and runoff. Although many farmers are familiar with and use some form of reduced tillage, the expertise and desire to implement more laborintensive and expensive conservation measures, such as grassed borders around field edges, is often lacking. Additionally, research on the value of these conservation measures has sometimes produced conflicting results. Sediment loss is frequently reduced by no-tillage, but soluble P losses may be enhanced due to buildups of P in surface horizons. Failing to incorporate manures, as required by no-tillage, may enhance P losses from these wastes, as discussed earlier (Table VI). This emphasizes the need for regulatory and advisory agencies that compel farmers to assume the costs and loss of land associated with conservation practices to assume the obligation to evaluate thoroughly the performance of these practices under field conditions. e. Refine and Utilize Monitoring Techniques There is a serious need to improve soil testing programs for P to achieve environmental, as well as agronomic, ends. As previously mentioned, many soil testing laboratories do not determine or report the actual value of extractable P. This presents a serious problem for individuals monitoring soils-i.e., how high is a “high” soil test, and at what level of extractable P is regulation of organic waste application implemented? Beyond this is the fact that, while some studies have shown that soil test extractable P may be well correlated with bioavailable P (Wolf et al., 1985), soil test extractants are at best crude estimates of potentially desorbable P. Further, a soil test extractant provides no real estimate of the capacity of the soil to sorb additional P. Other types of “soil tests” for P that bear further investigation include dilute salt solutions to estimate soluble P; P

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sorption isotherms, from which equilibrium phosphorus concentration (EPC,) at zero sorption values can be obtained; and tests better correlated with bioavailable P (e.g., algae-availableP) than routine soil test extractants. Sims (1993) recently reviewed this issue and stated that a concerted effort is needed to develop and evaluate, under field conditions, regional “environmental soil tests” for P that identify soils with sufficiently high levels of soluble, readily desorbable, and/or bioavailable P to be of environmental concern. An example of an approach that integrates all these factors is the Phosphorus Index System currently under development by a national working group that includes representatives from the USDA-SCS, university and federal agricultural experiment stations, and the Cooperative Extension. The objective of this working group is to develop a phosphorus indexing procedure that can identify soils, landforms, and management practices with the potential for unfavorable impacts on water bodies because of phosphorus movement. The development and use of the P index has been described in detail by Lemunyon and Gilbert (1993), but essentially it is a field-oriented matrix system that is designed for use by technical personnel in advisory agencies, crop consultants, and others. It assesses the potential for a field to be a significant source of nonpoint pollution with P by computing a weighted site index based on erosion, runoff, soil test P, and the method, source, and rate of P additions to soils. Once the P index has been determined for all fields on a farm, prioritized conservation and nutrient management activities can be planned and implemented.

VII. CONCLUSIONS The implementation of environmentally sound management programs for waste products will clearly be one of the greatest challenges faced by the poultry industry in the next decade. We see the following trends developing in this area. Comprehensive waste management plans are likely to be required for all levels of the industry (farm to processing plant) by environmental regulatory agencies, in response to public concerns and an expanding body of research documenting the impact of nutrients from poultry wastes on groundwaters and surface waters. The concentrated geographic nature of the poultry industry will result in a critical need for state or regional waste management plans that focus on redistribution of excess nutrients to areas where they are needed for crop production, particularly N and P. Directly related to this, an infrastructure that can process poultry wastes into value-added products that are economical to transport, easy to apply, and adaptable to a wider variety of cropping systems is likely to evolve. An example might

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include the production of pelletized broiler litter that has been enriched with commercial fertilizers, giving a higher analysis material suitable for use in turf, vegetables, or as the primary nutrient source for grain crops. This approach would provide opportunities to not only blend crop-specific grades of fertilizer, but to incorporate other products such as herbicides and nitrification inhibitors. The most critical research need at present is an accurate assessment of the impact of high-P soils resulting from long-term amendment with poultry wastes on surface water quality. If P should replace N as the land-limiting element for poultry waste application, major logistical difficulties will face the poultry industry, as many soils would not be available for use in land application programs. Management strategies to control the loss of soluble (as well as sedimentbound) P will become more important due to the excessive P levels common in soils that are routinely amended with poultry litters and manures. Past research has clearly documented the potential groundwater contamination by NO; leaching from soils amended with poultry wastes. Losses of N in runoff are also of concern in areas where litters and manures are applied to erodible pasturelands. Research on N management for poultry wastes is likely to focus on more intensive management practices, such as better application timing and the use of soil and plant N testing to improve N use efficiency. Other research may be directed toward the use of increased processing (composting, pelletizing) to alter the rate and extent of N transformations in poultry wastes, thus increasing plant recovery of N. Research on other aspects of poultry waste management, such as dead poultry disposal and the fate of pathogens, trace elements, pesticides, and antibiotics, will be needed to ascertain the true environmental significance of these issues. Current research is simply inadequate in scope to answer questions about the short-term or long-term effects of these waste constituents on soil, plant, or water quality.

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Steele, K. F., and McCalister, W. K. (1991). Potential nitrate pollution of ground water in limestone terrain by poultry litter, Ozark Region, U.S.A. NATO ASI Ser., Ser. G 30, 209-218. Stephenson, A. H., McCaskey, T. A., and Ruffin, B. G . (1990). A survey of broiler litter composition and potential value as a nutrient resource. Biol. Wastes 34, 1-9. Stephenson, E. L., Bond, P. L., Miller, M. A,, and Meinecke. C . F. (1985). Poultry coccidiostats compared for effect on chick performance and efficacy against coccidiosis. Arkansas Frrrm Res. 34(6), 9. Strebel, 0.. Duynisveld. W. H. M., and Bottcher, J. (1989). Nitrate pollution of groundwater in western Europe. Agric. Ecosysr. Environ. 26, 189-214. Stuedemann, J . A . . Wilkinson, S. R., Williams, D. J . , Ciordia, H., Ernst, J. V., Jackson, W. A , . and Jones, J. B., Jr. (1975). Long-term broiler litter fert tion of tall fescue pastures and health and performance of beef cow. Managing Livest. Wastes. Proc. Int. Symp., 3rd. UrbanaChampaign, IL, pp. 264-268. Tietjen, C. (1975). Influence of antibiotics and growth promoting feed additives on the manuring effect of animal excrements in pot experiments with oats. Managing Livest. Wastes, Proc. In!. Symp., Jrd, Urbana-Champaign, IL, 1975, pp. 328-330. Tufft, L. S . . and Nockels. C. F. (1991). The effects of stress, Escherichia coli. dietary EDTA, and their interaction on tissue trace elements in chicks. Poult. Sci. 70, 2439-2449. Uehara, G . . and Gillman, G . P. (1981). “The Mineralogy, Chemistry, and Physics of Tropical Soils with Variable Charge.” Westview Press, Boulder, CO. U.S. Department of Agriculture (USDA) (1991). “Nitrate Occurrence in U S . Waters.” USDA. Washington, DC. U.S. Environmental Protection Agency (USEPA) (1985). National primary drinking water regulations: Synthetic organic chemicals, inorganic chemicals, and microorganisms: Proposed Rule. Fed. Regist. 50,46935-47022. Vaithiyanathan. P., and Correll, D. L. (1992). The Rhode River watershed: Phosphorus distrihution and export in forest and agricultural soils. J . Environ. Qua[. 21, 280-288. Vandepopuliere. J. M., Johannsen. C. J . , and Wheaton, H. N. (1975). Manure from caged hens evaluated on fescue pasture. Managing Livest. Wastes. Proc. Inr. Svmp., 3rd, UrbanaChampaign. IL, 1975, pp. 269-270. Vandepopuliere, J. M., Lyons, J . J . , and Fulhage, C. D. (1992). Broiler litter sampling reveals needed information. Poult. Dig. 51(8), 14- 18. Wallingford, G . W., Powers, W. L., and Murphy, L. S . (1975). Present knowledge on the effect of land application of animal waste. Managing Livest. Wastes. Proc. Int. Symp., 3rd. UrbanaChampaign, IL. lY75, pp. 580-582. 586. Wasti, S. S . , Shaw, F. R., and Smith, C. T. (1970). Detection of residues of rdbon in manure of Rhode Island Red hens. J . Econ. Entomol. 63, 1355- 1356. Weaver. W. D., Jr., and Meijerhof, R. (1991). The effect of different levels of relative humidity and air movement on litter conditions, ammonia levels, growth, and carcass quality for broiler chickens. Poult. Sci. 71, 746-755. Webb, K. E., Jr., and Fontenot, J . P. (1975). Medicinal drug residues in broiler litter and tissues from cattle fed litter. J . Anim. Sci. 41, 1212-1217. Weil, R. R.. and Kroontje. W. (1979). Physical condition of a Davidson clay loam after five years of heavy poultry manure applications. J . Environ. Qual. 8, 387-392. Weil, R. R., Kroontje, W., and Jones, G . D. (1979). Inorganic nitrogen and soluble salts in a Davidson clay loam used for poultry manure disposal. J . Environ. Qua/. 8, 86-91. Weil, R. R., Weismiller, R. A . , and Turner, R. S. (1990). Nitrate contamination of groundwater under irrigated coastal plain soils. J . Environ. Qual. 19, 441-448. Westing. T. W.. Fontenot, I. P., McClure, W. H., Kelly, R. F., and Webb, K. E., Jr. (1981).Mineral

POULTRY WASTE MANAGEMENT

83

element profiles of animal wastes and edible tissues from cattle fed animal wastes. Livest. Waste: Renewable Resource, Proc. Int. Symp.. 4th, Amarillo, T X , 1980, pp. 81 -85. White, R. E., and Beckett, P. H. T. (1964). Part I. The measurement of phosphate potential. Plant Soil. 20, 1- 15. Wilkinson, S. R. (1979). Plant nutrient and economic value of animal manures. J . Anim. Sci. 48, 121-131. Wilkinson, S. R., Stuedemann, J. A.. Williams, D. J., Jones, J. B., Jr., Dawson, R. N., and Jackson, W. A. (1971). Recycling broiler house litter on tall fescue pastures at disposal rates and evidence of beef cow health problems. Managing Livest. Wastes. Proc. Int. Symp., 3rd. Urbana-Champaign. IL, 1975, pp. 321-324, 328. Wills, L. E . . Mullens, B. A , , and Mandeville, J. D. (1990). Effects of pesticides on filth fly predators (Coleoptera: Histeridae, Staphylinidae; Acarina: Macrochelidae, Uropodidae) in caged layer poultry manure. J. Econ. Entomol. 83(2), 45 1-451. Wolf, A. M., Baker, D. E., Pionke, H. B., and Kunishi, H. M. (1985). Soil tests for estimating labile, soluble, and algae-available P in agricultural soils. J. Environ. Qual. 14, 341-348. Wolf, D. C. (1992). Impact of human and animal waste on water quality. Spec. Rep.-Arkansas Agric. Exp. Sin. 154, 17-22. Wolf, D. C., and Daniel, T. C. (1989). Water quality. Arkansas Farm Res. 38(6), 4. Wolf, D. C., Gilmour. J. T., and Gale, P. M. (1988). Estimating potential ground and surface water pollution from land application of poultry litter. 11. Arkansas Water Resour. Res. Cent., Publ. 137. Wood, C . W., and Hall, B. M. (1991). Impact of drying method on broiler litter analyses. Commun. Soil Sri. Plant Anal. 22, 1677- 1688.

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RAINWATER UTILIZATION EFFICIENCY IN RAIN-FEDLOWLAND RICE Pradeep Kumar Sharma and Surjit K. De Datta* I Ubon Rice Research Center Ubon Ratchathani 34000, Thailand and International Rice Research Institute Manila, Philippines 2 Office of International Research and Development Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061 and International Rice Research Institute Manila, Philippines

I. Introduction 11. Constraints 111. Potentials TV. Efficient Utilization ofRainwater A. Soil Management Practices B. Water Harvesting C. Agronomic Practices V. Research Priorities VI. Summary References

I. INTRODUCTION The dominant ecological factor that distinguishes different kinds of rice cultures is the surface hydrology of rice fields. Rain-fed rice depends entirely on the local rainfall for its water. The crop receives water either directly from the rain or indirectly through supplemental irrigation with stored rainwater. Thus, the term ruinfed describes a situation wherein the catchment and the command area have the same or comparable climates, particularly rainfall. The term lowfund describes an area where land remains submerged for a considerable period during rice growth, with water depth not exceeding 50 cm [De Datta, 1981; International Rice Research Institute (IRRI), 19841. The submergence may be caused by the topography, the nature of the soil, or special land management practices intended to achieve water stagnation in the field. Rain-fed lowland rice, therefore, refers to rice grown in bunded fields and submerged with rainwater for at least a part of the growing season, with access to a supplemental irrigation system (De Datta, 1981): the crop may use on-farm

85 Advances in Agronomy. Volume 52

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

86

PRADEEP K. SHARMA AND SURJIT K. DE DATTA

conserved rainwater for part of its water requirements. Upland rice also uses water from local rains, but unlike rain-fed lowland rice, it grows in unbunded fields where water does not stagnate and it has no access to irrigation. Another difference between upland and rain-fed lowland rice is the method of planting. Whereas upland rice is always dry seeded, rain-fed lowland rice may be either dry seeded in a dry-tilled field or transplanted in puddled fields (De Datta, 1981). Rain-fed lowland rice covers about 38 million ha, which is 28% of the rice area of the world (Garrity er af., 1986), contributing an estimated 19% of the global rice supply. Of world’s rain-fed lowland rice area, 95% is in Asia. A majority of the rice-growing areas in South and Southeast Asia are rain-fed lowlands (Fig. 1). India has the largest rain-fed lowland rice area, with about 16.5 million ha, which is about 40% of the total rice area in India (Pande and Reddy, 1984; De Datta, 1986). Eastern India alone (states of Assam, West Bengal, Bi-

Figure 1 Predominantly rain-fed lowland rice areas in south and southeast Asia.

EFFICIENT UTILIZATION OF RAINWATER BY RICE

87

Table I Rain-Fed Lowland Rice Areas in South and Southeast Asian Countriesa Area (lo00 ha)

Country

Total rice area

Rain-fed lowland rice area

Rain-fed lowland

143 5679 805 5716 2932 12,779 807 277 1415 2086 222 153 1378

76 65

(%)

~~

Bhutan Thailand Nepal Bangladesh Myanmar Eastern India b Cambodia Laos Philippines Vietnam Sri Lanka Malaysia Indonesia

I89 8677 1262 10,012 5317 26.763 203 1 695 3515 5573 760 735 8204

64 57 55

48 40 40 40 37 29 21 I1

“Adapted from Huke (1982). bEastern India includes the states of Assam, West Bengal, Bihar, Orissa, Madhya Pradesh, and Uttar Pradesh.

har, Madhya Pradesh, and Uttar Pradesh) leads all other countries by a wide margin in the area of rain-fed lowland rice, with about 15 million ha (Table I). In Thailand, an area of -6.2 million ha (62% of total rice area) is planted in rain-fed rice, and is the second largest in Asia. In all rain-fed areas, the actual rice yields are generally much lower than the potential yields because of several soil, environmental, and socioeconomic constraints. One key factor in increasing the production of rain-fed lowland rice is to increase the rainwater utilization efficiency in these areas.

11. CONSTRAINTS Rain-fed lowland rice environments are highly diverse and unpredictable, with insufficient or excess water as the major limiting factors. Most of the monsoonal Asian countries receive 50-90% of the annual rainfall during May-September

88

PRADEEP K. SHARMA AND SURJIT K. DE DATTA

(Fagi et al., 1986), and in most areas these rains are concentrated within a short span of 4-6 weeks per monsoonal season (Abeywardene, 1987). High-intensity rains, exceeding the infiltration rates of soil, are not only lost as runoff but may also cause serious soil erosion. Spatial and temporal fluctuations are the characteristics of monsoonal rains. One such example, showing variations in the time of onset and withdrawal of monsoon and its distribution during the cropping season for the Tarai region of Nepal and northern India, is shown in Fig. 2. Saenjan et al. (1990) analyzed rainfall data collected at 40 locations in northeast Thailand for 30 consecutive years. The coefficient of variation for the total seasonal rainfall, which is a measure of the reliability of rainfall at a given location, varied between 40 and 72%. Variations in the onset and termination of monsoons have technical as well as production consequences. Late onset delays land preparation and crop establishment of rain-fed lowland rice, and early termination causes drought stress at the reproductive stage. The uncertainty in the frequency and amount of rainfall also Rainfall (mm)

C V % 125 93 70 74 66 200 149 106 73 123 99 198

Figure 2 (A) Annual rainfall distribution illustrating the extent of variability (dashed line) associated with monsoon onset and withdrawal periods, and (B) rainfall distribution and variability common to the Terai region of Nepal and northern India (Chang et al., 1979).

EFFICIENT UTILIZATION OF RAINWATER BY RICE

89

Table I1 Approximate Probability of Deficient Rainfall during the Monsoon in Various Meteorological Subdivisions of India“ Recurrence of highly deficient rainfall

Meteorological subdivisions Assam West Bengal, Madhya Pradesh, Konkan, coastal Andhra Pradesh, central Maharashtra, Kerala, Bihar, Orissa Southern interior Mysore, eastern Uttar Pradesh, Vidarbha Gujrat, eastern Rajasthan, western Uttar Pradesh, Tamil Nadu, Kashmir, Rayalaseema, Telengana Western Raiasthan

Very rare, once in 15 years Once in 5 years Once in 4 years Once in 3 years Once in 2-5 years

“From Kateswaram ( 1 974). bRainfall deficiency is equal to or greater than 25% of normal rainfall.

makes predictions about rainfall difficult. One rice crop may suffer from both drought and excess water during the same growing season. Table I1 shows the probability of drought occurrence in India. Based on the recurrence interval of highly deficient rainfall, every part of India, except Assam and the adjacent states, faces the probability of a drought once in 5 years. Some provinces, such as Gujrat and Rajasthan, and regions such as Rayalaseema and Telengana, are subject to drought once in about 3 years. It has generally been accepted that rain-fed rice grows best in areas receiving more than 200 mm of rainfall per month for a minimum period of 3 months, without dry periods exceeding 7- 10 days (Krishnamoorty, 1979; Hundal and Tomar, 1985; Abeywardene, 1987). The 200-mm rainfall per month is based on 6-7 mm of daily evaporation. On this basis, IRRI (1974) developed a breakdown of the climatic zones of the rice-growing regions of Southeast Asia (Table 111). If a monthly rainfall of 200 mm is considered as the lower limit for lowland rice, then rain-fed areas belonging to climatic zones 11-3, 111-1, 111-2 and IV require serious efforts toward increasing the effectiveness of the normal rainfall, particularly if year-to-year fluctuations in the monthly rainfall are sharp. An international committee divided the rain-fed lowland ecosystem into five subecosystems based on two major hydrological stresses, i.e., inadequate or excess moisture (IRRI, 1984): 1. Rain-fed shallow, favorable environments, areas in which drought or submergence is not a serious constraint and crop management practices are essentially similar to those in fully irrigated systems.

90

PRADEEP K. SHARMA AND SURJIT K. DE DATTA Table I11 Climatic Zones in Southeast Asian Rice-Growing Regions“

Climatic zone I 11-1 11-2

11-3

111-1

111-2

IV

Description

Utilization

More than 9 consecutive wet months with 200 mm of rainfallimonth From 5 to 9 consecutive wet months with 100-200 mm of rainfalhnonth From 5 to 9 consecutive wet months with 100-200 mm of rainfalhonth during the remaining part of the year, and with another rainfall peak From 5 to 9 consecutive wet months with at least 2 months of rainfall
Year-round cropping with two crops of puddled rice Year-round cropping with one crop of puddled rice Suitable for multiple cropping; farmers are likely to grow two crops of puddled rice Possible to grow two crops in a year

Limited possibility of growing two crops in a year Limited possibility of growing two crops in a year

Not suitable for any type of agriculture

~

“IRRI (1974).

2 . Rain-jed shallow, drought-prone environments experience frequent and severe water deficits at any growth stage. The rainy period may be short (90- 110 days) or may continue for longer periods, but with highly uncertain distribution. 3. Rainfed shallow, submergence-prone areas frequently experience shortterm flooding that may damage or destroy the crop when specifically adapted varieties and crop management practices are not used. 4. Rain-fed shallow, drought- and submergence-prone ricelands may experience both water deficits and short-term flooding on a frequent basis. 5 . Rain-jed medium-deep, waterlogged ricelands accumulate water at depths of 25-50 cm for a substantial portion of the growing season. Based on the source of the water supply, rice-growing areas exist in three topographic sequences (O’Toole and Chang, 1978; Greenland and Bhuiyan, 1982): pluvial, phreatic, and fluxial. Rainfall is the only source of water in the pluvial toposequence; rainfall, seepage water, and the water table are sources in the phreatic toposequence; and rainfall, seepage water, the water table, and runoff are sources in the fluxial toposequence. Pluvial and phreatic toposequences

EFFICIENT UTILIZATION OF RAINWATER BY RICE

91

have drought constraints, whereas floods may occur in fluxial toposequences. Most of the rain-fed lowland rice areas are in the fluxial and phreatic toposequence categories, and only some are in the pluvial category. Based on soil texture, growing period, and amount of rainfall, Garrity et al. (1986) estimated that 25% of the rain-fed lowland rice areas in South and Southeast Asia are favorable, 21% are intermediate, 32% are drought prone, and 22% are highly drought prone. Mackill (1986) estimated that 12% of the Asian lowlands are submergence prone and that 8% are drought and submergence prone. In addition to hydrological constraints, there are limitations imposed by an insufficient potential growing season, high temperatures (>35"C), and decreased solar radiation. There are various soil-related constraints, such as coarse texture; low water-holding capacity; low organic matter content; low CEC; low buffering capacity; N, P, K, and Zn deficiencies; Fe toxicity; and salinity, alkalinity, and organic and acid sulfate conditions (Garrity et af., 1986; Goswami and De Datta, 1986). Coupled with these are socioeconomic problems that deter high-level rice production. Most rain-fed lowland rice farmers are poor and cannot take risks. Because of the high risks of drought and flooding and unstable rice yields, they usually invest very little in fertilizers, herbicides, pesticides, etc., for rice production in rain-fed lowland areas (Pa., 1979; Alcantara et af., 1984; De Datta, 1986). Adoption of modern rice cultivars and associated production technologies has been limited and nonexistent in some areas. Lack of rice cultivars responsive to improved management yet tolerant of the important biophysical stresses of the rain-fed lowland areas is another major limitation to growing rain-fed lowland rice. De Datta (1984) stressed the need for identifying constraints in relation to specific target environments in order to make rain-fed technology viable.

1II.POTENTIALS A large gap exists between rice yields in rain-fed and irrigated areas. For example, 60% of the rain-fed rice contributes to only about 40% of the total rice that is produced in India (Krishnamoorty, 1979). Also, rain-fed lowlands dominate the worldwide rice-growing regions, and thus are the areas that will allow potential future increases in the world's rice production, if there is an effort to focus on technologies that will stabilize yields, require a minimum of purchased inputs, and minimize risks to the environment. One key step in this direction is to improve rainfall effectiveness and water-use efficiency of rain-fed lowland rice. This can be achieved by proper soil, water, and crop management techniques, and the following discussions review some of the findings, possibilities, and challenges that are significant.

92

PRADEEP K. SHARMA AND SURJIT K. DE DATTA

IV.EFFICIENT UTILIZATION OF RAINWATER Physiologically, water-use efficiency refers to the ratio of total biomass production (or economic yield) and total crop water use. There are two aspects of increasing the efficiency of rainwater use under rain-fed situations: (1) raising the yield of the rain-fed crop when rainfall is normal and ( 2 ) preventing yield loss when rainfall is inadequate and drought occurs frequently. The technology for improving the efficiency of rainwater use in rain-fed lowlands aims at ( 1) increasing in situ rainwater interception, soil infiltrability, and profile water storage, ( 2 ) decreasing nonproductive water losses, such as percolation and seepage, (3) collecting runoff water for providing supplementary irrigation at critical growth stages, if needed, and (4) properly managing rice crops for efficient utilization of conserved soil moisture. These objectives can be achieved by employing various soil and water management practices.

A. SOILMANAGEMENT PRACTICES 1. Puddling versus Dry Seeding Either of two planting systems for growing rain-fed lowland rice may be used, depending on the hydrological situation of the area. In one method, the soil may be puddled and inundated and then planted with rice. This technique is used in areas that either receive high rainfall or receive supplemental irrigation from stored rainwater. The second system starts with dry-seeded rice and ends up with a wetland rice culture. It is practiced in areas with relatively low rainfall. In this system, rice is dry seeded in the early part of the monsoon; when heavy rains arrive, the system is converted to a wet area by impounding water. Dry seeding is practiced in a limited area in Asian countries (De Datta et af., 1979). To decrease soil permeability, to remove weeds, and to reduce crowding of seedlings, the field is wet-plowed in both directions with a native plow, preferably under standing water, when the seedlings are at the three- to four-leaf stage. At the same time, the gaps are filled with the seedlings from the crowded parts of the field. This practice is called “halod” in Himachal Pradesh, “biasi” in Madhya Pradesh, and “beusani” in Orissa (states of India), and “gogo rancah” in Indonesia. This method takes advantage of early rainfall and allows time for a second crop of rice to be grown. The major problems with this system are vigorous weed growth, increased risk of drought damage, poor stand establishment, and low grain yield. For lowland rice, puddling is the most common technique of land preparation. Puddling destroys soil aggregates and peds, creating plastic mud, and thus elimi-

EFFICIENT UTILIZATION OF RAINWATER BY RICE

0

93

=0.982

A = 0.972 A ~0.982

=0.908

I

0

I

2

I

4

I

6

8

1

Puddling depth (cm)

0

Figure 3 Effect of depth of puddling on water flux through soils of different texture (Is, loamy sand; 1, loam; sil, silt loam; cl, clay loam) (Sharma and Bhagat, 1993).

nates most macropores, which transmit water. The remaining macropores are partially filled by dispersed fine particles (Sharma and De Datta, 1986; Adachi, 1990). Consequently, there is a drastic reduction in percolation losses of water and nutrients (Sharma and De Datta, 1985, 1986; Sharma etal., 1988). Sharma and Bhagat (1993) reported a nonlinear reduction in water flux through soils with an increase in puddling depth (Fig. 3). This benefits the rice plant by increasing grain yields and decreasing water requirements, thus, improving water-use efficiency. In a study at IRRI on a clay soil (Alfisol), rice grown on puddled soil used half as much water as rice grown on nonpuddled soil (Fig. 4). Rice production on puddled soil was 2.5 times more efficient in water use than rice grown on nonpuddled soil because of decreased water percolation losses and greater soil moisture retention in puddled soil (De Datta and Kerim, 1974). In a clay loam soil (Typic Argiudoll), the average daily water use for transplanted rice during a 90-day irrigation period was 501 mm day-' with zero tillage, 263 mm

94

PRADEEP K. SHARMA AND SURJIT K. DE DA'TTA Cumulative amount of water (mm) 1200 NONPUDDLED SOIL

A

lo00

800 600

400 200 0

20

40

60

20 80 1( 0 Day8 after planting

40

60

80

1( 0

Figure 4 Comparison of cumulative water applied, evapotranspiration, and percolation loss in puddled and nonpuddled soils continually flooded at 5 cm (De Datta and Kerim, 1974).

day-' with minimum tillage, and 164 mm day-' with puddling (Sharma et al., 1988). Puddling increased water-use efficiency by 5 and 1.7 times over zero and minimum tillage (Table IV). In another experiment, puddling significantly deTable IV Tillage Effects on Rice Grain Yield, Total Water Use, and WaterUse Efficiency in a Clay Loam Soil '

Tillage Zero' Minimum" Puddling' CD (5%)f

Grain yield (Mg h a - ' )

Total water use (mm)b

Water-use efficiency (kg grains ha-' mm-l)

2.6 4.3 4.5

45,090 23,670 14,760

0.06 0.18 0.30

0.9

-

-

"Adapted from Sharma el a/. (1988). 'Computed for a 90-day period. "One spray with paraquat (at I kg a.i. ha I ) followed by a 3-day submergence before transplanting. 'One rotovation followed by a 3-day submergence before transplanting. eTwo animal-drawn moldboard plowings followed by three harrowings. 'CD. Critical difference. ~

95

EFFICIENT UTILIZATION OF RAINWATER BY RICE Table V

Tillage Effects on Leaching Losses (Computed for 77-Day Period) and Nutrient Uptake of Rice in a Clay Loam Soil“ Nutrient loss (kg h a - ’ ) Treatment Nonpuddled Puddled CD(I%)’

Nutrient uptake (kg ha - )

’

NH4+

P

K

N

P

K

8.67 I .57

1.62 0.33

0.86 0.21

71 14

74 110

15 25

138 209

2.61

0.34

0.19

30

20

6

36

NO,-

“Adapted from Sharma and De Datta (1985). bCD, Critical difference.

creased leaching losses of major nutrients, i.e., N, P, and K, and increased their uptake by rice plants (Table V). Puddling increases soil water retention, usually below -0.01 MPa water potential (Sharma and De Datta, 1986). Evaporation losses from puddled soils are also low because of the increased soil microporosity. The high water retention capacity of puddled soil may increase the energy required to evaporate the same quantity of water, compared to an upland soil. The higher unsaturated hydraulic conductivity of puddled soils (because of the increased microporosity) may also help in keeping the surface soil moist longer by transporting more profile water upward to the soil surface. Studies have shown that puddled soils maintain higher water potentials than do nonpuddled soils under conditions of moderate moisture stress (Fig. 5). Thus, puddling is a preferred technique of land preparation in areas experiencing low to moderate drought situations (De Datta and Kerim, 1974; De Datta, 1981; Sharma et al., 1987; Mambani et al., 1990). But under conditions of severe moisture stress, puddled soils shrink, there is an increase in the mechanical resistance to growing roots, cracking occurs, and consequently rice roots are damaged. In a rain-fed lowland field study, Thangaraj et al. (1990) found that soil mechanical impedance as low as 0.01 MPa inhibited root growth and values greater than 0.3-0.5 MPa decreased root growth and extension by 75%. In this case rice may perform better on granulated than on puddled soil (Sanchez, 1973; Mambani et al., 1990). However, more research is needed to substantiate these effects in relation to soil type and rainfall pattern. Rice responses to tillage vary with soil texture and climatic water balance. In one study (Sharma et al., 1987), the grain yield of rain-fed lowland rice was significantly affected by tillage in a sandy loam but not in a clay loam soil with a shallow water table (Table VI). Studies have shown that intensive tillage is

PRADEEP K. SHARMA AND SURJIT K. DE DATTA

96

Matric suction (kPa)

-

B

Nonpuddled Puddled ----

50 -

I

20

40

60

80 100 40 60 Days after transplanting

80

100

Figure 5 Matric suctions in puddled and nonpuddled (A) clay loam soil at 200 mm depth (De Datta and Kerim. 1974) and (B)sandy loam soil at 150 mm depth (Sharma er uf.. 1987) under rainfed conditions.

required for rain-fed lowland rice in highly permeable, coarse to mediumtextured soils under drought-prone environments, but not in low-permeability soils under adequate climatic water balance (Sharma and De Datta, 1985; Sharma etal., 1987, 1988; Mambani et al., 1989, 1990). Krishnamoorty (1979) discussed four types of contingencies based on the amount and reliability of rainfall received: 1. Rainfall during the vegetative and reproductive phases is adequate and assured. It is the ideal system. Puddling would probably be the best tillage system. 2. Rains during the vegetative phase are inadequate and unreliable, but rains Table V1 Tillage Effects on Grain Yield of Rain-Fed Lowland Rice in Relation to Soil Texture"

'

Grain yield (Mg ha - ) Treatment

Sandy loam

Clay loam

Puddling Dry seeding CD ( 5 % ) b

4.0 2.8 0.6

5.3 4.8 Not significant

"Adapted from Sharma et al. (1987). *CD, Critical difference.

EFFICIENT UTILIZATION OF RAINWATER BY RICE

97

during the reproductive phase are adequate and reliable. In this case there are two alternatives. First, dry tilling of rice followed by conversion into wetland later in the season; second, puddling of the field when rains are sufficient and transplanting photoperiod-sensitive varieties. 3. Rains are adequate and reliable during the vegetative phase but inadequate and unreliable in the reproductive phase. Again, two alternatives are available. First, transplanting rice in the puddled field and providing supplemental irrigation by storing runoff from the early rains; second, by selection of short-duration varieties. 4. Rainfall is inadequate and uncertain during both vegetative and reproductive phases. Under this situation rice is an extremely risky crop. Other crops having low water requirements should be planted.

2. Soil Compaction The physical properties of medium to coarse-textured soils, low in active clay, change little with puddling (Lal, 1985a,b; Sharma and De Datta, 1986). For such soils, Ghildyal (1969) suggested soil compaction as an alternative to puddling. Compaction refers to the increase in soil bulk density caused by a static or transient load applied normal to the soil surface. During compaction soil particles become more closely in contact. Bulk density increases, total porosity decreases, but water retention and residual pores (pores <50 pm) increase at the expense of water transmission pores. Consequently there is a reduction in infiltration and in percolation losses of water and nutrients (Agrawal, 1991), and there is an increase in the water-holding capacity of soil (Gulati et al., 1985). Abo-Abda and Hussain (1990) obtained a 13-42% reduction in infiltration of a sandy soil due to surface compaction. The saturated hydraulic conductivity and water fluxes through soils are log-linear and inversely related to the bulk density (Gardner and Chong, 1990; Sharma and Bhagat, 1993). The relationship between water flux and degree of compaction in some soils is shown in Fig. 6 . Compacted soil layers also have relatively low evaporation losses, especially when the soil water content is below field capacity (El-Kommos, 1989), probably due to increased microporosity. Soil compaction may occur at surface or subsurface levels. It involves dry plowing of land followed by compaction, using farm machinery such as tractors or tractor-driven or manually operated rollers, etc., at approximately Proctor soil moisture content. The compacted soil may be dry seeded by dibbling or by opening a furrow line for dry seeding, tilled shallow before dry seeding, or shallow puddled for transplanting. Many studies have shown that moderate soil compaction increases rice yield (Varade and Ghildyal, 1967; Ghildyal and Satyanarayana, 1969; Kumar et al., 1971; Mahajan et al., 1971; Varade and Patil, 1971; Gupta and Kathavate, 1972,

98

PRADEEP K. SHARh4A AND SURJIT K. DE DATTA 10-~

- 10-5 . -E In X

c 8 c 10-S

-

,

values o

0.970

0.968 A 0.991 A 0.940 D 0.972

16’ 1 1.0

I

1.1

I

I

I

I

1.2 1.3 1.4 1.5 Bulk density (Mg m3)

I

1.6

Figure 6 Effect of degree of compaction on the water flux through soils of different textures (Is, sandy loam; I, loam; sil, silt loam; cl, clay loam) (Sharma and Bhagat, 1993).

1974; Bhan and Padwal, 1976; Kar et al., 1976; Patel and Singh, 1979, 1986; Reddy and Hukkeri, 1979, 1983; Singh et al., 1980; Bhadoria, 1986). It may result, among other factors, from better root-soil contact (Varade and Ghildyal, 1967). Ogunremi et al. (1986) reported a 20% increase in rice yield with soil compaction over puddling in a sandy loam soil. Mathan and Natesan (1990) obtained an 18% increase in rice grain yield by compacting a Vertisol from I . 1 1 to 1.33 Mg m-3. Bhadoria and Dutta (1984) found a 50-60% increase in the yield of upland rice by compacting a laterite soil. Some data are given in Table VII. Two observations emerge from these studies. First, most of the compaction studies have been made on irrigated rice; studies on rain-fed lowland rice are almost nonexistent. Although the purpose of soil compaction in irrigated and rain-fed situations is essentially the same, i.e., reduction in percolation losses, under rain-fed situations the depth to the compacted layer below the soil surface is an important consideration. The compact layers at relatively shallow depths may have adverse effects on rice under prolonged dry spells. Second, there have been few investigations on optimum compaction levels for rice, and conclusions are diverse. According to some workers, a bulk density as high as 1.80 Mg m - 3 does not adversely affect rice growth and yield in loamy sand to sandy loam soils (Ghildyal, 1969; Singh et al., 1980; Patel and Singh, 1986). Contrary to this, Varade and Ghildyal (1967) obtained a reduction in rice yield when the bulk

EFFICIENT UTILIZATION OF RAINWATER BY RICE

99

density of a sandy loam soil increased above 1S O Mg m-3. Our studies show a bulk density of 1.70 Mg m-3 as critical for root growth and grain yield of rainfed rice in a loamy sand soil. At this bulk density, the saturated hydraulic conductivity of the soil was 2.8 X m sec-! (P. K. Sharma and S . K. De Datta, unpublished data). In a silty clay loam soil, the critical bulk density limit was observed to be 1.63 Mg m-3 (Ghildyal and Satyanarayana, 1969). Such variations in results emphasize the need for more research to determine critical limits of soil compaction for optimum rice growth in relation to soil type and hydrological situations. Strong interactions exist among soil type, water content, bulk density, and root growth. According to Ogunremi (1991), the optimum bulk density for dry matter production, 100-grain weight, panicle length and panicle weight per hill, root density, and grain yield was 1.5 Mg m-3 under nonflooded conditions and 1.6 Mg m-3 under flooded conditions. Excessive soil compaction led to a reduction in rice yield (Ogunremi et al., 1985). Subsurface soil compaction may be achieved either by using heavy machinery, such as road rollers, pneumatic tire rollers, tamping rollers, and bulldozers, at the soil surface and then recultivating the surface soil layer (Yamazaki, 1988), or by directly compacting the subsoil after removing the top soil layer (Mallick et al., 1977; Somani and Kumawat, 1986; Yadav and Somani, 1990). The effec-

Table VI1 Effect of Different Land Management Practices on Grain Yield and Water-Use Efficiency of Irrigated Rice

Soil texture Loamy sand Sandy loam Sandy clay loam

Water-use Bulk efficiency density Grain yield (kg ha (Mg I I - ~ )(Mg h a - ’ ) m m - ’ )

’

Tillage Puddling Compaction CD ( 5 % ) Dry seeding Compaction CD (5%) Dry Seeding Puddling Compaction CD (5%) Dry seeding Puddling Compaction CD ( 5 % )

1.75 -

1.42 I .63 I .so 1.45

1.62 -

8.42 8.82 0.39 1.80 2.75 0.19 1 .so 2.70 2.90 0.30 4.65 5.03 5.18 0.39

3.40 4.08 0.24 -

0.60 1.06 1.41 4.36 5.70 6.03

Ref Patel and Singh (1986)

Bhadoria (1986)

Reddy and Hukkeri (1983)

Bhan and Padwal (1976)

100

PRADEEP K. SHARMA AND SURJIT K. DE DATTA

tive depth of soil compaction varies with the soil type, machine type, soil moisture content, and the extent of soil manipulation before compaction. Four to six passes with a 6-ton road roller at Proctor moisture content may compact soil up to a 30- to 35-cm depth (Yamazaki, 1988). The latter technique is in use in Japan. Mallick et al. (1977) obtained a 25-44% saving in irrigation water, without any adverse effect on rice yield, by compacting the 20- to 40-cm soil layer to as high as 1.75 Mg m-’ bulk density. Some information is available on the longevity of subsoil compaction effects. McKibben (197 1) observed that deep frost conditions reduce compaction effects over a period of time. But Gaultney et al. (1982) and Voorhees (1983) reported limited effects of natural forces, such as wet-dry and freeze-thaw cycles, on compacted soil layers. Voorhees et al. (1986) observed subsoil compaction effects after four seasons of freezing and thawing, whereas Blake et al. (1976) observed such effects even after 10 seasons. Hakansson et al. (1987) and Lowery and Schuler (1991) concluded that, depending on clay content, the compaction effects may persist for 5 years or more. Mathan and Natesan (1990) observed residual effects of compaction on rice in a Vertisol over five growing seasons. According to Logsdon et al. (1992), compaction persisted for 7 years in the 35to 60-cm zone of a clay loam soil. The choice between surface and subsurface compaction would depend on the type of cropping systems being followed. In a rice-rice cropping system subsurface compaction has an advantage over surface compaction in that (1) it is not repeated every season and (2) if below the root zone, it does not affect the rice root system. Surface compaction above the critical limit would adversely affect the rice roots. In a rice-upland crop cropping system, however, subsurface compaction may interfere with the roots of upland crops following rice growth.

3. Fallow Land Management A considerable amount of soil moisture is lost through evaporation and transpiration by weeds if the land remains fallow for a few weeks during the dry season before the onset of monsoons. If this moisture is conserved in situ, it will help in the early establishment of rain-fed rice and reduce the chances of the crop suffering from considerable drought damage. Studies conducted at IRRI in the Philippines suggest that soil mulches created by shallow (10 cm) or deep (20 cm) tillage, residue mulch, and chemical weed control during the fallow period are effective in conserving soil moisture (Bolton and De Datta, 1979; Hundal and De Datta, 1982). Weed-free plots lost no more than 5 cm of water from a 1.05-m soil profile, whereas the loss was 25 cm in conventional weedy-fallow plots (Fig. 7). According to Bolton and De Datta (1979), tillage at the end of a previous wet season conserved so much soil mois-

EFFICIENT UTILIZATION OF RAINWATER BY RICE

y

101

soil water content (cm3/cm3

1.25 a50 0.7

1.25 0.W 0.7!

Rototillrd

Plowed and rototilled

Straw-mulched

Bore soil

Weedy control

Figure 7 Soil water depletion in 6 weeks (3 April-15 May) under various dry season soil management systems (Hundal and De Datta, 1982).

ture that it enabled crop establishment 3 weeks earlier than if the soil was prepared at the beginning of the following wet season.

4. Organic Amendments The water-holding capacity (WHC) of soils depends principally on (1) the number and size distribution of soil pores and (2) the specific surface area of soils. Pore size distribution affects the WHC mainly at higher water potentials, such as those at field capacity, where the WHC is a function of soil structure. At lower water potentials, close to the permanent wilting point, the WHC is a function of soil texture, and it also depends on the specific surface area of soil particles. Organic matter affects both soil properties. It increases soil pores favorable for water retention and the specific surface area of soils. The water-holding capacity of organic matter is very high, although much of the water is retained at potentials below the permanent wilting point (Feustal and Byers, 1936; Jamison, 1953). Thus, when added to soil, organic matter dilutes material of low water retention with that of high retention.

102

PRADEEP K. SHARMA AND SURJIT K. DE DATTA

Organic matter increases aggregation and decreases the bulk density of soils (Biswas et al., 1971; Khaleel et al., 1981; De Datta and Hundal, 1984; Sharma and Aggarwal, 1984; MacRae and Mehuys, 1985; Clapp et al., 1986). This results in increased total porosity and the alteration of the pore-size distribution; the relative number of water retention pores increases. This is especially true in coarse-textured soils (Volk and Ullery, 1973). An increase in the WHC at lower water potentials due to an increase in the specific surface area of soils on addition of organic matter has also been reported by various workers (Volk and Ullery, 1973; Gupta et al., 1977; Rajput and Sastry, 1988a). Niskanen and Mantylahti (1987), using data from 60 soils, obtained the following relationship (rz = 0.84) between the specific surface area (m2g-I), clay (%), and organic carbon content (OC) (96)of soils: Specific surface area = 2.69

+

1.23 clay

+ 8.69

OC

(1)

Equation (1) shows that the effect of organic carbon on the specific surface area is about 7 times that of the clay content. Khaleel et al. (1 98 1) concluded that about 80% of the observed variations in percentage increase in water retention may be explained by soil texture and increases in organic carbon. Coarse-textured soils, in general, show the greatest increase in water retention at both field capacity and wilting point due to additions of organic matter (Clapp et al., 1986). Unlike the WHC, the plant-available water capacity (PAWC), i.e., the water retained between field capacity and wilting point, is affected little or not at all by organic matter. This is due to organic matter raising the water retention of soils at both lower and higher tensions, and decreasing their bulk density. The decreased bulk density tends to counterbalance any increase in the PAWC on a mass basis. Therefore, when moisture content is computed on a volume basis, increases in the PAWC may not be as dramatic or may be nonexistent (Khaleel et al., 1981; MacRae and Mehuys, 1985; Clapp etal., 1986). Most of the studies that show positive correlation between the PAWC and the soil organic matter have computed water retention on a mass basis (Biswas and Ali, 1969; Epstein et a f . , 1976; Gupta and Larson, 1979; Lal, 1979; De Kimpe et al., 1982). In order to evaluate the real effects of organic matter on the PAWC, the moisture content should be determined on a volume basis. Diverse opinions have emerged regarding the effect of organic matter on the PAWC in relation to soil texture. According to one group of workers, organic matter benefits the PAWC in coarse-textured soils only. Jamison (1953), using the data of Peele et al. (1948), concluded that organic matter increased the PAWC of sandy soils having < 15% clay. As soil texture became finer, organic matter had less effect on the PAWC; factors other than organic matter dominated in determining the PAWC. In another study, Jamison and Kroth (1958) found that organic matter influenced the PAWC only in soils of medium to low clay

EFFICIENT UTILIZATION OF RAINWATER BY RICE

103

content (13-20%). Earlier, Coile (1938) had also concluded that organic matter increased the moisture equivalent of coarse-textured soils more than their permanent wilting point (PWP), and the effect on water retention in general decreased with textural fineness. Several other studies have reported increases in the PAWC (on a volume basis) with increases in organic matter content of sandy soils (Salter et af., 1965; Biswas et af., 1971; Kumar et af., 1984; Bhadoria, 1987; MacRae and Mehuys, 1987; Ramunni et af., 1987; Rajput and Sastry, 1988a,b; Tester, 1990). But contrary to this, many researchers did not find any improvement in the PAWC of sandy soils due to addition of organic matter (Petersen et af., 1968; Hartmann and De Boodt, 1974; Singh et af., 1976; Gupta et al., 1977; Kladivko and Nelson, 1979). According to another school of thought, it is the fine-textured soils that benefit most in their PAWC from organic matter additions (Khaleel et al., 1981; Clapp et af., 1986). These reviews conclude that in fine-textured soils increases in water retention due to increases in organic matter are greater at field capacity than at wilting point. This effect is probably the result of increased aggregation, producing a greater number of larger size pores that cannot drain under gravity. In coarse-textured soils, on the other hand, organic matter produced a larger increase in water retention at the PWP than at field capacity, perhaps due to an increase in the number of smaller pores not draining at - 1500 kPa. Consequently, the PAWC increased in fine-textured and not in coarse-textured soils due to organic matter buildup. Russel et al. (1952) reported about a 0.012 cm3 cm-j increase in the PAWC of a silt loam soil due to addition of 40 Mg ha-' manure. However, Petersen et af. (1968), Morachan et af. (1972), Epstein (1975) and Weil and Kroontje (1979) did not find any improvement in the PAWC due to organic matter additions in silt loam to clay loam soils. Sommerfeldt and Chang (1986) observed a decline in the PAWC with an increase in organic matter content of a clay loam soil. Many researchers have reported improvements in the PAWC due to organic matter, irrespective of soil texture. Unger (1975) observed an increase in the PAWC by about 1.8% (volume basis) for each per cent increase in organic matter for soils ranging in texture from sandy to clay. Mbagwu (1989) also obtained an increased PAWC with manure additions (2-10%) in sandy loam, sandy clay loam, and clay soils; the effect, however, decreased with textural fineness. Studies reveal that the influence of organic matter on the WHC is comparatively more at lower rather than at higher tensions, irrespective of soil texture (Salter and Williams, 1963; Biswas and Ali, 1969; Biswas et af., 1971; Sharma and Nath, 1979; Joe, 1990). On the basis of data from 144 rice soils, Joe (1990) concluded that every 1% increase in organic matter increased field capacity and the PWP by 2.21 and 1.01%, respectively (Table VIII). Water retention at the PWP is largely affected by the clay content of soils (Biswas and Ali, 1967). Such diversities in conclusions may arise due to the varied nature of soil or-

104

PRADEEP K. SHARMA AND SURJIT K. DE DATTA Table VIII

Multiple Regression Equations Relating Moisture with Clay, and Organic Matter Content in 144 Paddy Soils" hopefly

Horizon

Field capacity

Surface Subsoil All horizons Surface Subsoil All horizons

Wilting point

Regression equation

Y Y Y Y Y Y

= 0.767X1

O.597Xl = 0.634X, = O.543Xl = 0.412XI = 0.456Xl

=

+ 1.693X2 + 13.689 + 2.14IX2 + 12.198 + 2.210X2 + 13.074 + O.725X2 + 1.645 + 1.172X2 + 2.154 + 1.009X2 + 2.033

r

0.818**

0.841** 0.815**

0.800** 0.843** 0.812**

from Joe ( 1990). bMoisture ( Y )is expressed as a percentage (w/w); X I is clay (%); X2 is organic matter content (%).

a Adapted

ganic matter, the different sources of organic amendments used, the different durations following organic matter additions, after which the data were collected, and the different ecological situations under which the experiments were conducted. The same organic matter contents in soils may yield different results depending on whether the organic matter was directly incorporated into the soil or was built up in situ over a period of time (Biswas and Ali, 1969). Profile water storage also depends on the water transmission characteristics of the soil. The higher the infiltration and saturated hydraulic conductivity, the higher the water intake and the less the runoff. Soil water transmission characteristics are closely associated with the soil organic matter status (Wischmeier and Mannering, 1965; Allison, 1973; Khaleel et al., 1981; Joe, 1990). In general, although fine-textured soils show increases in infiltration and saturated hydraulic conductivity with increases in their organic matter content due to improvement in aggregation (Tiarks et al., 1974; Gupta et al., 1977; Mathers and Stewart, 1984), sandy soils show a decline (Das et al., 1966; Biswas et al., 1971; Kumar et al., 1984; MacRae and Mehuys, 1987; Rajput and Sastry, 1988a; Bhagat and Acharya, 1989; Bhagat, 1990). The decline in infiltration and saturated hydraulic conductivity is associated with the reduction in water transmission and increase in water retention pores of sandy soils. If the organic matter is hydrophobic in nature, whatever the soil texture, it would decrease the hydraulic conductivity of soils (Weil and Kroontje, 1979). Gupta et al. (1977) observed a decrease in the diffusity of a sandy soil with an increase in organic matter content. They argued that this behavior of sandy soils is important with respect to water storage, particularly in wet tropics and arid zones. Lower unsaturated hydraulic conductivity and diffusivity decrease the water loss due to evaporation. Although it is encouraging to note that profile water storage and the PAWC of

EFFICIENT UTILIZATION OF RAINWATER BY RICE

105

soils can be improved by increasing soil organic matter content, the challenge is to build up the organic matter content, especially in wet tropics, where most of the lowlands exist. Also, substantial increases in organic matter content are required before improvements in hydrophysical properties of soils are visible. Different types of organic amendments that can be used are farm yard manure, animal wastes, crop residues, green manures, composts, night soil, sewage, sludge, and organic wastes from industries. Asian farmers mostly use cattle dung as the organic source; composts and green manures are also common. The decomposition rate of these materials in soil depends on (1) the chemical composition of the material (C:Nratio), (2) soil temperature, (3) soil moisture, (4) method of application (surface applied, soil incorporated, etc.), and (5) rate of application. A rule of thumb is that a material resistant to ready decomposition is necessary if soil organic matter levels are to be increased or maintained (MacRae and Mehuys, 1985). According to Warman (1980), plant materials low in N (<1.5% N on a dry mass basis) are most effective for this purpose. Organic materials added to soil practically burn up in humid tropics and subtropics (Singh, 1962). Wetting and drying processes, as in a rain-fed lowland situation, further accelerate the rate of decomposition (Amato et al., 1984). Sometimes, addition of fresh organic matter to a soil accelerates the decomposition of native organic matter, and the net result is a reduction in the organic matter content of the soil, rather than a buildup (Singh, 1962; Khaleel et a l . , 1981). Therefore, moderate applications of organic materials in tropical soils help mainly the current season rice crop, with negligible residual long-term effects. For the buildup and maintenance of soil organic matter, regular applications of large quantities of organic materials are required (Khaleel et al., 1981; Eck and Unger, 1985; MacRae and Mehuys, 1985). The availability of organic materials in most of the rain-fed areas is very restricted. Most of the rain-fed lowland soils do not support a satisfactory stand of green manure crops without the use of organic and/or chemical fertilizers. These problems may discourage the use of organic amendments for improving profile water storage of sandy soils in tropical areas. We did not find any published study where organic amendments were used to improve water-use efficiency in rain-fed lowland rice. In one study, Ladha et al. (1984) successfully used farm yard manure (10 Mg ha-') to increase the WHC of a sandy soil planted in soybeans. This technique has potential in rain-fed lowland rice, provided practical and cost-effective ways of getting organic amendments are identified and developed.

5. Percolation Barriers Soil dressing and subsurface barriers of various kinds, such as asphalt, bitumen, polyethylene sheets, concrete, bentonite clay, and subsurface compaction,

106

PRADEEP K. SHARMA AND SURJIT K. DE DATTA

have been successfully used for reducing percolation losses in lowland rice fields in sandy areas. In irrigated areas they have been used to decrease the total water use, without affecting the rice yield, whereas in rain-fed areas their use has been to increase rainwater-use efficiency by increasing rice yield per unit of water use. Soil dressing refers to the mixing of fine-textured soil with the plow layer of a coarse-textured rice field soil. Dry soil may be spread uniformly over the rice field and mixed with the plow layer by puddling, or a thick slurry of clay soil may be sprayed on the rice field using slurry pumps. About 250-650 m3 soil ha-' may be required for soil dressing. Irrigating rice fields with muddy water is another way of soil dressing. Where it is difficult to get good clay for dressing, bentonite powder is used. Soil dressing has been described in detail by Yamazaki (1988). Fujioka et al. (1963) incorporated bentonite in the top 25 cm of soil to reduce percolation in a volcanic ash rice soil. Rout et al. (1989) obtained a 13.5% reduction in the water requirement of transplanted rice in a sandy loam soil with the incorporation of bentonite clay (500 kg ha-') before puddling. The clay increased water-use efficiency of rice from 3.60 to 4.22 kg ha-' of rice mm- of water. However, Mallick et al. (1977) did not find any effect of bentonite clay (0.5-1%, on a soil mass basis) on the percolation rate through a sandy loam soil. Patel and Singh (1986) suggested the mixing of 2-4% clay soil (clay >70%, dominating in smectites), with or without pretreatment with sodium bicarbonate, with the 0- to 10-cm soil layer to decrease percolation losses in a loamy sand soil planted to lowland rice. Somani and Kumawat (1986) and Yadav and Somani (1990) used clay mixing (mixing of soil having >40% clay), with or without soil compaction, to improve the WHC of sandy soils. Rao et al. (1972) investigated the effects of bitumen and concrete as subsurface barriers on water requirement and yield of irrigated rice. These barriers increased rice yields, decreased water requirement, and increased the water-use efficiency of rice (Table IX). The depth of the barrier had little effect on rice yields. Similarly, Pande (1975) observed that percolation on lateritic sandy clay soils was negligible with subsurface barriers of asphalt, bitumen, cement, or polyethylene sheets, but was as high as 100-120 rnm day-' without subsurface barriers. Patel and Singh (1986) used polyethylene sheets at a 30-cm depth to decrease percolation losses in a loamy sand soil planted in lowland rice. Erickson et ai. (1968) used asphalt as a subsurface barrier in a sandy soil planted in rain-fed lowland rice in Taiwan. The asphalt increased rice yields 10fold or more (Table X). The depth of the asphalt layer had little effect on rice yield. The depth of the barrier becomes important if rains recede early or if drought periods are prolonged. Under these situations, the barriers at shallow depths may become harmful to the rice crop. Parashar (1978) obtained 1.14 Mg ha-' less yield of rain-fed lowland rice than that obtained for the control (2.89 Mg ha-')

EFFICIENT UTILIZATION OF RAINWATER BY RICE

107

Table IX Effect of Subsurface Barriers on Grain Yield, Water Requirement, and WaterUse Efficiency of Rice" ~

-~

Water-use Grain efficiency yield Water use (kg ha (Mgha-') (mm) mm-')

Treatment ~ _ _ _ _

~

~

Control Barrier depth 0.2 m 0.3 m 0.4 m Puddling Polyethylene sheet at 0.3 m

Ref

~

4.98~

3173

1.57

Rao et al. (1972)

7.20a 6.31b 7.60a 7.50a

965 854 869 4030

7.46 7.39 8.75 1.86

Patel and Singh (1986)

7.82a

3440

2.27

"Means followed by the same letter do not differ significantly at the 5% level of significance.

when polyethylene sheets were placed at a 22.5-cm depth. The reduction in yield was attributed to the shallow root zone (22.5 cm), which was insufficient to hold water to carry the crop to maturity under conditions of prolonged drought. Similar results were obtained by Garrity and Vejpas (D. P. Garrity and C. Vejpas, personal communication). During a season of low rainfall, polyethylene sheets at 25- and 40-cm depths resulted in rice yields of 0.61 and 1.40 Mg ha-' as compared to the control yield of 1.62 Mg ha-l. The root zone moisture content in these treatments at harvest was 2.2,4.0, and 8.7%, respectively. These studies Table X Rice Yields as Affected by an Asphalt Barrier in Soil" Treatment

Grain yield (Mg h a - ' )

No barrier Asphalt depth 0.2 m 0.3 m 0.4 m 0.6 m

CD(I%)' OFrom Erickson et al. (1968). bCD, Critical difference.

0.40 4.32 4.79 4.85 5.38 0.56

108

PRADEEP K. SHARMA AND SUKJIT K. DE DATTA

indicate that shallow placement of barriers is risky under erratic and low-rainfall situations, and that a sufficient root zone must be provided to allow rice plants to survive during a drought. Also, short-duration varieties of rice should be selected for cultivation. Embanking plots with thick and strong earthen bunds, or reinforcing them with polyethylene sheets, also helps in checking seepage losses of rainwater. Parashar (1978) obtained 0.99 and 1.14 Mg ha- increases in grain yield of rainfed lowland rice compared to the control yield of 2.89 Mg ha-' by adopting these two techniques, respectively. Rout et al. (1989) obtained a 45% reduction in the water requirement of transplanted rice on a sandy loam soil by using polyethylene lining along the four bunds up to a 60-cm depth. The water-use efficiency increased from 3.61 to 6.32 kg ha-' rice mm-' of water. Chakrabarti et al. (1991) found that plastering the inner side of the field bund with mud and a polyethylene sheet barrier was an effective means of reducing lateral seepage of water in rice fields. Percolation barriers, thus, have no doubt proved useful in increasing the efficiency of rainwater in rain-fed rice culture, but cost considerations make their field-scale use prohibitive. It is also cumbersome to remove soil to 30- to 40-cm depths in order to install the barriers. Cost-effective and field-oriented techniques need to be developed. Soil compaction technology appears to have an edge over the use of barriers in that it can be easily applied in the field. However, it needs to be tested over a wide range of soils and climates.

B. WATERHARVESTING Water harvesting is an important component of rain-fed farming. About 10- 14% of total rainfall, depending on soil and rainfall characteristics, may be lost as runoff. If harvested judiciously, this runoff can greatly help boost rice production. Rainwater can be harvested in different ways. One way is to construct thick, strong, raised bunds to impound rainwater in situ (Borthakur, 1983). This technique is very popular with the farmers of the Madhya Pradesh state of India. They construct about 60-cm-high bunds to impound rainwater in relatively large plots (about 40 X 30 m). They also store rainwater in upper land areas for use as supplemental irrigation through gravity flow, if drought occurs at critical growth stages of the crop. In this way, they can harvest a good crop of rice even with about 1200 mm of annual rainfall (N. K. Awadhwal, IRRI, personal communication). Sharma and Modgal (1984) were able to increase rice yield by 15-26% over the unbunded control by providing plot bunds in a clay loam soil. The yield increase was 33-187% when paired rice rows were alternated with 150-mm-high ridges. Bhuyian et al. (1979) used 2- to 12-cm spillway heights to impound all of the 10-mm daily rainwater in the rice fields.

EFFICIENT UTILIZATION OF RAINWATER BY RICE

109

A ridge-furrow cropping system, called “Sorjan,” is popular in Java and in tidal wetland areas in Indonesia. Raised beds about 3 m in width alternate with furrows that vary in depth and width depending on flooding depth during the rainy season. Raised beds grow high-value upland crops; rice is grown in ditches that act as water reservoirs during drought. Sengar et al. (1989), in India, increased the grain yield of rain-fed rice in a Vertisol from 1.2 to I .5 Mg ha- ’ by growing rice in sunken beds with a 9-m water-harvesting width (WHW) rather than in flat beds without the WHW. The concept of the toposequence in cropping in terraced and sloped lands is another type of runoff farming. In this concept, crops requiring well-drained conditions (e.g., Zea mays L.) are grown in the upper terraces while rice is grown in the lower terraces (or lower portions of the hill slope) where runoff concentrates. Another method employs the microcatchment technique of water harvesting. A microcatchment (0.5 to 1000 m2 area) consists of two elements: (1) the runoff area (also called the contributing or catchment area) and (2) the runon area (also called the infiltration basin, basin area, watershed area, or command area). The runoff area either remains fallow, and is sometimes treated to yield maximum runoff, or is planted in a row crop having a relatively low water requirement. The runon area is planted in rice. The amount of runoff depends on rainfall characteristics, such as the intensity and duration, and the soil properties, such as texture, permeability, degree and length of slope, vegetation cover, and the duration of the established runoff area (Sharma, 1986). Bhushan (1979) successfully used conservation bench terraces (CBTs) for growing rain-fed rice on a 2-5% slope in the low Himalayan hill region of northern India. He tested four width ratios of the runoff and runon area, i.e., 0: 1, 1 : 1 , 2 : 1, and 3 : 1 . The rice yields in level benches with runoff areas were 21, 47, and 88%, respectively, greater on three ratios than the benches without a runoff area. The major advantage of the CBT system was the reduction in the chances of crop failure. Under Bhushan’s experimental conditions, the probability of getting rice yields < 1.4 Mg ha-l was decreased from 50% with a 0: 1 ratio to 16% with a 3: 1 ratio. Similarly, the probability of achieving yields >3.5 Mg ha-’ increased from 0% in the 0: 1 CBT system to 50% in the 3 : 1 CBT system. During high-intensity rains, runoff may be collected and stored in suitable reservoirs for use as supplemental irrigation during drought periods. The provision of a small pond in a corner of a field to collect runoff, and its utilization as lifesaving irrigation or during critical periods, is an age-old practice of the farmers in eastern India that is now rarely employed (Borthakur, 1983). In Indonesia, successful pond systems have been demonstrated in the Sengkon, Lambok Island. According to one study, reported by Fagi et al. (1986), small farm ponds with a storage capacity of 400 m3 water cost about U.S. $400-600, and can increase the efficiency of rainwater utilization from 55 to 70%.

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C. AGRONOMIC PRACTICES The adoption of suitable agronomic practices is equally important in increasing the efficiency of rainwater use in growing rain-fed lowland rice. These include selection of crop variety, time of planting, plant populations, crop geometry, efficient fertilizer use, weed control, and insect control. These practices should be adjusted according to the rainfall distribution pattern of the area. Generally, early-maturing cultivars of rice should be used in rain-fed areas. Proper time of planting not only helps in preventing drought damage but also makes growing two crops possible in areas having a relatively long rainy season. Optimum plant density for rain-fed systems is likely to be less than for irrigated crops. A higher plant density produces more foliage and a quicker loss of soil moisture through evapotranspiration. Proper fertilizer management also plays an important role in increasing rice production and in water-use efficiency in rain-fed lowlands. Nitrogen is the most important nutrient under rain-fed conditions. Many reports have been published on the effective and efficient management of nitrogen in growing rain-fed lowland rice (Borthakur, 1983; Aragon ef al., 1984; De Datta, 1986; Goswami and De Datta, 1986). The yield of rain-fed rice was found to increase at the rate of 20 kg grain per kg of N with 40 kg N ha-l (Borthakur, 1983). About 40-60 kg N ha-', in split applications, has been suggested as economical and optimal for rain-fed lowland rice. Reports (Tables XI and XII) indicate that relatively higher fertilizer applications, especially N , can to some extent offset the adverse effects of moisture stress (Sen and Das Gupta, 1969; Islam and Ullah, 1973; Aragon and De Datta, 1982) and of delayed planting (Goswami and De Datta, 1986). Another reason for maintaining a sufficient supply of N under rain-fed lowland conditions is that alternate wetting and drying of soils leads to losses of both native soil nitrogen and of fertilizer N. Yield reductions, ranging from 20 to 80%, have been reported due to uncontrolled weed growth in different rice cultures (De Datta, 1986). Poor land preparation and relatively higher rates of fertilizer applications greatly affect weed infestation. The benefits from added fertilizers can be optimally achieved only if weeds are controlled effectively. Various weed control methods may include proper land preparation, hand weeding, mechanical weeding, use of chemicals, regulating plant spacing, increasing seed rate, and changing cultural practices (Moody et al., 1986). Mitra and Reddy ( 1986) have suggested the following technology for improving rice production in rain-fed lowland areas of the eastern and northeastern states of India: 1. Use of a 60- to 80-kg seed rate for direct seeding or 6-8 seedlings per hill in transplanted rice.

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Table XI Effect of Different Levels of N on Rice Grain Yield in Relation to Soil Water Regimes" N level (kg ha - ) Water regimes wa Wb

wc Wd

Mean (CD, 5%

=

2.93)

0

45

90

Mean

16.9 20.9 33.1 36.2 26.8

20.6 27.0 37.0 51.0 33.9

27.9 28.5 39.3 33.8 32.4

21.8 25.5 36.5 40.3

"Adapted from Sen and Das Gupta (1969). Rice grain yield is expressed as gramdplant. Soil water content increased gradually from field capacity to submergence in treatments from W, to Wd; in each treatment, different water regimes were maintained at different growth stages. Standard error = t 2.1; CD, 5% = 3.39, for entries in the table body.

2. Transplanting of tall (70-75 cm) and aged (40-60 days old) seedlings in bunches in low-lying areas where the land is inundated with early monsoon showers. 3. Use of a double transplanting technique, i.e., transplanting with clonal tillers detached from the mother plants of an early transplanted crop (30-40 days after first transplanting), in low-lying areas where the rice fields are inundated to a depth of 40-70 cm. Clonal tillers have the advantage of being environmentally rehabitable.

Table XI1 Effect of Date of Planting and Fertilizer Level on Rice Grain Yield" Treatmentb

Grain yield (Mgha I)

aAdapted from Goswami and De Datta (1986). b D I , Normal sowing date (29 July, 1982); D z . delayed sowing (30 August, 1982); F , , 100-26-33 kg NPK h a - ' ; F z . 50-13-17 kg NPK h a - ' .

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4. Use of 40-20-20 NPK ha-' fertilizers to grow a healthy crop capable of withstanding the ravages of hostile water conditions.

V. RESEARCH PRIORITIES 1. Site-specific constraints should be identified to develop target-oriented technology. 2. Cost effective and practical techniques for in situ water harvesting and improvement of profile water storage should be developed. The possibilities of using percolation barriers should also be studied. 3. Identification and characterization of different materials that can be used as organic amendments in rain-fed lowlands should be carried out. The longterm effects of different organic amendments on water retention and plantavailable water capacity should be examined. 4. Detailed investigations should be conducted on the effects of plant nutrition on rice under drought situations, and fertilizer recommendations should be made based on soil type, moisture regime, and ecological situations. 5 . The use of drought-responsive varietal traits in the development of cultivars suited to rain-fed environments should be intensified. 6 . Yield prediction models based on long-term rainfall data and soil fertility status should be developed.

VI. SUMMARY Nearly one-third of the world's lowland rice is rain fed; however, rain-fed rice comprises only one-fifth of the global rice produced. Insufficient or excess water is the major constraint in the production of rain-fed lowland rice. Coupled with this are problems of poor agronomic practices followed by farmers, socioeconomic constraints, and lack of technology suited to rain-fed lowlands. This article reviews the existing knowledge about improving rice yields per unit quantity of rainwater available. Broadly speaking, there are two systems of rice production in rain-fed lowland areas, wet culture and dry culture, with subsequent conversion to wet culture late in the season. The choice between the two methods depends on the rainfall pattern. Subsurface soil compaction before planting rice appears to be a practical way to decrease percolation losses and to improve profile water storage in coarsetextured soils. Other possibilities of improving the water retention capacity of

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soils and decreasing percolation losses are based on the use of organic and inorganic amendments and of subsurface barriers. Water harvesting is an important component of rain-fed technology. Rainwater stored in suitable reservoirs can provide supplemental irrigation to rice during prolonged dry spells. Proper land shaping is another means of in situ water harvesting. Selection of rice cultivars and adoption of sound agronomic practices are equally important considerations in improving rainwater-use efficiency of rain-fed lowland rice.

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Pande, H. K. (1975). Water management practices and rice cultivation in India. In “Symposium on Water Management in Rice Field,” Jpn. Tech. Rep. 13. Tropical Agricultural Research Centre, Ministry of Agriculture and Forestry, Japan. Pande, H. K., and Reddy, M. D. (1984). Fertilizer use efficiency and water management in rice. Ferr. News 29(4), 17-26. Parashar, K. S . (1978). Study on the use of polythene sheets in checking water losses for growing rice under rainfed conditions. Indian J. Agron. 23, 387-388. Patel, M. S., and Singh, N. T. (1979). The effect of soil compaction on growth and water use efficiency of rice. Indian J. Agron. 24, 429-43 1. Patel, M. S., and Singh, N. T. (1986). Influence of soil compaction, polyethylene barrier and clay incorporation on the yield and water requirement of rice in light-textured soils. Indian J. Agric. Sci. 56, 868-872. Peele, T. C., Beale, 0. W., and Lesnesne, F. F. (1948). Irrigation requirements of South Carolina soils. Agric. Eng. 29, 157- 158. Petersen, G . W., Cunningham, R. L., and Matelski, R. P. (1968). Moisture characteristics of Pennsylvania soils: 11. Soil factors affecting moisture retention within a textural class-Silt loam. Soil Sci. SOC.Am. Proc. 32, 866-870. Rajput, R. P., and Sastry, P. S. N. (1988a). Effect of soil amendments on the physico-chemical properties of sandy loam soil. 11. Structural and hydrophysical properties. Indian J. Agric. Res. 22, 209-216. Rajput, R. P., and Sastry, P. S. N. (1988b). Effect of soil amendments on the physico-chemical properties of sandy loam soil. 111. Static and water retention properties. Indian J. Agric. Sci. 22, 197-202. Ramunni, A , , Scialdone, R., and Pignalosa, V. (1987). Agronomic properties of a volcanic ashderived soil as affected by uncomposted organic materials. Plant Soil 102, 247-251. Rao, K. V. P., Varade, S . B., and Pande, H. K. (1972). Influence of subsurface barrier on growth, yield, nutrient uptake and water requirement of rice. Agron. J . 64, 578-580. Reddy, S. R., and Hukkeri, S. B. (1979). Soil, water and weed management for direct seeded rice grown on irrigated soils in north-western India. Indian J . Agric. Sci. 49, 427-433. Reddy. S. R., and Hukkeri, S. B. (1983). Effect of tillage practices on irrigation requirement, weed control and yield of lowland rice. Soil Tillage Res. 3, 147- 158. RON, D., Taha, M., and Acharya, N. (1989). Some practical measures for minimization of seepage loss in contour basins under transplanted rice. Environ. Ecol. 7, 803-805. Russel, M. B., Klute, A , , and Jacob, W. C. (1952). Further studies on the effect of long-time organic matter additions on the physical properties of Sassafras silt loam. Soil Sci. Soc. Am. Proc. 17, 156- 159. Saenjan, P., Gamier, B. J . , and MacLean, P. A. (1990). Patterns of wet season rainfall in northeast Thailand. In “Remote Sensing, Soil and Water Management in Northeast Thailand,” Tech. Rep. Ser., Khon Kaen University, Thailand. Salter, P. J., and Williams, I. B. (1963). The effect of FYM on the moisture retention characteristics of a sandy loam soil. J . Soil Sci. 14, 73-81, Salter, P. J., Williams, I. B., and Harrison, D. J. (1965). Effect of bulky organic manure on the available water capacity of sandy loam. Exp. Horric. 13, 69-75. Sanchez, P. A. (1973). Puddling tropical rice soils. 11. Effects of water losses. Soil Sci. 115, 303308. Sen, P. K . , and Das Gupta. D. K. (1969). Studies in water relations of rice. 111. Effect of varying water regimes and levels of nitrogen on the growth and yield of rice. Indian J . Agric. Sci. 39, 1000- 1009. Sengar, S . S., Tomar, A. S., Rajput, R. P., and Raghav, H. S. (1989). Effect of water harvesting on rainfed rice in a vertisol. O n z a 26, 33-36.

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Sharma, K. D. (1986). Runoff behaviour of water harvesting microcatchments. Agric. Warer Manage. 11, 137-144. Sharma, P. K., and Aggarwal, G. C. (1984). Soil structure under different land uses. Catena (Cremlingen-Destedt, Ger.) 11, 197-200. Sharma, P. K., and Bhagat, R. M. (1993). Puddling and compaction effects on water permeability of texturally different soils. J . Indian SOC.Soil Sci. 41, 1-6. Sharma, P. K., and De Datta, S. K. (1985). Puddling influence on soil, rice development and yield. Soii Sci. Soc. Am. J . 49, 1451- 1457. Sharma, P. K., and De Datta, S. K. (1986). Physical properties and processes of puddled rice soils. Adv. Soil Sci. 5, 139- 178. Sharma, H. L., and Modgal, S. C. (1984). Studies on agronomic control of soil moisture in upland rainfed rice crop. Hirnachal J . Agri. Res. 10, 23-26. Sharma, D. P., and Nath, J. (1979). Soil moisture retention characteristics for Hissa Major Command area. Haryana Agric. Univ. J . Res. 9,43-52. Sharma. P. K., De Datta, S . K., and Redulla, C. A. (1987). Root growth and yield response of rainfed lowland rice to planting methods. Exp. Agric. 23, 305-313. Sharma, P. K., De Datta, S . K., and Redulla, C. A . (1988). Tillage effects on soil physical properties and wetland rice yield. Agron. J . 80, 34-39. Singh, A. (1962). Studies on the modus operandi of green manures in tropical climates. Indian J . Agron. 7, 69-79. Singh, K. D., Kar, S., and Varade, S. B. (1976). Structural and moisture retention characteristics of lateritic soil as influenced by organic amendments. J . Indian SOC.Soil Sci. 24, 129- 13I . Singh, N. T., Patel, M. S . , Singh, R., and Vig, A. C. (1980). Effect of soil compaction on yield and water use efficiency of rice in a highly permeable soil. Agron. J . 72, 499-502. Somani, L. L., and Kumawat, B. L. (1986). Interactive effect of claylization and subsurface compaction on the physical properties of a sandy soil and yield of bajra crop. Trans. Indian SOC. Desert Technol. Univ. Cent. Desert Stud. 11,49-53. Somrnerfeldt, T. G., and Chang, C. (1986). Soil-water properties as affected by twelve annual applications of cattle feedlot manure. Soil Sci. SOC. Am. J . 51, 7-9. Tester, C. F. (1990). Organic amendment effects on physical and chemical properties of a sandy soil. SoilSci. Soc. Am. J . 54,827-831. Thangaraj, M., O’Toole, J. C., and De Datta, S. K. (1990). Root response to water stress in rainfed lowland rice. Expl. Agric. 26, 287-296. Tiarks, A. E., Mazurak, A. P., and Chesnin, L. (1974). Physical and chemical properties of soil associated with heavy applications of manure from cattle feedlots. SoilSci. SOC.Am. P roc. 38,826-830. Unger, P. W. (1975). Relationships between water retention, texture, density and organic matter content of west and south central Texas soils. Exp. S m . Misc. Publ. 1192C, 20. Varade, S. B., and Ghildyal, B. P. (1967). Mechanical impedance and growth of paddy in artificially compacted lateritic sandy loam soil. J . Indian SOC. Soil Sci. 15, 157- 162. Varade, S. B., and Patil, E. A. (1971). Influence of soil compaction and nitrogen fertilization on growth of rice. Riso 20, 219-223. Volk, V. V., and Ullery, C. H. (1973). Disposal of municipal wastes on sandy soils. In “Report to the Boeing Company,” p. 50. Dep. Soil Sci., Oregon State University, Corvallis. Voorhees, W. B. (1983). Relative effectiveness of tillage and natural forces in alleviating wheelinduced soilcompaction. Soil Sci. Soc. Am. J . 47, 129- 133. Voorhees, W. B.. Nelson, W. W., and Randall, G. W. (1986). Extension and persistence of subsoil compaction caused by heavy axle loads. Soil Sci. SOC. Am. J . 50,428-433. Warman, P. R. (1980). The basics of green manuring. MacDonaldJ. 41, 3-6. Weil, R. R . , and Kroontje, W. (1979). Physical condition of a Davidson clay loam after five years of heavy poultry manure applications. J . Environ. Qual. 8, 387-392.

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Wischmeier, W. H., and Mannering, J. V. (1965). Effect of organic matter content of the soil on infiltration. J. Soil WaferConserv. 20, 150-152. Yadav, B. L., and Somani, L. L. (1990). Interactive effect of soil compaction and mixing of heavy textured soil in a loamy sand soil on the physical properties of sandy soil and yield of cluster bean. Trans. Indian SOC.Desert Technol. 15,43-48. Yamazaki, F. (1988). “Paddy Field Engineering.” Agricultural and Land Development Programme, Asian Institute of Technology, Bangkok, Thailand.

WETLAND SOILS OF THE

PRAIRIE POTHOLES J. L. Richardson, James L. Amdt, and John Freeland Department of Soil Science North Dakota State University Fargo, North Dakota 58105

I. Introduction A. Background B. History 11. Climate, Basic Hydrologic Concepts, and Wetland Classification A. Climate B. Hydrologic Aspects of Wetland-Groundwater Interactions in the PPR C. Wetland Classification 111. Geologic Factors A. Parent Materials B. Erosion and Sedimentation IV.Water Quality A. Land-Use Patterns and Water Quality B. Soil Landscape and Salinity V. Wetland Soil Properties A. Salinity B. Organic Matter C. Calcium Carbonate Occurrence and Formation D. Texture Vl. Soil Sequences A. Fens: Histosols B. Recharge, Flowrhrough, and Discharge Wedand Soils VII. Soils on Prairie Pothole Edges VIII. Conclusions and Future Work References

I. INTRODUCTION Wetlands and soils are important, productive resources in the northern prairie region of North America. However, the maintenance of natural prairie wetlands for nonagricultural uses, such as wildlife habitat and groundwater and surface 121 Advances in Agronomy, Volume f2 Copyright 0 1994 by Academic Press, Inc. All rights of reproduction in any form reserved

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water management, often conflicts with traditional uses of soil for dryland agriculture (Leitch, 1989). The societal importance of natural prairie wetlands for ecosystem support and water management has been recognized in the controversial “swampbuster” provisions of the 1985 farm bill. Many important wetland functions, however, have been poorly defined for the public. As an example, a little appreciated function of prairie wetlands is flood abatement (Hubbard and Linder, 1986; Richardson and Arndt, 1989). Drainage of large numbers of wetlands results in less water stored on the landscape. Wetland drainage increases the catchment area of adjacent streams and drains (Moore and Larson, 1979) and can aggravate downstream flooding (Novitzki, 1978; Brun er al., 1981). There are considerable difficulties in defining exactly the benefits of such nonagricultural wetland uses. Because the benefits of wetland conversion for agriculture are tangible and immediate, wetland drainage for agricultural use and to improve cropping efficiency is still occurring at a rapid rate on the prairies. We feel an understanding of wetland functions as reflected in hydric soils and hydric soil development is necessary to manage the prairie wetland resource appropriately for both societal and individual benefit. It is to this end that this discussion is directed. Three reviews of prairie wetlands have been published (Adams, 1988; van der Valk, 1989; Hubbard, 1989). Only Hubbard (1989) discussed wetland soils; we are expanding his review considerably.

A. BACKGROUND “Prairie potholes” are numerous water-filled depressions characteristic of the glaciated portion of central North America that was once grassland. Although prairie wetlands are occasionally found in Wisconsin, Texas, Illinois, Nebraska, Oklahoma, and Missouri, we are confining our discussion to the wetlands of the prairie pothole region (PPR) that extends from the prairie-forest line north of Edmonton, Alberta, southward to the end of the Wisconsin-aged Des Moines lobe in central Iowa (Fig. 1). Wetlands in the PPR are mostly kettle-type depressions formed on a till surface that has not yet developed an integrated network of surface drainages. The depressions vary in size from less than 0.5 ha to several hectares and usually contain surface water for some period of time during the year. A few are permanent lakes. Water accumulates in prairie wetlands as a function of complex interactions between topography, vegetation, and climate as they influence the local hydrology. Hydrology, considered as the sum of all the factors influencing the chemistry, movement, and distribution of groundwater and surface water, is a unifying principle of soil development that has been overlooked, although it is essential in understanding wet soils (Richardson et al., 1992). Winter (1988; 1992) makes a solid case for groundwater hydrology as a unifying concept for wetland ecology in general.

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WETLAND SOILS OF PRAIRIE POTHOLES

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Zoltai (1988) points out that two important hydrologic factors, climate and topography, really explain the existence of wetlands in any landscape. Depressions collect water; level areas do not have enough slope to create appreciable runoff, and low areas such as floodplains have runon water from adjacent uplands in addition to periodic flooding. In the PPR where integrated drainage networks are lacking, topography is the main control on the movement of groundwater and surface water in wetlands, and exerts a significant influence on hydric soil development. The glaciated landscape of the PPR is a mosaic of closed system catchments that vary in size, topographic position, and relationship to the groundwater. Runoff as well as groundwater recharge and discharge are focused on the wetland depressions occupying these catchments. Interdepressional uplands are usually not involved in direct transfers of water to and from the water table (Lissey, 1971) because low rainfall characteristic of the region confines recharge to areas where the vadose zone is thin, e.g., in and around wetlands (Winter, 1983). PPR wetlands typically form relatively large, complex wetland systems connected to each other by groundwater flow. Sediment stratigraphy of the area around a wetland and the climatic factors of precipitation, evapotranspiration, and freezing will in turn impact soil development (Arndt and Richardson, 1988; 1989a,b; 1992). We base PPR wetland soil development on hydrologic processes and conditions created by a climatic gradient as impacted by topography, sediment lithology, and stratigraphy.

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B. HISTORY The landscape of the PPR is very young, with most of the surface sediments consisting of Wisconsin-age drift that dates from 14,000 to 9000 years before present (YBP). Spruce forests followed the ice margin as continental glaciers gradually wasted northward during the close of the Pleistocene Epoch. As the climate moderated and became drier, the forests were gradually replaced by prairie parkland, which was replaced by true prairie around 6000 YBP (Ritchie, 1976). Thus the native vegetation in the PPR has been dominantly grass for the last 6000 years. Prairie grasses create dark-colored fertile soils that are high in organic matter; nearly all the soils in the region are Mollisols (Soil Survey Staff, 1975). The northern prairies were opened to settlement at the turn of the century. Farming is currently the dominant land use, with wheat and sunflowers the typical cash crops in the north, and corn and soybeans dominant in the south. In the hummocky landscape of the PPR, wetlands are often drained because naturally wet soils are a hindrance to crop production (Leitch, 1989). Poor aeration in wet soils restricts crop growth due to lack of sufficient oxygen for root respiration. Only plants that are adapted to long periods of poor soil aeration can survive (Bartlett, 1961, 1986; Gambrel1 and Patrick, 1978). Additionally, large equipment becomes mired when crossing wetlands except when the wetlands are quite dry. However, temporarily and seasonally ponded wetlands are abundant in the PPR and are usually flooded during spring planting. The necessity of continually traveling around these wetlands with farm machinery increases production costs and reduces the efficiency of tillage operations (Leitch, 1989). Early studies of the soils in prairie wetlands focused on ephemeral, seasonal, and temporary wetlands. Examinations of more permanent wetland soils were not numerous. The soils in temporarily and seasonally ponded wetlands were found to be calcareous on the pond periphery and leached in the pond interiors (Redmond and McClelland, 1959). In Iowa, however, wetlands underlain by upland loess deposits did not have distinct calcareous edge soils, but did have leached centers exhibiting extremely well-developed argillic horizons (Ulrich, 1949; 1950).

II. CLIMATE, BASIC HYDROLOGIC CONCEPTS, AND WETLAND CLASSIFICATION A. CLIMATE The PPR has a cool continental climate characterized by cold winters, hot summers, and extreme variations in both temperature and precipitation. Temperatures may range from - 40 to 40°C annually. The precipitation regime

+

WETLAND SOILS OF PRAIRIE POTHOLES

12s

varies from semiarid in the west to humid in the east. As an example of the variation in average yearly precipitation in the PPR, Richardson et al. (1991) had three sites representative of semiarid, subhumid, and humid regions. Mean yearly precipitation (20-year norms) was 34 cm in semiarid regions, 50 cm in subhumid regions, and 85 cm in humid regions. Yearly variations are also extreme. Droughts and pluvial cycles are the norm. The westerly winds that typically prevail in central North America provide little precipitation to the PPR, because these air masses, which originate in the Pacific Ocean, lose most of their moisture on the west side of the Rocky Mountains. Most of the precipitation in the PPR occurs in the spring and summer, the result of weather systems occasionally bringing in moist air from the Gulf of Mexico. The frequency of weather patterns that bring moist Gulf air decreases as one moves west in the PPR, explaining the west to east gradient in precipitation (personal communication, Dr. John Enz, State Climatologist for North Dakota). The interactions between precipitation, temperature, and evapotranspiration (ET) are important factors in the water budget of wetlands, and can influence wetland frequency on the landscape. Given the same landscape and landforms, high precipitation coupled with low ET favors the development of wetlands because water inputs are maximized and ET losses minimized. Conversely, low precipitation coupled with high ET inhibits the development of wetlands because ET losses are maximized (Zoltai, 1988). In the PPR, potential yearly evapotranspiration (PET) generally exceeds mean yearly precipitation, with the ratio between PET and average precipitation being highest in the southern and western portions of the region, and decreasing northward and eastward. The impacts of high PET coupled with low precipitation on the water budget of prairie wetlands are great. Shjeflo (1968) noted that on average usually more than 35% of the water lost from wetlands in North Dakota is evapotranspired, but that the ET loss as a percentage of the total water loss was greatly influenced by the dominance of seepage inflow over seepage outflow of groundwater. ET losses are a smaller percentage of the total water loss in wetlands dominated by seepage outflow, whereas ET losses can go up to 100% of total water losses in wetlands dominated by groundwater seepage inflow. Millar (197 1) observed a positive correlation between shoreline :wetland area ratios and wetland evapotranspiration, and noted that smaller and shallower ponds with a large shore1ine:pond area ratio tended to have higher evapotranspiration rates and were only seasonally persistent. In summary, the high PET: precipitation ratio tends to mitigate against high wetland density and permanence in the PPR. Permanent lakes are few compared to the more humid glaciated regions north and east. Many wetlands of the PPR are only seasonally to semipermanently ponded, and wetland density decreases from east to west. A climatic factor that does favor the formation of wetlands in the PPR is the timing and distribution of surface runoff. Prairie wetlands receive a significant

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portion of their water volume as surface runoff during spring snowmelt (Hubbard and Linder, 1986), when frozen ground minimizes infiltration, and low temperatures and dormant plant communities minimize ET losses (Shjeflo, 1968; Lissey, 1971; Sloan, 1972). Shjeflo (1968) determined that snow accounts for at most 25% of total yearly precipitation, yet it accounts for at least 50% of the water that reaches the wetland. Generally, a wet winter means a wet year for PPR wetlands. Whereas intense summer thunderstorms may result in runoff, infiltration in the upland areas of the catchment after thaw, coupled with high evapotranspiration rates, minimizes runoff during the growing season.

B. HYDROLOGIC ASPECTSOF WETLAND-GROUNDWATER INTERACTIONS INTHE PPR Typical PPR wetlands lack integrated drainage networks. However, the presence of overflow or high-water outlets in some PPR wetlands indicates that partially closed systems are more common during unusually wet climatic cycles or more pluvial periods, such as that which existed immediately after deglaciation. Under present climatic conditions, the lack of effective water removal by channels accentuates the importance of groundwater as a component of the PPR wetland water balance (Winter, 1988, 1992; Richardson et al., 1992). Surface runoff in closed systems or partly closed systems is depression focused-i.e., flow converges on the depression occupying the lowest portion of the closed catchment. Convergent flow thus brings water (and the material it transports) to a point on the landscape from the surrounding catchment. Once the water is collected in depressions, some equilibrium level in the wetland water balance is reached where sediment saturation persists long enough to develop anaerobic conditions in the soil zone favorable for the growth of hydrophytes (Cowardin et al., 1979). This equilibrium level strikes a tenuous balance between the input components of precipitation, overland flow, and seepage inflow, and the output components of evapotranspiration and seepage outflow. Because precipitation and temperature are so variable in the PPR, the exact elevation where saturation persists long enough for hydrophytes to grow is extremely variable, and can change several feet in horizontal distance from year to year. Thus, the presence of hydrophytes represents a short-term indicator of wetness at best (Stewart and Kantrud, I97 1). Several soil characteristics better indicate long-term saturation status (the hydrologic regime) because they integrate the effects of anaerobic conditions and water movement over time. Important morphological characteristics of the soil that are often-used indicators of wetness include ( 1 ) soil mottling, which reflects the distribution pattern of redox-sensitive soil constituents, (2) high levels of organic matter, which build up due to slow rates of decomposition in anaerobic environments, and (3) the distribution of evaporites in the

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soil, which can indicate the intensity and direction of saturated and unsaturated water flow. The spatial relationship of soil horizons within and between pedons is also an often-used indicator of water movement in soils. We feel the hydrology of PPR wetlands is reflected most clearly in the associated hydric soils. Because groundwater fluxes in PPR wetlands are so important in explaining the development and morphology of hydric soils in the PPR, we will examine the characteristic groundwater hydrology in some detail.

1. Darcy’s Law Groundwater movement can be analyzed and simulated by Darcy’s law and its extensions (Freeze and Witherspoon, 1967). Darcy’s law (4 = Kdh/dl) indicates that groundwater flow velocity ( 4 ) is the result of the presence of a hydraulic gradient ( d h / d l ) in sediments with a characteristic hydraulic conductivity ( K ) , Darcy’s law applied to saturated conditions predicts that flow will increase if the hydraulic gradient or hydraulic conductivity increases. We will first consider the influence of topography and climate on the spatial distribution of hydraulic gradients in the PPR.

2. Distribution of Hydraulic Gradients: Recharge and Discharge in the PPR Seeps or hillslope wetlands often occur below steep slopes because the water table gradient is also steep, favoring groundwater discharge (Fig. 2) (Winter, 1988). Many hydrology texts and mathematical simulations of groundwater movement assume that groundwater recharge generally occurs in upland areas adjacent to depressions and streams (e.g., Toth, 1963; Freeze and Witherspoon, 1967; Winter, 1983). Under these conditions it is universally assumed that the water table will become a subdued replica of the surface topography: groundwater mounds will form under topographic highs and groundwater depressions will be associated with topographic lows (wetlands, lakes, or streams). Using this model all wetlands will be foci of groundwater discharge (Lissey, 1971). The assumption of upland-focused recharge does explain many of the hydrologic characteristics of wetlands in the glaciated humid regions near the PPR. Because of an excess of precipitation relative to evapotranspiration, upland recharge often occurs. Most streams in the humid areas are effluent (receive groundwater), and it has been suggested that wetlands in humid regions typically receive water from the groundwater flow system as well (Richardson et al., 1991, 1992). Many wetlands in this situation fill and overflow, and form the “deranged” surface drainages characteristic of glaciated humid areas north and east of the PPR. However, low precipitation and high evapotranspiration limit the development of surface drainages in much of the PPR. Instead, PPR wetlands typically form

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J. L. RICHARDSON ET AL. Recharge

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2

DISTANCE (km) Recharge

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DISTANCE (km)

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Figure 2 Recharge, flowthrough, and discharge wetlands with mineralogical controls and soil types fresh to saline wetlands in Nelson County, North Dakota. After Arndt and Richardson (1988, 1989b) and Richardson et al. (1992).

relatively large complex wetland systems connected to each other by groundwater flow. Whereas upland groundwater recharge may be characteristic of humid areas where precipitation exceeds potential evapotranspiration, several researchers have indicated that groundwater recharge is much more complex in subhumid to semiarid areas. Wetlands in the PPR have been described as surficial expressions of the water table (Sloan, 1972), the free water surface at atmospheric pressure that can be located at, below, or above the hydric soil surface. In the absence of integrated surface drainages, it is apparent that groundwater recharge, groundwater movement, and groundwater discharge are intimately associated with PPR wetlands. Lissey ( I 97 1) noted that groundwater recharge and discharge are focused on depressions in hummocky areas of the PPR. He also observed that interdepressional uplands are relatively inactive regarding water transfers to and from the water table. This depression-focused nature of groundwater and surface water movement is a direct result of hummocky topography and a subhumid to semiarid climate. In the PPR, groundwater recharge occurs first where the vadose zone is thinnest (Winter, 1983). Because the vadose zone in uplands is thick in

WETLAND SOILS OF PRAIRIE POTHOLES

129

the PPR and precipitation is meager, much of the summer infiltration occurring on upland slopes never reaches the water table. Two additional factors act to reduce recharge in the uplands for PPR wetlands: (1) hydraulic conductivity of surface sediments is often ansiotropic, favoring lateral water movement over vertical movement, and (2) spring snowmelt and runoff occur when the soil is impermeable due to the presence of frost layers. Groundwater movement in the PPR has necessitated the development of several hydrologic concepts and terms suited to the region. The groundwater system interaction with the surface water in PPR wetlands is expressed as recharge, discharge, orjowthrough, depending on the dominant process at each site (Fig. 2). Recharge wetlands recharge groundwater within the wetland basin. When rapid overland flow is discharged to a recharge-type depression, infiltration into and percolation through the pond bed eventually recharges the groundwater, producing a water table mound that slowly dissipates through lateral and downward groundwater movement (Miller et al., 1985; Knuteson et al., 1989). In the PPR, wetlands that are usually dry by midsummer receive the majority of their water as spring snowmelt and recharge shallow groundwater aquifers (Richardson et al., 1991). Discharge wetlands receive groundwater that is discharged into the wetland basin. Evaporative discharge is the upward capillary flow of water from a near-surface water table in response to hydraulic gradients set up by higher evapotranspiration rates at the soil surface (Fig. 3). Flowthrough wetlands both recharge the groundwater system and receive groundwater as discharge. On a landscape scale, water can be thought of as moving laterally through flowthrough wetlands (Fig. 2). The groundwater recharge and discharge characteristics of PPR wetlands are strongly influenced by climate, and reflect a climatic gradient from the humid east to the semiarid west (Richardson et al., 1991). As examined above, semipermanently ponded wetlands in humid areas are typically discharge type (Fig. 4A). However, in subhumid areas, semipermanent wetlands typically receive and yield water to the water table (Fig. 4B). In semiarid regions, recharge to groundwater is common in nearly any wetland basin (Fig. 4C) (Miller et al., 1985; Mills and Zwarich, 1986; Richardson et al., 1991). Flow reversals may occur with changes in hydraulic gradient. Recharge wetlands may briefly become discharge wetlands if the water table in adjacent uplands rises above usual levels, and discharge wetlands can become recharge wetlands if the water table is drawn down and a subsequent precipitation event floods the basin. Flow reversals affecting entire wetlands, or parts of wetlands, are frequent occurrences in the PPR. When considered on a small scale, such as that involving the wetland edge, several flow reversals may occur in one season. In Fig. 4A, the presence of the wetland on the downslope side of the water table indicates a discharge condition. After a rain, edge-focused recharge as discussed by Winter (1983) and Arndt and Richardson (1993) results in groundwater

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J. L. RICHARDSON E T A .

A Bearden

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B

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Distance (m)

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Figure 3 (A) Wetland recharge and edge soils (after Knuteson et al., 1989). The solid arrows are saturated flow and the open arrows represent unsaturated flow from the water table to the drier soil surface. The Bk-horizon here is from a short flow distance and lacks gypsum and salinity. (B) Flownet illustrating a recharge wetland based on (A) and Knuteson et al. (1989) and Richardson et al. (1992). Note that the equipotential lines are high in the surface and decrease with depth.

mounding at the edge (Fig. 4B) that produces a shallow, localized reversal of flow landward of the groundwater mound. Because of the increase in hydraulic gradient pondward of the mound, groundwater discharge to the pond is also

131

WETLAND SOILS OF PRAIRIE POTHOLES A Humid region

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Postprecipitationwet meadow mound Direct discharge

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enhanced. Such reversals may be unimportant when considering large, deep flow systems; however, reversals have been shown to be very important when examining soil morphology and salinity dynamics in wetland soils (Arndt and Richardson, 1993). Flow reversals induced by phreatophyte transpiration at the wetland edge were examined in detail by Meyboom (1966) in wetlands surrounded by willow (Salix spp.). After the spring recharge, groundwater mounds typically developed under these wetlands. However, transpiration by the phreatophytic willow resulted in the formation of a water table depression around the wetland periphery, which becomes a groundwater discharge site (Fig. 4C). Because high rates of evapotranspiration are typical of the wetland periphery, the wetland edge often becomes the location of focused discharge during significant drawdown

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periods (Fig. 4C). Soils formed under such complex hydrologic conditions are discussed by Wilding et al. (1963), Mills and Zwarich (1986), Seelig el al. (1990, 1991), and Richardson et a1.(1992).

3. The Influence of Sediment Hydraulic Conductivity Changes in recharge and/or discharge produce flow reversals as discussed above. The influences of sediment characteristics affecting hydraulic conductivity (K) are less obvious but have a strong influence on groundwater movement. The important components of K that influence groundwater movement in the PPR are the texture and the anisotropic nature of unconsolidated surface sediments. Anisotropy is a term that indicates that hydraulic conductivity within the sediment is not the same in all directions. Preferential lateral flow (also called interflow, throughflow, and stormflow) results from higher hydraulic conductivity in the horizontal when compared to the vertical dimension. Preferential lateral flow is the result of several factors. Compaction resulting from the weight of overlying materials progressively increases sediment bulk density and reduces porosity with depth. Other factors include plant root macroporosity, textural changes, and the presence of bedding planes, frozen or partially frozen soil, and restrictive layers such as an argillic horizon (Kirkby and Chorley, 1967; Zaslavsky and Sinai, 1981). Preferential lateral flow in surface sediments reduces groundwater recharge in sloping uplands and influences the distribution of evaporites and soil morphology of hydric soils (Steinwand and Richardson, 1989). Texture and stratigraphy are features of surface sediments that directly influence K , PPR wetland groundwater topography, and the magnitude of groundwater flow. In lacustrine sediments and fine-textured tills, high hydraulic gradients are often observed because the very low hydraulic conductivity associated with fine textures limits the quantity of flow. As an example, a groundwater mound with an unusually steep gradient was found to be associated with a recharge wetland in a very low-relief landscape with fine-textured lacustrine sediments (Knuteson et al., 1989). The relief of the groundwater mound was several times greater than the relief of the land surface. The persistence and magnitude of the mounding under the recharge wetland were associated with depressionfocused recharge and fine-textured sediments. The mounding explained the major differences noted in soil type and leaching regime over short distances (Fig. 3). Conversely, coarse-textured sediments have higher hydraulic conductivity than do fine-textured sediments. The rapid movement of groundwater in these sediments prevents the development of steep hydraulic gradients (Winter, 1986). Significant groundwater flows often occur in sand-dominated landscapes, although very steep hydraulic gradients will be absent. We speculate, based on our field observations, that the presence of coarse textures enhances upland-focused recharge, and discharge-type wetlands are common.

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4. Flownet Examples of Recharge, Flowthrough,

and Discharge Wetlands A flownet is a mesh of equipotential lines and associated streamlines that indicates the direction and magnitude of groundwater flow. By convention, equipotential lines indicate constant values in head, and adjacent lines indicate equal head drops. Streamlines indicate the linear path of water flow. Streamlines always meet equipotential lines at right angles, and groundwater flow is always from higher to lower magnitude equipotential lines. All streamtubes formed by adjacent streamlines have equal amounts of flow per unit time. Thus fast groundwater flow is indicated where streamlines converge. Conversely, where streamlines diverge, groundwater flow slows. Closely spaced equipotential lines indicate the presence of large hydraulic gradients. Conversely, where equipotential lines are spread far apart, low gradients are found. Figure 2 illustrates the groundwater-surface water interactions of recharge, flowthrough, and discharge wetlands using flownet analysis. Recharge occurs as water collects in a depression and infiltrates into the soil. The magnitude of equipotential lines associated with recharge wetlands decreases from the surface maximum (Fig. 2a), indicating the downward flow (Fig. 2b). Because the water is snowmelt and runoff derived, intermittent ponding with fresh water leaches the soil and promotes the development of an argillic horizon (Amdt and Richardson, 1988). In Fig. 3b we also illustrate a recharge wetland condition. The equipotential lines in the flowthrough wetland are perpendicular to the surface and decrease in magnitude from left to right (Fig. 2a). Lateral water flow from left to right is indicated (Fig. 2b). Leaching of minerals will not play an important role in soil development in flowthrough wetlands because the presence of continuously saturated conditions and brackish water limits the development of an argillic horizon. The magnitude of the equipotential lines steadily decreases as the discharge wetland (20) is approached (Fig. 2a), indicating groundwater seepage (discharge) to the wetland (Fig. 2b). These discharge wetlands concentrate salts in many cases. The evaporites tend to accumulate in soils at the pond margin. Evapotranspiration on the edge of wetlands or any other landscape position with a shallow water table creates a condition of abundant soil water loss. This evaporative discharge allows evaporite minerals such as calcite and gypsum to accumulate in the periphery of flowthrough and discharge wetlands (Arndt and Richardson, 1988; Steinwand and Richardson, 1989).

C. WETLAND CLASSIFICATION Several systems are currently used to classify wetlands in the PPR. The most important classification systems include the Canadian system (Zoltai, 1988), the

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United States Fish and Wildlife system (Cowardin et al., 1979), the Stewart and Kantrud (1971, 1972) system, and an extension of the Stewart and Kantrud (1971, 1972) system that incorporates hydrology and landscape position (Arndt and Richardson, 1988). We will examine the last three systems in some detail below.

1. United States Fish and Wildlife Service System The Cowardin et al. (1979) wetland classification was developed for use nationwide by the United States Fish and Wildlife Service. It has been adopted as the standard system of wetland classification in the United States. The Cowardin et al. (1979) classification is hierarchical in nature. The highest level of classification consists of five systems based on major ecological type: they are marine, estuarine, lacustrine, riverine, and palustrine. Within each system, wetlands are further differentiated into subsystems, classes, and dominance types based on criteria that include ponding permanence, dominant vegetation, water chemistry and pH, and soil type. In essence, the Cowardin et al. (1979) system is designed to discriminate ecologically important zones within a wetland; however, basins can often be classified at the highest levels. In the PPR virtually all “pothole”type wetlands would be classified into the Palustrine System. Basins would be classified into the Palustrine Emergent class, based on the presence of emergent vegetation. Zones within the wetland are classified at the more detailed subclass and dominance-type levels. Specifically, Cowardin et al. (1979) define wetlands and palustrine systems as follows: Wetlands are lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. For purposes of this classification wetlands must have one or more of the following three attributes: (1) at least periodically, the land supports predominantly hydrophytes; (2) the substrate is predominantly undrained hydric soil; and (3) the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season of each year. The Palustrine System includes all nontidal wetlands dominated by trees, shrubs, persistent emergents, and emergent mosses or lichens, and all such wetlands that occur in tidal areas where salinity due to ocean-derived salts is low. It also includes wetlands lacking such vegetation, but with all of the following four characteristics: (1) area less than 8 ha (20 acres); (2) active wave-formed or bedrock shoreline features lacking; (3) water depth in the deepest part of basin less than 2 m at low water; and (4) salinity due to ocean-derived salts is low. The Palustrine System was developed to include vegetated wetlands traditionally called by names such as marsh, swamp, bog, fen, and prairie (Cowardin et al., 1979). It also includes the small, shallow, permanent, or intermittent

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water bodies often called ponds. Palustrine wetlands may be situated shoreward of lakes, river channels, or estuaries; on river floodplains; in isolated catchments; or on slopes. They may also occur as islands in lakes or rivers. The erosive forces of wind and water are of minor importance except during severe floods.

2. The Stewart and Kantrud System The Stewart and Kantrud (1971, 1972) system was developed specifically as a research tool for the PPR. It uses plant community composition criteria to describe and define distinctive wetland basins. The Stewart and Kantrud system is based on zonal plant communities that, in addition to hydrologic variables, reflect salinity and pond permanence. Wetlands are assigned class numbers from I to V depending on the plant species and zonation present. In classes I through V, higher numbers indicate the presence of central vegetation zones that reflect increasingly wet conditions: from central low prairie communities through central wet-meadow, shallow-marsh, and deep-marsh communities, to central open water (Figs. 5 and 6 ) . Subgroupings based on species composition reflect salinity tolerances: from fresh, through brackish, to saline subgroups (Table I).

3. Hydrologic Classification of Wetlands We believe that a wetland basin classification based on pond permanence and salinity is the best method to make ecologically meaningful delineations for most wetlands in the PPR. The separations are easier, more accurate, and require less time than other systems. Arndt and Richardson (1988) outlined a field-oriented basin-delineation system that inferred recharge-flowthrough-discharge hydrology by relating soil morphology to the observations of Stewart and Kantrud (1971, 1972), Sloan (1972), and Lissey (1971). Stewart and Kantrud (1971) identified pond permanence and salinity as defined by wetland plant communities, and Sloan (1972) and Lissey (1971) related salinity, topographic position, and pond in duration to groundwater recharge-discharge relationships. They found that, although wetlands can be defined or classified by hydrologic function as recharge, flowthrough, and discharge wetlands, these wetlands actually form a continuum on the landscape. The emphasis is on dominant flow conditions, because seasonal and climatic variation creates intermittent reversals. Recharge and discharge wetlands are relatively easy to distinguish. Recharge wetlands are temporarily to seasonally ponded with fresh water and have leached soil profiles consistent with their hydrologic function. Discharge wetlands are semipermanent to permanently ponded, and often have saline soils. Efflorescent salt crusts are often evident around the pond edge. Even though they are permanently ponded, the catchment area for the pond is small relative to the pond itself, indicating a large amount of groundwater discharge. Flowthrough wetlands “bridge the gap”

A

Class 111 (seasonal)

Shallow marsh

B

Class IV (semi-permanent)

Prairie pothole wetlands Figure 5 Diagrammatic model of a class III (seasonal) and a class IV (semipermanent) wetland without open water, and a class IV (semipermanent) wetland with open water and fen. After Stewart and Kantrud (1971).

136

137

WETLAND SOILS OF PRAIRIE POTHOLES Wet Meadow

I

Zone

Wet Meadow Zone

I I

I

I

Scolochloa

I

I

.

Shallow Marsh Zone

Scirpus

I

I

I

I I

I I

I

Figure 6 Generalized cross-section of vegetational zonation in a semipermanent wetland, with a late spring water table. After Richardson and Bigler (1984).

between the extremes of pure recharge and pure discharge. As such they exhibit a range of recharge or discharge dominance, and a corresponding range in salinity. Flowthrough wetlands in a recharge-flowthrough-discharge continuum include the following consistent indicators: (1) they are intermediate in groundwater and pond water EC (i.e., brackish); (2) they are generally semipermanent in ponding duration; (3) catchment area to pond surface area ratios indicate that they have both recharge and discharge components to their water balance; and

Table I Ranges in Specific Conductance of Surface Water in Plant Communities as Indicators of Differences in Average Salinity" Plant community Fresh Slightly brackish Moderately brackish Brackish Subsaline Saline

Normal range (dS/m) 0.4-0.5 0.5-2.0

2.0-5.0 5.0- 15 15-45 45-100+

"After Stewart and Kantrud (1971).

Extreme range (dS/m) 0.04-0.7

0.3-2.2 2.0-8.0 1.6-18 5-70 20-100+

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(4) they contain hydric soil profiles, usually located at the wetland edge, that contain calcite and gypsum and can be saline. A few comments about the effects of drainage on wetland classification are needed. In the PPR wetland, artificial drainage of wetlands is often inefficient and changes the hydrology to more ephemeral conditions (Hubbard et al., 1988). The quick removal of water by either tile or ditch drainage alters the total amount of water that infiltrates and recharges the system. Also, created wetlands will be recharge wetlands until the water table is altered to place the created wetland in a groundwater flowpath.

111. GEOLOGIC FACTORS

A. PARENTMATERIALS The PPR is an extensive Wisconsin-aged glacial terrain that has a mantle of relatively fine-textured till draped over sedimentary rocks of Mesozoic and Cenozoic age (Bluemle, 1971). Winter (1989) states that most tills in the United States are sandier than the tills found in the PPR. In addition, the characteristic relief in the PPR is usually greater than in other glaciated terrains. Unusually high relief is associated with the Coteau du Missouri in central North Dakota and other areas with stagnant-ice end moraines (Bluemle, 1971). Typically tills of the PPR contain substantial amounts of dolomite and calcite derived from glacial erosion of locally derived marine shales and Paleozoic bedrock sequences exposed north and west of the region. The presence of these minerals buffers the soil at slightly alkaline. Many of the tills are loams and clay loams; the term often used to describe the typical till is calcareous clay-loam till (Bluernle, 1971). Ablation (stagnant or dead ice) tills are sandy loams; the coarser texture is presumably a result of ablation processes. Mineralized groundwater discharging into glacial sediment from underlying bedrock, and postglacial oxidation of high-sulfur shales entrained in the till, have produced abundant interstitial sulfate salts in the glacial sediments (Winter et al., 1984; Hendry et al., 1986; Arndt and Richardson, 1989b). The central and western areas that have a characteristic calcareous shaley till have a unique geochemistry (explained in detail below). Commercially exploitable deposits of Na and Mg sulfates are located in the PPR, with the largest deposits located in the Canadian province of Saskatchewan in large saline wetlands (Last and Schweyen, 1983). Thin lacustrine and glacio-fluvial sediments are occasionally superimposed on the glacial terrain. The lacustrine sediments vary from coarse-textured beach and deltaic sediments to finer textured offshore sediments that are silt loams to silty clays (Lord, 1988). Most of the glacio-fluvial sediments are coarse-textured sands to sandy loam and lie in outwash plains and terraces. Chemically and mineralogically the finer sediments resemble the tills from which they originated,

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but the coarse-textured sediments contain unweathered feldspars, carbonates, and ferromagnesium minerals. Wind erosion of many of the coarse-textured Late Pleistocene beaches, deltas, and outwash areas resulted in sand dunes and coarse aeolian sediments of Holocene age (Lord, 1988). Wetlands are interspersed in the aeolian areas.

B. EROSION AND SEDIMENTATION Wetlands in closed systems trap clastic sediments displaced by mass wastage and erosion of the surrounding catchment. Evaporite sediments, especially calcite and gypsum, precipitate in soils as a result of evapotranspiration and are often incorporated with clastic sediments. Organic material may also be a significant portion of the total soil mass when rates of plant tissue production exceed removal by various processes such as decomposition, fire, or herbivory. Soil parent materials, basin morphology, climate, and nutrient availability all influence the quantity and composition of wetland sediments accumulated over time (van der Valk, 1989). PPR wetlands are geologically ephemeral systems. Basin in-filling, either through natural processes or culturally related activities, will eventually replace the wetland ecosystem with a terrestrial ecosystem. A variety of sedimentological evidence helps reconstruct paleoenvironments and patterns of deposition in prairie potholes, including fossils, carbon and cesium-137 dating, and textural analyses. Okland (1978) examined and radiocarbon-dated fossil assemblages to reconstruct paleoecologic conditions in a dried lake in central North Dakota. The Late Quaternary sediments contained fossil plants and mollusks resembling the modern boreal forest communities of northern Minnesota. Radiocarbon-dated wood indicated the area was surrounded by spruce forest from about 12,600 to 10,500 years ago. Fossils in younger, overlying strata provided evidence of increasingly warmer and drier conditions approaching modern times. Okland suggested that a sandy layer within a Holocene silty claystone unit might have been deposited during the Hypsithermal, an especially warm and dry episode that began approximately 8200 years ago and lasted for 2700 years. Walker ( 1966) made detailed observations of sediments in two Iowa wetlands developed in Des Moines lobe till. He found that both organic and inorganic sediments increased in thickness from the basin edge to the center. The particle sizes became finer from the upland to wetland centers. Calcium carbonate was absent in the soils on the surrounding slopes but was concentrated on the wetland edge. A similar study conducted by Malo (1975) in North Dakota found nearly identical morphology and relationships, with CaCO, accumulating exclusively around wetland edges. Steinwand and Richardson (1989) ascribed this pattern to groundwater discharge and evaporative edge-focused discharge (Fig. 3). Walker (1966) determined that interbedded clastic and organic sediments ac-

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cumulated in the wetland under alternating depositional environments. Periods of drought promoted soil erosion and inorganic sediment deposition, while organic deposits accumulated under more humid, quiescent conditions. Drought presumably lowered the amount of vegetative cover and increased decomposition of the organics, allowing maximum erosion of exposed soils from the upland with little deposition of organic material in the wetland to mask the accumulation. Walker estimated that the surrounding basins contributed sediment to the wetlands at rates ranging from 1 to 20 cml1000 years. Climate and basin slope and size were apparently the chief determinants of erosion and sedimentation rates. Little doubt exists that conventionally tilled fields maximize erosion when contrasted to consistently vegetated slopes. Ritchie and McHenry (1978) used bomb fallout cesium- 137 to assess erosion and sedimentation rates in several watersheds in the north central United States, and concluded soil eroded far faster in cultivated versus noncultivated watersheds. Studies in cultivated and noncultivated wetland watersheds in eastern South Dakota (Martin and Hartman, 1987) provide evidence of both sediment and nutrient accumulations in cultivated areas. Wetlands surrounded by cultivated fields accumulated inorganic sediment and phosphorus at twice the rates of wetlands surrounded by grasslands. Phosphorus is a major plant nutrient that travels with the sediment (Neely and Baker, 1989). Although higher nutrient concentrations in cultivated watersheds may accelerate plant production, the increase in organic matter seemingly does not lead to pronounced litter accumulation. Neely and Baker (1989) provide evidence indicating that decomposition rates increase with elevated nitrogen and phosphorus fertility levels. Martin and Hartman (1987) found that sediments from wetlands in cultivated watersheds had a greater inorganic fraction than did sediments from wetlands surrounded by grasslands. Two separate processes might account for lower organic matter proportions in the wetlands surrounded by cultivated watersheds: higher inorganic sediment loading due to basin erosion, and accelerated decomposition associated with higher nutrient levels. The increased inorganic sedimentation pattern apparently associated with cultivation resembles the pattern Walker (1966) attributed to drought. Likewise, the relatively organic-enriched sediments found in the South Dakota wetlands surrounded by grasslands (Martin and Hartman, 1987) resemble organic deposits that Walker (1966) associated with humid climate conditions in Iowa. Pennock and de Jong (1990) identified two key landscape parameters controlling erosion and sedimentation: plan and profile curvature. Using cesium- 137 redistribution techniques in the pothole region of southern Saskatchewan, they found erosion and deposition were largely a function of three-dimensional landscape shape. Generally, both profile convex and plan convex slopes eroded faster than did concave slopes. Observed in plan view, laterally convex shoulder and backslope positions were associated with higher rates of soil loss in arid and

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Ustic Boroll landscapes. In Udic Boroll regions, however, laterally concave backslopes tended to erode faster, apparently due to higher levels of precipitation and convergent flow patterns. Regardless of climate, the highest rates of sedimentation were found on concave footslopes. Convergent footslope positions appear to function as sediment traps. Extending this model to entire wetlands, we would expect sediments to erode from points and collect in bays. Pennock and de Jong pointed out the limitations of conventional erosion prediction models, such as the Universal Soil Loss Equation (USLE) (Wischmeier and Smith, 1978), which estimate landscape parameters based on straight-line slope length and gradient but not curvature. The USLE did not successfully predict the patterns of soil loss and sediment deposition in the PPR landscapes of Saskatchewan (Pennock and de Jong, 1990). Wetland sediments influence soil development and are important to our understanding of individual basin and regional history, but wetland soil evolution is driven more by subsurface water flow and materials transported by water through soils, rather than overland flow and deposition of clastic sediments (Arndt and Richardson, 1988).

IV.WATER QUALITY The dominant water quality considerations for wetlands in the PPR are (1) increased sedimentation from cultivation and (2) salinity associated with hydric soils, groundwater, and surface water (Neely and Baker, 1989; LaBaugh, 1989).

A. LAND-USE PATTERNSAND WATERQUALITY Movement of sediment from the uplands into lower portions of the closed catchments in the PPR has occurred continuously since deglaciation, but at rates that vary with short-term drought cycles and longer term climatic variations. Drought reduces vegetative cover and results in accelerated rates of erosion and sedimentation (Walker, 1966; Callender, 1969). Because soils in the region are inherently fertile, the upland portions of many wetland catchments are intensively cultivated, often to within the low prairie and wet meadow zones of the wetland itself. Under cultivation, vegetative cover is dramatically reduced from the native condition. After harvest the land is often plowed bare, then cultivated to prepare for the following year. The small amount of vegetative cover effectively mimics a drought condition. Increased rates of sedimentation in wetlands are often significant and sometimes dramatic (Neely and Baker, 1989). Because applied fertilizers are absorbed as N and P and move with the sediment, in-

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creased sedimentation due to management practices has been shown to lead to an increase in wetland eutrophication (Neely and Baker, 1989). Wetlands in rangeland may not be immune from the effects of management. Short-term grazing systems exhibited increased infiltration in upland portions of a wetland catchment as compared with long-duration grazing systems. Because increased infiltration in the uplands reduces runoff, and enhances and lengthens the period of groundwater discharge to the wetland, wetlands associated with short-duration management systems were ponded for longer periods than those associated with long-duration grazing in similar landscapes (Jim Kramer, Ecologist, Baukol Noonan Mining Company, Center, North Dakota, personal communication).

B. SOILLANDSCAPE AND SALINITY 1. The Influence of Hydrology

The soil landscape, a term we use to include land form morphology, sediment stratigraphy, and sediment mineralogy, interacts with climate in the PPR to produce a hydrogeologic setting that strongly influences water quality. The glaciated landscape as examined above is a mosaic of closed system catchments that vary in size, topographic position, and relationship to the groundwater. Groundwater recharge and discharge are focused on the wetland depressions occupying these catchments. Uplands between depressions are usually not involved in direct transfers of water to and from the water table because low rainfall characteristic of the region confines recharge to areas where the vadose zone is thin, e.g., in and around wetlands (Winter, 1983). The unique groundwater dynamics of the PPR result in the formation of numerous locally developed recharge-flowthroughdischarge groundwater flow systems that are isolated from regional flow systems and from each other (Toth, 1963; Mills and Zwarich, 1986). The development of groundwater mounds under upland depressions recharged by snowmelt and fresh surface runoff sets up flow systems in which groundwater movement is directed downward and laterally away from the points of recharge. Flow from point of recharge to point of ultimate discharge is often interrupted by movement into and out of brackish flowthrough wetlands intermediate in landscape position (Lissey, 1971). These wetlands can be thought of as evaporation ponds that further concentrate groundwater discharged to them. When this water reenters the groundwater flow system, it is often more saline than that received by the flowthrough pond. Ultimately the groundwater is discharged into low wetlands that are usually large, permanent, and saline. In this landscape, groundwater flowpath length from point of recharge to point of discharge can vary from meters (e.g., recharge occurring near a wetland edge) to kilometers or tens of kilometers

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when regional flow systems are considered. Residence time will vary similarly from hours or days to millennia. The resulting groundwater flowpaths associated with PPR wetlands can be very complex and are seasonally dynamic. Low relief and long slopes favor the development of long, relatively simple flowpaths whereas the steep, hummocky topography characteristic of many areas of the PPR (Bluemle, 1971) favors the development of many small local flowpaths isolated from each other and the regional flow system (Toth, 1963; Winter, 1988, 1989). Till stratigraphy, including the distribution, extent, and continuity of sand lenses and clay aquitards, can complicate groundwater movement further by redirecting flow and intensifying and focusing discharge. Flownet modeling has shown sand aquifers in particular to intensify discharge when associated with wetlands in discharge positions. The distinctive groundwater hydrology helps to explain the large range in dissolved solids found in PPR wetlands, as well as the role evapotranspiration and lithology plays in the chemistry and development of wetland salinity. Tills in the PPR are finer textured and lower in hydraulic conductivity than the tills characteristic of other regions of the United States and Canada (Winter, 1989). Higher levels of dissolved ions in groundwater have often been associated with long flowpaths and long residence times in a groundwater system due to accumulation of ions originally present in the sediments, as well as those released through weathering. Dissolved ions naturally accumulate to high levels in flowthrough and discharge PPR wetlands. Long flowpaths through poorly conductive till aquifers result in long aquifer residence times and maximal contact of groundwater with the aquifer media prior to discharge in the wetland. However, because of the nature of groundwater movement in the PPR, much of the groundwater flow occurs near the surface where evapotranspiration can actively concentrate groundwater solutions. The development of wetland salinity in the PPR is the result of evapotranspirative concentration of dilute solutions more than any other single factor.

2. Initial Chemistry of Dilute Solutions: Impacts of Lithology Geochemical characteristics of PPR till lithology impart a unique chemistry to dilute groundwater. In the PPR, dilute groundwaters [electrical conductivity (EC) < 0.5 dS/m] often have calcium-magnesium bicarbonate chemistry with high levels of sulfate; however, sulfate quickly becomes dominant in more concentrated solutions (Rozkowski, 1969; LaBaugh et al., 1987; Arndt and Richardson, 1989a). The high levels of sulfate found in dilute groundwater and surface water solutions in the PPR are the result of acid sulfate weathering processes involving specific lithologic components such as entrained Pierre Shale fragments of high-sulfur tills common in the area (Hendry et al., 1986). Unweathered tills of the PPR typically contain calcite and dolomite derived from glacial

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abrasion of calcareous Paleozoic Era bedrock exposures located north of the region, as well as locally derived Cretaceous marine shales that are high in reduced forms of organic and inorganic sulfur (Bluemle, 1971). The sulfatic groundwater chemistry results from oxidation to SO, of organic S and inorganic sulfides entrained in the shale. Concurrent calcite and dolomite dissolution to HCO,, Ca, and Mg buffers the acidity produced by this reaction, producing dilute solutions with Ca and Mg the dominant cations, and HCO, and SO, the dominant anions (Groenewold er al., 1983; Hendry et a l . , 1986; Mermut and Arshad, 1987). Several landscapes in the PPR, such as sandy outwash plains and proglacial and englacial lake beds, sandy deltaic deposits, and areas of sand dunes, have sediments whose lithology lacks the marine shale component that provides a sulfur source during weathering. The dilute groundwaters and surface waters associated with these landscapes often lack sulfate as a dominant anion and have Ca, Mg, Na, and K as dominant cations, with HCO, , CO, , and CI as the dominant anions. This chemistry results from the weathering of mafic and felsic minerals typically associated with sandy deposits.

3. The Development of Salinity in Soil and Water The composition of closed basin brines is the result of chemical changes imposed by the formation of evaporite minerals in solutions undergoing evaporation (Hardie and Eugster, 1970; Eugster and Jones, 1979). These compositional changes follow specific pathways that depend on the initial ratios of interacting ions that combine in solution to form binary salts. With further concentration, the precipitating mineral will remove both cations and anions from solution at a rate dependent on the rate of evaporation and the molar ratio of the ions in the mineral. The ion present in smaller quantity (mEq/liter basis) will remain fixed at the activity dictated by the solubility product of the mineral as the ion present in larger quantity increases in concentration, until saturation with a more soluble mineral is reached. Thus a solubility sequence of evaporite minerals whose composition is appropriate to the composition of the initial dilute solutions act as a sequence of “chemical divides,” directing the path of future chemical alterations as evaporation continues (Fig. 7).

4. Recharge Conditions In till landscapes of the PPR, wetland salinity is the result of the progressive concentration of dilute, sulfatic groundwater solutions as they move in the groundwater flow system. Pond waters and groundwaters in recharge wetlands are fresh because they result from recent precipitation events and have had minimal, short-term contact with soil and the underlying aquifer media, both of which are typically leached free of salts. In these seasonal ponds calcite and

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Chemical divides C a < C 0 3 pH>8.4 Ca>C03

I

I

Ca, Na, Mg Chlorides

I

Na and Mg Sulfates

Soda Na,C03

I

Evaporation k

Evaporites 1

4 1-

I

f

Dolomite Evaporation

I CaCO, Evaporation

First to precipitate

AT I

Solution

Figure 7 Closed basin brine sequences based on the Hardie and Eugster (1970) chemical divides.

gypsum are absent (Miller et al., 1985; Arndt and Richardson, 1988), EC values are low (Arndt and Richardson, 1988, 1989b), and the solutions are typically Ca/Mg (HCO,) type but with relatively high levels of sulfate (Arndt and Richardson 1988, 1989b), the development of which was examined above. In the absence of mineralogical controls exerted by calcite and gypsum, SO,, HCO, , Ca, and Mg concentrations are positively correlated to EC. Several chemical processes other than mineralogical controls are invoked to explain subtle variations in the dilute solutions characteristic of recharge wetlands. These controls include weathering and lithological differences, differences in ionic mobility, exchange relationships between the aquifer media and solute, and biological cycling (Rozkowski, 1969; Eugster and Jones, 1979; Last and Schweyen, 1983; Miller et al., 1985, 1989; Timpson et al., 1986; Arndt and Richardson, 1988, 1989b; LaBaugh, 1989).

5. Flowthrough Conditions Pond waters and groundwaters associated with flowthrough wetlands are brackish; however, the salinity is variable depending on the ratio of dissolved solids received by the wetland in groundwater discharge to dissolved solids lost in groundwater recharge (Sloan, 1972). As discussed above, groundwater discharging to flowthrough wetlands is a dilute Ca/Mg (HCO,) type of water with elevated levels of SO,. These dilute solutions are progressively concentrated in the hydric soil zone as well as in the pond water. Arndt and Richardson (1988,

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1989b) found that calcite is consistently present in hydric soils of wetlands identified in the field as flowthrough. Here, calcite exerts a mineralogical control on alkalinity that results in an increasing dominance of SO, with increasing EC of pore water and surface water solutions collected from flowthrough wetlands. Gypsum, a common evaporite associated with saline edges of flowthrough wetlands (Steinwand and Richardson, 1989), exerts a similar mineralogical control on the levels of Ca. The observed progressive increases in SO,, Mg, and Na dominance in pore waters and groundwaters collected from flowthrough wetlands result from eva~transpirationand calcite and gypsum precipitation. Calcite controls alkalinity, and gypsum controls Ca, as predicted by the Eugster and Jones (1979) model.

6. Discharge Conditions By definition, discharge wetlands lose little of their water to seepage outflow. Thus the brine concentration model is directly applicable to the groundw~ter discharging to these wetlands as well as to the surface water contained in them. Because of mineralogical controls exerted by calcite and gypsum earlier in the flowpath, groundwaters and pond waters associated with discharge wetlands are typically brackish to saline Mg/Na (SO,) solutions. Geochemical modeling of solution equilibrium indicates that the solutions are saturated with respect to gypsum and calcite, and gypsum and calcite are invariably present as stable evaporites in the sediments of discharge-type wetlands associated with calcareous, shaley till characteristic of the PPR. The remaining MgiNa sulfate salts that would potentially exert a control on the chemistry of saline solutions arrt extremely soluble. Arndt and Richardson (1989b) compared mineral saturation status to saturation extract EC in samples collected from recharge, flowthrough, and discharge wetlands in North Dakota. They found no evidence of mineralogical controls beyond gypsum acting to modify solution chemistry at saturation extract EC values up to 30 dSim; however, significant differences were found in the relative concentrations of Mg and Na between saline solutions collected during summer, and those collected during winter. These and similar observations have been attributed to temperaturedependent solubility differences between the stable MgiNa sulfate salts in solutions being concentrated by freezing over winter (Timpson et at., 1986; Beke and Palmer, 1989; Richardson et al. 1990). Several investiga~orshave examined the mineralogy of efflorescent crusts collected from sulfatic discharge wetlands. Thenardite (Na2S0,), epsomite (MgS0,.7H20), and konyite (Na~Mg(SO~).5H~O) are the dominant efflorescent minerals in summer, whereas mirabilite (Na,SO,lOH,O) and epsomite are dominant in salts collected during winter (Last and Schweyen, 1983; Keller et al., 1986; Timpson et al.. 1986; Henry et ul., 1084; Hendry et al., 1986; Beke and Palmer, 1989; Arndt and Richardson, 1985, 1986, 1988, 1989b, 1992, 1993). The double salt, konyite, converts to mirabilite

WETLAND SOILS OF PRAIRIE POTHOLES

147

and epsomite at temperatures below 20"C, and thenardite is stable only at temperatures above 25°C (Keller et al., 1986). Thus, during concentration by freezing over winter, the equilibrium relationships between thermodynamically stable mirabilite and epsomite need to be considered. At 30°C epsomite and mirabilite have similar solubilities. The solubility of each decreases with decreasing temperature; however, at 0°C mirabilite is one-fourth as soluble as epsomite. In freezing saline solutions, where sulfates of Mg and Na are high, mirabilite precipitates before epsomite. With further concentration, mirabilite precipitation will fix Na levels in exactly the same way that gypsum precipitation controls Ca levels. Resulting brines will become enriched in Mg and SO,.

7. Salinity as an Indicator of Climate and Hydrology Drought cycles increase wetland salinity in large regional discharge wetlands. Callender (1969) used sediment sulfate concentration in Main Bay of Devils Lake in central North Dakota to indicate extreme droughts since glaciation ended. Three such droughts have occurred that were drier than the drought of the 1930s. Conversely, Arndt and Richardson (1993) observed that in a local system, a discharge wetland that had been monitored for years changed to a recharge wetland during the 1988 drought. The end result was a decrease in salinity for this wetland. The salts were removed by the recharge waters during this drought. Steinwand and Richardson (1989) and Arndt and Richardson (1989b) had data that showed formation of calcite and gypsum with attendant increases in salinity in a series of recharge, flowthrough, and discharge wetlands in eastern North Dakota. In the flowthrough wetlands with brackish water, gypsum occurred around the entire pond except one bay. The peninsulas were highest in gypsum but the entire shoreline was saline. This increased salinity was attributed to concentration attendant on evaporative discharge. In the discharge ponds, all soils were saline and gypsiferous, but gypsum concentrations were less than those of the edge of flowthrough wetlands. Accumulations of secondary gypsum are indicative of groundwater discharge and reflect geomorphic and stratigraphic controls on water entering and leaving semipermanent ponds (Steinwand and Richardson, 1989). A small catchment area:pond surface area ratio suggests a small surface runoff impact to the total receipt of water by the pond. Water inputs include direct precipitation or groundwater discharge. Water losses are primarily by seepage or evapotranspiration. Pond edges are clearly the focus of water discharge. Salinity also reflects anthropogenic modifications of groundwater flow. Skarie et al. (1986) noted that drainage ditches can act as recharge wetlands and alter salinity around the ditch. The water table is raised locally around the ditch and moves the salt up into the soil by evapotranspiration. They observed that vegetation indicators suggested by Stewart and Kantrud (1971) predicted water permanence in the ditch.

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J. L. RICHARDSON ET AL.

Arndt and Richardson ( 1993) studied the effects of transient groundwater mounding on the accumulation and mobilization of salts within and between wetlands in the PPR of North Dakota. They examined the temporal variation in the salinity of pond water and near-surface groundwater in a variety of wetlands and through a sharp drought. The EC of groundwater collected from lysimeters in coarse-textured surface sediments was usually highest after draw down and steadily declined after recharge events. The EC values of groundwaters collected from fine-textured subsurface sediments were less variable. The greatest variation in the saturation status of calcite and gypsum was associated with the pond water and groundwater from the coarse-textured sediments that contained little gypsum or calcite. Zones that contained the minerals remained saturated with respect to them. Gypsum in particular was observed only below the zone of expected seasonal water level fluctuation. Apparently temporal and spatial salination patterns in near-surface groundwaters are dynamic and closely related to transient recharge events in semipermanent ponds. The EC and gypsum distribution patterns in both the groundwater and in soils indicate that salt mobilization can be rapid and is responsive to precipitation events. Water movement is primarily lateral through coarse-textured soils that likely formed in Holocene beaches on the edge of larger ponds. Salt accumulation or removal is sensitive to the intensity of recharge and drawdown events, especially in wetlands with coarsetextured edges.

V. WETLAND SOIL PROPERTIES Wetland soil properties that have the greatest influence on current land use include soil moisture dynamics, soil aeration, and salinity (Stewart and Kantrud, 1972; Arndt and Richardson, 1986; Fulton et al., 1979, 1986; Richardson and Arndt, 1989). Principal components analysis (PCA) is a statistical technique that reduces the number of variables in a data set by finding linear combinations of those variables capable of explaining most of the variability. Using PCA, Richardson and Bigler ( 1984) found that principal component factors reflecting salinity, organic matter accumulation, calcium carbonate dynamics, and texture, respectively in order of importance, explained most of the observed variations in PPR wetlands.

A

SALINITY

The salinity factor of principal components analysis included the amount of dissolved sulfate, Na, Mg, and electrical conductivity in saturation extracts

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(Richardson and Bigler, 1984). This combination of properties explains the majority of the variability of their study of wetlands. Richardson and Bigler (1984) attributed the salinity component to differences in groundwater flow. The longer the recharge-discharge flowpath the more the salinity accumulates in soils associated with discharge locations. Wetlands that are distant from their recharge zone tend to be more saline; wetlands in local groundwater sequences as studied by Arndt and Richardson (1988, 1989b) and Loken (1991) tend to be nonsaline near the source and become more saline in lower wetlands that lose water to evapotranspiration. Also, evaporation of edges accumulates salts as compared to other parts of the wetland (Steinwand and Richardson, 1989). This differential occurrence of evaporites is relatively consistent in PPR wetlands and could possibly assist in wetland delineation. Evaporites form in distinct sequences as noted in Fig. 7 (Hardie and Eugster, 1970; Doner and Lynn, 1977). These mineral sequences are evident at wetland edges (Arndt and Richardson, 1992). This topic was covered more extensively in Section IV.

B. ORGANIC MATTER Organic matter distribution varies more within a soil profile than between zones in wetlands (Richardson and Bigler, 1984). Profile differences within a wetland are important in that the pond centers tend to be “cumulic” (Richardson and Bigler, 1982; Arndt and Richardson, 1988). The soils of the deep marsh pond interior often have cumulic A-horizons over 1 m thick. The soils of the shallow marsh and wet meadow zones usually have only 50.5-m-thick Ahorizons. At high amounts of organic C(>0.5%) in the soil, the readily oxidizable organic matter is higher as a percentage of total C than in upland soils. When the OC is <0.5%, the readily oxidizable organic matter is lower than in upland soils (Richardson and Bigler, 1982). Richardson and Bigler believe that if the wetlands are drained, much of the organic C will oxidize and the remnant will be tightly bound to clay and will be inert. In the PPR, histic epipedons are common only in deep marsh vegetation zones; Histosols are often found in discharge wetlands low in sediment SO,. We believe that only deep marsh vegetation not subject to frequent drawdown and drying allows for organic sediments to accumulate. Anaerobic conditions in sediments of discharge wetlands with inflowing groundwater persist, promoting organic accumulation by a decrease in decomposition rates. In the PPR, oxidation of organic matter resulting from aeration during frequent drawdown and sulfate reduction during anaerobiosis consume much organic matter (Komor, 1992). From North Dakota westward, Histosols are not common except in fens. We ascribe the lack of Histosols to both frequent sediment exposure to air during drawdown and carbon mineralization during microbially mediated sulfate reduc-

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J. L. RICHARDSON ET AL.

tion. Fens seldom have a real drawdown and are low in SO,, (Malterer et al., 1986; Richardson ef al., 1987); these are the Histosols that occur in scattered locations in North Dakota and throughout the PPR (Holte, 1966; van der Valk, 1975; Vitt et al., 1975; Kratz et al., 1981; Malterer and Farnham, 1985).

C. CALCIUM CARBONATE OCCURRENCE AND FORMATION Exposure, warming, and evaporation of calcite-saturated water on discharge results in C02 outgassing that forces precipitation of calcite or aragonite (Doner and Lynn, 1977; Richardson eb al., 1987; Arndt and Richardson, 1992). Freezing can also concentrate solutions such that GaCO, can precipitate (Cerling, 1984) and can affect other minerals of the evaporite sequence as well (Arndt and Richardson, 1986). The classification of these calcareous soils into the Histosol order can be problematic because of the amount of authigenic calcite present (Richardson et al., 1987).

D. TEXTURE Unconsolidated glacial sediments are easily sorted by texture in PPR wetlands. Wave action quickly sorts out the fines and sands. Fines are transported to pond interiors whereas sands and gravels stay on the edges as lag or beach deposits (Callender, 1969; Bigler and Richardson, 1984; Arndt and Richardson, 1992). A second sorting mechanism is the translocation of clays that results in the formation of argillic horizons in recharge wetlands (Knuteson et al., 1989). Finally, a third sorting mechanism occurs by sediment segregation from the upland to pond center as observed by Walker ( I 966), Malo (1979, and Bigler and Richardson ( 1984).

VI. SOIL SEQUENCES A. FENS:HISTOSOLS A fen wetland, according to one definition, has wetland vegetation and alkaline to neutral soils and waters (van der Valk, 1975). The mineral components of the soil in fen wetlands are often plant debris, and chemical precipitates are delivered by groundwater, Frequently Ca levels are high (van der Valk, 1975). Fens are also called sedge wetlands and comprised a special category in the Stewart and Kantrud (1971) classification. Summer water levels are usually below the soil surface. In fact, many times the water levels remain below surface

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151

Figure 8 The Denbig fen from McHenry County, North Dakota formed on the edge of the Souris River entrenchment (Des Lauriers, 1990; Malterer er al., 1986). The discharge is from water bearing outwash sands and gravel.

because of the discharge or seepy nature of the water flow, even though these are clearly organic soils with constantly flowing water just below the surface. The alkaline nature results from the groundwater passing through calcareous sediments or rocks before discharging into the soil; the parent materials of the recharge area and the source rock of groundwater change these soils directly by adding ions. The vegetation is typically hydrophytic but these fens do have many indicator species that are calciphilolous and specific for fens. Because the groundwater carries dissolved constituents that precipitate in the soil zone, these wetlands are frequently called mineraltrophic wetlands (Vitt et al., 1975; Slack et al., 1980; Siegel and Glaser, 1987). The three most common fen types in the PPR are associated with wetland edge (Fig. 5C), valley edge (Fig. 8), and swales in outwash plains (Fig. 9) that start above a wetland but then flow to a typical wetland or lake. In nearby forested areas other streamlike fens are much larger (Siegel and Glaser, 1987). Frequently fens are partially open wetlands and occur in entrenched valleys or in incised landscapes. A valley edge fen, which is partially open, from McHenry County, North Dakota, is illustrated in Fig. 8. In valleys that intersect sandy or sand and gravel sediments, the edges of the valleys have seeps that become fen wetlands (Kratz et al., 1981; Malterer and Farnham, 1985; Malterer et al., 1986; Richardson et al., 1987). Valley edge fens are usually small; many are local seep zones or pockets of wetland vegetation on slopes of the valley. An outwash plain fen, illustrated in Fig. 9, is located in Kidder County, North Dakota. These fens occur in low areas in coarse-textured sediments such as glacial outwash. The water flows into these fens from the surrounding landscape and then through the fen. Stewart and Kantrud (1971, 1972) illustrate these fens

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J. L. RICHARDSON ETAL.

Cross section Figure 9 A fen formed in a swale in outwash sand and gravel deposits. Water discharges into the depression from three directions. This is based on the Tappen fen in Kidder County, North Dakota (Seelig and Gulsvig, 1988).

in Fig. 5C. Holte (1966), van der Valk (1975), and Vitt et al. (1975) observed these types of fens. The fens discussed by Siege1 and Glaser (1987) are similar but have peat uplands and the entire area of the fens is a regional discharge zone for groundwater.

I. Hydrology of Prairie Fens The three examples of prairie fens are given as models. These are diagrammed from fens located in central North Dakota in McHenry County, a valley edge (Fig. 8), and in Kidder County, an outwash plain (Fig. 9); the third fen, a wetland edge, is from the example in Stewart and Kantrud (1971) illustrated in Fig. 5C.

WETLAND SOILS OF PRAIRIE POTHOLES

153

The first fen in Fig. 8 is modeled after the Denbigh fen in McHenry County, North Dakota. The fen occurs on a slope on the edge of the Souris River and extends out onto the floodplain of the river. The fen discharges on the slope and has created a calcareous Histosol thicker than 4 m (Malterer et al., 1986). Many fens occur in sandy areas of outwash between wetlands. These sandy areas transmit water across wetland zones as illustrated in Fig. 5C. Arndt and Richardson (1988) and Steinwand and Richardson (1989) report on such a fen. The soil characteristics reflect local changes in materials and hydrology. The Kidder County fen lies in a swale in an outwash terrace near Tappen, North Dakota. In Fig. 9 we sketch the general water table configuration of the wetland in an east-west cross-section. The water discharges into the low swale from the north, east, and west and flows slowly south through the fen. These soils are calcareous Histosols. In fens, small bodies of open water frequently occur in long stringlike pools. Several scientists have noted these patterns or zones in fen vegetation of the PPR (Vitt et al., 1975; Holte, 1966; van der Valk, 1975; Slack et al., 1980; Siegel and Glaser, 1987). Vitt et al. (1975) and Slack et al. (1980) call these pools “flarks.” These areas, with flarks separated with strings or long mounds of vegetation stretching between pools of open water, appear as slow-moving streams in the lower portions of peat landscapes (Siegel and Glaser, 1987). These occur on the slope stream side of the fen that is away from the discharge points (van der Valk, 1975). Vitt et al. (1975) noted that as the water exits the ground and travels in the fen, the water becomes influenced by plants. The water tends to lose the minerals and nutrients and results in a more acidic soil, or “poor fen.” Apparently wealth is implied by the abundance of Ca: calcareous fens are super rich, high-Ca fens are rich, and low-Ca fens are poor. The next step would be acid bogs and would be a sign of penury; to our knowledge this does not occur in the prairies.

2. Calcareous Fens Calcareous fens with calcareous Histosols have been reported in Iowa, Minnesota, North Dakota, and the prairie provinces of Canada (Stewart and Kantrud, 1972; van der Valk, 1975; Vitt et al., 1975; Slack et al., 1980; Malterer and Farnham, 1985; Malterer et al., 1986; Richardson et al., 1987). This type of fen is formed by organic matter accumulation in wetlands fed by discharge waters that contain exceptionally high amounts of both Ca and bicarbonate ions. Due to low acreage and agricultural unsuitability, the classification and characterization of calcareous fens have been neglected. Their unique environmental conditions and broad geographic occurrence plus the number of rare and endangered plants make these wetlands and wetland soils important to heritage preservation programs. We admit to an exceptional fondness for these wetlands.

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J. L. RICHARDSON ETAL.

3. Mounds in Fens Holte (1966) and van der Valk (1975) in their description of fens in Iowa noted elevated mounds of vegetation they called “discharge cones” that result from concentrated groundwater discharge under the mound (Holte, 1966). Similar features called peat mounds have been described by Kratz et af. (1981) in Wisconsin. Kratz et af. (1 98 1) had abundant piezometric data that conclusively demonstrate that these are discharge sites.

4. Fens across Zones in Semipermanent Ponds Arndt and Richardson (1989b, 1992) and Steinwand and Richardson (1989) noted that the general geochemistry of a wetland differs at the point of discharge of small aquifers on the edge of semipermanent ponds (illustrated in Fig. 5C). Steinwand and Richardson (1989) noted the general increase of calcite and a decrease in gypsum in the fen zone of a saline pond. These aquifer discharge points around ponds are readily identified by the fenlike vegetation located in an area that contrasts to the typical concentric vegetation zones.

B. RECHARGE, FLOWTHROUGH, AND DISCHARGE WETLAND SOILS 1. Soils in Recharge Wetlands Redmond and McClelland (1959), Miller et al. (1985), Mills and Zwarich (1986), Hubbard et af. (1987), Amdt and Richardson (1988, 1989b), and Knuteson et al. (1989) studied soil development in ephemerally, temporarily, and seasonally ponded wetlands. Wetlands with these water permanence regions are usually associated with groundwater recharge in the PPR. During the spring, “depression-focused” (Lissey, 1971) melt water flows into recharge wetlands. Redmond and McClelland (1959) observed that frost in the soil accentuates overland flow by reducing percolation through the frost layer. Water that collects in the wetlands to form transitory ponds quickly infiltrates the soil and recharges the groundwater. As the water percolates through the soil, it leaches the solum free of carbonates and readily soluble ions. The soil frequently dries later in the summer. Any reflooding of dry wetland soils by summer storms produces only brief ponding events as the water rapidly infiltrates the pond bed. As stated in Soil Taxonomy (Soil Survey Staff, 1975), these are nearly ideal wetting and drying conditions needed for the formation of an argillic horizon. Richardson (1989) observed that in central North Dakota, calcareous tills have upland soils lacking argillic horizons but the small, nearby wetlands contain soils with well-

WETLAND SOILS OF PRAIRIE POTHOLES

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expressed argillic horizons. Therefore, soils that are characteristic of recharge wetlands are leached free of carbonates, and usually possess argillic horizons. Miller et al. (1985) found that groundwater flow and water table depth influenced wetland soils in a semiarid landscape near St. Denis, Saskatchewan. They attributed the formation of nonsaline, leached Argiaquolls in upland depressions to depression-focused recharge. These soils had deep sola and frequently contained eluvial horizons. Ponds edge soils were Calciaquolls and had groundwater mounds in the springtime. During the spring, solute-enriched groundwater apparently discharged into the wetland, forming a pond. Later in the summer, evapotranspiration caused carbonates to precipitate along the pond edge. These sites often had “willow rings” similar to those studied by Meyboom (1967) that caused water table depressions at the pond edges by evapotranspiration. Hubbard et al. (1987) investigated the soils in several PPR wetlands in subhumid eastern South Dakota. Seasonal ponds had leached interior soils with argillic horizons (Argiaquolls), and were ringed with Typic Calciaquolls. They observed two temporary wetlands that had the Argiaquolls in the center but the edges were Typic Haplaquolls that lacked carbonates in the A-horizon. In their study, the larger and more permanent wetlands were not recharge basins and had far different soils. Our point is that in seasonal and temporary ponds in subhumid and semiarid regions, groundwater recharge conditions prevail that produce an environment favorable for the formation of Argiaquolls or Argialbolls, These soil types were observed by Arndt and Richardson (1988, 1989b) and Loken (1991) in recharge wetlands within eight different North Dakota landscapes; their observations support the Miller et al. (1985) and Hubbard et al. (1988) studies. Knuteson et al. (1989) studied the formation of two adjacent soils, a Typic Argiaquoll and an Aeric Calciaquoll, found on the flat Glacial Lake Agassiz plain in North Dakota. Corroborating Hubbard et al. (1987), they observed that a leached recharge soil forms under ephemeral or seasonal ponds, where alternating wetting and drying conditions produced an argillic horizon. The Typic Argiaquoll, formed in a recharge wetland, had ponded water in the springtime and a near-surface water table throughout the years of the study. The bottom of the argillic horizon appeared to remain saturated throughout the year and was observed to contain ferrous iron; this indicates active reducing conditions within a hydric soil environment (Daniels et al., 1961; Richardson and Hole, 1979; Childs 1981; Childs and Clayden, 1986; Richardson and Daniels, 1993). The Bk horizon of the Aeric Calciaquoll contained magnesium calcite. Most of the carbonate found in the parent material was dolomite. Knuteson et al. (1989) modeled a set of flow processes (Fig. 3a) to explain the genesis of the spatially associated but contrasting argillic and calcic horizons. They suggested that ponding raises the water table under the pond to form a groundwater mound, an idea also demonstrated by Meyboom (1967), Hendry (1982), and Miller et al. (1985). Saturated water flow is downward and outward

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J. L. RICHARDSON ETAL.

from the mound, leaching the soil and recharging the local groundwater (Fig. 3a and b). Knuteson et al. (1989) measured an upward soil water flux from late spring through the summer in these ponds. The lower matric potential of the dry soil surrounding the saturated mound results in outward and upward unsaturated flow that encircles the leached soil of the pond center with a ring of secondary carbonate. By their calculations, a Bk-horizon should take 4000 years to form under these conditions. The enrichment rate is about 0.2 mol m-2 year- I in the soils studied. The flow systems are illustrated in Fig. 3a and b. Arndt and Richardson (1989a,b) observed flow reversals in recharge ponds. When the ponds were flooded, downward saturated flow removed most of the soil solutes. However, in the fall, the surface soils had higher salinities than the Bt-horizon, indicating a seasonal reversal of solute transport by upward unsaturated flow. Mills and Zwarich (1986) illustrate this movement below their temporary wetland with a flownet based on tensiometer and piezometer data (Fig. lo). The upward flow illustrated by the flownet clearly shows that ions should move in that direction.

102

-

3

g '0

95

2

94

m

iil

93

-

' 099.32

).29

-

I

3

Piezometer or tensiometer.. . 0

98.9

I I I

;

993 099.31

Water table ................... Equipotential line.. ...........,96"

91

0

10

20 30 40 50 60 70 80

90 100 120 130 140 150 160 170 180 190

Vertical distance (M)

Figure 10 An equipotential net (after Mills and Zwarich, 1986) with a recharge depression and a flowthrough depression (in the right). The equipotential lines include saturated and unsaturated flow. The high between the two is an area of evapotranspiration-driven discharge. Flow in the depression to the right is lateral or flowthrough.

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In subhumid and semiarid landscapes, wetlands that recharge the groundwater are expected to be more numerous than discharge types (Redmond and McClelland, 1959; Lissey, 1971; Sobecki and Wilding, 1982; Knuteson et al., 1989; Richardson et al., 1991, 1992). Miller et al. (1985) clearly demonstrated that in Ustic moisture regimes, water that recharges the water table leaches those soils and promotes the formation of an argillic horizon. Richardson et al. (1991) believe that in humid regions recharge wetlands are unusual, they are common in subhumid areas, and they are the most abundant wetland type in semiarid regions based on the soil morphology. Soil morphology reflects the hydrology and is the most inexpensive method of determining the long-term hydrology of a site. Seelig et al. (1991) presented evidence that recharge with small quantities of sodium lead to the development of “Solods.” The dominant process is recharge but late-season evaporation returns some salt back to the soil. Therefore they are more complex than simple recharge wetlands.

2. Soils of Flowthrough Wetlands Bigler (1981), Bigler and Richardson (1984), and Richardson and Bigler ( 1984) studied selected physical and chemical properties of flowthrough wetland

soils. Typically, flowthrough wetlands are semipermanent ponds with two distinct soil regions: the interior soils located in the deep marsh or open water zones and the soils of the edge in the wet meadow and shallow marsh zones. The interior soils usually classify as fine smectitic (calcareous) frigid Cumulic Haplaquolls. Minor variants that occur are (1) more sand or silt toward the pond edge and hence fine-loamy or fine-silty soils in the shallow marsh, (2) distinct stratification in the cumulic A-horizon creating a Fluvaquentic subgroup, and (3) noncalcareous soil profiles. Overall, the deep marsh and open water zones have less variability than the shallow marsh or wet meadow, which, texturally, can range from beach gravels to fine textures. Mollic and Typic Fluvaquents; Typic, Fluvaquentic, and Cumulic Haplaquolls; Typic Calciaquolls; and Typic Natraquolls all have been reported for the wet meadow and shallow marsh zones of semipermanent flowthrough wetlands (Bigler and Richardson, 1984; Hubbard et al., 1987; Arndt and Richardson, 1988). Hubbard et al. (1987) observed that all semipermanent ponds in their South Dakota study had Cumulic Haplaquolls in the interior regions with Calciaquolls or, occasionally, Natraquolls on the edges. The soils in the pond interior of a flowthrough wetland are relatively homogeneous because receipt, loss, alteration, or redistribution of sediment and secondary minerals is minimal. The semipermanent pond interior tends to stay wet, has slowly moving lateral water flow, and receives small amounts of dissolved material. Only growing plants and occasional influx of sediment exert change on the soil. The process of building a thick A-horizon is strong because organic C accumulates due to saturated, reduced conditions. In flowthrough wetlands, the water tables are relatively stable, and the water is usually brackish throughout

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the year. A generalized pattern of water table fluctuation is illustrated in Fig. 11. The wet meadow is nearly saturated to the surface in June, then the water table drops until late fall. The shallow marsh is usually ponded through June. The deep marsh soils in Fig. 11, both sites 3 and 4, are typically ponded through summer and remain very wet year round. The open water zone is typically flooded throughout the year. Richardson and Bigler (1984) observed an interaction between organic carbon, soluble Ca, and calcium carbonate equivalent (CCE). When aggregated into a principal component, these data explained about 22% of total variation in their study. Soil organic matter, which decreases steadily with depth, reflects the incorporation of biogenic materials, especially plants, into the mineral soil. Biological replenishment continually adds fresh material to the soil surface. Organic matter is lost in these water-logged soils at depth by slow microbial decomposition. The organic carbon is highest at the surface and the CCE is lowest at the surface. Oxidation of organic carbon produces CO, that would form carbonic acid and dissolve the calcite (Doner and Lynn, 1977; Arndt and Richardson, 1992). Soluble Ca is high at the surface [averaging 1.9 cmol( +) kg-'1 compared to deeper Il.0 cmol(+) kg-'1 in the profile (Richardson and Bigler, 1984). Near-surface soil solutions, relatively enriched in CO, , dissolved calcite near the surface; precipitation of calcite occurred at depth where there

I 2 WET MEADOW

SHALLOW MARSH

I 3 DEEP MARSH

I 4 DEEP

MARSH

I

5 o PEN

WATER

Figure 11 Water table levels at five vegetation zones in a flowthrough wetland recorded from June through October 1978, a typical year for the area of central North Dakota. Mean values from four flowthrough wetlands are used (after Bigler, 1981). The deep marshes at sites 3 and 4 have cattail (Typha sp.) and bulrush (Scirpus sp.), respectively.

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Table I1 Selected Data for Deep Marsh and Open Water Zones" Zone Deep marsh

Deep marsh

Deep marsh

Depth (cm)

Clay (%)

OrganicC CCEb (%)

(%)

0- 15 15-30 30-45 45-60 60-75 75-90 0- 15 15-30 30-45 45 - 60 60-75 75-90 0- 15 15-30 30-45 45-60 60-75 75- 90

44 45 39 41 35 34 44 44 43 40 47 46 42 42 42 42 47 46

3.1 1.7 1.3 1.1 0.9

14 15 15 16 13 13 12 14 16

1.1

6.0 2.3 1.3 1.5 1.2 I .2 6.5 3.5 2.6 1.8 1.4 1.1

16

14 12 I1 14 15 15 14 13

ECC (dS/m) 9.5 8.5 1.9 7.8 7.8 8.4 8.5 7.8 7.0 7.3 7.0 7.0 7.0 6.0 5.6 3.9 3.6 3.5

"From Richardson and Bigler (1984). 'CCE, Calcium carbonate equivalent. 'EC, Electrical conductivity of saturation paste extracts.

was less dissolved carbon dioxide. Whittig and Janitzky (1963) proposed a similar process. Texture does not vary much with depth in the deep marsh and open water soils (Table 11). These soils contrast with the wet meadow and shallow marsh soils that are textually heterogeneous. Edges of the ponds receive overland flow. The receipt of coarser particles on the edge and the removal of fines to the center create a distinct textural difference. Richardson and Bigler (1984) calculated that about 15% of the total variation in their wetland study was due to the textural variations between edge and interior soils. Salinity factors used by Richardson and Bigler (1984) explained 48% of the variation and was undoubtedly the most important component in their study. Flowthrough wetlands are usually brackish but can range from fresh to saline. Salinity then can be related to water flow. As dissolved salts become more concentrated as a result of evapotranspiration, the soil salinity increases. Individual profiles in a single flowthrough wetland may have highly contrasting salinities because some areas receive groundwater discharge and other areas recharge the groundwater.

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In Fig. 10, Mills and Zwarich (1986) illustrate various conditions of recharge and discharge in a flowthrough wetland using a flownet. In Fig. 10 the topographic high between the depressions has dominantly unsaturated flow that moves upward through the vadose zone. Solutes are carried to and precipitated near the land surface in this area of Calciaquolls. In the large depression on the left in Fig. 10, a flowthrough wetland with widely spaced perpendicular equipotential lines illustrates slow lateral flow. The upward unsaturated flow between two depressions occurred throughout the year even though the recharge basin had saturated downward flow and unsaturated upward flow. The flowthrough wetland had lateral flow that reversed direction. Between the recharge and flowthrough basins groundwater flow was upward all year except in early spring. We believe the data of Mills and Zwarich (1986) and Arndt and Richardson (1988, 1989b) indicate that soil salinity and evaporite mineralogy can be inexpensive field and laboratory alternatives to ascertain hydrologic functions in prairie wetlands.

3. Soils of Discharge Wetlands Other Than Fens Discharge wetlands in western semiarid regions are usually saline and hypersaline (Last and Schweyen, 1983). Southern Saskatchewan and portions of adjacent Alberta, North Dakota, and Montana have hundreds of saline and hypersaline lakes that range in size from minute to over 300 km2. Last and Schweyen (1983) distinguished two wetland categories based largely on size: playas and perennial lakes. Playas, as described here, are discharge-type PPR wetlands that are intermittently ponded. These small, closed basins are commonly elongate or riverine in shape and are usually associated with outwash channels or collapsed outwash channels. Playas and saline lake brines are mostly the Na, Mg, and SO, types, however, some lakes are rich in C1 and bicarbonate. The most significant soil-related processes include (1) cyclic flooding and desiccation of the surface, (2) formation of salt crusts, (3) wind displacement of salts, and (4) periodic detrital sedimentation (Last, 1984). Over time the amount of evaporites that have aggraded in these playas and lakes is large. Last (1984) has observed over 40 m of salt in some basins. In subhumid areas of the PPR, large saline lakes decrease in number but are still common. Most of these discharge wetlands vary from saline to hypersaline, but several are brackish. Devils Lake in northwest North Dakota is an example of a large, brackish lake; however, during the period 1930 to 1945, it was saline. Both salinity and lake level have fluctuated considerably since the basin formed at the end of the Pleistocene (Aranow, 1955; Callender, 1969). The soils that occur in the Devils Lake basin reflect relatively recent fluctuations in lake levels. In Ramsey County, North Dakota, for instance, the higher beaches have watersorted, gravelly sand Entisols (Bigler and Luidahl, 1986). Immediately below

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the high beaches lies a wave-cut zone with Aeric Haplaquents of medium to coarse textures over till. These soils support mostly range grasses, or are wooded. The senior author believes that water flows laterally over the till through the coarser sediments, creating the aquic soil moisture regime. Use for crop land is limited, as suggested by the soil series name (Mauvais, which is French for bad). Below the Mauvais soils, the slope gradient of the lake edge usually is much lower. The next lower surface is occupied by Psammaquents. Below this surface, or in the gently sloping bays, is a silty-clay Typic Fluvaquent. The lake has created a sedimentary sequence of surfaces and soils. Boone County, Iowa has roughly equal amounts of precipitation and evapotranspiration (Andrews and Dideriksen, 1981). In humid regions, water tables follow the topography; every depression receives water by discharge from the adjacent topographic high (Richardson et al., 1991). These discharge wetlands and their edges are calcareous and nonsaline. Relatively dilute groundwater, flowing short distances in calcareous but nonsaline parent materials, may produce a Bk-horizon. In the county nearly all soils of wetlands or wetland edges are calcareous, or carbonates exist within .05 m of the surface. In fact, 38% of the mapped soils in Boone County are poorly drained or wetter (Andrews and Dideriksen, 1981). Only one-quarter of these soils is noncalcerous and all are in poorly drained mapping units, not very poorly drained units. Clearly, the soils reflect the removal of carbonate from the higher positions and relocation to the wetter low areas.

VII. SOILS ON PRAIRIE POTHOLE EDGES In the PPR, three soil types have been observed in nonfen wetland edges: (1) the Calciaquoll without salinity and gypsum, (2) the Calciaquoll with salin-

ity and gypsum, and (3) the Natraquoll. The Hardie and Eugster (1970) closed basin brine evolution model helps explain the three different soil types. Edgefocused ground water discharge driven by high evapotranspiration rates promotes precipitation of CaCO, . The edge soil produced may be free of gypsum and more soluble salts. This soil type, illustrated as a type I11 edge in Fig. 12C, is formed around recharge wetlands and discharge wetlands having short flowpaths and parent material that lacks high concentrations of sodium and magnesium sulfates. After the calcite is precipitated, the solution may contain significant concentrations of either Ca or carbonate (Fig. 7). If more carbonate dominates, the pH will be elevated. In these conditions, Eugster and Jones ( 1 979), De Deckker and Last (1988), and Last (1990) observe that dolomite occurs in the sediments and

Wetland

B

Highest ET losses

Nonwetland

Wet edge effect type I I Wetland ET losses TYPha

White-topped ..

Grass

Highest ET losses

Nonwetland

t

FeS

Wetland

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soils, provided that the Mg:Ca ratio exceeds 8. Sherman et al. (1962) observed Bk-horizons dominated by dolomite in western Minnesota. We revisited many of these sites and determined that these horizons are dolomite dominated. Nearly all occur in regional discharge zones with underlying sands in Ca-depleted but Mg-rich soil solutions. Rostad (1975) observed aragonite in the marl of two wetlands in Saskatchewan. He also associated dolomite with the aragonite in a saline wetland. He believes aragonite converts to dolomite, not calcite, if the Mg content is high. De Dekker and Last (1988), in their study of Canadian and Australian lacustrine sediments and soils, further note that aragonite occurs as a stage in the calciteto-magnetite sequence of carbonates as Ca becomes depleted. These soils develop sodium carbonate in the salt crusts that form at the soil surface (Eugster and Jones, 1979; Keller e f a l . , 1986); the mechanism may be as described by Whittig and Janitzky (1963) (edge type I, Fig. 12A). If, on the other hand, the Ca is higher than carbonate in the solution after the initial carbonate precipitates, a gypsiferous saline soil forms (Steinwand and Richardson, 1989) (edge type 11, Fig. 12B). Arndt and Richardson (1993) observed that seasonality and climatic cycles are included in PPR hydric soil salinity.

VIII. CONCLUSIONS AND FUTURE WORK Much is still unknown about several important aspects of prairie wetland soils, ecology, biogeochemistry, and hydrology. We feel that needed research falls into four main categories: 1. Interdisciplinary research into wetland systems. More information is needed on how the geologic, hydrologic, and biologic components of prairie wetland systems interact on the landscape and within the prairie ecosystem. The importance of developing interdisciplinary research on several scales cannot be overemphasized. We have shown that groundwater hydrology coordinates hydric soilforming processes and vegetation-soil interactions within and between zones in a wetland, between wetlands in a recharge-flowthrough-discharge system, and between local and regional flow systems. In other words, we know that prairie wetlands are organized into landscape-oriented hydrologic systems that interact

Figure 12 (A) The edge of the wetlands with natric soils (after Whittig and Janitzky, 1963) is type I. (B) The gypsum-rich, saline edge Calciaquolls (after Steinwand and Richardson, 1989) are illustrated as type 11. (C) The third type (after Knuteson e t a / . , 1989; Arndt and Richardson, 1989a.b) is the short-flow or low-solute condition, illustrated here as type 111.

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at scales that range from interpedon to regional. Management and land use in the PPR is intensively agricultural, yet little information exists that examines the effects of management of lower level system components on higher level systems. For example, the controversy initiated by the swampbuster provisions of the 1985 farm bill indicates that drainage of seasonal recharge-type wetlands is desired by many landowners and is likely the most immediate of anthropogenic impacts on PPR wetland ecosystems. Yet the impacts of drainage of recharge wetland components on the hydrology and wetland functions of the affected recharge-flowthrough-discharge system are unknown. Hubbard et al. (1987) suggest that potential lowering of local and regional water tables produced by extensive drainage may increase drought severity in the PPR and result in longterm deterioration of overall agricultural productivity, but little concrete evidence exists. More specifically, we know that sedimentation in prairie wetlands is a natural process that can be accelerated by management decisions regarding cropping and grazing practices within the wetland catchment. Yet the magnitude of increased sedimentation and the influence of sediment chemistry and nutrient loading on wetland biota and hydrologic function are virtually unknown, let alone related to specific management practices. Even though many people acknowledge the severity of this problem, much of the evidence is anecdotal at best. If society determines that PPR wetland systems are an essential component of the prairie landscape that require preservation and management, we must produce the research that provides the knowledge to incorporate effectively these wetland systems into the whole management picture. 2 . Biogeochernistry of C , N , and S. Climatic and geologic factors have combined to produce a unique hydrogeology in the northern Great Plains that has implications for C, N, and S cycling in wetlands. Hydric soil characteristics in PPR wetland systems strongly influence and are influenced by interactions between microbiotic and macrobiotic communities; fluctuations in the movement, quantity, and quality of water; redox chemistry; and landscape controls. The edaphic factors involved include one of the strongest natural buffers (the calcite-gypsumwater system); organic matter decomposition (type, rates, and A-horizon development); C, N, and S partitioning into gas, solute, and inorganic and organic solid phases; and system-wide pH and Eh controls. All involve interactions between biotic communities, mineral equilibria, and hydrologic factors. SO, reduction and P o x i d a t i o n in particular are thought to be important in C and S cycling in PPR wetlands (Arndt and Richardson, 1993);however, the importance of S redox chemistry in the hydrochemical context of PPR wetland systems has not been investigated. 3 . Landform-oriented hydrogeologic studies. The PPR contains many distinct glacial landscapes, including hummocky stagnation moraine of fine-loamy to coarse-loamy till, rolling drift prairie consisting chiefly of dense lodgement till,

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and extensive sandy outwash plains, deltaic deposits, and finer textured lacustrine plains of low relief. In spite of the considerable variation in texture, lithology, and topography represented by these landscapes, wetland research in the PPR has not specifically recognized landform as an important controlling factor. In fact, much of the research reported here and in recent reviews has emphasized wetlands emplaced in hummocky, high-relief stagnation moraine. More research is needed that compares and contrasts pedologic, hydrologic, and biologic characteristics of wetlands stratified by these distinct landforms to assess the magnitude and type of differences in wetland function. 4 . Improved methodology.Recent advancesin computersimulationsof groundwater movement, solute transport, and solute-sediment interactions have provided powerful research tools to model the hydrogeologic complexity of PPR wetland systems. Several field-scale applications of such techniques have recently been reported; however, more are needed that stress model verification.

ACKNOWLEDGMENTS We acknowledge the helpful suggestions given to this manuscript by B. D. Seelig, R. B. Daniels, and Dan Hubbard.

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ShjeBo, J. B. (1968). Evapotranspiration and the water budget of prairie potholes in North Dakota. Hydrology of prairie potholes. Geol. Sum. Prof. Pap. ( U . S . ) 585-B. Siegel, D. I., and Glaser, P. H. (1987). Groundwater flow in a bog fen complex, Lost River peatland, northern Minnesota. J . Ecol. 743-754. Skarie, R. L., Richardson, J. L . , Maianu, A,, and Clambey, G. K. (1986). Soil and groundwater salinity along drainage ditches in eastern North Dakota. J. Environ. Qua/. 15(4), 334-340. Slack, N. G.. Vitt, D. H., and Horton, D. G . (1980). Vegetation gradients of minerotrophically rich fens in western Alberta. Can. J . Bor. 58, 330-350. Sloan, C. E. (1972). Ground-water hydrology of prairie potholes in North Dakota. Geol. Surv. Prof. PUP.( U . S . )585-C. Sobecki, T. M., and Wilding, L. P. (1982). Calcic horizon distribution and soil classification in selected soils of the Texas Coastal Prairie. Soil Sci. SOC.Am. J . 46, 1222- 1227. Soil Survey Staff (1975). Soil taxonomy. U . S . .Dep. Agric., Agric. Handb. 436. Steinwand, A . L., and Richardson, J. L. (1989). Gypsum occurrence in soils on the margin of semipermanent prairie pothole wetlands. Soil Sci. Soc. Am. J . 53, 836-842. Stewart, R. E., and Kantmd, H. A. (1971). “Classification of Natural Ponds and Lakes in the Glaciated Prairie Region,” Resour. Publ. 92. Fish Wildl. Serv., U.S. Dep. Inter., Washington, DC. Stewart, R. E., and Kantrud, H. A. (1972). Vegetation of prairie potholes, North Dakota in relation to quality of water and other environmental factors. Geol. Surv. USGS Prof. Pup. (U.S.)

585-D. Timpson, M. E., Richardson, J. L., Keller, L. P., and McCarthy, G. J. (1986). Evaporite mineralogy associated with saline seeps in southwestern North Dakota. Soil Sci. Soc. Am. J . 50,490-493. Toth, J. (1963). A theoretical analysis of groundwater flow in small drainage basins. Proc. Hydrol. Symp. 3,75-96. Ulrich, R. (1949). Some physical changes accompanying prairie, wiesenboden, and planosol soil profile development from Peorian loess in southwestern Iowa. Soil Sci. Soc. Am. Proc. 14, 287-292. Ulrich, R. (1950). Some chemical changes accompanying profile formation of the nearly level soils developed from Peorian loess in southwestern Iowa. Soil Sci. Soc. Am. Proc. 15, 324-329. van der Valk, A., ed. (1989). “Northern Prairie Wetlands.” Iowa State Univ. Press, Ames. van der Valk, A. G . (1975). Floristic composition and structure of fen communities in northwest Iowa. Proc. Iowa Acad. Sci. 82(2), 113- 118. Vitt, D. H.. Achuff, P., and Andms, R. E. (1975). The vegetation and chemical properties of patterned fens in the Swan Hills, north central Alberta. Can. J . Bor. 53, 2776-2795. Walker, P. H. (1966). Post glacial environments in relation to landscape and soils on the Cary Drift, Iowa. Iowa. Agric. Exp. S m . , Res. Bull. 549, 838-875. Whittig,.L. D., and Janitzky, P. (1963). Mechanisms of formation of sodium carbonate in soils 1. Manifestations of biologic conversions. J . Soil Sci. 14, 322-333. Wilding, L. P., Odell, R. T., Fehrenbacher, J. B., and Beavers, A. H. (1963). Source and distribution of sodium in Solonetzic soils in Illinois. Soil Sci. SOC.Am. Proc. 27,432-438. Winter, T. C. (1983). The interaction of lakes with variably saturated porous media. WaferResour. Res. 19, 1203-1218. Winter, T. C. (1986). Effect of ground-water recharge on configuration of the water table beneath sand dunes and on seepage in lakes in the sandhills of Nebraska. J . Hydrol. 86,221 -37. Winter, T. C. (1988). A conceptual framework for assessing cumulative impacts on the hydrology of nontidal wetlands. Environ. Manage. 12, 605-620. Winter, T. C. (1989). Hydrologic studies of wetlands in the northern prairie. In “Northern Prairie Wetlands” (A. van der Valk, ed.), pp. 16-54. Iowa State Univ. Press, Ames. Winter, T. C. (1992). A physiographic and climatic framework for hydrologic studies of wetlands.

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In “Aquatic Ecosystems in Semi-arid Regions” (R. D. Robarts and M. L. Bothwell, eds.), Natl. Hydro]. Res. Symp. Ser. 7, pp. 1277-148. Environment Canada, Ottawa, Ontario. Winter, T. C., Benson, R. D., Engberg, R.A , , Wiche, G. J., Emerson, D. G . , Crosby, A. A,, and Miller, J. E. (1984). Synopsis of ground-water and surface-water resources of North Dakota. Geol. Surv. Open-File Rep. (U.S.) 84-732. Wischmeier, W. H., and Smith, D. D. (1978). Predicting rainfall erosion losses: A guide to conservation planning. U.S. Dep. Agric. Sci. Educ. Admin., Agric. Handb. 537. Zaslavsky, D., and Sinai, G. (1981). Surface hydrology: 111. Causes of lateral flow. J . Hydraul. Div., Am. SOC. Civ. Eng. 107,35-52. Zoltai, S. C. (1988). Wetlands environment and classification. In “Wetlands of Canada” (C. Tarnocai, Chairman), National Wetland Working Group, Ecol. Land Classification Ser., No. 24, pp. 1-26. Environment Canada, Ottawa, Ontario.

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NEWDEVELOPMENTS AND PERSPECTIVES ON SOIL POTASSIUM QUANTITY/~NTENSITY RELATIONSHIPS V. P. Evangelou, Jian Wang, and Ronald E. Phillips Department of Agronomy University of Kentucky Lexington, Kentucky 40546

I. Introduction 11. Electrochemical Considerations 111. Quantity/Intensity k Fundamental Basis of Q/I B. Gapon Q/I Interpretation N. Basis of Molecular Interpretation of Quantity/Intensity A. Gapon-Derived Q/I Parameters B. Vanselow-Derived Q/I Parameters C . Interrelationship between & and Kv D. Influence of Anions E. Ternary Exchange Systems F. Exchange Reversibility V. Rapid Approaches for Quantity/Intensity Determinations A. ISE Theory and Its Applications B. Q/I Measurements VI. Experimental Observations and Future Quantity/Intensity Applications A. Experimental Observations B. Future Applications References

I. INTRODUCTION The term available as applied to nutrients is vague with respect to plants and other forms of life in soil systems. Some investigators refer to available nutrients as those that are extractable by a given extractant, such as the well-known ammonium acetate extraction, under a given set of conditions. These conditions include the ratio of soil to extractant, concentration of extractant, pH of extrac173 Adwaxes in Agronomy, Yolumr J2

Copyright 8 1994 by Academic Press, Inc. All rights of reproduction in any form reserved.

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V. P. EVANGELOU ET AL.

tant, and shaking period. This definition of an available nutrient also has empirical meaning when determinations by extractions are calibrated with yield components or nutrient uptake from extensive observations of field studies. Other researchers consider an available nutrient as representative of that quantity of a nutrient that is added to a soil, and which may be removed from the soil by a plant at any time when it is needed. The latter definition implies that the availability of a nutrient to a plant is completely independent of all other nutrients present in the soil, and, therefore, differential removal of these nutrients with respect to time from the soil would have no influence on the availability of the added nutrient. This definition has neither theoretical nor experimental support at the field level or at the excised root level (Epstein et al., 1963; Maas, 1969; Beckett, 1972; Doll and Lucas, 1974, and references therein; Le Bot et al., 1990). There is evidence in the literature demonstrating that the uptake of K + , for example, from soil and/or solution culture is dependent on the concentration of Ca2+ and/or Mg2+ (Elzam and Hodges, 1967; Maas, 1969; Zandstra and MacKenzie, 1968; Stout and Baker, 1981). Furthermore, there is evidence indicating that, for the range of Ca concentrations encountered in the soil solution (Adams, 1971; Curtin and Smillie, 1983), the uptake of K+ and Ca2+by excised roots appears to be competitive (Maas, 1969). In other words, K+ uptake is inhibited in the presence of increasing concentrations of Ca2+.Similar competitive effects have been demonstrated with increasing solution proton activity (Schofield, 1949). Woodruff (1955a,b) defined available nutrients in terms of free energy. Free energy is that which dictates the relative ease with which an ion will move from the solid phase (soil) to the soil solution phase, the surface of the plant root, or the surface of any soil organism. This potential energy of ions to move in the soil-soil solution media is described by the electrochemical (EC) potential /..&bEC, where b denotes any ion. This potential energy can, therefore, be viewed as the driving force of chemical reactions in soil. A number of investigators (Beckett, 1972; Olsen, 1968; Nye and Tinker, 1977) point out that the parameter /..&bEC has only theoretical meaning because it cannot be verified experimentally. However, Woodruff (1955a,b) pointed out that in soils one is often interested in the difference between the electrochemical potential of any two ions in the solid phase, where the majority of the plantavailable nutrients reside, instead of the absolute magnitude of the electrochemical potentials of the two ions involved. The difference in electrochemical potential between any two ions in the soil can be quantified employing the chemical equilibrium concept. On the basis of K-Ca exchange equilibria considerations, Woodruff (1955b) concluded that the term RT ln[(aK)l(ac,)”2],where R is the universal gas constant and T is temperature, is related to one chemical equivalent of potassium in

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS

175

the standard state replacing one chemical equivalent of calcium on the clay followed by the liberation of “certain calories of energy.” Woodruff (1955b), by selecting soil samples from different agricultural fields and calculating the AG (free energy change accompanying K-Ca exchange) from K+ and Ca2+analyses of the saturation extract, observed that when the AG values ranged between - 10.5 and - 12.5 kJ per equivalent there was a suitable balance between K + and Ca2+to provide an adequate supply of K and Ca to plants. He also concluded that AG values in the range of - 14.6 to - 16.7 kJ per equivalent were associated with K deficiencies, whereas AG values less than - 8.4 kl per equivalent were associated with excessive amounts of K+ in relation to the amount of Ca2+ present. Woodruff (1955b) stated that “the relationship between energy of exchange and RT ln[(aK)/(aca+,,)~~2] appears to be universal in scope and applicable to all soils.” Beckett (1964a,b) popularized the application of Woodruff’s findings and conclusions by introducing the theoretical basis of the quantity/intensity relationship and a clever experimental approach for quantifying this relationship. The idea behind Beckett’s approach was that the term ( U ~ ) / ( L I ~was ~ ) ~related ‘ ~ to some quantity of exchangeable potassium in the soil. Considering that a given crop responds to a certain (Beckett, 1972), then the magnitude of the latter ratio would reflect a different quantity of exchangeable K +, depending on the soil type. Thus, for a given crop, a single relationship between crop yield and (aK)/(uCa)li2 for all soils for which K + in soil solution is controlled by K-Ca exchange would be expected, whereas different crop yield response curves with respect to exchangeable K + would be obtained for each of the soils. Thus, Beckett started a revolution in that his approach had a profound effect on the manner in which people investigated soil K availability to plants (Singh and Jones, 1975, and references therein). However, after a number of years and voluminous studies by scientists throughout the world, the Q/I concept was not always successful in describing soil K availability to plants (Rasnake and Thomas, 1976). The universal application of the Q/I concept as Woodruff predicted did not necessarily materialize. The reasons for the failure of the Q/I concept to predict soil K availability in all soils are varied. This is discussed at the end of this article (Section VI). Despite the limitations of the Q/I concept for predicting soil K availability, K-Ca exchange evaluation is still a powerful tool for predicting and evaluating the mechanisms of exchangeable K + release to the soil solution (Beckett and Nafady, 1967a,b, 1968). A thorough evaluation of the Q/I concept, as it applies to soils, is presented by Beckett (1971, 1972, and references therein). The first paper (Beckett, 1971) deals with evaluation of the Q/I concept with respect to its potential to describe K availability in the soil, and the second paper (Beckett, 1972) deals with the utility of the Q/I concept in describing K availability to plants in soils.

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V. P. EVANGELOU ETAL.

In addition to these two reviews on soil K, two more reviews on soil K are highly recommended to those interested in K chemistry and dynamics: Sparks and Huang (1985) comprehensively deal with the physical chemistry of soil potassium, and Sparks (1985) discusses the kinetics of ionic reactions in clay minerals and soils, including the kinetics of K-Ca exchange behavior. The purposes of this article are to discuss the Q/I concept in relation to modem cation exchange theory on binary and ternary soil systems, to introduce the reader to concepts involving the theory and application of ion-selective electrodes to rapid estimations of Q/I parameters for soil suspensions, and to discuss the future application of the Q/I concept in predicting soil K availability vis-a-vis plants.

11. ELECTROCHEMICAL CONSIDERATIONS For a soil system containing a cation, when equilibrium between solid and solution phase is attained: PhEC

(SOlUtiOn) =

PhEC

(adsorbed)

(1)

The electrochemical potential ( N h E C ) of ion b in the solid (adsorbed) or solution phase is described by PhEC

= PLC

+

(2)

Z h 9 E

where pK is the chemical potential of ion b, E is the electrical potential, % is Faraday’s constant, and Z, is the charge of cation b. By substituting Eq. (2) into Eq. ( l ) , PbCs

+

Zh$ES

=

+

Z h s E L

(3)

where S and L denote solid and liquid, respectively. Rearranging, ~

K -sPbCL

=

Zh9(EL

-

ES)

(4)

=

zds(EL

-

ES)

(5)

Likewise, for cation d, WdCs

-

PdCL

The unmeasurable electrical potential difference between the solid and solution phases of any single cation may be eliminated when considering equilibrium state conditions. Combining and rearranging Eqs. (4) and ( 5 ) ,

Equation (6) demonstrates that the difference in the chemical potential, between any two ions in the solid phase is equal to the difference in the free energy of the

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS

177

two ions in the liquid phase. The chemical potential of any ions in the solution is given by PKL=

PPCL

+ RT

W ) L

(7)

where &,, is the chemical potential at the standard state, R is the universal gas constant, T is temperature (OK), and (b)Lis the activity of cation b in solution. By substituting Eq. (7) into Eq. ( 6 ) ,it can be shown that

where Apc denotes the difference in chemical potential between any two ions in the solid or in the solution phase. Equation (8) demonstrates that the difference in the chemical potentials, between any two ions on a given solid phase, i.e., clay, can be related to the activity of the two ions in the solution phase. By considering an equilibrium state between ions b and d in the solid and solution phases,

PLs+

P!CS

= PL,

+

P8,,

(9)

and based on Eq. (8), one may write an equilibrium expression by setting A p 0 [Eq. (S)] equal to the Gibb’s standard free energy (AGO) of the equilibrium reaction. This equilibrium reaction expression is as follows: AGO =

- RT ln[(b~z~/d~z~)(d~~zd/bbiZb)]

(10)

AGO = -RT In Keq

(11)

and because

where Keq is the thermodynamic exchange constant,

Keq =

[(b~~zb/d~Zd)(d~’Zd/b~’zb)]

(12)

Note that in order for Eq. (12) to be valid, the ion symbols b and d in the solid and solution phases have to denote activity. From this point on, for reasons of clarity and simplicity, ions b and d will be represented by K + and Ca2+.This K-Ca exchange equilibrium expression is given by

Equation (1 3) was first published by Vanselow (1932) and it is known as the K , expression. In this expression, mole fractions X of adsorbed ions ( X , , XcJ are assumed to represent adsorbed-ion activities. Equation (13) describes a binary exchange reaction at equilibrium involving K+ and Ca2+in a soil system, 1/2Ex,Ca

+ K+

ExK

+

1/2Ca2+

(14)

178

V. P. EVANGELOU ETAL.

where Ex is an exchanger phase with a charge of - 1 and K + and Ca2+ are solution ionic species. Note that according to Eq. (13), which describes an equilibrium state between K + and CaZ+in a soil/solution system, the absorbed quantities of K + and Ca2+ have to be described as mole fractions ( X I ) .This can be justified as follows: A criterion of chemical reaction equilibrium is as follows (Smith and Van Ness, 1987):

2

Yip,

= 0

where v, is the stoichiometric coefficient in a chemical reaction for species i and p, is the chemical potential for species i. The chemical potential p, of species i in solution is identical to the partial molar Gibbs energy G, and at constant T,

d p , = dG,

=

RTd 1nJ;

(16)

which relates p, and GI to the fugacity cf, in solution. Integration of Eq. (16) from the standard state of species i to a state of species i in solution gives p,

- GP

= RT ln(f,/fP)

(17)

where G? is the molar Gibbs energy for species i. The ratio f,/f p is defined as the activity a, in solution. For a gas, the standard statef: is the ideal gas state of pure i at a pressure of 1 atm. Thus for gas-phase reactions, a, = JfP = J because fP equals unity. For solids and liquids the usual standard state is the pure solid or liquid at 1 atm and the system temperature. From the preceding equations and definitions, p, = CP

+ RT In a,

(18)

and at thermodynamic equilibrium for a chemical reaction,

2 v,(G: + RT In a,) = o

(19)

from which it follows that

where II signifies the product over all species i in the chemical reaction and K,, is the equilibrium constant for the reaction. Also,

-RT In K ~ = ,

2 V ~ G=: AGO

(21)

The pure component Gibbs energy, GP, is a property of pure species i in its standard state and fixed pressure. It depends only on temperature. It follows from Eq. (21) that Keqis also only a function of temperature and AGO is the standard Gibbs free energy change of reaction.

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS 179

For any reaction at equilibrium, Keq = II(aj)&’1 = II(’/fp)va

and it follows that the activities ai are not completely defined without also defining the pure component reference states f P and Gp. This is necessary if one is attempting to explain variations in the Kv across the K-Ca exchange isotherm due to changes in the molecular makeup/behavior of clay surface functional groups (Sposito, 1977, and references therein). Note that W o ~ r u f f(1955a,b) and Beckett (1971) assumed that reactions between exchange and solution phases take place on a chemical equivalent basis, an assumption that is consistent with the Gapon exchange expression (Gapon, 1933). Sposito (1977) argued that because of this latter assumption, the Gapon exchange expression does not have direct molecular interpretation. Therefore, a reevaluation of the Gapon (1933) exchange expression with respect to the Vanselow (1932) expression is necessary to demonstrate validity of the direct molecular interpretation of the Q/I plot and ’s s~atementthat the term RT ln[(ffK)/(uK)/ its implications to W o ~ ~ f f(1955a,b) ( u ~ , ) ”is~related ] to a chemical equivalent of K + in the standard state replacing a chemical equivalent of Ca2+ on clay followed by the liberation of “certain calories of energy.” Based on the above statements, the true thermodynamic exchange equjiibr~um constant, K e g , for the reaction expressed in Eq. (14) at room temperature (22°C) and 1 atm pressure is represented by

where ac, and f f K are the activities of solution phase K + or Ca2+and aExCa are the activities of K + or Ca2+on the exchange phases. Activity ai is defined by the equation

ffErK

and

a, = yici (24) where aj is the activity coefficient of species i and c, is the concentration of species i. In order to define solution phase a i , its value is set to I , hence y i = 1, when solution ionic strength ( I ) approaches zero. For mixed electrolyte solutions when I > 0, the single-ion activity concept in~oducedby Davies is employed to estimate yi (Amacher, 1984; Sposito, 1984a). The activity component of the adsorbed or solid phase is defined by employing the mole fraction concept (Xi) introduced by Vanselow (1932) as previously discussed. According to Vanselow, for a heterovalent binary exchange reaction such as K+-Ca2+,assuming that the system obeys ideal solid-solution theory (Evangelou and Phillips, 1987), the activity term (aExi) is defined by @ExK

XK =

ExK ExK 4- Ex,Ca

180

V. P. EVANGELOU ET AL.

and aEx$3

LI

XCa

=

Ex2Ca ExK + Ex2Ca

where XK and X,, denote mole fractions of K+ or Ca2+ and Ex denotes the exchange phase with a valence of - 1. For a system where ideal solid-solution behavior is not obeyed, aExi

(27)

= gtx,

where g, denotes the adsorbed ion activity coefficient. On the other hand, equivalent fractions ( E , ) for K + and Ca2+,employed by the Gapon (1933) expression, are defined by EK

=

ExK ExK + 1/2Ex2Ca

Eca

=

1/2Ex2Ca ExK 1/2Ex2Ca

and

+

Equation (28) is used to estimate the exchangeable K percentage by simply multiplying EK times 100. For a binary system, the cation exchange capacity (CEC) of the soil is taken to be: CEC = ExK

+

1/2Ex2Ca

(30)

It is assumed that any other cations, such as exchangeable Na+ and/or H + , are present in negligible quantities and do not interfere with K+-Ca2+ exchange, or H + is tightly bound to the solid surface, giving rise only to pH-dependent charge (Sposito, 1981b). Commonly, the magnitude of K , is taken to represent the relative affinity of K + with respect to Ca2+by the clay surface (Shainberg et a f . , 1980; Sposito and Le Vesque, 1985). When Kv equals 1 at a given level of exchangeable K , the exchanger at that level of K+ loading shows no preference for either K + or Ca2+. On the other hand, a K, > 1 at any given level of exchangeable K + signifies exchanger preference for K + , and a K, < 1 at any given level of exchangeable K + signifies preference for Ca2+ . Vanselow (1932) and later Argersinger et al. (1950) noted that because experimental Kv values describing exchange reactions in soils and clay minerals do not remain constant for the entire exchange isotherm, but vary as a function of cation loading on the solid phase, such Kv values cannot be equated to thermodynamic equilibrium constants (Keq). +

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS

18 1

In order to give physical meaning to changes in magnitude of Kv , the triplelayer model (TLM) can be employed (Sposito, 1984b). The TLM model describes a negatively charged surface with three compartments or regions: region I, the inner-sphere region; region 11, the outer-sphere region; and region 111, the diffuse ion swarm region. The inner-sphere region is assumed to represent completely adsorbed cations that are dehydrated (i.e., an inner-sphere complex). It is known that such an adsorption mechanism does not involve to any significant degree Ca2+,but it does occur with K + in vermiculitic-type soil clays. Calcium commonly occupies region I1 and to a lesser degree region 111 (Sposito, 1984b). Region I1 is characterized by water tightly bound to adsorbed cations (i.e., an outer-sphere complex); anions are excluded. Region 111 is characterized by water tightly bound to adsorbed cations as well as water loosely bound to adsorbed cations; however, anions are not completely excluded. Note that the above physical interpretation does not include the molecular makeup of functional groups. This is not necessarily a limitation because such metal ion adsorption behavior could be attributed to all functional groups associated with clay surfaces, depending on type of cation, pH, and surface makeup.

111. QUAN"ITY/INTENSITY A. FUNDAMENTAL BASISOF Q/I Solution-exchange phase equilibria in soils have been studied extensively in soils for the purpose of determining how nutrients become readily available to plants (Le Roux and Sumner, 1968a,b; Rasnake and Thomas, 1976; Sparks and Liebhardt, 1981, 1982) or how readily cations are subject to leaching and chemical or microbiological transformations (Goldberg and Gainey, 1955; Avnimelech and Laher, 1977). Potassium, calcium, and magnesium are some of the cations that have undergone the most extensive testing in soil systems in terms of solution-exchange phase interactions (Beckett, 1964a,b; Moss, 1967; Le Roux and Sumner, 1968a,b; Nash, 1971; Lee, 1973; Sparks and Liebhardt, 1981, 1982, and references therein). Based on classical thermodynamic considerations, availability of nutrients such as K and Ca to plants in soil systems is related to the quantity and form of these nutrients in the solid phase (Beckett, 1964a). Quantity and form of the nutrients are related to the chemical potential (Beckett, 1964a). However, as pointed out earlier, it is not possible to measure directly the chemical potential of an ion in the solid phase (Guggenheim, 1950, 1952), but it is possible to measure the difference in the chemical potential between two ions in the solution phase at an equilibrium with the solid phase. The latter can then be related to the

182

V. P. EVANGELOU ETAL.

chemical potential difference of the two ions in the solid phase (Beckett, 1971, 1972; Nye and Tinker, 1977). An approach often used to predict the relative chemical potential of K+ in soils, as discussed previously, is the quantity/intensity (Q/I) relationship first theoretically justified by Woodruff (1955a,b) and experimentally introduced and interpreted by Beckett (1964a,b). A typical Q/I plot for a binary cation system as described by Beckett (1964a,b) is shown in Fig. 1 with the following components: AExK is the quantity factor ( Q )that represents changes (gains or losses) in exchangeable K +, ARK is the intensity factor (I) or activity ratio (AR) for K +, ExKO is the labile or exchangeable K + , ExK, are specific K + sites, A& is the equilibrium activity ratio for K + [aK/(aca)"2],and PBCK is the linear potential buffering capacity for K+. This relationship (Fig. 1) implies that the ability of a soil system to maintain a certain concentration of a cation in solution is determined by the total amount of the cation present in readily available forms (exchangeable and soluble) and the intensity by which it is released to the soil solution. The dynamics of the Q/I relationship with respect to K depletion or soil K enrichment have been discussed extensively by Beckett (1971, 1972), and the reader is referred to these readings for a thorough understanding of the Q/I concept. Woodruff (1955a,b), employing K-Ca exchange equilibria in soils, attempted to relate classical thermodynamics to soil exchangeable K + and Caz+ release to the soil solution, which he assumed would be related to ion availability to plants. This assumption can be justified as follows: Consider the exchange reaction given in Eq. (14). A modified Gapon ( K G )exchange expression for Eq. (14) may be written as KG = [(ExK)/(1/2&Ca)l [(&a)] [(ac,) "2/(a~)1

(31)

where aca and a K are single-ion activities for Ca2+ and K + . Note that Gapon (1933) employed concentrations for K + and Ca2+. Rearranging Eq. (31) and using the classical relationship between the Gibbs free energy of formation (AG) and the solution activity, AG

=

AGO

+ RT In ai

(32)

where AG is the free energy of formation, AGO is the free energy at standard pressure and temperature, R is the universal gass constant, T is absolute temperature, and ai is the activity of ion i in solution. Equation (32) gives

RT l n [ ~ , / ( a ~ , ) ' /= ~ ]RT ln(l/KG)

+ RT ln[ExK/(1/2Ex2Ca)]

(33)

Based on Eq. (33), Woodruff (1955b) concluded that the term

RT In[ a K / ( aca)"*I

(34)

SOIL POTASSIUM QUANTITYDNTENSITY RELATIONSHIPS

183

I

Figure 1 A typical quantity/intensity (Q/I) plot.

is related to one chemical equivalent of potassium in the standard state replacing one chemical equivalent of calcium on clay followed by the liberation of “certain calories of energy.” As stated earlier, Woodruff (1955b) concluded that the relationships between plant nutrition and the energy of exchange appear to be universal in scope and applicable to all soils. The assumptions made for accept~ / ~ ] to plant nutrition involve certain ing the premise that RT l n [ ~ ~ / ( a ~ .is) related predictable behavior in ion uptake by roots and certain predictable behavior of all the soil parameters that would be involved in regulating u ~ / ( u ~in ~the ) ~soil /~ solution. However, Woodruff’s (1955b) conclusion relating free energy of K-Ca exchange to soil K availability to plants may be weak because the validity of the assumptions that such conclusions were based on is not easily justified. A number of these assumptions, in a general way, are described by Beckett (1972) and the reader is directed to this review article. In general, what Beckett pointed out was that if indeed the term a K / ( a c a )or ~ ~ARK, Z , can be linked to plant nutrition, then one must accept the assumption that ARK reflects the chemical potential difference between K + and Ca2+in the solid and solution phases of the soil and in the soil solution and plant root uptake sites. Furthermore, no other cation is involved in any way in the control of ARK and its direct relationship to K-Ca exchange. Many studies have been carried out in the past that demonstrated that plant roots do acquire nutrients actively, e.g., using specific carriers, and not independently of other cations present (Epstein and Hagen, 1952; Epstein and Leggett, 1954; Epstein et al., 1963; Mass, 1969; Epstein, 1972). More recently, strong evidence was presented in the literature that the root cell wall plays a very important role in passively modifying the solution cation concentration and composition and thus in modifying the apparent potential of plants to absorb and bioaccumulate metals (Haynes, 1980, and references therein; Wang et al.,

184

V. P. EVANGELOU ET AL.

1992). Based on these concepts, one has to distinguish ion availability to plants from ion uptake by plants. Furthermore, some additional problems arise in defining ion availability to plants when one considers the complexities involved due to direct interactions of roots and/or root exudates with soil surfaces, thus modifying the potential of soil surfaces to control and/or deliver cations to roots and/ or to soil solution. When using the term RT l n [ ~ ~ / ( a ~to~ describe ) l / ~ ] K availability, it is implied that K+ release to the soil solution is facilitated through exchange by CaZ+only, although Beckett (1964a,b) did also introduce Mg2+,e.g., h[aK/(aCa+Mg)”2]; inclusion of Mg2+ in this manner implies that Ca2+and Mg2+are indistinguishable with respect to K + soil exchange, but they are not indistinguishable with respect to K’ uptake by plants. Data in the literature demonstrate the strong competitive nature of Mgz+ with respect to K uptake by plants (Le Bot et al., 1990; Hannaway et al., 1982, and references therein). Also, the term RT ln[aK/ (ac,+M,)’/2]implies that no other cations except Ca2+plus Mg2+are interacting with K + . This is highly questionable, because cations such as Na+, NH,’ , H + , and A13+ are always present in soil systems (Evangelou et al., 1986). The presence of such additional cations requires expansion of the term RT ln[aK/ ( a C a + Mg)’”]. Inclusion of additional cations requires the assumption that such a polycationic system is near an equilibrium state. However, in many soils this is highly questionable because a polycationic system would be more subject to kinetic and hysteretic effects due to increasing soil surface/cation specific interactions. For example, in some soils, such as those high in vermiculite, the kinetics of exchange rather than exchange equilibria control K availability (Sparks, 1985; Lumbanraja, 1991). Therefore, despite the deficiencies in using the term RT ln[~~/(a~~ (Woodruff, + ~ ~ ) l 1955b), ~ ~ ] it can be used as a relative index of soil potassium availability, e.g., comparing the potential of a given soil to release potassium to the soil solution with respect to another soil. Assuming that plant roots respond to solution chemistry and ARK is related to plant K availability (Beckett, 1972), by choosing “critical” ARK values, as referred to by Beckett (1972), the corresponding quantity of K in the soil system referred to as exchangeable K and the AG values can be calculated employing Eq. ( 3 3 ) . Equation (33) reveals that two components are contributing to AG based on the magnitude of ARK. One component is the Kc and the other component is the proportionality of exchangeable K to exchangeable Ca. The contribution of these two components to the AG based on the magnitude of ARK is inversely related. In other words, when KG increases, its AG contribution decreases, whereas when ExK/( 1/2ExzCa) increases, its AG contribution increases. consequently, as CEC increases, in order to maintain a “critical” ARK, ExK has to increase. These points are clearly demonstrated in Table I. The data

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS

185

Table I Contribution of Individual Components to AG Determined from ARKin Solution Phase" ARK (moli liter)

KG (mol/ liter) - I 1 *

CEC (cmol kg - I ) ExK/I/ 2EXzCa

10

0.01 0.03 0.05 0.01 0.03 0.05 0.01 0.03 0.05 0.01 0.03 0.05

38.7 113.9 186.2 38.1 113.9 186.2 38.7 113.9 186.2 38.7 113.9 186.2

Pg g

3.33 x 1.00 x 2.00 x 1.66 x 4.99 x 3.00 x 8.33 x 2.50 x 4.21 X 3.35 x 1.00 X 1.69 X

I O - ~ 10-2 10-2

10-4

IO-~ lo-' 10-4

3 3 3 6 6 6 12 12 12 30 30 30

20

-' K

AGo(KG) (kJ/equiv)

AG' (ExK/ 1/2ExzCa) (kJ/equiv)

AG (kJ/equiv)

+

77.4 227.8 372.4 17.4 227.8 372.4 77.4 227.8 312.4 11.4 227.8 377.4

- 2.72 - 2.72 -2.12 -4.44 -4.44 -4.44 -6.15 -6.15 -6.15 -8.41 -8.41 - 8.41

- 1 1.42 - 8.70 - 7.40 - I I .42 - 8.70 -1.40 - 1 I .42 -8.70 -7.40 -11.42 -8.70 -1.40

- 14.14 -11.42 - 10.12 - 15.86 - 13.14 - 11.84 - 17.57 - 14.85 - 13.56 - 19.83 -17.11 - 15.82

a A R ~= u K / o C a l l i 2 . ARK values were generated by the Gapon equation [see Q . (35)1employing a range of KG and exchangeable cation ratios commonly encountered in wils.

in Table I show that various AG values are obtained by different quantities of exchangeable K + (ExK) depending on the magnitude of CEC and K G . Therefore, levels of ExK necessary to attain an optimum AG appear also to be dependent on the magnitude of K G and CEC. As KG increases, so does the quantity of exchangeable K+ (ExK) in order to maintain an optimum AG. Furthermore, for the same reason, in order to maintain an optimum AG as CEC increases, ExK also increases. Further discussion on this appears in Section VI. The previous discussion implies that the quantity of exchangeable K + needed to grow a crop on a number of different soils would appear to vary depending on the magnitude of both KG and CEC.

B. GAPONQ/I INTERPRETATION The term ARK, associated with potential soil K + release to the soil solution, is related to the magnitude of the potential buffering capacity, PBCK . The latter has previously been justified by the Gapon equation (Beckett, 1965b; Le Roux and Sumner, 1968a). The interrelationship between ARK and PBCK (also see Fig. 1) has been described in detail by Evangelou and Karathanasis (1986). For the binary K-(Ca + Mg) interaction, where the term (Ca + Mg) indicates a

V. P. EVANGELOU ETAL.

186

system in which Ca and Mg are treated as one ionic species (Beckett, 1964a; Le Roux and Sumner, 1968a), the Gapon equation can be written as follows (Gapon, 1933; U.S. Salinity Laboratory Staff, 1954; Carson and Dixon, 1972; Knibbe and Thomas, 1972, Rasnake and Thomas, 1976): KG = [ E x K ] ( a c , + ~ , ) ‘ /I/2Ex2Ca ~/[ + 1/2EX,Mg](a~)

(35)

where KG denotes the Gapon cation exchange selectivity coefficient in (liters mol-l)l/?, Ex is the quantity of exchangeable ions (cmol, kg-I), and a is the single-ion activity (mol liter-’) for the cations shown. Rearranging Eq. (35) and solving for ExK results in [ExK]

= [ 1/2Ex,Ca -t ~ / ~ E X ~ M ~ ] K , ( U K ) / ( U , ~ + M ~ ) ” ~

(36)

Equation (36) can be further written as [ExK]

=

KG[1/2Ex2Ca

+

1/2Ex2Mg]AR,

(37)

where

+ +

For most agricultural soils under K fertilization, [ExK] << [1/2Ex,Ca 112 Ex,Mg] and Eq. (37) then becomes a linear equation, where [ExK] is the dependent variable, ARK is the independent variable, and KG[I/2Ex2Ca 1/2 Ex,Mg] is the slope. This slope can also be expressed as the derivation of [ExK] with respect to ARK. d[ExK]ldAR, = K,[ 1/2ExzCa

+

1/2Ex2Mg]

(39)

The right-hand side of Eq. (39) is the PBC, (Fig. I ) , where PBCK = KG[1/2Ex2Ca+ 1/2Ex,Mg]

(40)

If one considers an equilibrium state following increases or decreases in [ExK] by addition or removal of K + from the soil, and keeps in mind that [ExK] << [l/2Ex2Ca + 1/2Ex2Mg],Eq. (39) can be rearranged as follows:

*

+A[ExKl = KG{[I/2Ex,Ca + 1/2Ex2Mg] [(AExK)]}ARK,- [ExK”]

(41)

where

and [ExKO] = [ExK] in soil at equilibrium. The plus or minus designations in Eqs. (41) and (42) indicate addition of K + ( + ) or removal of K + ( -). After the addition or removal of K + (still assuming that [ExK] << [1/2Ex,Ca + 112

SOIL POTASSIUM QUANTITYANTENSITY RELATIONSHIPS

187

+

Ex,Mg]), Eq. (41) is still a linear equation, where KG[1/2Ex2Ca 1/2Ex2Mg] f A[ExK] is the slope and [ExKO] is the constant of the linear equation. Equation (40) now becomes PBC,, = slope

=

KG([l/2Ex2Ca

+ 1/2Ex,Mg] 2 A[ExK])

(43)

It follows from Eq. (41) that as ARK, + 0, A[ExK] + [ExKO]. The value of [ExKO] in Fig. I indicates available K+ (labile or exchangeable) (Sparks and Liebhardt, 1981). On the other hand, when A[ExK] + 0, ARK' + ARKo. The value of ARKorepresents the equilibrium activity ratio of K + for the soil sample (Sparks and Liebhardt, 1981); see also Fig. 1. The straight line section of the Q/I plot in Fig. 1 represents the linear buffering capacity of the soil (PBCK,)[see Eq. (43)], as long as KG remains constant. For most agricultural soils under K fertilization, the contribution of [ExK] and/or A[ExK] to the CEC is insignificant; i.e., [ExCa ExMg] >> A[ExK] and Eq. (43) reduces to

+

PBC,

=

PBCK

=

(CEC)KG

(44)

When ExK, ExCa, and ExMg all represent a significant portion of the soil CEC, ExK

=

KG[CEC - ExK]ARK

(45)

where [CEC - ExK] = [1/2Ex2Ca

+

1/2Ex,Mg]

(46)

and d[ExKlldAR~

7

KG(CEC)I(l

+ KCARK)'

=

PBCK

(47)

Equation (47) points out that for a given KGvalue, a given CEC, and relatively small ARK values, the product of KG ARK is near zero and Eq. (47) reduces to Eq. (44). However, when using the same KGvalue, the same CEC, and significantly larger ARK values, so that the denominator of Eq. (47) is significantly greater than 1, the PBCK better fits a curvilinear function asymptotically approaching some upper limit. The validity of the CEC KG product in evaluating the PBCK when ExK is an insignificant portion of the CEC is evident in Figs. 2 and 3, which represent soils of different CEC and KG values (Rasnake and Thomas, 1976). Although CEC fails to predict the PBCK of these soils (R2 = 0.330, Fig. 2), a plot of (CEC) KG versus PBCK indicates a much better fit of the data points to a straight line with a slope of 0.754 and R2 = 0.952 (Fig. 3). This relationship fails only at ARK values near zero where an exponential increase in KG values causes a proportional increase in PBCK (Fig. 1) (Carson and Dixon, 1972; Knibbe and Thomas, 1972). Generally, however, it could be said that a number of different soils in

188

V. P. EVANGELOU ET AL. 0

9

I

I

I

I

I

I

1

II

13

15

17

19

21

23

25

CEC, cmolc kg-'

Figure 2 Relationship between CEC and PBCK of six Kentucky soils. After Evangelou and Karathanasis (1986).

60

70

I

I

80

90

1

100

I

110

I

I

I

120

130

140

I

150

160

CEC . K G , cmol, k g - " L mol-')"'

Figure 3 Relationship between (CEC)K, and PBCKof six Kentucky soils for various CEC and K, values. After Evangelou and Karathanasis (1986).

SOIL POTASSIUM QUANTITY/INTENSITYRELATIONSHIPS

189

+

Mg) interactions could which K release in solution is controlled by K-(Ca have the same PBCK because (1) they also have equal CEC and K G values or (2) high or low CEC values are offset by equally low or high KG values, respectively. Even though a theoretical interrelationship between PBCK and ARK has been demonstrated, direct molecular interpretation of this interrelationship, e.g., that ARK is related to the chemical potential of adsorbed K (ExK), needs further verification. Theoretical considerations of the thermodynamics of exchange reactions show that the Gapon exchange coefficient ( K G ) (Gapon, 1933) has no direct molecular interpretation because it considers that cations react with an exchanger in equivalents (Sposito, 1977). Based on this, the Q/I relationship cannot be assigned direct molecular interpretation, i.e., PBCK represents a certain class of exchange sites or that ARKis related to the relative chemical potential of K+ on the soil exchange phase, because the relationship is derived from the Gapon equation (Beckett, 1964a,b; Woodruff, 1955a,b). However, direct molecular interpretation could be assigned to the Q/I plot if the latter is demonstrated to be consistent with the Vanselow exchange equation ( K , ) (Vanselow, 1932). The Vanselow exchange equation can be used for molecular interpretation because it considers that cations react with an exchanger in moles (Sposito, 1977, and references therein).

IV.BASIS OF MOLECULAR INTERPRETATION OF QUANTITY/INTENSITY A. GAPON-DERIVED Q/I PARAMETERS From the Gapon expression for the K-Ca exchange reaction [Eq. (14)] we can derive PBCK by first solving for ExK [see also Eq. (36)]: EXK = KG(CEC)(ARK)[l

ARKKGI-’

where ARK

=

u K / ( u ~ ~ ) ” ~

Taking the derivative of Eq. (48) with respect to ARK,

and when ARK

-

dExK/dARK = KG(CEC)[l

KGARK]-~

0, PBCK = limit dExK/dARK ARK+O

=

KG(CEC)

(48)

190

V. P. EVANGELOU ETAL.

For some agricultural soils, ExK is considered to represent an insignificant portion of the CEC and the condition ARK+ 0 is considered to be met. Under this condition, the PBCK is linearly related to the (CEC)K,. This was discussed in the previous section.

B. VANSELOW-DERIVED Q/I PARAMETERS The Vanselow expression for the reaction described in Eq. (14) is K v = [XK/(Xc,)l’Z](AR~)-’

(51)

Note that the true Vanselow equation (Vanselow, 1932) is as follows: KV

=

(52)

[(XK)2/(XCa)l[(aCa)/(aK21

Thus, the K , of Eq. (51) equals ( K v ) l ‘ * [Eq. (52)], where the terms XK and Xc, are the mole fractions of K + and Ca2+ on the exchanger. The mathematical expression of mole fraction for K and Ca on the exchanger is given by Eqs. (25) and (26). Equation (51) is solved for ExK by substituting into it X K

= ExK/[1/2CEC

+ 112ExKI

(53)

and Xca

=

[1/2CEC - 1/2ExK]/[1/2CEC

+ 1/2ExK]

(54)

Equation (5 1) then takes the form ExK = (CEC)KvAR,[4

+ (KVARK)~I-”~

(55)

Taking the derivative of Eq. (55) with respect to ARK, dExKldAR, = 4Kv(CEC)[(KvAR,)’

+ 4]-3’2

(56)

and the linear potential buffering capacity of the soil for K+ is PBC, = limit dExKldARK = (Kv/2)CEC

(57)

ARK-0

Based on Eqs. (50) and (57), it is clear that PBCK

=

limit dExK/dARK = K,(CEC) = (KV/2)CEC

(58)

ARK-0

C. INTERRELATIONSHIP BETWEEN K GAND Kv Additional support for Eq. (58) can be demonstrated by providing mathematical interrelationships between KG and Kv . By rearranging Eq. (48) and dividing ExK by CEC and solving for ARK, we obtain ARK = EK[KG(1 - EK)]-’

(59)

SOIL POTASSIUM QUANTITY/INTENSITYRELATIONSHIPS

191

Also, from Eq. (55) ARK

2EK[Kv(l - EK’)”’]-~

=

(60)

where EK is the equivalent fraction of K+ in the exchanger. For the binary K-Ca system, the equation is EK = ExK/[ 1/2Ex2Ca

+ ExK]

(61)

By equating Eqs. (59) and (60) and solving for KGone obtains Kc

=

Kv[l

+ E K I ” ’ [ ~ (-~ EK)”2]-I

(62)

By taking the limit as EK+O one obtains KG = Kv/2. To this point it has been shown that limit dExKldARK = Kc(CEC) = (Kv/2)CEC ARpO

and that limit Kc = Kv/2 k-0

One also needs to show that limit KG = Kv/2 AReO

The limit of Eq. (63) can be shown by solving Eqs. (48) and ( 5 5 ) for EK and equating EK of the two resulting equations to obtain EK = (KGARK/[l

+ KCARK]) = (KVARK)/[4

(ARKKv)’]”’

(64)

Solving for K G , KG

=

Kv/[(4

+ (ARKKV)2)1’2- KvARK]

(65)

and taking the limit of KGas ARK + 0, one obtains limit KG = Kv[l ARpO

+ KvARK][4 + (ARKKV)’]-”*= Kv/2

(66)

As was shown above, as ARK+ 0, KG = Kv/2 and the relationship PBCK = (CEC)Kc = 1/2(CEC)Kv is therefore valid. In an analogous evaluation involving Na-Ca exchange, Sposito (1977) concluded that the traditional Gapon (1933) expression and the Vanselow (1932) expression will not be very different numerically if the exchangeable sodium percentage is <20. A similar sensitivity analysis on the comparison between KO and Kv has been carried out by Evangelou and Phillips (1987, 1988). In summary, the Q/I relationship for a binary system has direct molecular interpretation (Beckett, 1964a,b; Woodruff, 1955a,b). Also, according to the

192

V. P. EVANGELOU ETAL.

Gapon and Vanselow exchange equations, a plot of ExK versus ARK for the entire exchange isotherm will give a curvilinear function approaching the CEC asymptotically as long as CEC and KG or Kv are constant (Evangelou and Phillips, 1987, 1988). In order to demonstrate the relationship between Q/I and the entire K-Ca exchange isotherm, Evangelou and Karathanasis (1986) solved the Gapon equation [Eq. (35)] for a wide range of K+ surface coverage using a computer successive approximations technique for a number of soils characterized by different CEC and KG values. The data generated by such computer simulations are presented in Figs. 4, 5 , 6 , 7 , and 8. The plots in Fig. 4 demonstrate that an approximately straight line relationship, which is the equivalent of PBCK of a Q/I plot [Eq. (44)], is applicable to low K-loading systems. It is also apparent from these plots that at lower CEC and KO values the effect of increasing ARK on decreasing the magnitude of the slope of the plots is more prominent [Eq. (47)]. Figure 4 illustrates that the slope of any Q/I plot representing a soil system is a function of CEC, K G , and ARK, hence the magnitude of K loading. The dependence of the slope of the plots (Fig. 4), not only on CEC but also on KG at low K surface coverage (where these slopes are equated to PBCK), is shown in

10

c

2 0

0.0

85.0

170.0

255.0

AR, x lo3 (mol

340.0

425.0

510.0

c')''~

Figure 4 Computer-simulated data showing the relationship between ARKand ExK for CEC and KG ranges representative of those reported in the literature (Rasnake and Thomas, 1976; Knibbe and Thomas, 1972). (Regions 1, 11, and 111 represent different K-loading levels plotted in Fig. 7). After Evangelou and Karathanasis (1 986).

SOIL POTASSIUM QUANTITY/INTENSITYRELATIONSHIPS

I40

--

120

2

80

\ N

Ll

t

193

R 2 =0.999

20

0

0

20

40

60

80

100

120

140

160

CEC . K G ,cmol, kg-ll L rnol-')''' Figure 5 Relationship between (CEC)K, versus PBCK from computer-simulated data. After Evangelou and Karathanasis (1986).

Fig. 5 . This figure was made by graphically extrapolating PBCK values from slopes near the origin of isotherms similar to those shown in Figs. 4 and 8 and plotting them against (CEC) K G . A 1 : 1 linear relationship is evident between PBCK and (CEC)KG at low K loadings as was pointed out in Eq. (44). Similar dependencies of PBCK on CEC and KG at low K loadings are also shown in Fig. 6, where increases in PBCK as a function of CEC are greater at increasing KG values. The 1 : 1 linear relationship between PBCK and (CEC)KG fails when ExK becomes a significant portion of the CEC. To further demonstrate these points, computer-simulated Q/I plots for three K-loading regions (I, 11, and 111) of one soil in Fig. 4 are plotted in Fig. 7. The data reveal that the value of the apparent PBCKappears to vary with the fraction of CEC occupied by K + and its magnitude decreases with increasing K loading. Literature data substantiate this finding, which is often attributed to higher KG values observed at low K loadings (Le Roux and Sumner, 1968a; Rasnake and Thomas, 1976). The apparent PBCK dependence on K loading (Figs. 4 and 7) is actually due to increasing ARK values. This can be demonstrated with Eq. (47). For constant CEC and KG values and relatively high ARK values, the denominator in Eq. (47) becomes significantly >1; hence the slope = PBCK progressively decreases

194

V. P. EVANGELOU ETAL.

I20

-2

1_1

-

; 90

-\ t

r 0,

-

0

60

Ei Y

0

m

a 30

0 0.0

4.0

2.0

6.0

8.0

10.0

12.0

14.0

CEC, cmolc kg-I

Figure 6 Computer-simulated data showing the relationship between CEC and PBC, at three different Kc values. After Evangelou and Karathanasis (1986).

f

0.5 7- 0.4 x

5" 0.3

-

- 0.93

R 2 * 0.986

0

Y

A

Y=

45X -0.25 R 2 a0.997

W

Ya

27X-0.34 R'80.997

941 x

a

CEC = 15 cmol, kg-1 K, =6 ( L m o l - l ) l ' p

5

5W

Q

Oe2 0.1

-

I )

0

60

80

loo&

0

140

160

-0.1

-0.2

f AR,IX

103(mol

c')"'

Figure 7 Computer-simulated Q/Iplots for a soil at three different K loadings (1, 11, and 111) illustrated in Fig. 4. After Evangelou and Karathanasis (1986).

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS

8.0

0

0

CEC = 16.7 cmolc kg-l K, =1.93 (Lmol-’)”2)

+

CEC =11.1 cmol, kg-1 Ko = 2.82 ( L rno 1-l) 112

85

170 A R x ~

2 55

340

195

I 425

lo3 (mol L - ’ ) ’ ’ ~

Figure 8 Computer-simulateddata showing the relationship between ARKand A[ExK] for a soil with different CEC and KG values due to no-till and conventional soil management. After Evangelou and Karathanasis (1986).

(the relationship becomes curvilinear). This was tested by substituting CEC, KG (Fig. 7), and ARK values (Fig. 4) for the three ExK regions (I, 11, and 111) in Eq. (47). The calculated slope of the lines was found to agree well with the slopes of the regression equations of Fig. 7. Although monovalent-divalent cation exchange reactions are solely described by nonlinear isotherms (Fig. 4), the ranges of ARK commonly found in Q/I plots reported in the literature (Beckett, 1964a,b; Rasnake and Thomas, 1976; Sparks and Liebhardt, 1981) are small enough so that changes in ExK as a function of ARK can be appropriated as a linear function. This approximation is more applicable when dealing with low ARK values than when dealing with high ones. At higher ARK values, ExK becomes a significant portion of the CEC and PBCK values are only applicable for the narrow range of ARKspecified by the experiment (Fig. 7). Implications of the concepts demonstrated in Fig. 6 with respect to no-till and conventional-till soil systems (Blevins et al., 1983) are worth mentioning. In a no-till system, accumulation of organic matter causes an increase in CEC (Evangelou and Blevins, 1985). However, because of the lower affinity of the organic matter for the monovalent cation (low KG),the PBCK value may remain unchanged in no-till systems, depending on relative increases or decreases in CEC or KG values, respectively (Salmon, 1964; Evangelou and Blevins, 1988). This point is illustrated in Fig. 8, which represents actual CEC and KG values for notill and conventionally tilled soil systems (Evangelou et d . , 1986). The two soil

196

V. P. EVANGELOU ETAL.

systems, despite difference in CEC and KG values, exhibit similar PBC, values up to a K loading of 1.5 cmol kg- I .

D. INFLUENCE OF ANIONS Although a great deal of research has been presented on Q/I studies in the literature and this work has been summarized by Beckett (1971, 1972, and references therein) and Bertsch and Thomas (1985, and references therein), data on the influence of the type of anion associated with the metal cations on Q/1 interrelationships are lacking. However, the influence of anions on solution cation chemistry and their influence on cation exchange is well known. Ion exchange equilibria are affected by the types of anions used (Babcock and Schultz, 1963; Rao et al., 1968) owing to the pairing properties of divalent ions (Tanji, 1969; Adams, 1971). Tanji (1969) demonstrated that approximately one-third of the dissolved CaSO, of a solution in equilibrium with gypsum (CaSO, . 2H20) is in the CaS0,O pair form. Magnesium behavior is similar to Ca2+because Mg2+pairing ability with is approximately equal to that of Ca2+ (Adams, 1971). Longenecker (1960) observed that Na2S0, displaced appreciably more exchangeable Mg2+ than did equivalent amounts of NaCl or NaNO, . Similarly, Dutt and Doneen (1963) observed that the concentration of Na+ in the saturation extract of Ca-saturated Yo10 clay loam was less when saliNa2S0, than with an equivalent amount of NaCl only. nized with NaCl By investigating Na-Ca exchange reactions in C1- or SO,2- solutions, Babcock and Schultz (1963) and Rao et af. (1968) found that exchange selectivity coefficients for the two cations were different. But they demonstrated that this difference was nearly eliminated when solution-phrase activities and CaS0,O were considered. The tendency for Ca2+ and Mg2+ to form pairs with SOZ- might have an influence on Q/I relationships for K + in soil systems. Because a certain number of Ca2+and Mg2+ ions are presented as uncharged sulfate pairs and as such are not expected to participate in exchange reactions with K + , this will allow a greater proportion of the available K' (solution plus exchangeable K+)to exist in the exchangeable form. Evangelou (1986) investigated the influence of C1- or SO,2- anions on the Q/I relationships for K + in several soils. The data from these studies demonstrated that correcting the solution data for single-ion activity considering ion pairing and single-ion solution activity did not compensate for the experimental differences in the Q/I relationships between the chloride and sulfate systems. Evangelou (1986) concluded that these differences may be due to competitive interactions of CaCl+ with Ca2+ and/or K + and KSO,- with SO,2- for exchange sites and possibly S0,2--specific adsorption onto the solid surfaces of the soil. Sposito et al. (1983) have shown that external clay sur-

+

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS

197

faces adsorb charged pairs of CaCl+ and MgCl+ in Na+-Caz+-Cl- and Ma+Mg2+-C1- clay suspension systems. Mehlich (1981) and Rajan (1979) reported that specific adsorption of anions onto soil solid surfaces resulted in an increase in the CEC of the soil. Pratt et al. (1962) showed that on increasing the CEC, the magnitude of the KG decreases. Changes in the CEC and KG appear to directly influence the magnitude of PBC, . San Valentin et al. (1973) and Sparks and Liebhardt (1981) reported that on increasing the pH-dependent CEC of a soil through liming, the PBCK also increased. Evangelou and Karathanasis (1986) demonstrated that, for soils in which ExK is an insignificant portion of the CEC, the PBCK is directly proportional to the product of KG and CEC. Further studies on Q/I relationships on various soils by Karmarkar et al. (1991) employing chloride and perchlorate solutions showed that differences in the Q/I relationships were most likely accounted for by K+-CaCl+ external clay surface interactions rather than by surface charge changes. However, similar observations were not made for all soils tested. These findings suggested that additional unknown factors were influencing cation exchange reactions due to the type of anion added. The influence of anion on K+ rate of adsorption, mobility, and retention were investigated by Sadusky and Sparks (1991) on two Atlantic Coastal Plain soils. They reported that the type of anion had little effect (if any) on the rate of K+ adsorption but had an effect on the amount of K+ adsorbed. They found that the amount of K + adsorbed in the presence of a particular accompanying anion was of the order SiO, > PO, > SO, > C1 > C10,. These findings strongly imply that Q/I relationships are affected by the anion present in the soil.

E.TERNARY EXCHANGE SYSTEMS A great deal of research has been reported on soil clay minerals and soils as exchanger surfaces using both theoretical and experimental approaches (Vanselow, 1932; Gapon, 1933; Argersinger et al., 1950; Sposito, 1981a,b; Sposito et al., 1983; Jardine and Sparks, 1984). These studies consider that clays and/or soils are two-ion exchange systems (binary). Field soils, however, are at least three-ion exchange systems (ternary) (Adams, 1971; Curtin and Smillie, 1983). It is sometimes assumed that data for binary exchange reactions can be employed to predict ternary exchange reactions. In order for this assumption to be valid, one has to accept that binary exchange selectivity coefficients are independent of exchanger phase composition. However, Chu and Sposito (1981) have shown at a theoretical level that one cannot predict exchange-phase/solution-phaseinteractions of a ternary system solely from data obtained from binary exchange. These researchers argued that it is necessary to obtain experimental data from systems with three ions. Ternary exchange behavior at the theoretical level was studied by Wohl

198

V. P. EVANGELOU ETAL.

(1953), Hardy (1953), Currie and Curties (1976), and Chu a d Sposito (1981). Experimental data for ternary exchange involving NH,-Ba-La and Na-Rb-Cs on montmorillonite and Na-K-Cs on attapulgite were reported by Elprince and Babcock (1975) and Elprince et al. (1980). Additional exchange studies dealing with Na-Ca-Mg were reported by Wiedenfeld and Hossner (1978), Sposito et al. (1983), and Sposito and Le Vesque (1985). Farmers often use NH, and K salts as fertilizer sources. Even though applied NH, has a short life span in agricultural soils (1 -3 weeks or more depending on rates of nitrification), the K-NH,-Ca exchange interaction controls the distribution of these cations between the exchange and solution phases during that period. Thus, the availability of K and NH, in the solution phase would be affected by all ions present. There is much evidence in the soil science literature pointing out that the behavior of NH,+ in soils is related to the ability of the soil to adsorb NH,' . For example, Fenn et al. (1982) and Avnimelech and Laher (1977) have reported that NH, volatilization from soil is related to the cation exchange capacity. Furthermore, there is evidence suggesting that the rates of NH,+ oxidation in suspensions of various clay minerals appears to be regulated by the ability of the clay to adsorb NH,+. For example, Goldberg and Gainey (1955) reported that the availability of NH,+ to the nitrifying bacteria in liquid cultures containing different soil minerals appeared to be directly related to the quantity of unabsorbed NH,+ or the quantity of NH,+ ions spontaneously released from the soil mineral by exchange with other cations in the medium. Similarly, Kai and Harada (1969) reported that the substrate for nitrifying organisms was NH4+ in solution and not NH4+ adsorbed to the clay surfaces. Despite the suggestions that the soil surface could be implicated in the regulation of NH,+ losses and transformations, studies describing the exchange behavior of NH4+ in soils are lacking. Some investigationsof NH, exchange chemistry in soils with respect to K have been reported by Opuwaribo and Odu (1978), Pasricha (1976), Pasricha and Singh (1977), Evangelou and Blevins (1985, 1988), and Rappaport and Axley (1984). Opuwaribo and Odu (1974) demonstrated that application of K' to an illitic soil stimulated NH4+-Nfixation and Rappaport and Axley (1984) reported that the application of KCI with urea [(NH,),CO] diminished NH, losses and, consequently, increased the yield of sorghum-sudangrass. This evidence suggests that NH4+ behavior in soils is influenced by the addition of K + . Because of this influence, one also has to assume that K-Ca exchange behavior in soils would also be influenced by the addition of NH,' . Thus, the relative chemical potential of adsorbed monovalent cations would depend on the number and type of cations present in the soil system. This was demonstrated by Lumbanraja and Evangelou ( 1990). The Q/I plot of a ternary (three-ion) soil system can be explained by the

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS

199

Vanselow (1932) exchange expression (Evangelou and Phillips, 1987, 1988, and references therein). It should be noted that for exchangeable K+ fractional loads of c0.20, the situation encountered in most agricultural soils, the Vanselow equation and the Gapon equation are indistinguishable (Kc = 1/2Kv).For more information on this the reader should refer to Evangelou and Phillips (1987, 1988, and references therein). Because Mg2+ behavior is similar to that of Ca2+ (Beckett, 1964a; Sposito et al., 1983), the chemical symbol Ca is meant to represent (Ca Mg) in the following equations. For the exchange reactions

+

+ i+ % Ex, + 1/2Ex,Ca + j + Exj + 1/2Ex2Ca

1/2Ca2+

(67)

1/2Ca2+

(68)

where Ex, denotes the exchangeable form of monovalent ionic species i and j , the Vanselow expressions are Kv, = [xi/(X~,)”21[(ARi)-’l

(69)

KVZ= [Xj/(X,a)”21[(ARj)-’l

(70)

and

where Kv, and Kv2 are the Vanselow exchange selectivity coefficients for reactions (67)-(68), respectively, X, denotes the mole fraction in the exchanger, and ARij is the equilibrium activity ratio. The activity ratios for species i and j are ARi = (Ci)(Yi)/(C~~)”2(Y~a)”2

(71)

The component C, denotes the dissociated concentration of the cations in the solution phase, and yWarepresents the single-ion activity coefficient of the ionic species in the solution. The equation describing exchangeable ionic species i , in the presence of j and Ca (ternary system) (Lumbanraja and Evangelou, 1990) is

+

Ex, = (Kv,)(CEC)(ARi)/[4 (KvlARJ2 (KvzARj)2 Kv,Kv2AR,ARj]~~2

+

+

(73)

and the linear potential buffering capacity for i (PBC,) of the ternary system at a constant AR, is the partial derivative of Eq. (73). PBCi

=

(dExi/dARj),Rj

- CEC(Kv,)[4 + (ARjKV# [4

+ (KvlARj)’ +

+

(74)

1/2(Kv,KvzARiARj)] (KvZARj)’ KVIKv2ARjARjl3/’

+

Equations (73) and (74) could be applicable across the entire K-Ca exchange isotherm of a given soil if Kv, and Kv2have been empirically determined to be

2 00

V P. EVANGELOU ET AL.

constant and independent of exchanger phase composition. It is also assumed that for a given soil, the CEC is constant and independent of exchanger phase composition. Equation (74) also demonstrates that the ability of a soil to buffer the soil solution with respect to i at a given constant AR, is dependent on CEC, the Vanselow exchange selectivity coefficients for i-Ca and j-Ca exchange, and the relative concentration of the three cations (i, j, and Ca) in the soil solution. For a ternary system such as K-NH,-Ca, the equation that describes PBCK at constant ARNH4(Lumbanraja and Evangelou, 1990) is PBCK

=

(dExK/dARK),RNH4 = (dExK/dARK)ARNH4

(75)

and PBCNH, at constant ARK is PBCNH, = (~EXNH,/~ARN,)AR, = (dExNHJdARNH,)ARK Under such conditions, the limit of Eq. (75) as ARK PBCK = lim (aExK/dARK)AR,,, = ARK-0

ARK-0

--j

(76)

0 is

CEC * Kvl (77) [4 4- (KVZARNH~)~]~’*

and the limit of Eq. (76) as ARNH, + 0 is

Equation (77) demonstrates that when ARK + 0, PBCK will depend on the CEC, K,, , and K,, and on the magnitude of the constant ARNH4value. Assuming that CEC remains constant and for a constant ARNH4 value, PBCK will depend directly on Kvland inversely on KV2.A high Kvr, which indicates a high affinity for K, manifests itself as an extremely high slope on the low portion of the Q/I plot. Furthermore, in accordance with Eq. (77), an increase in ARK or ARNH4 would suppress PBCK (upper portion of ARK or Q/I plot) assuming CEC, Kvl and KV2are constant over the appropriate activity ratio range. A similar discussion applies to Eq. (78), which represents the change in exchangeable NH,+ at a constant ARK. In contrast to the PBCK for the K-NH,-Ca ternary exchange system [i.e., Eq. (77)], the PBCK for the K-Ca binary system, as ARK + 0, is described by Eq. ( 5 8 ) , which, for convenience, is restated: PBCK = (1/2)(Kv,)(CEC) This equation demonstrates that, when ARKapproaches zero, PBCK will depend on the product of K,, and CEC only. Assuming that CEC remains constant, then the PBCK is dependent on K,, only. A high Kvl indicates a high affinity for K and manifests itself as an extremely high slope on the low portion (ARK --j 0) of the Q/I plot. Furthermore, an increase in ARK would have a suppressing effect

SOIL POTASSIUM QUANTITYANTENSITY RELATIONSHIPS 20 1 on PBCK (upper portion of the Q/I plot) for a constant CEC and K,, (Evangelou and Phillips, 1987). The same applies to the NH4-Ca exchange system. Based on the above theoretical analysis, it appears that the addition of NH4+ or any other monovalent ion to a binary system such as K-Ca, under a constant CEC, K,, , and KV2, suppresses the PBCK and, consequently, increases the AR", Figure 9 shows that, indeed, NH4+ suppresses PBCK and increases ARE. In order to evaluate whether the experimental difference in the PBCK between binary and ternary systems is predictable, Eq. (73) was employed by Lumbanraja and Evangelou (1990) as follows:

+ (KvIARK? ExK = (KvI)(ARK~)CEC[~ + Kv,Kv2 ARK~ARNH,,~] -'I2

+

(KV~ARNHJ)'

(79)

where K,, and Kv2denote the experimentally determined binary Vanselow exchange selectivity coefficients for K-Ca and NH,-Ca exchange, respectively, and ARKt and ARNH,, are experimentally determined from a ternary (t) exchange system. The values K,, and K,, are obtained from the slope of binary Q/I plots, as ARK or ARNH4t 0 [Eq. (32)] (Evangelou and Phillips, 1988). Data obtained from a soil were used to solve Eq. (79) and the results drafted as Q/I plots. The PBCK values were determined from the slopes of these plots. The results are presented in Fig. 9 and they show that the predicted ExK-ARK relationship is not in agreement with the experimental relationship, which indicates that the binary constants are not adequate for predicting the ternary exchange system (Chu and Sposito, 1981).

-

0

0,005

0.010

0.015

0.020

A R ~(mol~-1)1/2

Figure 9 Potassium quantitylintensity (Q/I) plots of Eden soil in the absence (binary) and presence (ternary) of NH4 [ternary prediction is from experimental binary Vanselow exchange selectivity coefficients for K-Ca and NH,-Ca exchange, and from experimental ternary activity ratios (AR) for K and N R ; E x K is exchangeable K].After L u m b m j a and Evangelou (1990).

2 02

V. P. EVANGELOU ET AL.

-

2.0

-

I

0 Binary (NH4-Ca)

A Ternary (NH4-K-Co)

- Predicted Ternary (NH4-K-Cal 0

0.005

0.010

0.0I 5

0020

A R ~ ~ 4 , c ( m V2 ~ ~

Figure 10 Ammonium quantity/intensity (Q/I) plots of Eden soil in the absence (binary) and presence (ternary) of K (ternary prediction is from experimental binary Vanselow exchange selectivity coefficients for K-Ca and NH4-Ca exchange, and from experimental ternary activity ratios for K and NH4; ExNH4 is exchangeable NH,). After Lumbanraja and Evangelou (1990).

The results of the NH, Q/I plots in the presence and absence of added K + are presented in Fig. 10. These data demonstrate that the addition of K + greatly enhanced the adsorption of NH,' on the solid phase of the soil. A single slope for the Q/I plot (Fig. 10)of the NH,-Ca system indicates a single-site adsorption system. However, in the presence of added K, at least a two-site adsorption system is indicated by two slopes. The first slope, due to its magnitude, is attributed to high-affinity sites activated by the addition of K+.The second slope parallels that of the NH,-Ca system in the absence of K + , suggesting a similar type of reaction. To evaluate the possible causes for the difference in PBCNHI between the binary and ternary system, Eq. (73) was employed by Lumbanraja and Evangelou (1990) as follows: -I- ( K v ~ ( A R N H ~ ~ ) ~ ExNH4 = (KV,)(ARNH,~)CEC[~ + (KVIARK,)* + KVIKV2ARKtARNH4t1

(80)

The results for the solution of Eq. (SO) (Lumbanraja and Evangelou, 1990) as presented in Fig. I0 show that the predicted ExNH4-ARNH4 relationship greatly underestimates the experimental relationship. These findings suggest that the addition of K + influences the surface behavior of the soil. Detailed clay surface mechanisms for these processes are presented in Lumbanraja and Evangelou ( 1990). The point that is being demonstrated by the data in Figs. 9 and 10 is that the

SOIL POTASSIUM QUANTITYANTENSITY RELATIONSHIPS 203

Q/I relationship is highly dependent on the presence of a third cation. Depending on the mechanism of surface reaction of the third monovalent cation with the clay surface, the Q/I relationship may or may not be predictable. For example, in the case of K-Ca exchange in the presence of NH,+ as the third ion, the Q/I relationship for K was qualitatively predictable. On the other hand, in the case of NH,-Ca exchange in the presence of K + as the third ion, the Q/I relationship for NH,+ was greatly unpredictable. This is explained on the basis of specific adsorption of K + by the clay surface, which in this case was vermiculite, a mineral known for its high specificity for K+.

F. EXCHANGE REVERSIBILITY In the quantityhtensity (Q/I) concept, it is assumed that the shape and form of the Q/I plot is identical between the adsorption and desorption modes (Fig. 1 la). However, Nye and Tinker (1977) suggested that the desorption process may be affected by hysteresis. An example of an adsorption-desorption Q/ I relationship affected by desorption hysteresis is shown in Fig. 1Ib. The graphi-

Figure 11 (a) Idealized adsorption-desorption of any monovalent cation; (b) adsorptiondesorption behavior of a monovalent cation with a large hysteresis effect on the desorption process. After Arnold (1970).

2 04

V. P. EVANGELOU E T A .

cal relationship presented in Fig. 1 l b shows that the difference between the quantity of adsorbed X + and the quantity of desorbed X+(X,) is the quantity of X + fixed, X,(Arnold, 1970). In this case, the tem$xed denotes exchange reaction nonreversibility under conditions described by the Q/I experiment. Often, in the literature, the term Jixed denotes nonextractability of the cation by concentrated salt solutions (Quirk and Chute, 1968; Barshad, 1954; Lurtz, 1966; Opuwaribo and Odu, 1978). Arnold (1970) points out that an isotherm demonstrating a hysteresis effect can exhibit two different shapes (linear and curvilinear). If the desorption isotherm is linear, it indicates that the cation (X) is being desorbed from low-affinity sites only. If the desorption isotherm is curvilinear, the cation (X) is being desorbed from low- and high-affinity sites. The hysteresis effect appears to be affected by length of equilibration period. Nye and Tinker (1977) point out that a true hysteresis effect would persist no matter what length of time was given to establish a true equilibrium. In contrast, if the difference between an adsorption and a desorption isotherm is eliminated by extending the equilibration time, this would be considered a relaxation effect (Everett and Whitton, 1952). Distinction between hysteresis and relaxation may not be important. That the 24-hour desorption period often employed to establish Q/I relationships has practical meaning is important. In order for such Q/I data to have meaning with respect to soil K availability to plants, a 24-hour equilibration period for the desorption process is assumed to be adequate time for the soil to replenish the K in the soil solution, during periods of high K demand by the plant. However, such data are currently not available. Examples of K + and NH,+ adsorption-desorption plots for a two-cation system (K-Ca; NH,-Ca) and a three cation system (K-NH,-Ca) are shown in Figs. 12 and 13. These data show that there is a significant hysteresis or relaxation effect in the 24-hour desorption process. This hysteresis/relaxation effect, in the case of K-Ca exchange, is more pronounced without added NH,+ than with added NH,+ in the desorbing solution (Fig. 12). This behavior difference suggests that the hysteresis effect in this soil may be due to ion size. In the absence of NH,+ , the K+ would have to be desorbed by Ca2+.However, because Ca2+ has a larger crystalline radius than the K+, it cannot enter the collapsed vermiculitic interlayer, where it is assumed the K + is located (Rich and Black, 1964). However, because NH,+ has a relatively small unhydrated radius and a low negative hydration energy (Bohn et al., 1985), it can displace the K + from the vermiculitic interlayer. Potassium desorption in the presence or absence of added NH,+ appears to be associated with two types of K + adsorption sites, low-affinity sites, which most likely represent planar sites, and high-affinity sites, which most likely present specifically adsorbed K+ and/or interlayer trapped K + . Structural K + is also a type of high-affinity site. These apparent high-affinity sites maintain ARK at a +

SOIL POTASSIUM QUA"/INTENSITY

RELATIONSHIPS

205

1.0

-0

0 0

J

-5

x

w

0

A 0 Adrorptlon

+ Derorption -.5

Tornory K Adsorption

0 Derorption (Co2+constontl X Desorption (Coz+and NHtconrtant)

I

0

I

0.005

0.010

0.015

AR, (mol L-')'/*

Figure 12 Potassium quantity/intensity( Q I I ) plots of adsorption-desorption in the absence (binary) and presence (ternary) of added N&+ of the Eden soil (pH 4.91) (Lumbanraja, 1991).

constant level or, in other words, they display a very large capacity to buffer K + in the soil solution. Figure 13 demonstratesthat NH,+ adsorption is enhanced by the addition of K + . Opuwaribo and Odu (1974) reported a similar finding. They

0 Adsorption

+ Dosorption Ternary NH

A

Adsorption 0 Dosorption

0

0

0.005

0.010

0.015

0.020

Figure 13 Ammonium quantity/intensity ( Q I I ) plots of adsorption-desorption in the absence (binary) and presence (ternary) of added K+ and in a constant of Ca*+of the Eden soil (pH 4.92) (J. Lumbanraja and V. P. Evangelou, unpublished data).

206

V. P. EVANGELOU ET AL. Table I1

Quantityhtensity Relationships for Adsorption and Desorption of Potassium in the Absence (Binary) and Presence (Ternary) of Added Ammonium Desorption

Soil Eden

pH

System

4.29

Binary (Ca constant) Ternary (Ca constant) Ternary (Ca + NH4 constant) Binary (Ca constant) Ternary (Ca constant) Ternary (Ca + NH4 constant) Binary (Ca constant) Ternary (Ca constant) Ternary (Ca NH4 constant) Binary (Ca constant) Ternary (Ca constant) Ternary (Ca NH4 constant) Binary (Ca constant) Ternary (Ca constant) Ternary (Ca NH4 constant) Binary (Ca constant) Ternary (Ca constant) Ternary (Ca NH4 constant) Binary (Ca constant) Ternary (Ca constant) Ternary (Ca NH4 constant) Binary (Ca constant) Ternary (Ca constant) Ternary (Ca NH4 constant) Binary (Ca constant) Ternary (Ca constant) Ternary (Ca + NH4 constant)

4.92

7.26

+

Lowell

4.00

+

5.42

+

6.82

+

Nicholson

4.07

+

5.22

+

7.10

Adsorption

R’ Kf KdU R2 (Q/I (cmol (cmol (Q/1 plot) PBCK* k g - I ) k g - ’ ) plot) PBCKb 0.970 0.992 0.996 0.950 0.974 0.990 0.987 0.997 0.992 0.972 0.983 0.996 0.982 0.952 0.986 0.988 0.987 0.997 0.973 0.975 0.984 0.990 0.989 0.989 0.979 0.965 0.989

77.1 71.8 81.0 74.8 99.9 113.7 125.6

1.01 0.44 0.19 1.10

0.25 0.00 1.78

104.5

1.10

120.5 61.6 56.3 64.2

0.84 0.64 0.18

44.5

67.6 90.2 134.8 108.4 127.1 24.5 22.1 27.0 24.7 29.6 34.3 50.7 41.2 48.7

-0.11

0.77 0.31 -0.13 0.96 0.68 0.34 0.12 0.12 0.05 0.34 0.15 0.01

0.62 1.20 1.45 0.74 1.15 1.39 0.67 1.35 1.62 0.61 1.07 1.36 0.74 1.13 1.57 0.73 1.01 1.35 0.52 0.53 0.60 0.64 0.71 0.85

0.999

202.8

-

-

0.994 0.999

113.7 241.9

-

-

0.990 0.979

142.3 453.4

-

-

0.996 0.996

156.7 105.6

-

-

0.999 0.997

69.6 123.4

-

-

0.996 0.997

94.4 243.5

-

-

0.990 0.999

147.8 52.4

-

0.969 0.997

-.

37.3 42.6

-

-

35.3 120.1

0.10

1.01

0.994 0.989

0.22 -0.01

0.88 1.03

0.995

-

-

68.5

(ExK (first 24 hours of equilibrium) - y intercept(Kf). bPBCK = (cmol, kg-l)/(mol liter-’)”*.

“Kd =

showed that the addition of K + in some Nigerian soils caused an increase in NH4+ fixation, but NH,+ desorption in the presence and absence of initially added K + is virtually negligible. The desorption process of the three-ion system also appears to be pH dependent. The soils that had been adjusted with Ca(OH), to higher pH values and with HCl to lower pH values were used to carry out Q/Idesorption experiments in the binary and ternary systems. The effects of pH on Q/I desorption are shown by comparing the data in Table 11. These data point out that the decrease in pH

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS 207 values causes not only a decrease in the adsorption of K+ (note PBC decreases with decreasing pH), but also a decrease in the K+ fixation (Kf) (note the yintercept decrease with decreasing pH). Data on PBCK for adsorption and desorption indicate that the binary and ternary systems have lower PBCK values in soils with lower pH values than in those with higher pH values. The data in Table I1 show that fixed K+ (K,), which is represented by the y intercept, is higher in the high-pH soil than in the low-pH soil, except for the Nicholson soil. For example, in the Eden soil at pH 7.26, the Kf in the absence of added NH,+ (binary) is about 1.78 cmol kg-I. Conversely, on the same soil (Eden) but at pH 4.29, the Kfvalue is roughly half, 1.01 cmol kg-’. In the case of the Nicholson soil, however, the Kf of the binary system does not seem to differ significantly with respect to pH. The data (Table 11) also show that, in the case of the Eden and Lowell soils, the PBCK and ExK or Kf remain the same. This behavior could be due to the quantity and composition of the vermiculitic minerals that contain Al-hydroxy interlayers. Scott et al. (1957), Rich and Black (1964), and Barshad and Kishk ( 1970) demonstrated that the presence of Al-hydroxy interlayers tends to increase the potential of a soil to fix K +. The data in this study support these conclusions only for the Eden and Lowell soils, but not for the Nicholson soil. This is expected because conductimetric titration data for the Eden and Lowell soils (Lumbanraja, 1988) provide evidence of Al-hydroxy interlayers (Kissel and Thomas, 1969; Rich, 1970). The Nicholson soil, on the other hand, reveals a conductimetric titration plot that does not provide evidence of Al-hydroxy interlayers (Lumbanraja, 1988). The observation made in this study with respect to increasing Kfalong with increasing pH (the Eden and Lowell soils) appears to be explained by the possible presence of Al-hydroxy species, as suggested from the conductimetric titrations (Lumbanraja, 1988). The Nicholson soil, on the other hand, showed no K, change with respect to pH and there was no supportive evidence for the presence of Al-hydroxy species using conductimetric titration analysis (Lumbanraja and Evangelou, 1991). Hanway and Scott (1956), Hanway et al. (1957), and Rich and Black (1964) proposed a “pinching effect” by K+ on the interlayer edges that could account for N&+ fixation. The data presented by Lumbanraja and Evangelou (1990) do not lend support to such a conclusion. For example, it is shown that N&+ is not desorbed by Ca2+in the absence of added K + . Furthermore, X-ray diffraction shows that NH,+ addition did not collapse the vermiculitic interlayers in comparison to K + addition. The diffractogram is presented in Fig. 14. It demonstrates that the addition of NH,+ in the presence of Ca2+but in the absence of K + resulted in the maintenance of a 14-A peak that did not collapse to 10 A. In addition, in the presence of added K+, Ca2+,and NH,+ the 14-i%peak did not

208

V. P. EVANGELOU ETAL. I

15

10

5

2

Degree 2 9

Figure 14 X-Ray diffractogram of the Eden soil showing the effect of K + and NH,+on the 148, peak (Lumbanraja 1991).

completely collapse. This suggested that adsorption of NH,+ took place in unique sites that did not affect the d-spacing. The exact nature and mechanism of NH,' adsorption on such sites was not apparent. In order to demonstrate that the absorbed NH,' at low ARNHlvalues with and without added K' in the Q/I solution is indeed "fixed" (Fig. 13) or chemisorbed despite interlayer expansion, the techniques of Bremner (1959) and Bremner et al. (1967) were employed by Lumbanraja (1988). The results are reported in Table 111. These data show that 1 M KC1 cannot extract all the absorbed NH,+ in the presence or absence of added K + in the Q/I solution. Furthermore, the added K + in the Q/I solution appears to decrease the quantity of KC1-extracted NH,' . This nonextractable NH,+ is recovered, however, with a mixture of hydrofluoric and hydrochloric acids. An explanation for the high-affinity adsorption of the NH, ion on the clay surface could be justified by the difference in the adsorption mechanisms for K + and NH," on the surface composing the interlayer. In the case of K + , its attraction is multidirectional. Thus, due to its low negative hydration energy (Bohn et al., 1985), collapse of the vermiculite interlayer is induced. On the other hand, NH,+ has a tendency to expand the vermiculitic interlayer possibly due to the ability of NH,+ to chemisorb onto surfaces (James and Harward, 1964; Mortland, 1968). In fact, it is noticed that, in the presence of added K + , the adsorption of NH,+ on high-affinity sites is enhanced (Fig. 13). This could be due to

SOIL POTASSIUM QUANTITYIINTENSITY RELATIONSHIPS 209 Table 111 Adsorbed NH4 Extracted with 1 M KCI and Fixed NH4Extracted with a Mixture of 1 M HF and 1 M Hcl" Original soil Soil Eden

Lowell

Nicholson

pH 4.29 4.92 5.78 7.26 4.00 5.42 5.47 6.82 4.07 5.22 5.81 7.10

Fixed E x N H ~ NH4' 0.05

0.61

0.05

0.60

0.05

0.59

With added K in NH4+ Q/Isolution ExNH,

Fixed NH4+

Added NH4+

0.25 0.22 0.18 0.17 0.48 0.62 0.47 0.21 0.69 0.82 0.55 0.54

2.18 2.55 2.61 2.64 1.29 1.61 1.43 1.96 0.41 0.50 0.79 0.82

1.87 2.19 1.99 2.21 1.37 1.65 1.49 1.68 0.49 0.86 0.73 0.83

Without added K in the NH4+ Q/I solution +

Fixed E x N H ~ NH4+ 0.68 0.52 0.32 0.29 0.90 1.23 0.91 0.50 1.14

0.76 0.91 0.50

1.59 2.05 2.25 2.55 0.88 0.72 1.07 1.82 0.09 0.56 0.79 1.07

Added NH4' 1.75 1.97 2.09 2.34 1.32 1.64

1.53 1.86 0.68 0.89 1.04

1.08

"The added NH4+ is the difference between the NH4+ initially present in the Q/I solution and the NH4+ analyzed after the 24-hour equilibration. All values in cmol kg - I.

the potential of K + to dehydrate the interlayer, thus increasing the potential of NH,+ to coordinate itself as NH, on cations such as Ca, Mg, or A1 (James and Harward, 1964; Mortland, 1968).

V. RAPID APPROACHES FOR QUANTITY/INTENSITY DETERMINATIONS The traditional methods of obtaining Q/I information are considered too tedious for use in routine soil testing (Quemener, 1979; Bertsch and Thomas, 1985). These methods involve a lengthy procedure of equilibration and laborious analytical measurements of soil filtrates by flame spectrometry or other methods. Recently, advances in ion-selective electrode (ISE) technology have resulted in the development of potentiometric methods of determining K + in soil extracts and suspensions (Bailey, 1976; Talibudeen and Page, 1983; Yu, 1985). Advantages provided by ISEs include (1) simple, nondestructive analyses, (2) direct measurements of ion activities or concentrations, (3) automated analyses, (4) con-

2 10

V. P. EVANGELOU E T A .

tinuous monitoring capabilities, and ( 5 ) in situ determinations of ions. Clearly, these unique advantages of ISE can lead to a new way of characterizing the various forms and reactions of K in soils and minerals.

A. ISE THEORY AND ITSAPPLICATIONS An ISE has been defined by the Commission on Analytical Nomenclature of the International Union of Pure and Applied Chemistry (1976) as an electrochemical sensor, the potential of which is linearly dependent on the logarithm of the activity of a given ion in solution. The response of an ISE to its primary ion in the presence of interfering ions can be described by the extended Nicolsky equation (Bailey, 1976):

where E is the electrode potential and is the standard potential of the electrode; R, T, and 9 are the gas constant, absolute temperature, and Faraday constant, respectively; Z, and a,, and Z, and a, are the valence and activity of the primary ion i and interfering ions j, respectively; and kr' is the potentiometric selectivity coefficient, which defines the ability of an ISE to distinguish between the primary and interfering ions in the same solution (Bailey, 1976). For a situation where a, >> k ~ ' ( ~ , ) ~ the ~ ' ' Nicolsky ~, equation is reduced to the classical Nernst equation,

E

=

EP

RT + z, -In a, 9

which, for a system with a monovalent cation in a solution at a temperature of 295.2"K predicts a 58.6-mV potential response for each 10-fold change in ai. Apparently, the successful measurement of an ion in a mixed system, e.g., soil solution, is very much dependent on the effectiveness of the selectivity of the ISE for that particular ion. For the K + ISE (K-ISE), major improvement in its selectivity came about when the highly K+-selective neutral macrocycle valinomycin was discovered (Moore and Pressman, 1964) and incorporated into polyvinyl chloride (PVC) (Fielder and Ruzicka, 1973) to make the electrode membrane. Such an electrode has been reported to yield selectivity ratios of 10,000:1, 1000: 1, and 100: 1 for K + over Na+, H + , and NH4+, respectively (Bailey, 1976). In the past 40 years, there has been a great deal of literature on the application of the K-ISE in characterizing soil K. Mortland (1961) first used a cationic glass

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS 2 1 1 electrode (CGE) to determine Mg-exchangeable K+ in soil suspensions. Since then, CGEs have been used to determine K+ activities in aqueous soil suspensions and pastes (Krupskiy et d., 1974a,b), the rate of K+ exchange on clay minerals (Malcolm and Kennedy, 1969), and the K status at the soil-root interface (Xu and Liu, 1982). As the new generation of valinomycin-based Kselective electrodes (VKEs) have become commercially available, the widespread use of K-ISEs in soils research has been reported. Banin and Shaked (197 1) studied K + activity-concentration relations in soil water extracts, Farrell and Scott (1987) determined the concentration of soluble and exchangeable K in soil extracts, and others measured K + diffusion coefficients (Wang and Yu, 1989) and intraparticle diffusion kinetic constants (Ogwada and Sparks, 1986). Finally, K-ISEs alone (Para and Torrent, 1983) or in combination with other ISEs (Wang, 1986, 1990; Wang et al., 1988, 1990; Yu et al., 1989) have been used to expedite potassium Q/I and activity ratio (ARK) determinations.

B. Q I I MEASUREMENTS In the ISE-simplified Q/I procedure described by Parra and Torrent (1983), a single K-ISE in an electrochemical cell with liquid junction was used to measure the concentration of potassium (C,) in soil suspensions based on a successiveaddition procedure, whereas values of ARK were estimated from the empirical expression

+

ARK = (11.5 - 0 . 3 b ) C ~ 22

X

(83)

where ARKis the activity ratio in (mol liter-’)”*, CKis the potassium concentration in mol liter-) of equilibrated soil solutions, and b is the CEC (cmol) based on the weight of soil samples used. This method is significantly quicker than the conventional procedure employing spectrophotometry, especially because equilibration time is cut down to 10 minutes in comparison to the traditional 10-24 hours. Although this approach yielded Q/I results that were comparable to those based on analyses of soil extracts by atomic absorption spectrophotometry (AAS), the universal applicability of this method in estimating ARK values is apparently under question due to the large variation in soil properties of the various soils (Wang, 1986; Wang er al., 1990). It has been recommended that short equilibration periods (10 to 30 minutes) be used to minimize the effects of microbial activity and release of nonlabile K+ during the determination of Q/I relationships (Moss and Beckett, 1971), and of the CRK [concentration ratio: CK/(CCa+Mg)l/*]term instead of ARK (Evangelou et al., 1986; Wang, 1986). Therefore, Wang et al. (1988) later modified the successive-addition procedure of Para and Torrent (1983) to keep the equilibration

2 12

V. P. EVANGELOU E T A .

period short and characterize the Q/I relationships by introducing direct measurements of CRK values with Ca- and K-ISEs in an electrochemical cell with or without liquid junction. The cells with and without liquid junction, referred to as single-ISE [ISE(S)] and dual-ISE [ISE(D)] methods, consist of electrochemical cell arrangements of the type DJRE(Li0Ac) I( CaCl,

(0.01 M ) ,

KCl (0-5 mM) 1 M-ISE

(A)

and Ca-ISE I CaCI, (0.01 M ) ,

KCI (0-5 mM) I K-ISE

(B)

respectively, where M is K+ or Ca2+. The electromotive force (emf) values of these cells were measured and the related concentrations [CM] and ratios [ C,/( Cca)”z]were calculated from

E,(M) = EO,’(M) + 58.6 LOg[(CM YJ”Zu]

-

E,

(84)

and EB = E!’

+ 58.6 ~ o g [ ( C , / C 6 ) ( r K / r ~ ~ ) ]

(85)

in which P ‘ ( M ) includes the standard electrode potential of the M-ISE and the potentials of the internal reference element and inner liquid junction of the DJRE(Li0Ac) assembly; EE’ = EX’(K) - ER’(Ca); 58.6 is the slope (Nernst) factor at 295.2”K; yM and ZMdenote the activity coefficient and valence of the cations, respectively; and E, is the outer liquid-junction potential between the LiOAc salt bridge and the CaCI,-KCI test solution. The CaCI,-KCl solutions specified for cells A and B were used to characterize the ISEs, but were replaced by suspensions of the soils in the same salt solutions for the Q/I measurements. In the above application of the DJRE(SB) assembly, the inner liquid-junction potential between the internal reference element and the salt bridge was considered constant (Hefter, 1982); thus, EX’(M) and Eg’ are constant terms in Eqs. (84)and (85). In addition, the effects of Ej were minimized by using a concentrated solution of nearly equitransferent LiOAc (10 M ) for the salt bridge (Farrell, 1985) and by dominating the ionic environment of the test solutions and soil suspensions with CaCI, . On the other hand, in the ISE(D) method C, was estimated from CR, based on the assumption of a constant Ccain the equilibrated suspension. Although these two methods demonstrate good agreement with the traditional (spectrophotometric) methods (Wang er af., 1988), some uncertainties associated with the E, effect and C, calculations that are inherent in the ISE(S) and ISE(D) methods, respectively, have been reported (Wang et af., 1990). To eliminate these uncertainties, Wang er af. (1990) introduced a new electrochemical cell ternary arrangement referred to as ISE(T). In this method, K-, Ca-, and C1-ISEs were combined: Cl-ISE I CaCl,

(0.01 M),

KNO,

(0-5 mM) 1 M-ISE

(C)

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS 2 13 1.6 *

-

- AAS 0

-0

ISE(S)

ISE(D) A lSE(f1

r 01

-0.4

0

i

I

t

0.01

0.02

003

CR, (mol

0.04

L-’

Figure I5 Q/I relationships determined with the ISE(S), ISE(D), and ISE(T) methods in soil suspension; the AAS method in filtrate; and the successive-addition procedure. After Wang et al. (1988, 1990).

This eliminates the liquid-junction potential and yields emf values (Ec) that are related to the cation concentrations by the equation

Ec = Egr

58.6 b g ( c M ‘ f~)’”~(Cc1 ’ Ycl)

(86)

The components of cell C and Eq. (86) are defined by the same terms as those used with the other ISE methods, but reflect the inclusion of a CI-ISE as the reference electrode. By assuming that the C1- activity in the solutions and soil , Eo’ terms of Eq.(86) are also assumed suspensions is constant, the C,, * T ~and to be constants, and the CMvalues can be estimated from the measured emf values. Rapid, sequential determinations of E,(K) and Ec(Ca) were carried out and the measured emf values were combined to obtain CRK values from the equation

Ec(K) - Ec(Ca) = &’(K) - EOf‘(Ca) + 58.6 l o g ~ c ~ / c ~ ~ ) ( ~ (87) K/Y~~) Figure 15 shows the comparison of potassium Q/I relationships for Iowa Clarion soil determined with the above three ISE methods of characterizing soil suspensions along with atomic abso~tions~ctrophotometric(AAS) determinations of soil filtrates based on the successive-additionprocedure. Whereas the overall agreement between the Q/I curves is excellent when CRK values are K0.02 (mol liter-l)l’z, the Q/I curve tends to diverge at higher CRK values for the ISE(S) method. This difference can also be seen in Table IV, which shows the results of regression analyses comparing the A[ExK] and CRK values obtained with the various ISE and AAS methods. A[ExK] and CR, values obtained with the ISE(S) method demonstrate largest deviation from a I :I relationship among the comparisons. Clearly the ISE(S) method is subject to a source of error

2 14

V. P. EVANGELOU ET AL. Table IV

Results of Linear Least-Squares Regression Analysis Comparing the ISE Methods of CharacterizingSuspensions and the AAS Method of Characterizing Filtrates for Iowa Clarion Soila Linear regression Variable

Comparison’

Slope

A[ExK] (cmol kg-’)

ISE(S) vs. AAS ISE(D) vs. AAS ISE(T) vs. AAS ISE(S) vs. AAS ISE(D) vs. AAS ISE(T) vs. AAS

0.933 1.039

CRK (mol liter-’)”’

1.040 1.055

0.993 0.993

Intercept 1 -6 3 3 -I -I

x 10-2 X

x 10-3 x x x 10-5

r

0.998 0.999 0.999 0.999 0.999 0.999

‘After Wang er al. (1990). *ISE, Ion-selective electrode; S, D, and T, single, dual, and ternary; AAS, atomic absorption spectrophotometry.

that is not encountered with either the ISE(T) or ISE(D) method. Such error has been attributed to the physical blockage of the liquid junction that exists in the electrochemical cell for the ISE(S) measurements (i.e., cell A), rather than E, effects (Wang et al., 1990). On the other hand, although the ISE(D) method demonstrates no essential difference in measuring Q/I curves as compared with AAS (Fig. 15 and Table IV), caution must be taken when this method is used. This is because in the ISE(D) method the K concentration and thus A[ExK] is calculated based on the assumption that added Ca concentration in the equilibrated soil solutions is not changed (Wang er al., 1988). However, such an assumption was later shown to be invalid for general application (Wang et al., 1990). In contrast, the assumption of constant added CI concentration in the ISE(T) method was found to hold for most temperate soils, in which the permanent negative-charged clay particles are dominant. It is for this reason that the ISE(T) method has been recommended for rapid determination of potassium Q/ I relationships with soil suspensions (Wang et al., 1990). The Q/I parameters based on least-squares regression quadratic equations using only the data obtained with the CRK values <0.01 (mol liter-’)’’* (Wang er al., 1988) were compared in Table V. It is clear that agreement between the ISE and AAS results was excellent, especially in the CQ and P B Q results (no significant differences between all methods). In addition, the ISE methods were generally found more reproducible than the traditional spectrophotometric approach method in terms of low coefficients of variation for the Q/I parameters (Wang et al., 1990). However, only ISE(T) demonstrates overall excellent agreement with the AAS method (Fig. 15; Tables IV and V).

SOIL POTASSIUM QUANTITY/INTENSITYRELATIONSHIPS 2 15 Table V Parameters of the Q/I Relationships Determined with ISE Suspension and AAS Filtrate Methods and a Successive Addition Procedure for Iowa Clarion Soil” Q/I parameterb,‘

Q/ I method

CRKO

PBCKod

KLd

ISE(S) ISE(D) ISE(T) AAS

0.0030a 0.0031a 0.0031a 0.0030a

75.5a 73.5a 72.3a 73.8a

0.279ab 0.294a 0.274ab 0.257b

“After Wang et al. (1990). ’CRKo = (mol liter-’)’’*, PBCKO = cmol kg-’/(mol liter-1)’’2, and KL = cmol kg-I. ‘Within columns, means followed by the same letter are not significantly different at the 5% probability level by Duncan’s multiple range test. d P B C Kis~ defined as d(AExK)/d(CRK) at CRKO(Wang et al., 1988); and K L is the sum of ExK, and ExK, (Beckett, 1964b; see Fig. 2).

VI.EXPERIMENTAL OBSERVATIONS AND FUTURE QUANTITY/IN”ENSITY APPLICATIONS A. EXPERIMENTAL OBSERVATIONS The use of Q/I relationships to describe potassium status in soils is based on Woodruff’s observation that K availability to plants can be characterized by the free energy of K-Ca exchange with respect to solid-solution reactions. Beckett (1972) later listed a number of assumptions that need to be met in order for Woodruff‘s observation to be valid. The reader is referred to Beckett’s (1972) article for a detailed description of all the assumptions. One of these assumptions is that the rate of uptake of K by the plant root must be regulated by “the difference in the equivalent free energies of K and Ca as offered to the uptake sites.” Evidence that this assumption could be met under certain conditions is demonstrated from ion uptake data of excised maize roots obtained by Maas (1969). These data are presented in Fig. 16 and show uptake of K by excised maize roots from solutions with various concentrations of KCl and in the presence of 10 mmol, liter-’ CaCI,. Figure 16 demonstrates that the uptake of Ca2+in the presence of K+ is a competitive process. The data also show the rate of uptake of K maximized in the AG range predicted by Woodruff (1955b). Such experimental evidence of K uptake by excised maize roots provides additional support to the

216

V. P. EVANGELOU ET AL.

-

70

-, -

-Woodruff's predicled range of A G for optimum K a v o l l o b i l i t y to p l a n t s

a 70

0

50

-? 2 0) 0

'im

+

(u

0

u

10

-

1-

w"

1

I

I

I

'0

0

1

2

4

6

8

10

J*iiI

I

K;C mmol C' 1

-9.37 -7.61 -11.13 -11.84 -12.84 14.56

-

I

I

-6.57

-5.77

L -5.19

AG, kJ/Equivalent

Figure 16 The effect of increasing K + concentration on the uptake of K in 24 hours. The concentration of Ca2+was 10 mmol, liter-' and the pH was 6. Modified from Mass (1969).

validity of Woodruff's observation that K availability could be described by the free energy of K-Ca exchange as long as all other plant growth soil factors are not limiting and are held constant (Beckett, 1972). The use of Q/I in predicting K availability to plants in soils has been extensively tested in the past (Beckett, 1972; Bertsch and Thomas, 1985, and references therein). However, it has been shown that the Q/I relationship does not enjoy universal application, because a single relationship for all soils between K uptake by a given crop and ARK does not exist, perhaps due to the nature of the soil components regulating ARK. Recall that the term ExK is related to KG(CEC)(ARK) [Eq. (48)].By rearranging Eq.(48)and substituting the relationship AG = - RT In K G we can show that

RT ln[ARK] = RT In ExK

+ RT ln(l/KG) + RT ln(l/CEC)

(88)

Equation (88) reveals that when ExK represents an insignificant portion of the CEC of a soil (Evangelou and Karathanasis, 1986), RT In[ARK]is determined by three terms, namely, quantity of exchangeable K+, magnitude of K G , and magnitude of CEC. The latter two components represent the PBCK of the soil. Experimental evidence on the role of PBCK on K availability to plants was summarized by Khasawneh (1971). He pointed out that in the case of two soils with the same K G and the same quantity of exchangeable K + ,but with one soil having a CEC lower than the other, the ARK would be higher in the soil with the lower CEC. Consequently, the soil with the lower CEC would allow greater K uptake

SOIL POTASSIUM QUANTITYDNTENSITY RELATIONSHIPS 2 17 by a given crop, assuming all other plant growth factors were held constant. A similar argument could be made by varying the KG and holding the CEC constant. On the other hand, when two soils have identical ARK values but the CEC and/or KG of one soil are higher than those of the other, then the quantity of exchangeable K+ will be higher in the soil with the higher CEC and/or KG and this soil will allow for a greater K uptake by a particular crop, if all other plant growth factors were held constant. Similar data and arguments were presented by Mengel (1982, and references therein). For the same reasons Beckett (1964a) stated that ARK should be used as a comparative measure of K nutrition only for soils of similar Ca status. The above concepts demonstrate that soil K availability to plants is described by the interrelationship between ARK and PBCK. Therefore, ARK could be related to K uptake by plants, assuming that it is able to predict soil PBC, during the growth period of plant(s). Presently, such a prediction is not possible. For example, in some soils ExK may represent a significant portion of the CEC and therefore the relationship between ARK and ExK is a curvilinear one (Evangelou and Karathanasis, 1986). In such case, the ability of a soil to supply K to plant roots is rather complex. Le Coux and Summer (1968b) showed that during a K uptake period of 30 days and employing Japanese millet as a test plant, soil PBCK changed and, furthermore, there was no single ARK-K uptake relationship for all three soils tested. Grove et al. (1987) showed that the relationship between concentration ratio for K (CRK) and relative yield of soybeans in three Kentucky soils was excellent, whereas the relationship between extractable K by neutral, molar ammonium acetate and relative soybean yield was relatively poor. Possible reasons for the strong correlations between CRK and relative soybean yield for all three soils include a fairly similar soil mineralogy and, consequently, similar Ca status or PBCK values. It was of interest to note that two of the soils exhibited indistinguishable Q/I plots. For these two soils, as expected, the relationship between relative yield and ammonium acetate-extractable K appeared similar to the relationship between relative yield and CRK . On the other hand, the third soil exhibited very low PBCK but a relatively large quantity of high-affinity K. These results indicate that if soils have similar PBCK values, ARK or CRK alone can effectively describe behavior of K uptake. However, as Rasnake and Thomas (1976) pointed out, for many different soils, because different sites of adsorption for K+ exist and because these sites vary drastically in their affinities for K + , single determinations of concentration or activity ratios or exchange coefficients of soils are not sufficient for predicting K availability to plants. Rasnake and Thomas (1976) reported that the K+ for six Kentucky soils changed in the range of two- to sixfold from the period before cropping “Midland” bermuda grass [Cynodon dactylon (L.) Pers] to after cropping (Table VI). The causes for these large changes in the magnitude of the K + appear to be related to mineralogy and

2 18

V. P. EVANGELOU ET AL. Table VI Gapon Exchange Coefficient of Several Kentucky Soils before and after Cropping" Gapon coefficient (moI/Iiter) Soil Commerce sil Collins sil Melvin sil Huntington sil (1) Huntington sic1 (11) Nolin sil

Before cropping

After cropping

6.4

16.4 43.2 27.5 39.5 25.0 16.5

8.0 10.0

5.5 9.5 6.5

"After Rasnake and Thomas (1976).

to the chemistry of the soil system along with physical processes such as wetting and drying (Beckett, 1964b; Carson and Dixon, 1972, and references therein; Knibbe and Thomas, 1972). Natural soils are polycationic systems and for this reason Eq. (33) [or Eq. (SS)] does not satisfactorily describe soil PBC, . In polycationic systems, ions such as Mg2+,Na+, NH,', H 3 0 + ,H + , A13+ and the Al-hydroxy species need to be taken into consideration in order to predict ARK and PBCK . At the present time accomplishing such a task is impossible for a number of reasons. Exchange constants of polycationic systems are not necessarily similar to binary exchange constants (Lumbanraja and Evangelou, 1990, and references therein). Furthermore, ions such as H + and A13+and the numerous Al-hydroxy species appear to act as soil potential-determining ions (Sposito, 1984b; Lumbanraja and Evangelou, 199l , and references therein). Currently, our understanding of such reactions at the level where we can accurately predict them is extremely poor, especially in polycationic systems. Even greater difficulties in describing the above reactions are encountered when organic surfaces are involved. Researchers have attempted to describe ARK in the presence of more than two ions. For example, at the suggestion of Beckett (1964a,b, 1965), researchers routinely include Mg2+and Ca2+when calculating ARK. Addition of Ca2+and Mg2+ has thermodynamic justification because the two cations are nearly indistinguishable with respect to surface exchange reactions, although exceptions to this have also been reported in the literature (Sposito and Le Vesque, 1985; Sposito and Fletcher, 1985). Incorporation of Mg2+ into the calculation of ARK is correct with respect to the mathematical treatment of exchange reactions; however, its implication with respect to K+ uptake by plants is a different matter. It is well known that in some plants Mg2+uptake is highly controlled by K + levels,

SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS

2 19

but the reverse is not always true (Stout and Baker, 1981). Furthermore, competitive plant uptake effects between Ca2+ and Mg2+ may also influence the ARK-K uptake relationship (Khasawneh, 1971, and references therein). Tinker (1964) included A13+ in calibrating ARK in a manner similar to that used by Beckett (1964a,b) to include Mg2+.However, incorporation of AP+ into the ARKas proposed by Tinker (1964) so that ARK = [K+/(Ca2+ Mg2+)”’ (A13+)’/3] is strictly empirical. This may only allow one to evaluate the possible influence of A13+on K + uptake in a particular study involving a soil or a number of soils. In terms of modeling K availability in a soil system, A13+ inclusion in the above manner needs experimental justification. Clay surface-adsorbed A1 and its hydroxy species act as potential-determining ions, thus as pH changes and dissolved A1 in solution changes, so does the apparent soil surface electric potential (Lumbanraja and Evangelou, 1991, and references therein). This affects the magnitude of KG or Kv for all nonhydrolyzable exchanging metal-ion species in the pH range studied. The component that describes ion availability of a given ion is the electrochemical potential gradient of that ion between the solid phase and the solution phase as well as between the solution phase and the root surface phase, or the electrochemical potential difference between the soil surface phase and the root surface phase. The Q/I approach allows us to estimate the difference in the relative chemical potential of an ion between the solid and solution phases. This difference depends on the magnitude of the difference in the electrical potential between the solid and solution phases. The latter difference cannot be calculated nor can it be measured directly, and in a soil system it is highly transient depending on surface-specific reactions and solution ionic strength changes. This difficulty may add a major limitation to the potential of applying Q/I data obtained under certain conditions [e.g., pH, ionic strength, Ca concentration, exchange phase makeup, and soil solution anion type(s)] to a soil that will undergo a number of changes during a single growing season. The purpose of this review is to deal with the Q/I concept alone; however, it is very difficult for one to separate plant nutrient availability effects due to soil K chemistry from plant nutrient physiological effects (Haynes, 1980, and references therein). The Q/I concept does not deal with physiological effects, but it appears to be influenced by them. One should be familiar with plant nutrition concepts in order to have a better understanding of Q/I and its limitation in predicting ion uptake. Nevertheless, even if nutritional effects are assumed to play a limited role in K uptake under a given set of experimental conditions, the ability of a soil to replenish K + in the soil solution as described in this review is a highly complex process and definitely not solely dependent on K-(Ca Mg) exchange alone. Additional factors controlling K availability, as discussed in this review, include (1) hysteresis or exchange irreversibility effects, (2) anion effects, (3) multica-

+

+

+

220

V. P. EVANGELOU ETAL.

tion effects, (4) potential-determining ion effects, and (5) kinetic effects. There is no doubt that these factors need further investigation in soils in order to predict K availability. Furthermore, similar investigations must be carried out with respect to plant-root surfaces to determine the concentrations of particular ions that the plant root surfaces encounter in soil and how the root surface interacts with a particular soil, or soil solution (Wang et al., 1992). In addition to understanding the interactive chemistry between soil and soil solution and/or plant root surfaces, K availability is also influenced by the diffusivity of the soil medium, and consequently by the soil moisture content (Mengel, 1982, and references therein).

B. FUTUREAPPLICATIONS Aside from the fact that Q/I plays a fundamental role in understanding K availability to plants, an additional purpose is its use in the environmentally sound management of fertilizers involving soil surface-solution interactive components. These components determine soil solution composition and thus the potential fate of plant nutrients, e.g., leaching, uptake, and transformations. Recent breakthroughs in highly resistant ion-selective electrodes will allow them to be attached directly to farm implements and thus used in monitoring/ regulating fertilizer applications. For example, a NO,- electrode of this type is currently under field evaluation at the Princeton Experiment Station of the University of Kentucky and the preliminary results look promising (L. W. Murdock, personal communication). Apparently, newly developed ion-selective electrode methods for rapidly and accurately estimating soil Q/I relationships in soil suspensions (Wang, 1990; Wang et al., 1990) could also be used in the field for monitoring/regulating the application of K’ , NH,+ , and/or Ca2+ plus Mg’+. The ultimate future application of Q/I (as computing power becomes less costly and understanding soils and plant root systems as polycationic systems advances) would be in modeling soil-plant systems on a real-time basis.

ACKNOWLEDGMENTS The authors wish to dedicate this article to Drs. Woodruff and Beckett for their major contribution to understanding potassium chemistry in soils and soil potassium availability to plants.

REFERENCES Adams. F. (1971). Ionic concentrations and activities in soil solutions. Soil Sci. Soc. Am. Proc. 35, 420-426.

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MORPHOLOGICAL AND PHYSIOLOGICAL TRAITS ASSOCIATED WITH WHEAT

YIELDINCREASES INMEDITERRANEAN ENVIRONMENTS Stephen P. Loss and K. H. M. Siddique Division of Plant Industries Department of Agriculture, Western Australia South Perth, Western Australia 61 5 1, Australia

I. Introduction 11. Constraints in Mediterranean Environments A. Rainfall B. Solar Radiation C. Temperature D. Growing Season 111. Biomass Production and Partitioning A. Phenology B. Growth and Morphology W. Water Use A. Water-Use Pattern and Early Vigor B. Xylem Diameter C . Glaucousness D. Abscissic Acid Accumulation E. Osmoregulation F. Carbon Isotope Discrimination V. Radiation Use A. Interception B. Radiation-Use Efficiency VI. High-Temperature Stress VII. Use for Breeders VIII. Concluding Comments References

I. INTRODUCTION For several thousands of years, humans have been selecting wheats that are adapted to specific environments and cropping practices (Bell, 1987). With im229 Advances m Agronmny, Volume 52 Copyright 0 1994 by Academic Press, Inc. All rights of reproduction in any form reserved.

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provements in crop management and plant selection, wheat spread from the mediterranean climate of west Asia to most other parts of the globe and it is now one of the most widely adapted plants in the world. Modern wheats perform best in the temperate regions of Europe and North America, where yields are higher and less variable than in the region where wheat originated and in other similar parts of the world that experience a mediterranean-type climate-west Asia, north Africa, South Africa, southern Australia, and in southwest North and South America (Fig. 1). It is somewhat ironic that plant scientists are currently trying to improve the adaptation of modern wheats to the environment where they originated. Nevertheless, about 10- 15% of the world’s wheat is produced in mediterranean-type environments. An important factor contributing to the widespread adoption of wheat was the recognition of the significance of the environment to adaptation. For example, after European settlement of Australia in 1788, the first two wheat crops failed miserably partly because the European wheats did not cope with the long, hot, and dry Australian summer (Macindoe, 1975). Australian settlers then began introducing from South Africa, India, and the mediterranean region earlymaturing wheats, which were better adapted to warm climates. With the development of plant breeding techniques during the twentieth century, breeders all over the world began to modify crops to increase yields, using

Figure 1 The distribution of mediterranean environments in the world

MEDITERRANEAN WHEAT YIELD INCREASES

23 1

mainly an empirical approach, that is, by trial and error. The progress of breeders in mediterranean environments has been slower than in other regions (Slafer et al., 1993), probably because of the limitations that the mediterranean environment places on plant growth, in particular, water stress. Yield increases associated with the genetic improvement of wheat have been demonstrated by comparisons of old and modern cultivars grown under the same conditions, and the mean yield increase attributed to wheat breeding in Western Australia is 6 kg ha-’ year-’ (Perry and D’Antuono, 1989), about one-eighth of that measured in Europe (Austin e t a ! . , 1980) and North America (Dalrymple, 1980). Breeders have successfully combined many desirable traits in cultivars, primarily by selecting for yield; however, except for two traits, time to anthesis and plant height, breeders have not been convinced of the value of selecting for other morphological and physiological traits recognized by physiologists as important in determining grain yield (Whan et a l . , 1993). Many studies have identified traits that have contributed to increased wheat yields in the past (Austin et a l . , 1980; Cox et a l . , 1988; Perry and D’Antuono, 1989; Kirby et a l . , 1989, Siddique et al., 1989a,b; Loss et al., 1989; Slafer et al., 1990; Siddique et a l . , 1990a,b; Slafer and Andrade, 1993), and the authors of these studies proposed that further improvements in many of these traits may lead to future yield increases. In addition, on the basis of physiological research, plant scientists have identified new unexploited traits that may increase wheat yields, for example, narrow xylem vessels (Richards and Passioura, 1981) and osmoregulation (Morgan, 1983). In the past, most physiologists and breeders operated independently, but recently a new level of cooperation has arisen. Future yield improvements may be hastened by a better understanding of factors that control growth, development, and yield of cereals (Shorter et a l . , 1991), and physiologists are helping breeders develop the most appropriate plants for particular environments. Richards ( 1 982) termed this breeding approach “analytical,” rather than empirical. This article reviews the wheat physiology/breeding work relative to the constraints of dryland cropping in mediterranean environments and explores opportunities for additional yield improvement associated with morphological and physiological traits. Other reviews have dealt with related aspects (Simmons, 1987; Ludlow and Muchow, 1990; Bidinger and Witcombe, 1989); however, these discussed a number of crops in a number of environments. By addressing wheat in mediterranean environments, we specifically review the progress of breeders and physiologists working in these regions and develop more concrete conclusions. Relatively few physiological studies have been conducted in mediterranean environments, therefore we occasionally draw on data from other environments. Improvements in disease and pest resistance, and increased tolerances to salinity, waterlogging, acidity, and mineral toxicities, are important contributions

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made by plant breeding; however, these are mainly localized stresses. Maintaining and improving grain quality is also an important breeding objective. Such types of breeding subprograms may also involve considerable physiological understanding, but they are not considered in this article. We begin by discussing the environmental constraints to crop growth in the parts of the world that experience mediterranean-type climates.

11. CONSTRAINTS IN MEDITERRANEAN ENVIRONMENTS Mediterranean climates are basically characterized by long, hot, dry summers and short, mild, wet winters. Cereals are mainly grown under dryland conditions in these areas, and although dryland implies unirrigated, water-limiting situations, the growth of cereals is not always limited by lack of water in mediterranean environments. Cereals are planted soon after the first autumn rains and they undergo vegetative growth in winter. They switch to reproductive growth as temperatures and photoperiods increase in spring, and they mature in early summer.

A. RAINFALL The constraints to cereal growth vary in mediterranean environments, but inadequate rainfall is usually the most limiting factor (Nix, 1975; Fischer, 1979). Cornish (1950) reported that 70-80% of the variation of yield in South Australia was due to variation in annual rainfall, and similar relationships exist in North Africa, west Asia and Western Australia (Srivastava, 1987; Blum and Pnuel, 1990; Karimi and Siddique, 1991a). According to Aschmann (1973), mediterranean environments receive between 275 and 900 mm annual rainfall, with the majority (>65%) in winter. Figure 2 illustrates the winter-dominated rainfall pattern at six sites that experience mediterranean climates, three in the Northern Hemisphere and three in the Southern Hemisphere. In general, winter rainfall exceeds crop demand because of mild temperatures, low evaporation, slow growth rates, and the high reliability of these rains. The coefficient of variation of the midwinter rainfall is about 6% at Merredin, Western Australia, whereas summer rainfall has a coefficient of variation of about 15%. Hence, intermittent drought during winter is rare, and, on the contrary, waterlogging can be a problem on some soil types in wet years. During spring, rainfall becomes less frequent, temperatures and vapor pressure deficits (VPDs) increase, and soil moisture is usually exhausted by the time

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-E E

2,

E

Ec

f

Figure 2 Mean monthly rainfall and temperature at six locations with mediterranean climates; (a) Merredin, Australia (31"20'S 118"17'E); (b) Cape Town, South Africa (3396's 19"29'E); (c) Rancagna, Chile (34"IO'S 7Oo45'W); (d) Aleppo, Syria (36"Il'N 37'13'E); (e) Rabat, Morocco (34"OO'N 6"50'E); and (f) Davis, California (38"32'N 121'45'W). Sites in the Southern Hemisphere are for January-December and those in the Northern Hemisphere are for July-June. Numbers in parentheses are the number of years of records used to calculate the means. Data from Wernstedt (1972).

the crop reaches maturity. This is often referred to as terminal drought and its timing varies according to the last spring rains, temperatures, soil type, and crop growth.

B. SOLARRADIATION Solar radiation has a large influence on temperature and evaporation regimes, and hence crop growth. In most mediterranean environments, midday solar radiation is about 6- 10 MJ m-* day-' in midwinter (Fig. 3), and it is unlikely that radiation limits crop growth, especially because temperatures and crop leaf areas are low at this time. In spring, however, when the leaf area index (LAI) is about 3, the lower canopy of the crop becomes shaded by the upper leaves and ear, and

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Month

Figure 3 Mean monthly climatic data for Aleppo, Syria (-) and Merredin, Australia (---); (a) solar radiation, (b) maximum and minimum temperatures, and (c) pan evaporation. Months are January-December for Merredin and July-June for Aleppo.

solar radiation may limit photosynthesis of the lower leaves. In midsummer, the elevation of the sun is high, there is a low incidence of cloud cover, and mean midday solar radiation is about 25-30 MJ m-* day-'.

C.TEMPERATURE As well as lack of rainfall, cereal growth can also be constrained by both high and low temperatures in mediterranean environments. Temperatures follow trends in solar radiation (Fig. 3). Summer maximum temperatures range between 25 and 40°C along western coasts and between 30 and 45°C inland and in the more easterly regions. Even if water was available in summer, as temperatures increase cereal development and respiration increase, while assimilation rates reach a plateaux, and thus growth at high temperatures (>30"C) is suboptimal. In most mediterranean environments, at least 1 month has an average temperature below 15°C (Fig. 2) and less than 3% of the year experiences minimum temperatures below 0°C (Aschmann, 1973). Mean monthly minimum tempera-

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tures in midwinter range from about 0 to 7°C and in some regions, especially inland areas, temperatures fall below 0°C during individual nights. Vegetative growth rates of cereals are restricted by low temperatures in midwinter and minimum temperatures are generally not low enough to cause long-term freezing damage to cereals during their vegetative stages of development. In northern Syria, severe frosts occasionally reduce the leaf area of cereal crops; however, the effect of frosts on grain yield during the vegetative stages is usually small because of compensation in later growth (Harris et al., 1989). Frost damage to the wheat stem and ear during early spring is an important constraint to wheat yields in mediterranean environments (Harris et al., 1989; Loss, 1989). Wheat becomes more susceptible to freezing damage as it enters its %productive stages of development, and the period from ear emergence to 2 weeks after anthesis is the most susceptible stage. Minimum temperatures may fall below - 2°C in early spring, killing the ear and/or restricting the movement of assimilates in the stem. Frosts can cause devastating yield losses to wheat crops that reach anthesis early in individual years. At these late stages of the life cycle, there is little opportunity for recovery from frost damage, although there is some compensation in the growth of unaffected grains and if spring conditions remain mild and moist, plants may be able to produce late tillers.

D. GROWING SMON In mediterranean environments, the period of crop growth is usually restricted by lack of rainfall, water deficits, and high temperatures at the start and end of the season. Potential evaporation (Epan) exceeds rainfall for a large proportion of the year. The timing of the first autumn rains can vary considerably, and sowing times may vary from year to year over a period of 8- 10 weeks. Developments in machinery and weed control have enabled farmers to sow soon after the first autumn rains and maximize autumn water use (Perry et al., 1989; Anderson and Smith, 1990; Kerr et al., 1991); however, because of the limitations of rainfall and evaporation, there is little scope for improving yield by extending further the period for crop growth. With the adoption of early sowing, there is also the risk of an extended period of dry weather after the initial autumn rain, and under such conditions, early sown crops can be subjected to water stress soon after emergence (Ken and Abrecht, 1992). Winter can change abruptly into spring and the termination of the growing season varies considerably depending on rainfall, temperatures, and soil type. Soil type is an important factor affecting the moisture status of a crop and soils can vary considerably within a small area. For example, there is often a variety of soils in Western Australia within a transect of 100-200 m, ranging from deep infertile leached coarse-textured soils on the higher parts of the landscape to

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poorly drained clay loams in the valley floors, including transitional and duplex soils. Most crops in Western Australia depend largely on current rainfall because of the poor water-holding capacity of many soils, particularly in low-rainfall regions. Crops grown on fine-textured soils, such as those common in northern Syria and South Australia, tend to rely more heavily on stored soil moisture. Rainfall largely determines the length of the growing season in mediterranean environments and the pattern and efficiency of water use has a large effect on wheat yields. Ludlow (1989) groups strategies of plant adaptation to waterstressed environments into three categories. Reduced life cycle of the plant to match the average growing season is termed escape. To maximize long-term yields in mediterranean environments, it would be ideal to have a wheat cultivar that not only tolerates years when the terminal drought is early, but one that also takes advantage of years when spring rains and mild temperatures extend the growing season. Maximizing water uptake and minimizing water loss are termed avoidance, whereas mechanisms that enable a plant to cope with reduced water content are termed drought tolerance. These strategies are useful in mediterranean environments and wheat plants are capable of all three. We will now examine the morphological and physiological traits that have or are likely to increase wheat growth and yield through reduced water, radiation, and temperature stresses. We also discuss how these traits have been measured and possible selection techniques for these traits in breeding programs.

111. BIOMASS PRODUCTION AND PARTITIONING Donald and Hamblin (1 976) define grain yield (GY) as the product of the biomass produced and the harvest index (HI; the proportion of the aboveground biomass that is partitioned to the harvested grain).

GY

=

Biomass x HI

Biomass can be increased by agronomic manipulation (early sowing and increased seed rate and fertilizer) or by genetic means. However, unless the increased biomass is matched to the life cycle of the crop, there is a risk of exhausting water sources before maturity is reached. In this article, we only consider the genetic paths to increased biomass. In the past, genetic increases in wheat yields around the world have largely been associated with changes in HI, whereas increases in biomass production have been small or negligible (Deckerd et al., 1985; Cox et al., 1988; Perry and D’Antuono, 1989; Austin et al., 1989; Siddique er al., 1989a; Safer and Andrade, 1993). Results from Mexico and Canada are exceptions (Evans, 1987;

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Hucl and Baker, 1987). Up to the 1970s, biomass production had not increased for wheats bred by The International Maize and Wheat Improvement Centre (CIMMYT), but more recent yield increases in cultivars bred under irrigation have been associated with increased biomass under water-limited situations (Evans, 1987). Also, Hucl and Baker (1987) found a positive correlation between GY and biomass production when comparing old and modem Canadian spring wheats. Figure 4 illustrates changes in GY, biomass, HI, and maturity with the year of release of Western Australian wheats. This figure is based on the data of Perry and D’Antuono (1989), but also includes three new cultivars not included in their study. The inclusion of these latest cultivars indicates that the rate of increase in GY between 1965 and 1990 is greater than in the preceding 100 years, and the rate of biomass increase associated with breeding is small (Fig. 4b). Most of the GY increase can be attributed to increased HI (Fig. 4c), and the duration from sowing to anthesis has decreased with selection for yield (Fig. 4d). This and other studies (CIMMYT, 1991; Slafer et al., 1993) also demonstrate that modern cultivars outperform older cultivars even in dry environments with low yield potentials. Several authors (Donald and Hamblin, 1976; Richards, 1987; Turner and Nicholas, 1987) suggest future wheat yields can be increased by increasing biomass production. Certainly, there is the potential for increased biomass production in mediterranean environments in some circumstances; barley is capable of producing more biomass and grain than wheat using the same amount of water (Siddique et al., 1989b; Lopez-Castaneda, 1992; Gregory et a!., 1992; Simpson and Siddique, 1993). However, as with increased biomass caused by agronomic manipulation, genetically increased biomass is not always translated into increased GY. Where lack of water is the major limitation to growth, it appears that it will be difficult to increase the biomass production of wheat significantly, especially when soil water is completely exhausted at maturity. Under these circumstances, higher biomass will be translated into higher yield when rainfall is used more efficiently for photosynthesis and it may be difficult to improve these fundamental physiological processes with conventional breeding methods. We will discuss in more detail the potential for increasing water-use and radiation-use efficiencies later and deal with assimilate partitioning first. The genetics of HI is probably more easily modified than biomass production, but given the nature of moisture stress during grain filling, large increases in HI are unlikely in mediterranean environments (Siddique et al., 1989b; Hadjishristodoulou, 1991). Aspects of the genetic, physiological, and environmental regulation of partitioning of assimilates were reviewed by Snyder and Carlson (1984), Gifford er al. (1984), and Wardlaw (1990). Germination, ear initiation, terminal

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S . P. LOSS AND K. H. M. SIDDIQUE

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Figure 4 Mean cultivar grain yield, biomass, harvest index and maturity score of wheat cultivars released between 1860 and 1990 in Western Australia when grown under the same conditions. Maturity score is the time from sowing to anthesis relative to Gamenya = 100, which takes about 110 days. Data from Perry and D'Antuono (1989) for cultivars up until 1979 (28 experiments each) and from Siddique et al. (1989a.b). Regan et al. (1992), and Loss et al. (1989) for the three latest cultivars (five experiments each).

spikelet, and anthesis act as physiological switches for the allocation of assimilates to different organs of the plant, hence phenology (i.e., the duration of each of these development phases) and assimilate partitioning are closely related.

MEDITERRANEAN WHEAT YIELD INCREASES

239

A. PHENOLOGY Change in phenology is the single most important factor that accounts for increased wheat yields in Australia (Perry and D' Antuono, 1989; Richards, 1991). Early Australian pioneers and wheat breeders recognized that matching the crop life cycle to the length of the growing season is one of the most important factors influencing crop growth and yield. As we will discuss in more detail, changes in phenology have had secondary effects on assimilate partitioning, pattern of water use, and other traits. Several reviews (Kirby and Appleyard, 1987; Simmons, 1987; Hay and Kirby, 1991) have detailed the current understanding of wheat development, and here we outline only the main features applicable to mediterranean environments. The switches from one stage of development to the next are determined primarily by genes sensitive to photoperiod, and both high and low temperatures. In general, there has been a trend to select for less sensitivity to photoperiod and vernalization, especially in mediterranean environments, thereby advancing development and reducing the time to reach anthesis (Hake and Weir, 1970; Austin et a l . , 1980; Davidson et al., 1985; Perry et al., 1987; Cox et a l . , 1988; Van Oosterom and Acevedo, 1992). Of the regions that experience mediterranean climates, wheat phenology has been described in the most detail in Western Australia. In several comprehensive studies, Kirby and Perry (1987), Kirby et al. (1989), Siddique e t a l . (1989a,b), and Loss e t a l . (1989) observed the development pattern of old and modern wheats bred in Western Australia, and illustrated how the life cycle of wheat has changed with selection for yield. Hence, we give frequent examples from these studies.

1. Vegetative Development The rates of leaf initiation and emergence in wheat are relatively constant when plotted against thermal time (i.e. accumulated temperature, as defined by Weir et a l . , 1984), although photoperiod and cultivar can have a small effect on these rates. The rate of primordia initiation is about one every 50"Cd (degree days, above a 0°C base), and the rate of leaf emergence is about one every 100"Cd (Kirby and Perry 1987). Tillers are initiated in the axils of the leaves and, if conditions are suitable, the first tiller appears after 2.5-3 leaves have emerged. Subsequent tillers appear at intervals equal to about one phyllochron (the interval in thermal time between the appearance of one leaf and the next). Ear initiation signals the end of vegetative development and the start of reproductive development. In general, modern Australian cultivars have faster rates of vegetative development than old cultivars, including faster rates of leaf appearance, shorter durations of vegetative growth, fewer leaves, and, hence, fewer tillers (Kirby et

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Kulin 1986 Garnenya 1960 Purple Strau 1860 Thermal time from sowing (OCd)

Figure 5 Duration between sowing, double ridge (DR), terminal spikelet (TS), anthesis (A), and physiological maturity (PM) for Western Australian wheats (Kulin. Gamenya, and Purple Straw) grown at Perth under irrigation. Year of release is indicated. Data from Kirby ef al. (1989) and Loss er al. (1989).

al., 1989; Siddique et al., 1989a,b). For example, the phyllochron intervals varied between 97 and 126"Cd for Western Australian wheats, and a modern barley cultivar had a phyllochron interval of 84"Cd. The longest duration of vegetative development was shown by the cultivar Purple Straw, released in the 1860s, which reached double ridge 958"Cd after sowing (Fig. 5). In contrast, the cultivar Kulin, released in 1986, reached double ridge 424"Cd after sowing, and, consequently, Kulin produced only 8 leaves on the main stem while Purple Straw produced 14. Roots have been less well researched than shoots because of the difficulties in root collection and measurement, hence our understanding of root growth and development is less complete than that for the shoot, particularly in mediterranean environments. This is not to say that roots are less important. In fact, root growth is an important component of the adaptation of wheat to dryland environments. The relationships between root and shoot development have been described by Klepper et al. (1984) and were recently reviewed by Klepper (1992). Root growth, with reference to mediterranean environments, is described later in this article.

2. Ear Initiation Ear initiation begins with spikelet initiation, progresses during a period of leaf growth, and ends at terminal spikelet formation. Double ridge is easier to recognize than ear initiation, and for practical purposes these have been considered the same. The rate of spikelet initiation is faster than that of leaf initiation (Kirby and Perry, 1987). Modern wheats have faster rates of spikelet initiation than do old cultivars; one every 12"Cd for Kulin and 33"Cd for Purple Straw (Kirby et al., 1989; Siddique et al., 1989b). Old cultivars have some vernalization requirement

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241

and when grown in areas with effective vernalizing temperatures, they have shorter durations between ridge and terminal spikelet than do modern cultivars-238"Cd for Kulin and 167"Cd for Purple Straw (Kirby et al., 1989). However, in warm areas with similar photoperiods, the vernalization requirements of the old cultivars are met more slowly and their duration between double ridge and terminal spikelet is extended, whereas the duration in modern cultivars is decreased (Fig. 5). Bingham (1969) suggested extending the period of ear development to increase sink capacity of the grain, and, interestingly, this has occurred with selection for yield in Western Australia. The period between double ridge and anthesis was 184"Cd longer in Kulin than in Purple Straw when grown in cool parts of Western Australia (Siddique et al., 1989b). However, warm temperatures during winter can cause rapid rates of development in wheats with little or no vernalization requirements and this results in reduced numbers of spikelets and number of grains per spike (Warrington et al., 1977; Shpiler and Blum, 1986).

3. Floret Initiation and Stem Elongation Floret development starts just before terminal spikelet formation, and coincides with the beginning of stem internode elongation. During this critical stage of development, the potential number of grains and the yield potential of the crop are determined, while there is an overlap of leaf, stem, root, and ear growth. Under favourable conditions, the central spikelet of a developing ear can produce up to 10 floret primordia, but only two to four survive and set grain in mediterranean environments (Siddique et al., 1989a; Slafer and Andrade, 1993). The number of tillers reaches a maximum at terminal spikelet and declines until anthesis. While old cultivars produce many more tillers than do modern cultivars, tiller survival is much lower-35% in old and 5 1 % in modern cultivars (Siddique et al., 1989b). The duration between terminal spikelet and anthesis of modern cultivars was shorter than in old cultivars (Kirby et al., 1989)-825"Cd for Kulin and 927"Cd for Purple Straw (Fig. 5 ) .

4. Anthesis Anthesis signals the end of vegetative growth and the start of grain filling, and its timing can have a large effect on cereal yields in mediterranean environments. The timing of anthesis is particularly important for determinate plants, such as cereals, because they only have a single opportunity for producing grain, as opposed to indeterminate plants, which are able to produce flowers over a considerable period of time. The time of anthesis that produces the highest longterm yield is a compromise between sowing time and the risks of frost, low biomass production, disease, high temperatures, and drought during grain filling. Several studies have demonstrated that cereal yields increase when anthesis is

2 42

S. P. LOSS AND K. H. M. SIDDIQUE

advanced because of decreased high temperature and moisture stresses during grain filling (Fischer and Kohn, 1966; Woodruff and Tonks, 1983; Kerr et ul., 1991). In addition, metabolic energy is required for the storage and retranslocation of assimilates (Geiger and Fondy, 1980), and once the minimum required structures of the plant have been produced, that is, the stem and leaves; then it is more efficient to partition growth directly into ears and grain, rather than producing additional vegetative growth and retranslocating the assimilates to the grain at some later stage. Anthesis has been advanced by genetic means and by the adoption of sowing very soon after the first autumn rains, when temperatures are warmer than in winter, hence improving early growth. Unlike ear initiation or terminal spikelet development, anthesis is easily visible and there has been a conscious selection for early anthesis in many environments (Halse and Weir, 1970; Austin et al., 1980; Davidson et ul., 1985; Perry and D'Antuono, 1989; Cox et al., 1988; Van Oosterom and Acevedo, 1992). Consequently, the duration between sowing and anthesis has decreased considerably (Fig. 5). In the study of Loss er al. (1989), Purple Straw flowered after 2132"Cd, whereas Kulin flowered after 1416"Cd. Early anthesis can also have detrimental effects. Unfortunately, during the period after emergence, the ear is very susceptible to frost damage, and as discussed earlier, frosts can cause devastating yield losses. Rapid development may also reduce the amount of biomass produced at anthesis, the number of sites for grain filling, and, hence, potential yield (Fischer, 1979). As was shown for sunflowers in Spain (Fereres et al., 1986) and sorghum in Texas (Blum and Arkin, 1984), very early-maturing plants may have restricted rooting depth and water use. In some early-sown crops that flower early, the high temperatures during vegetative growth increase the crop's susceptibility to diseases, especially on the early-emerging flag leaf (Wilson, 1989). Breeding for rapid development, that is, less photoperiod sensitivity and less vernalization requirement, has caused more variation in date of anthesis. In individual years and at locations where the optimum period of anthesis is very narrow, small variations in temperature can change crop development such that the crop reaches anthesis beyond the optimum time. For example, in Western Australia, farmers should sow early when the first autumn rains commence early to make use of the available moisture and warm temperatures (Perry et af., 1989; Kerr et al., 1991); however, there is a need for midseason cultivars that reach anthesis during the optimum period from these early sowing times, particularly in years when winter temperatures are above average. This could be achieved by incorporating a small vernalization requirement into existing cultivars (Anderson and Smith, 1990). Cultivars with a small vernalization requirement are less affected by temperature variations than are cultivars that require no vernalization, because vernalization prevents rapid pre-ear initiation development should an early break of the season be followed by a warm winter (Loss et al., 1990; Van

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243

Oosterom and Acevedo, 1992). Ludlow and Muchow (1990) also suggested that a greater sensitivity to photoperiod will overcome the effect on the timing of anthesis of year-to-year variations in temperature. With a large photoperiod sensitivity, anthesis is mainly triggered by daylength at a particular time of the year, irrespective of temperature. In contrast, yields may be increased by very early anthesis in dry areas with a low risk of frost, especially in seasons when autumn rains are delayed and sowing is later than average. Very early anthesis, that is, less than 95 days from sowing (<1 100-130O0Cd), may be appropriate for areas of North Africa (Van Oosterom and Acevedo, 1992) and Western Australia.

5. Grain Development The development of wheat culminates with the formation of grain. Grain filling can be divided into three phases. After anthesis there is a short period of exponential growth, sometimes referred to as the lag phase, during which time the cells of the endosperm divide rapidly and the potential size of the grain is determined. During the second phase, starch is deposited in the endosperm and the rate of growth is constant when expressed as thermal time. The final phase begins when lipids are deposited in the phloem strands supplying the grain and the growth rate declines until maximum grain weight is achieved. The process of grain growth can be considered as two components-rate, which is reflected in the rate of biochemical reactions involved in the synthesis of starch and protein, and duration, which is a reflection of the developmental program of the grain (Jenner et al., 1991). Loss et al. (1989) and Austin et al. (1989) studied grain growth and development of old and modern wheats under irrigation in a mediterranean and a temperate climate, respectively. Cultivars with a short duration from sowing to anthesis, which was the case for most modem cultivars, also had a long duration of grain growth, i.e., about 800°Cd, or 200"Cd longer than the oldest cultivars (Fig. 5). This may explain why modern cultivars are more able to exploit seasons and environments where conditions in spring are mild and grain filling is not terminated by drought and high temperatures. Modern cultivars that reached anthesis quickly also had shorter lag phases than did old cultivars-6% of the duration of grain growth for Kulin and 18% for Purple Straw. We will discuss changes in the rate of grain growth later.

B. GROWTH AND MORPHOLOGY Breeders have changed the structure of cereals considerably, both indirectly through changes in phenology and directly through the introduction of dwarfing genes.

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S. P. LOSS AND K. H. M. SIDDIQUE

1. Leaves and Tillers One path to improved wheat yields is to increase HI by decreasing the proportion of biomass in leaves and tillers (Donald and Hamblin, 1976). Selection of wheats for Western Australia and southwestern Iran has favored cultivars that produce fewer main stem leaves and fewer tillers than the old cultivars (Siddique ef al., 1989b; Ehdaie and Waines, 1989). Modern cultivars produce only primary tillers associated with the first two or three leaves. These tillers have three or more of their own leaves, their own nodal roots, and they are largely independent of the subtending leaf for assimilate supply, hence their survival rate is high, about 50%. Older wheats produce many more tillers and subtillers than do modern cultivars, of which 35% survive to produce grain (Siddique et al., 1989b). Given the inefficiencies in retranslocation (Geiger and Fondy, 1980), tiller death represents a net loss of assimilate by the plant and increases the water used before anthesis. Siddique el al. (1989b) suggested that additional increases in yield in mediterranean environments may result from further decreases in tiller number and increases in tiller survival, particularly in low-rainfall areas where biomass production is low. They acknowledged that this may also reduce the ability of the plant to take advantage of years when conditions are favorable and tiller survival is inherently high. The single mainstem wheat, or uniculm, was first proposed by Donald ( 1 968) as an important characteristic for dry environments. Islam and Sedgley (1981) found that wheats that were surgically restricted to two shoots per plant used more water after anthesis and produced greater GY s than did unrestrictedtillering wheats. Contrasting results were observed by Marshall and Boyd (1985), who found that two Israeli biculms, despite having larger ears, yielded 25% less biomass and 30% less grain than conventional cultivars from Western Australia. In addition to the differences in rainfall and soil types between the two studies, the comparison within the Marshall and Boyd (1985) study was probably complicated by the poor adaptation of the Israeli cultivars outside of Israel. More recently, Whan et al. (1989) and Yunusa and Sedgley (1992) reported no advantage of limited-tillering wheat breeding lines in water use or GY under dryland conditions in Western Australia. Reduced tillering was also of no advantage in the semiarid wheat-growing regions of western Canada (Hucl and Baker, 1991) or for barley in South Australia (McDonald, 1990). In these cases, the lack of yield increases with reduced-tillering wheats is probably related to their larger leaves and to the absence of changes in leaf area index or biomass when compared to conventional wheats (Richards, 1988). In addition, reducedtillering wheats tend to have other inefficient assimilate partitioning, that is, large specific leaf weights, high stem and ear densities, and a high proportion of the ear as chaff. Turner and Nicolas (1987) proposed that rapid, vigorous seedling growth should be advantageous on coarse-textured soils in water-limiting environments

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WHEAT YIELD INCREASES

2 45

because of more efficient use of water. Seedling growth has important consequences for the pattern of water use and is discussed later in this article.

2. Roots Root growth is strongly influenced by moisture and nutrient availability, soil type, and cultivation (Hamblin et al., 1990). During seedling and tillering growth, more assimilate is usually partitioned to roots than to shoots, but after anthesis root growth is reduced, and when grown under favorable conditions, roots frequently account for as little as 10%of the total crop biomass at maturity (Lupton et al., 1974). Under drought conditions, however, roots may comprise as much as 60% of the total crop biomass at maturity (Gregory et al., 1984; Hamblin et al., 1990; Siddique et al., 1990b). Hamblin and Tennant (1987) argue that rooting depth or rate of root elongation is a better selection criteria for maximizing water uptake than either root length, weight, or density in the mediterranean environment of Western Australia. Cereal root densities in the top 30 cm of soil are very high, compared with grain legumes (Gregory, 1988), and this has been interpreted as evidence that less roots in the surface soil may increase rooting depth and cereal yields (Richards, 1991); however, differences in root morphology and physiology are also important. Hamblin and Tennant (1987) also measured greater root lengths for cereals when compared to grain legumes, but water uptake was better correlated with rooting depth than with total root length. Water uptake per unit root length was greater in the grain legumes than in the cereals, probably due to lower axial resistances in the xylem vessels of the grain legumes. Smucker (1984) suggests that cereals adopt a “conservative rooting strategy,” typified by an extensive root morphology that uses water slowly. In contrast, legumes tend to have an “opportunistic rooting strategy,” which is characterized by a less extensive root system that uses water rapidly. Cereals have evolved from a predominantly nodal to a predominantly seminal root system, particularly when grown at high densities (MacKey, 1986). Seminals develop earlier and deeper, and are finer and more efficient at water uptake per unit dry weight than are nodal roots (Passioura, 1976). Seminal roots also have a higher resistance to water flow and tend to conserve soil water more than nodal roots. Producing more roots, particularly deeper roots to obtain more water, appears to be a logical drought avoidance mechanism; however, roots are a major sink for assimilates, requiring twice as much assimilate to produce the same amount of biomass as shoots (Passioura, 1983). In cases where a small amount of water is stored in the subsoil, the cost of producing deep roots to obtain this water may be less than the extra assimilate that can be produced from the additional transpiration and photosynthesis. In m e d i t e ~ n environments, e~ soil moisture is almost always exhausted at maturity (Cooper et al., 1987; Siddique et al., 1990b; Gregory et al., 1992),

246

S. P. LOSS AND K. H. M. SIDDIQUE

and cultivars that produce less roots, particularly in the top soil, may be at an advantage. In fact, this is what has occurred with selection for yield in Western Australia (Siddique et al., 1990a,b). Modern wheats produce less root dry matter and lower root: shoot ratios than do old wheats, which probably relates to their earlier ear sink development (double ridge), fewer tillers, and fewer adventitious roots associated with the tillers. Patterns of root and shoot growth for old and modern wheats are illustrated in Fig. 6. Modem cultivars have root densities of 10 cm ~ m in-the~ top 10 cm of soil, about half the density of the old wheats. Passioura (1983) estimated that root densities of 0.5 cm ~ m are- adequate ~ for removing all the water stored in soils, although higher densities may be required for nutrient uptake. In the past, roots for nutrient uptake were less important because nutrient uptake could be improved by increasing fertilizer application, but, recently, more economical use of fertilizer and reduced groundwater pollution from fertilizer leaching are considered desirable. MacKey (1973) claimed that because root growth tends to mirror shoot growth, semidwarf cereals may have reduced root growth when compared to tall cereals; however, the studies of Siddique et al. (1990b) and Holbrook and Welsh (1980) demonstrate this is not the case in dry environments. The growth of shoots by modern semidwarf and old tall cultivars is similar, and modern cultivars produce deep roots earlier than do old cultivars (Siddique et al., 1990b). By anthesis, the roots of old and modern wheats reach the same depth. MacKey (1986) suggested that root growth in modern wheats is reduced soon after anthesis because they

0

200 300 Root dry matter (g.m-2)

100

400

Figure 6 Patterns of root and shoot growth for Western Australian wheats grown at Merredin. Terminal spikelet (TS)and anthesis (A) are indicated with arrows. Reproduced with permission from Siddique eta!. (19Wa).

MEDITERRANEAN WHEAT YIELD INCREASES

247

have fewer lower leaves that provide the roots with assimilates, compared to old wheats. Although advocated by Bums (1980), Passioura (1983), and Richards (1991), it is not known if yields can be increased by further reductions in the growth of roots in the surface soil. Using a simulation model, Miglietta et al. (1987) predicted that for a silty clay-loam, increasing rooting depth below 80 cm increased water uptake early in the season but left little stored in the soil for use later in the season, hence yields were reduced. Techniques for measuring rooting depth are unsuitable for screening large numbers of breeding lines, and we are aware of only one example where rooting depth has been successfully incorporated into a wheat breeding program. In the semiarid region of western Canada, Hurd et al. ( I 972) selected parents for deep and prolific root systems in sloping plastic boxes and produced a deep-rooted and high-yielding line that was later released as a commercial cultivar. Using similar techniques, others have observed a wide variation in wheat root growth (Derera el al., 1969; MacKey, 1983; Sharma and Lafever, 1992). Other techniques for examining roots, such as hydroponics, herbicide placement at various soil depths, and minirhizotrons, have also been examined (Gregory, 1989), but these are also unsuitable for selection in routine breeding programs.

3. Grain Growth Temperatures above 30°C are common during grain filling in mediterranean environments, causing an increase in the rate of grain growth. However, this increased rate does not compensate for the reduction in duration of grain filling (Sofield et al., 1977; Wardlaw et al., 1980, 1989) and, consequently, grain sizes are smaller in mediterranean than in temperate environments. In fact, selection for yield in Australia has favored cultivars with many more grains but smaller grain weights (Perry and D’Antuono, 1989). Cultivars that are able to fill their grain quickly may reach physiological maturity before moisture stress limits grain growth (Bruckner and Frohberg, 1987), and in areas of high frost risk, cultivars with delayed anthesis and fast grain growth rates reduce the risk of frost damage without increasing the risk of moisture stress during grain filling (Loss et al., 1989). The proportion of GY that is derived from assimilates produced after anthesis varies from 70 to 95% depending on the degree of moisture stress (Rawson and Evans, 1971; Austin et al., 1977; Bidinger et al., 1977; Pheloung and Siddique, 1991; Kobata et al., 1992). Although modem cultivars have fewer leaves, the rate of grain growth is not reduced because most of the assimilate used for grain growth is produced by the upper canopy. These assimilates are derived mainly from the spike, the flag leaf, and its sheath (Austin and Jones, 1975; Rawson et al., 1983), and it has long been recognized that awns can also substantially

248

S. P. LOSS AND K. H. M. SIDDIQUE

increase spike photosynthesis and yields under dry conditions (Atkins and Norris, 1955; Bremner and Rawson, 1972; Evans et al., 1972; Olugbemi et al., 1976). Under postanthesis moisture stress, remobilized assimilates that were produced before anthesis make a considerable contribution to GY. These assimilates are nonstructural soluble carbohydrates that are stored in the stem before anthesis and during the lag phase of grain growth. They are rapidly depleted during grain filling, and other structural substances are also remobilized as the vegetative parts of the plant senesce. Modern semidwarf cultivars are more efficient at remobilizing dry matter assimilated before and after anthesis compared to old tall cultivars, and under irrigated conditions the old tall cultivars retained some of the stored assimilate in the stem at maturity (Pheloung and Siddique, 1991). Hence, modern cultivars are better able to take advantage of favorable conditions after anthesis, and further increases in the efficiency of remobilization could increase HI and GY. The application of chemical desiccants on leaves and stems has been used to simulate postanthesis water stress and to screen genotypes for their ability to retranslocate assimilates to the grain (Blum et af., 1983; Nicolas and Turner, 1993; Regan et al., 1993). Selection for large grains in breeding populations treated with the chemical dessicant effectively increased GY under postanthesis moisture stress with no change in the number of days to ear emergence or plant height, whereas selection without the chemical desiccant treatment did not improve grain filling under postanthesis stress (Blum et al., 1991). This method is best conducted in the absence of leaf diseases and under irrigated conditions or in wetter environments (Nicolas and Turner, 1993; Whan et al., 1993). Under very dry conditions, GY is hardly affected by the desiccant and there is little discrimination between genotypes. Chemical desiccants are also more convenient to apply to genotypes of similar maturity because the treatment can be applied to all genotypes in a single application, otherwise the desiccant has to be applied individually to each genotype after they reach anthesis (Regan et al., 1993). Loss et al. (1989) and Austin et al. (1989) examined the growth of individual grains of old and modern wheats under well-watered conditions. Although there were significant differences in the rates of grain growth between cultivars, the rates were not related to grain size; several old cultivars had slow grain growth rates and large grain sizes. Large grains have been associated with a low number of grains, and hence these genotypes are of no yield advantage (Siddique et al., 1989b; Slafer et al., 1993). However, Carlton (personal communication, 1993) had some success in combining high growth rates with high grain numbers in breeding populations in Western Australia. The effects of grain growth rate, duration, and number must be considered simultaneously to increase GY. The

MEDITERRANEAN WHEAT YIELD INCREASES

2 49

broadsense heritability of grain growth rate ranged from 60 to 92%, depending on the genetic background, and grain growth rate was stable over generations (F2-FJ. In contrast to the frequent sampling used in other studies, Carlton (personal communication, 1993) took only two samples during the linear phase of growth to provide an estimate of grain growth rate. This technique was able to differentiate between breeding lines without being too laborious, and considering the high heritability of grain growth rate, it may be useful for the selection of parents.

4. Harvest Index and Ear Growth Harvest index is important for agronomists and breeders concerned with crop production, and because water stress usually occurs after anthesis in mediterranean environments, wheat yields are often characterized by a low HI when compared to temperate environments (Austin et al., 1980). In mediterranean environments, the HI has risen from about 23% for old wheats to about 38% for modern cultivars and is well correlated with yield increases (Perry and D’Antuono, 1989; Siddique et al., 1989a,b; Ehdaie and Waines, 1989; Slafer et al., 1990). The increase in HI of modern compared to old cultivars can be attributed to earlier anthesis and reduced investment in the stem and, to a lesser extent, the roots. The worldwide introduction of the dwarfing genes from the Japanese cultivar ‘Norin 10’ has changed assimilate partitioning and increased HI. This has been clearly demonstrated with wheat isolines that contain differing numbers of the dwarfing genes (Brooking and Kirby, 1981; Fischer and Stockman, 1986; Siddique et al., 1989a; Youssefian et al., 1992). The time of maximum stem, root, and ear growth coincide, and it has been proposed that ear growth is limited by competition for assimilates at this stage (Siddique et al., 1989a; Slafer et al., 1990; Slafer and Andrade, 1993). This is also supported by results of shading during this time, which reduces ear growth and floret survival (Thorne and Wood 1987; Savin and Slafer, 1991). As a result of the shortage of assimilates or some hormonally mediated process, about 50% of the florets in old, tall wheats die before ear emergence, whereas in modern semidwarfs the stem weight is reduced and considerably more assimilates are available for investment in the ear. Consequently, semidwarfs set more grains per ear and per unit area, and increased yields are strongly related to increased grain number (Perry and D’Antuono, 1989; Ehdaie and Waines, 1989; Slafer et al., 1990; Slafer and Andrade, 1993). Although many studies indicate that HI and yield are well correlated, selection for the HI in simulated breeding populations was less effective at increasing GY than was selecting for yield (Whan et al., 1982).

S. P. LOSS AND K. H. M. SIDDIQUE

250 0.5

3

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1

.

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.

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.

0

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Figure 7 Relationships between ear: stein ratio and harvest index for a range of old and modern Western Australian wheats grown at Wongan Hills, Perth, and Merredin. Reproduced with permission from Siddique er ul. (1989a).

The ear:stem ratio, that is, the size of the ear relative to the stem, is an index of the competitive strength of the ear and it is highly correlated with the HI, grains per ear, and GY (Siddique et al., 1989a; Slafer et af., 1990; Slafer and Andrade, 1993). The relationships between ear:stem ratio and HI vary between sites (Fig. 7); however, the ranking of cultivars remains the same. The ear:stem ratio gives a better indication of yield potential than does HI because it is determined early in the life cycle and, unlike HI, it is not affected by environmental stresses after anthesis. When ear dry weight is about 1 mg, stem dry weight varies from 50 mg in modern cultivars to 250 mg in old cultivars (Siddique et a l . , 1989a). The effects of selecting for higher ear: stem ratio have been investigated in breeding populations and ear: stem ratio offers promise as a predictor of HI and yield potential (Siddique and Whan, 1994). Broad sense heritabilities for ear: stem ratio ranged from 50 to 90% and this trait was strongly correlated between generations and sites. Ear: stem ratio is a highly stable characteristic and has the potential for effective selection in early generations. Selections for high ear: stem ratio in the F2 generation resulted in greater HI and GY values in some lines than in their parents (Siddique and Whan, 1994). Unlike HI, ear:stem ratio is independent of GY because it does not contain GY as a component of the ratio. It is unlikely that ear: stem ratio will be adopted as a routine measurement in breeding programs, because it is laborious and time consuming. However, this trait could be used to identify superior parental genotypes and early generation

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selections from special crosses, because it is an important physiological attribute that reflects the ability to partition assimilates (Siddique and Whan, 1994).

IV.WATER USE We have already discussed how rainfall is the most limiting factor in dryland mediterranean cropping, but we only mentioned in passing how plant morphology and physiology can affect water use. Passioura (1977) proposed that in water-limited environments, biomass production is a function of the water used by the crop (WU) and the efficiency with which it is converted into biomass (WUE); Biomass

=

WU x WUE

Hence, GY = WU

X

WUE

X

HI

This model has become a framework for examining ways to improve crop yields, especially in water-limited environments (Acevedo, 1987; Turner et al., 1989; Ludlow and Muchow, 1990; Richards, 1991). WU is usually considered as soil evaporation (E) plus transpiration (T), and water runoff and drainage are often negligible in dryland areas and hence are ignored. A frequent feature of crops grown in mediterranean environments is that despite differences in biomass production, all the soil water is depleted by maturity (Siddique et al., 1990b; Cooper et al., 1987; Gregory et al., 1992). In mediterranean environments, E usually accounts for about 40% of the WU, most of which is lost early in the season when the crop biomass and ground cover are small (Cooper et al., 1983; French and Schultz, 1984; Siddique et al., 1990b). WUE is the amount of dry matter that is produced per unit of WU. It sometimes refers to the efficiency of WU for grain production; however, we shall restrict its use to biomass, unless specified otherwise. Any mechanism that reduces E and increases T will usually increase WUE because T is closely tied with photosynthesis and biomass production. When water deficits cause stomata1 closure, T and CO, assimilation rates are decreased. Increases in WUE will only increase growth and yield when WU and HI are maintained. For example, in a time of sowing experiment, Connor et al. (1992) observed that maximum WUE did not correspond to maximum growth or yield because late-sown crops, which had the highest WUE, also used much less water than early-sown crops. Increasing the proportion of WU that is used through T can be achieved agronomically, for example, by early sowing, mulching the soil, narrow row spacing,

2s2

S. P. LOSS AND K. H. M. SIDDIQUE

or encouraging early growth with fertilizer application (Cooper et al., 1987; Anderson, 1992). Here we only consider the genetic mechanisms of increasing T and WUE.

A. WATER-USE PATTERN AND EARLY VIGOR In mediterranean environments, the pattern of water use through the season is almost as important as the size of the water supply in determining GY (Passioura, 1983). The efficiency of transpiration and WUE are high when the VPD between the leaf and the air is low, and hence in mediterranean environments, plants use water most efficiently in winter when humidity is high and temperatures are low. Consequently, yields may be increased by increasing the proportion of water transpired during winter and this can be achieved genetically, through early vigor and/or appropriate phenology. French and Schultz ( 1984) suggested increasing preanthesis WU to improve yields of wheat in South Australia, although this is not the case elsewhere in Australia. Nix and Fitzpatrick (1969), Passioura (1977), and Richards and Townley-Smith (1987) advocated more postanthesis WU, whereas Siddique et a / . (1990a) showed a strong negative relationship between preanthesis WU and GY in Western Australia. This result was largely related to the early maturity of the modern wheats, so that even though their leaf conductances and transpiration rates were higher than those of the old wheats in winter (20-80 days after sowing), the durations from sowing to anthesis were shorter for the modern wheats and so they used less water before anthesis (Fig. 8). These WU patterns reflect the patterns of crop growth rates observed by Karimi and Siddique ( 1991b). The WUE for biomass production did not improve with selection for yield (about 45 kg ha-‘ mm-I), but as a result of earlier anthesis and improved HI, the WUE for grain production increased by 46% (Siddique et al., 1990a). Passioura (1 983) suggested an optimum ratio of pre- and postanthesis WU of 2: 1 in eastern Australia. French and Schultz (1984) measured 2.5: 1 in the mediterranean environment of South Australia, and in a drier part of the Western Australia wheatbelt, higher ratios (3-5: 1) have been measured (Rickert et al., 1987; Siddique et al., 1990a). It is apparent that the ratio of water used before and after anthesis in each environment relates to the severity of moisture stress in the postanthesis period. Siddique et al. (1990a) showed that modern wheat cultivars rapidly decreased their leaf conductances when the soil moisture content began to fall in spring, whereas the old wheats had a gradual decrease in leaf conductances. They described modern wheats as “opportunistic” in that they develop rapidly, reach anthesis early, and use water rapidly when it is most available, but they markedly reduce their water uptake when soil moisture becomes limiting. In contrast, the

MEDITERRANEAN WHEAT YIELD INCREASES *.OI

253

Kulin

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-

Garneya

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9 1.5

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-

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.-

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E

I-

0.5

0.0

0

40

80

120

160

Days After Sowing Figure 8 Transpiration patterns of old and modern Western Australian wheats grown at Merredin. Average annual rainfall is illustrated and arrows indicate anthesis data. Data from Siddique er al. (1990a).

“conservative” old cultivars are less effective at avoiding drought. They reach anthesis later, use water slowly when soil moisture is high, and slowly reduce their water uptake when soil moisture becomes limiting. On coarse-textured soils, rainfall is in excess of crop use during early winter and a significant proportion is lost through E and deep drainage. By selecting plants for rapid early growth and leaf area development, and a prostrate growth habit that covers the soil surface, drainage and E can be reduced significantly. In addition, plant growth is exponential in nature, so that a small increase in growth during the vegetative stages of development will often cause a considerable increase in transpiration and biomass production during the later stages, and if harvest index can be maintained, yields will also be increased (Richards, 1987). Turner and Nicolas (1987) observed a strong positive relationship between GY and aboveground biomass at the five- to six-leaf stage, and Siddique et al. (1989b) showed that compared to wheat, barley had a fast emergence rate after sowing and a more rapid development of leaf area that was related to its high biomass production and GY. Van Oosterom and Acevedo (1992) also found a relationship between ground cover and vigor and barley GY in Syria under terminal drought conditions. However, in other experiments, the relationship between early vigor and GY was equivocal (Acevedo, 1987; Siddique ef d., 1990a,b; Damisch and Wiberg, 1991). In Western Australia, Regan el al. (1992) found no correlation between the early biomass production of introduced geno-

2 54

S. P. LOSS AND K. H. M. SIDDIQUE

types and their GY; however, these genotypes were not adapted to local conditions and thus were inappropriate to assess this relationship. Whan et al. (1993) examined the biomass production and GY of the progeny of these genotypes when crossed with a number of standard local cultivars. High biomass was associated with high GY at dry sites but no relationship was evident at wet cool sites. Broad sense heritabilities for biomass production varied from 60 to 80%, depending on the cross. Clearly, the choice of parents and the target environment will play an important role in the success of selecting for improved early biomass production and GY. Increased rooting depth associated with early vigor may improve water extraction in coarse-textured soils where water is stored at depths of greater than 1 m. Studies with temperate grasses suggest that improved pasture yield and high growth rates in winter are associated with earlier commencement of reproductive development (Kemp, 1988; Kemp et al., 1989); however, recent studies with wheat found that rapid early dry matter production was not related to stage of development (Regan et al., 1992; Rawson, 1991). A better understanding of the effects of early vigor on WU and GY will enable physiologists and breeders to pinpoint the environments where early vigor is likely to be of benefit. Determining aboveground biomass by destructive sampling is a timeconsuming and labor-intensive task, and visual ratings of early vigor are unable to categorize breeding lines accurately. Spectral reflectance measurement in the visible, near-infrared, and mid-infrared regions is a new and promising technique for estimating early vigor in breeding programs (Smith et al., 1992; Elliott and Regan, 1993). This technique has the advantages of being nondestructive, rapid, and accurate, but further work is required to investigate the effects of environmental factors and genotypic differences in morphological characters such as tiller number and growth habit on the reflectance measurements.

B. XYLEMDIAMETER Passioura (1972) proposed that where crops rely heavily on water stored in the soil and the soil water is almost exhausted by anthesis, slowing the rate of water extraction during vegetative growth should increase the amount of water available after anthesis, and hence improve HI and yield. Richards and Passioura (1981) calculated that decreasing the metaxylem diameter in the upper part of the seminal wheat roots to less than 60 p m would increase the hydraulic resistance and slow the rate of water uptake from the soil. They developed a selection technique wherein roots of seedlings grown in tubes were washed, cut, and examined under a microscope. They crossed a landrace wheat from Turkey that had narrow xylem vessels with local cultivars and selected progeny with narrow xylems, which out-yielded the standard cultivars by up to 11% under stored

MEDITERRANEAN WHEAT YIELD INCREASES

255

moisture conditions in subtropical eastern Australia (Richards and Passioura, 1989). Biomass production was slightly increased and HI was greater in the narrow xylem wheats than in the standard cultivars, suggesting increased postanthesis WU and a longer period of grain filling. Narrow xylem progeny and standard cultivars produced similar GYs at the wet sites because nodal roots of the narrow xylem genotypes proliferated in the top soil, so that water uptake was not impaired. Little moisture is stored in the coarse-textured soils of Western Australia, where crops tend to rely heavily on current rainfall; hence this “conservative” drought-avoidance strategy is probably of little use in this environment. It may be of value for other mediterranean environments with fine-textured soils, especially in years with early terminal drought, but it has yet to be tested in these areas.

C. GLAUCOUSNESS Studies with isogenic lines have shown that under postanthesis water stress, epicuticular wax or glaucousness increases radiation reflectance, reduces leaf temperature, increases T, and hence increases leaf survival (Johnson et al., 1983; Richards et al., 1986). Glaucous wheats out-yielded nonglaucous wheats by 7% under drought conditions and suffered no yield penalty under irrigated conditions. Photosynthesis was reduced in wheats with glaucousness but not to the extent of transpiration, hence WUE was increased and HI was unaffected. Glaucousness develops mainly on the leaf sheaths and on the abaxial surfaces of leaves, and it reaches a maximum at flag leaf emergence (Richards et al., 1986). Water stress enhances the development of glaucousness, but the amount under stressed and unstressed conditions is positively correlated, hence it can be selected for under either condition (Nizam Uddin and Marshall, 1988). Glaucousness is simply inherited, being controlled by a single gene, with minor genes controlling intensity (Richards, 1983). Leaf pubescence or hairs in sorghum also have effects on WUE and yield under water stress similar to the effects of glaucousness (Baldocchi et al., 1983), but this has not been examined in wheat.

D. ABSCISSIC ACIDACCUMULATION Abscissic acid (ABA) plays an important role in the regulation of plant water relations, and its synthesis and accumulation are a natural drought avoidance mechanism in most plants (Turner, 1986). Within a few minutes of a reduction in the turgor pressure of leaf and root cells, ABA rapidly accumulates in the leaves, where it causes stomata1 closure, reduced transpiration, and decreased photosynthesis. In the growing points, ABA inhibits cell expansion and division.

256

S. P. LOSS AND K. H. M. SIDDIQUE

Other effects of ABA include advancing the rate of plant development and increased assimilate partitioning to the roots. When the water stress is relieved, ABA concentrations decrease while transpiration and growth recommence. Henson and Quarrie (1981) developed a detached-leaf test to screen for ABA accumulation under moisture stress and found a threefold variation in the rate of ABA accumulation within wheats. High-ABA lines were selected in the F2-F4 generations and tested at F, under irrigated and pre- and postanthesis drought in the United Kingdom. This trait was simply inherited (Quarrie, 1981). High-ABA wheat lines were smaller, flowered earlier, had fewer spikelets per ear, and used less water (Innes and Quarrie, 1987). High-ABA lines out-yielded low-ABA lines by about 5% in the test environment and had an improved WUE for grain production, probably because of lower leaf conductances that were sufficient to reduce water loss but not photosynthesis. In contrast, Read et al. (1991) tested these lines in Oklahoma and found that the lines selected as “low ABA” from the detached-leaf test had slightly higher ABA contents than did the high-ABA lines. The low-ABA lines also had lower stornatal conductance, much greater biomass, and higher WUE. Genotypes that are specifically selected for variation in ABA accumulation are being studied to predict more reliably the role of ABA in regulating WUE in the field. Although rapid ABA accumulation may improve yields under pre- and postanthesis drought, short periods of ABA accumulation in the ear near anthesis cause pollen sterility and drastic reductions in grain set (Morgan and King, 1974; Morgan, 1980; Saini and Aspinall, 1982). Consequently, high-ABA lines have a risk of major failure in most environments, unless greater pollen tolerance to ABA can be found and utilized.

E. OSMOREGULATION Osmoregulation or osmotic adjustment is a decrease in cell osmotic potential due to the accumulation of solutes rather than to a decrease in cellular volume (Turner and Jones, 1980). Under water stress, plants that accumulate solutes within cells maintain turgor pressure, stornatal opening, transpiration, photosynthesis, and growth, and in this way, plants tolerate mild dehydration. The maintenance of turgor pressure can prevent large increases in ABA near anthesis, thereby maintaining seed set. Although osmoregulation does not affect WUE, it enables roots in a dry surface soil to survive until the next rainfall event, or alternatively, it can maintain root growth and increase WU if water is stored at depth (Morgan and Condon, 1986). The rate of leaf senescence may also be reduced, hence retranslocation of assimilates is enhanced and HI is improved. Blum (1988) concluded that osmoregulation is one of the most important and effective components of drought resistance. In field experiments in eastern Australia, segregating lines selected for high

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257

osmoregulation in the glasshouse showed greater turgor maintenance and yielded 7-50% more than lines with low osmoregulation (Morgan, 1983; Morgan et al., 1986). This advantage was evident in a range of moisture regimes, indicating that this trait is environmentally robust and has no detrimental effects in highrainfall years. The capacity for high osmoregulation appears to be controlled by one or two genes and is simply inherited. Because cell expansion involves the accumulation of solutes to maintain turgor pressure as the cell volume increases, plants can be selected for high osmoregu~ationby measuring coleoptile expansion under water stress in petri dishes (Morgan, 1988). This technique is rapid and inexpensive, and hence is suitable for routine screening of breeding populations. Wheats with a high capacity for osmoregulation have not been tested in mediterranean environments and more work is required in this area.

F. CARBON ISOTOPE DISCRIMINATION The CO, in the atmosphere contains about 1% of the naturally occurring carbon isotope, 13C, but during photosynthes~splants discriminate against I3C in favor of the lighter isotope, I2C. The amount of discrimination is largely determined by the ratio between intercellular and atmospheric partial pressures of C 0 2 and hence indicates the efficiency of transpiration and photosynthesis. Carbon isotope discrimination (A) is a measure of the ratio of I3C and I2C in piant material compared with the same ratio in the atmosphere and depends on the balance between stomatal conductance, photosynthetic capacity, and transpiration efficiency ~ F ~ q and u hRichards, ~ 1984). The relationship between A and GY varies according to the environment. Increased salinity, decreased water availability, soil compaction, and increased VPD all cause lower A in plant material because of their effects on stomatal conductance or photosynthetic capacity (Condon et al., 1992). Also variations in E, WU patterns, root growth, and boundary layer conductances can affect the relationship between A and growth (Richards et al., 1993; Turner 1993). In a diverse range of wheat lines grown in eastern Australia, biological yields and GYs were positively correlated with A measured on the peduncle when grown in a wet environment but negatively correlated in a dry environment (Richards, 1991). No relationship was evident in a dry environment in Western Australia (Turner et al., 1989). In nothern Syria, A measured on grain samples was ~ s i t i v e l ycorrelated with barley GYs at dry sites and showed no relationship at wet sites, presumably because of the lower stomatal conductances under stress and the large contribution of preanthesis assimilate to GY at the dry sites (ICARDA, 1987; Craufurd et al., 1991). Differences in the relationships between A and yield may also relate to differences in the plant samples taken for measuring A in these studies. The A measure may reflect the improved WUE of early-vigor crops that pro-

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duce a rapid groundcover early in the season when the VPD is low. In California field experiments, A was negatively correlated with WUE, positively correlated with biomass, and there was also a strong negative relationship between A and days to ear emergence (Ehdaie et al., 1991). At a dry site in northern Syria, where early anthesis was a large advantage, A was also negatively correlated with days to ear emergence (Craufurd et al., 1991). Read et al. (1991) found that A was positively related to GY under drought conditions in Oklahoma, and the relationship had a greater slope in wheats with high ABA accumulation. Richards et ul. (1993) conclude that low-A genotypes should yield more in longduration environments when soil E is a small component of evapotranspiration or in environments where there is little variation in VPD during periods of rapid growth. Hence, the use of A to select for high GY in mediterranean environments is probably limited. This trait can be measured quickly and accurately with a mass spectrometer and shows considerable genetic variation in wheat (Condon et al., 1987). Condon and Richards (1992) measured a high heritability of A (68-97%) and a small genotype by environment interaction. The heritability of A was highest and the genotype by environment interaction was lowest when measured on plant samples taken during seedling and tiller development, the stages least likely to encounter moisture stress. The coefficient of variation of A is about 2%, which is about one-fifth of that for GY (Richards and Condon, 1993).

V. RADIATION USE Biscoe and Gallagher (1977) proposed that biomass production can be defined by the amount of radiation intercepted (RI) and the radiation-use efficiency (RUE), that is, the efficiency of the conversion of this radiation to dry matter. Biomass = RI x RUE Hence, GY

=

RI x RUE x HI

The use of radiation and water is linked together in photosynthesis, but, in general, radiation is much less limiting than water in mediterranean environments. In fact, excessive radiation can have detrimental effects during postanthesis moisture stress. Therefore, radiation use may be less easily manipulated to improve biomass production compared to WU. The amount of RI is related to leaf appearance, size, orientation, tillering capacity, and senescence, and RI can influence leaf temperature, T, and WUE.

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A. INTERCEPTION Canopy structure has a large effect on RI. During the seedling stages of development, photosynthesis is limited by the ability of the plant to intercept radiation. As the crop develops, the green area index (GAI) increases until the crop intercepts all the radiation and photosynthesis reaches a maximum. Crops with prostrate habits generally give rapid groundcover and have a high RI value during the early stages of plant development, crops with erect habits have greater radiation penetration and illumination of the lower canopy after anthesis. The duration of the leaf area has been found to be highly correlated with GY in temperate environments (Fischer and Kohn, 1966; Bingham, 1977; Borojevic et al., 1980). Siddique et al. (1989b) and Yunusa et al. (1993) showed that old wheat cultivars had more leaves with a prostrate orientation, a greater GAI, and a greater fraction of groundcover than modem wheats. These results suggest that old cultivars had a greater RI, but paradoxically the efficiency of radiation interception, as measured by the extinction coefficient, was greater for modern cultivars probably because of their large leaf size and erectophile leaf habit, which caused less shading once stem elongation commenced. Yunusa et al. (1993) demonstrated that the increased RI of modern wheats was partly due to their large-awned ears, which constituted the majority of the GAI at the top of the canopy. Awns are better placed than leaves to intercept radiation and to dissipate unused radiation because they are narrow structures at the top of the canopy. Awn weight and length and flag leaf area were positively correlated with GY in Jordanian wheats (Al-Shalaldeh and Duwayri, 1986), presumably due to effects on RI. Under moisture stress, wheats with erect leaves yielded better than prostrate lines because they intercepted less radiation and had more favorable water relations (Innes and Blackwell, 1983). Leaf movements, such as paraheliotropism, rolling, and wilting under water stress, reduce RI and T (Begg, 1980). Clarke (1986) and Ludlow and Muchow (1990) consider these traits as essentially survival mechanisms and proposed that they are likely to have little effect on yield in terminal drought situations. However, leaf rolling, a familiar response in cereals, can reduce effective leaf area and transpiration by up to 50% and the amount during early growth was significantly correlated with biological and GY in northern Syria (ICARDA, 1987). In later stages of growth, leaf rolling is indicative of susceptibility to moisture stress and loss of turgor, and delayed leaf rolling is an important selection criteria for drought avoidance in rice (O’Toole and Cruz, 1979).

B. RADIATION-USE EFFICIENCY Comparisons of wild diploid and domesticated hexaploid wheats indicate that the photosynthetic rate per unit leaf area has fallen considerably in the course of plant domestication and selection (Khan and Tsunoda, 1970; Austin et al., 1982)

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and that photosynthetic rate is negatively correlated with leaf area (Evans and Dunstone, 1970; Rawson er al., 1983; Morgan and LeCain, 1991). Within modern field crops, yield and photosynthetic rate are poorly correlated (Evans, 1975) and selection for wheats with high photosynthetic capacity often results in reduced leaf area or thicker leaves and no increase in RUE and yield (Rawson et al., 1983). The higher photosynthetic rate of the diploid wheats appears to be related to genes affecting chlorophyll protein complexes, and Austin er al. (1988) are attempting to incorporate these genes into bread wheats, while breaking the link with leaf area. Walker and Sivak (1986) and Evans (1987) saw a remote chance of using genetic engineering to improve the activity of the rate-limiting photosynthetic enzyme, ribulose- 1,5-bisphosphate carboxylase-oxygenase (rubisco). There have been no reports in the literature of success in this area. Unlike comparisons of diploid and hexaploid wheats, Blum (1990) showed that selection for yield among Israeli wheats had increased RUE that was associated with increased photosynthesis and WUE. Yields were positively correlated with the GAI after anthesis as a result of early anthesis. The cultivars that reached anthesis earlier had a longer duration of photosynthetic area during grain filling and a higher yield than the later cultivars. Similarly, Siddique et al. (1989b) demonstrated that the modem Australian cultivars had a longer green area duration in the postanthesis period than the old cultivars. Recent studies (N. Watanabe, personal communication) using a subset of Australian wheat cultivars (Siddique et al. 1989b) showed that selection for yield has decreased the amount of chlorophyll associated with the core complex in photosystem I1 relative to the total amount of chlorophyll. However, the rate of CO, assimilation, total chlorophyll content, and N content per unit leaf area has increased from old (Purple Straw) to modern cultivars (Kulin). In contrast, Gent and Kiyomoto (1985) did not measure any difference in the net CO, assimilation rates per unit leaf area of New York wheats, although they only compared two winter cultivars. In the study of Siddique et al. (1989b), modern cultivars converted the intercepted photosynthetically active radiation to aboveground biomass more efficiently compared to older wheats. The RUE increased from 1.08 g MJ-I for the old cultivars to 1.31 g MJ-’ for the modern cultivars and this appeared to be related to the reduced investment in root biomass by the modern cultivars. Similar RUE values were measured by Gregory et al. (1992). Increased RUE may be related to early vigor because photosynthesis is more efficient at low VPD and RUE decreases with the increasing VPD during the growing season (Stockle and Kiniry, 1990). High levels of solar radiation can damage proteins in the chloroplasts and impair photosynthesis when the electron transport systems are inhibited by feedback mechanisms caused by a low demand for photosynthate. Wheat is most susceptible to this type of damage, known as photoinhibition, during early summer when growth rates are limited by water and heat stress and radiation levels

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are high. Light green leaves have a higher capacity for photosynthesis at near light saturation point and are less susceptible to photoinhibition than darker leaves. The development of a light green color was positively correlated with GY under drought in durum wheats and barleys in northern Syria (ICARDA, 1987, 1988). Landrace wheats and adapted lines were darker during their vegetative growth stages when radiation levels were low and became paler as radiation increased, whereas lines from South Australia and Europe maintained an intermediate color throughout the season. Respiration is a large component of assimilate use in plants, and studies with several species have established a link between respiration and biomass production (Heichel, 1971; Wilson, 1975; Jones and Nelson, 1979; Gifford et ul., 1984). Winzeler et al. (1989) measured higher rates of respiration and lower rates of growth in wheat than in rye or triticale. The mechanism for more efficient respiration is unknown and there have been no reports in the literature of wheat genotypes with reduced respiration. In the future, respiration enzymes may be modified using genetic engineering to increase the amount of assimilate available for growth, and hence increase RUE and WUE. Clearly, a better understanding of the respiratory pathway and its effect on crop production is required before this could be attempted. Most methods for measuring photosynthesis or RUE are currently laborious and time consuming, and unsuitable for use in breeding programs. However, strong relationships between leaf N content and photosynthetic rate have been demonstrated in maize, rice, soybean (Sinclair and Horie, 1989), and wheat (N. Watanabe, personal communication), and selecting for high N content may increase the efficiency of photosynthesis.

VI.HIGH-TEMPERATURESTRESS Under dryland conditions, high-temperature, radiation, and moisture stresses often occur simultaneously, and heat stress alone rarely plays a role in reducing wheat yields. In mediterranean environments, high-temperature stress is an important constraint after anthesis. As temperatures rise, photosynthesis reaches a maximum at about 20°C (Al-Khatib and Paulsen, 1984) while respiration continues to increase, hence the assimilates available for growth are reduced (Gusta and Chen, 1987). However, if radiation and moisture sources are increased, the rate of photosynthesis increases with increasing temperature and biomass production is largely unaffected by high temperature (Rawson, 1988). Short periods of high temperature near anthesis can dramatically reduce wheat yields. In a study by Saini and Aspinall (1982), well-watered wheat plants were exposed to 30°C for 3 days before anthesis, and floret fertility was reduced by

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80%. As already mentioned, although high temperatures increase the rate of grain filling, the duration of grain filling is reduced considerably and grain weights are decreased (Sofield et al., 1977; Wardlaw et al., 1980, 1989). Wheat cultivars from the Middle East and tropical areas are more tolerant of high temperatures than are cultivars from North America and Europe, whereas Australian and Indian cultivars are intermediate (Shpiler and Blum, 1986; Wardlaw et al., 1989). Wheat and a number of other plants respond to heat stress by producing a large number of low-molecular-weight proteins, but the significance of these heat-shock proteins is poorly understood (Lindquist, 1986) and the physiological and biochemical basis for heat tolerance needs to be investigated further. Chlorophyll fluorescence may be a useful technique for screening wheats of similar maturity for high-temperature tolerance (Moffat et al., 1990).

VII. USE FOR BREEDERS We have discussed many morphological and physiological attributes that have or are likely to contribute to increased wheat yields, and yet relatively few breeding programs are currently selecting for specific physiological traits. Several authors advocate the analytical approach to plant breeding (Richards, 1982; Rasmusson and Gengenbach, 1983; Blum, 1983, Acevedo and Ceccarelli, 1989; Whan et al., 1993), although many have had variable success. There are several reasons for the lack of adoption of the analytical approach by breeders. First, empirical breeding programs have been very successful at producing consistent increases in yield, especially in high-yielding environments (Turner and Begg, 1981; Slafer et al., 1993). Breeders have not been convinced that a physiological approach will give better results, and they believe that improvements in field experimentation and computerization will ensure continued success of the empirical approach. As evident from this article, many traits highlighted by plant physiologists as useful for breeders have not been unequivocally proved to increase yield. Given the complex physiological process that determine yield and the large genotype by environmental interaction of yield, it is often easier to show that a trait improves a short-term plant function or characteristic under stress (e.g., root growth, T, or HI), rather than improving yield itself. Passioura (198 1) suggests that often physiologists begin their studies at too low a level of organization (e.g., molecular or cellular) and in controlled environments that are of little relevance to the breeder who is interested in the GYs of crops in the field. In addition, organ number and size are often inversely related (Grafius, 1978) so that selection for a morphological trait may result in compensation in some other trait and expected yield increases are often not achieved (e.g., reduced tillering lines may have larger leaves and WU is unaffected).

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Frequently, the yield increases associated with a particular trait are small, and breeders have not been convinced that selecting for the trait is more efficient than selecting for yield. For example, Whan et al. (1982) showed that selecting for harvest index was less effective at improving GY than direct selection for GY. Passioura (1981) argued that there is a low probability that a single plant trait will have a sufficient effect to cause a statistically significant increase in yield. Ceccarelli et al. (1991) argued that selection for a single trait is often unsuccessful, particularly in unpredictable environments where the frequency, timing, and severity of stresses are unknown. In these situations, different combinations of many traits may produce the same GY. Also, genes for the desired traits are sometimes found in exotic stock that are poorly adapted to the target environment and crossing with this stock incorporates other deleterious genes that reduce growth and yield in early crosses. The use of isolines has been advocated by several authors for proving increased yields with the inclusion of a trait because they overcome any genetic effects that other traits may have on yield (Pugsley, 1983; Rasmusson and Gengenbach, 1983; Richards, 1991; Shorter et al., 1991). Although isolines generate information on the effect of a trait in a particular genetic background, they are sometimes of limited use to breeders who want to incorporate the trait into many different backgrounds (Acevedo and Ceccarelli, 1989). In addition, it is costly and time consuming to develop isolines and they can only be produced for traits controlled by one or two genes. Many traits are measured with complex, time-consuming techniques that are unsuitable for screening large numbers of progeny in breeding programs. Consequently, most traits are evaluated in a small number of genotypes without testing in breeding populations. Some physiological techniques have been modified, and although not as accurate, they may provide a useful method for plant screening (e.g., osmoregulation in coleoptiles) (Morgan, 1988). Other more complicated and time-consuming techniques are only useful for screening a small number of genotypes for use as parents. Some traits can be combined in a single measurement. For example, infrared thermal sensing of canopy temperatures can be used to screen for deep roots, maintenance of higher leaf water potentials, increased stomata1 conductance, and general drought avoidance (Blum et al., 1982, 1989; Pinter et al., 1990), provided variations in measurements due to time of day, weather, and groundcover can be avoided (Turner, 1986). The high degree of environmental influence on the expression of some traits also poses problems for selecting for the trait within breeding programs. And yet, the environmental effects on GY are usually greater than those on morphological and physiological traits, hence the low heritability of GY. Often the use of irrigation, time of sowing, rain-out shelters, or artificial water stress treatments, such as chemical desiccants, is required to examine the trait under a range of moisture stresses. Many breeders are unsure of the conditions under which they should select for stress adaptation. A review of widely adapted wheat geno-

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types by Ceccarelli ( 1989) concluded that in environments where the average GYs are greater than 2-3 t ha-’, selection for traits is successful under optimum conditions. But where yields are less than 2 t ha-’, direct selection in the target environment is the most efficient strategy. In these harsh environments, yield stability is often a high priority. It would be a mistake to ignore the contribution that modeling can make to this area of breeding/physiology. Our ability to assess accurately the interaction of the numerous processes over a crop life cycle is limited and the development of models can remove much of the “hunch taking” in selecting relevant physiological traits for breeding (Moorby, 1987; Shorter et al., 1991). New genotypes are tested in the field for several seasons before release, and using historical weather and other data, models can also be used to predict how genotypes perform in other seasons and sites and in a changing climate. Many simulation models have been used successfully for wheat (Stapper, 1984; Weir et al., 1984;Ritchie et al., 1985; Hammer et al., 1987). For example, Stapper and Harris (1989) used the SIMTAG simulation model to illustrate how long-term, early-maturing wheats have higher yields in short-seasonenvironmentsthan do late-maturing wheats, and

I

’‘O (a) Breda

I I

0.0I 0

I

200

I

400

I

I

600

800

Grain yield (gm-2)

The cumulative frequency distribution of predicted yields of two maturity types, early (4, at (a) Breda (275 mm annual rainfall) and (b) Jindiress (479 mm annual rainfall), Syria. Reproduced with permission from Stapper and Harris (1989). Figure 9

(---) and late

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vice versa, in long-season environments (Fig. 9). Models can also predict much more complicated crop growth processes and interactions.

VIII. CONCLUDING COMMENTS Wheat is one of the most domesticated plants in the world and yet we are still trying to improve its performance in the mediterranean environment where it originated. The physiology of wheat is probably better understood than any other plant and the contribution that physiology has to make to wheat breeding is probably greater than for other crops. Water stress is a major limitation to wheat growth and yield in mediterranean and other environments, and physiologists have a role to play in improving WUE, RUE, and yield in these environments. Empirical breeding approaches have been very successful and we do not suggest that breeders abandon this methodology. However, over the last 15 years, wheat breeders in mediterranean environments have relied heavily on germ plasm for CIMMYT and they are concerned that the germ plasm may become too narrow and that yield may be approaching a plateau. It is important that some attempt is made to develop new parental lines with the potential for increasing yields significantly. As advocated by Whan et al. (1993), we believe this is best achieved by using a physiological approach to identify parental genotypes with superior traits. Rasmusson (1987) and Acevedo and Ceccarelli (1989) wisely suggest that breeding programs make a modest investment in the physiological breeding approach of about 15-25% of the total breeding resources. Given the equivocal evidence associated with some traits and the difficulty of selection, we believe these resources are probably most efficiently used in a parental identification subprogram. Because of the relatively small number of genotypes involved in parental screening, some of the more laborious physiological techniques can be used. Crosses should be made between superior parents and locally adapted cultivars and the progenies tested for yield and other standard measurements (e.g., grain quality) in the routine evaluation procedure. Some of the crossbred material rejected by the evaluation procedure may be useful parental germ plasm. In order to reap the large potential benefits that molecular biology has to offer plant breeding, breeders will require greater cooperation from physiologists because the genetic engineering technologies are of little use to breeders unless they understand the cellular and molecular processes that control stress tolerance and yield. Molecular biology can also assist the selection of traits that are expensive or slow to measure, that are sensitive to environmental factors, or that can only be measured in the mature plant. DNA markers that are visually recognized

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in the seedling can be linked to the genes for the trait, thereby increasing the efficiency of selection dramatically. We predict that, in the future, the most successful breeding programs will involve breeders, physiologists, agronomists, modelers and molecular biologists.

ACKNOWLEDGMENTS The authors thank the National Committee on Crop Improvement and Protection of the Grains Research and Development Corporation for financial assistance. We thank Mohan Saxena for the detailed climatic data for Aleppo and we are also grateful to Michael Perry, Neil Turner, and Richard Richards for useful comments on the manuscript.

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Index A

D

Abscissic acid, accumulation in wheat, 255-256 Adsorption, phosphorus, in soils, 42-45 Agricultural waste management system, site analysis, 60-61 Agronomic practices, efficient rainwater use, 110-112 Ammonia groundwater contamination, 4-5, 7-9, 33-35 volatilization, 25-27 Ammonium adsorbed, 208-209 exchange chemistry in soils, 198 quantityhntensity plots of adsorption-desorption, 205, 208-209 Anthesis, wheat, 241-243 Antibiotics, in poultry wastes, 11, 56-58 Arkansas, nutrient management and water quality, 8-1 1

Darcy’s law, 127 Decay series, 65-66 Delaware, nutrient budge, 21 Delmarva Peninsula, nutrient management and water quality, 7-8 Denitrification, 30-3 1 Desorption, phosphorus, in soils, 42-45 Discharge, prairie pothole region, 127- 132 Drought, terminal, 233

E Electrochemical potential, ions in soil, 174, 176 Erosion phosphorus losses, 45-5 1 wetlands, 139-141 Eutrophication, 4-5, 10 Evapotranspiration, wetlands, 125

F C Calcium, exchange equilibria with potassium in soil, 174-175, 177 Calcium carbonate, occurrence and formation in wetlands, 150 Carbon biogeochemistry in wetlands, 164 isotopes, discrimination in wheat, 257258 Cation exchange capacity, soils, 180 Climate salinity as indicator, 147- 148 wetlands, 124- 126 Coccidiostats, in poultry waste, 56-58 Conservation bench terraces, 109 Corn, response to nitrogen, 32-35

Fallow land, management, 100-101 Fens, 150- 154 across zones in semipermanent ponds, I54 calcareous, 153 hydrology, 152-153 mounds in, 154 Flownet, 133 Forages, response to nitrogen, 31 -32

G Gapon cation exchange selectivity coefficient, 186 Gapon exchange expression, 179 Gibbs energy, molar, 178 Gibbs free energy of formation, 182, 184- 185

277

278 Glaucousness, wheat, 255 Green area index, 259 Growing season, mediterranean environments, 235-236

H Harvest index, 236-237, 249-25 1 Heavy metals, in poultry wastes, 11-12 Hydraulic conductivity, sediments, effects on wetlands, 132 Hydraulic gradients, distribution, in wetlands, 127-132 Hydrology prairie fens, 152- 153 salinity as indicator, 147- 148 Hysteresis, adsorption-desorption isotherm, 204

I Immobilization, nitrogen, 29 Ion-selective electrode theory, 210215

N Nitrification, 29-30 Nitrogen biogeochemistry, wetlands, 164 management for poultry wastes, 23-35. 6568 ammonia volatilization, 25-27 corn response, 32-35 denitrification, 30-31 forage response, 31 -32 forms, 23-24 losses from drying, 24-25 mineralization-nitrification-immobilization, 27-30 transformations in storage and handling, 24 rice grain yield and, 110- 1 I 1 use efficiency, 67-68 Nutrient budget, farm-wide, 69 Nutrients available, 173- 174 management, water quality and, 4- 11 Arkansas, 8- 1 1 Delmarva Peninsula, 7-8 off-farm, 70

L 0 Land-use patterns, water quality and, 141142 Leaching, phosphorus losses, 45-5 1 Leaves, wheat, 244-245 Lithology, effects on prairie pothole region, 143- 144 Lowland ecosystem, rain-fed, subecosystems, 89-90

M Mediterranean environments distribution. 230 growing season, 235-236 rainfall, 232-233 solar radiation, 233-234 temperature, 234-235 Methemoglobinemia, 4 Microbes, in poultry waste, 58-59 Microcatchment technique, 109 Mineralization, nitrogen, 27-29 Monsoons, 88-89

Organic matter in lowland rice fields, 101- 105 in wetlands, 149- 150 Osmoregulation, wheat, 256-257

P Palustrine System, 134- 135 Percolation barriers, 105- 108 Pesticides, in poultry wastes, I I , 56-58 Phenology, wheat, 239-243 Phosphate sorption capacity, 45 Phosphorus adsorption isotherm, 42-43 leaching to groundwater, 48-49 management for poultry wastes, 35-51, 6871 adsorption and desorption, 42-45 concentration and form in soil, 36-40 losses by erosion, runoff, and leaching, 45-51

INDEX retention and movement in soils, 40-51 movement in soil, 140 subsurface transport, 49, 51 water contamination, 5-6 Photosynthesis, radiation interception and, 259 259 Plant-available water capacity, 102- 104 Plant wilting point, 103 Potassium, soil calcium exchange equilibria, 174- 175, 177 quantitylintensity relationships activity ratio, 183- 185, 190- 191, 218-219 adsorption-desorption relationship, 203-206 ammonium exchange chemistry, 198 anion effects, 196- 197 atomic absorption spectrophotometric determinations, 213 cation exchange capacity, 216-217 concentration ratio, 217 concept, 175 Eden soil, 201-202 X-ray diffractogram, 207-208 electrochemical considerations, 176- 18 I electrochemical potential, 174, 176 exchange reversibility, 203 -209 experimental observations, 2 15- 220 fundamental basis, I8 1 - 185 Capon-derived parameters, 189- 190 Capon exchange coefficient, 217-218 Capon interpretation, 185- 189 hysteresis effect, 204 interrelationship between Kc and Kv, 190-196 ion-selective electrode theory, 210-215 linear least-squares regression analysis, 214 measurements, 21 1-215 potassium concentration and uptake, 215-216 potential buffering capacity, 186- 190, 199-200 ternary exchange systems, 197-203 activity ratios, 199 typical plot, 182- 183 Vanselow-derived parameters, I90 Poultry industry, 2-3 confinement housing, 14- 16

2 79

Poultry waste, 1-72 analysis, appropriate use, 20-22 antibiotics in, 56-58 coccidiostats in, 56-58 composition, 17-20 dead poultry disposal, 12- 13 environmental impacts, 59-60 excessive manure effects, 19-20 filter strips, 48 management programs, 59-71 nitrogen management plans, 65-68 phosphorus management, 68-72 production, handling, storage, and treatment, 63 site analysis and selection, 60-62 transfer and application, 63 utilization, 64 manure production, 16 microbial population, 58-59 nitrogen management, 23-35, 65-68 pesticides, antibiotics, and heavy metals, 11-12, 56-58 phosphorus management, 35-51. 68-72 properties, 17-20 resource value, 59 storage, 15- 16, 63 trace elements in, 51-56 transportation, 63 types, production operations and, 14- 17 water quality and nutrient management, 4-11 Prairie pothole region, see also Wetlands characteristics, 121- 123 edges, soils, 161-163 groundwater flowpaths, 142- 143 movement, 129 history, 124 parent materials, 138- 139 recharge and discharge, 127-132 till lithology, geochemical characteristics, 143- 144 water quality, 141- 148 land-use patterns and, 141- 142 soil landscape and salinity, 142- 148 wetland-groundwater interactions, hydrologic characteristics, 126- 133 Darcy’s law, 127 flownet, 133

2 80

INDEX

hydraulic gradient distribution, 127- 132 sediment hydraulic conductivity effects, 132 Presidedress soil nitrate test, 66-67

R Rain mediterranean environments, 232-233 monsoonal, 88 -89 utilization efficiency, 85- 113 agronomic practices, 110- 112 research priorities, 112 soil management practices fallow land, 100- 101 organic amendments, 101- 105 percolation barriers, 105- 108 puddling versus dry seeding, 92-97 soil compaction, 97-100 tillage effects, 94-96 water harvesting, 108- 109 Recharge, prairie pothole region conditions, 144- 145 hydraulic gradient distribution, 127- 132 Rice growing regions. climatic zones, 90 rain-fed lowland, 85-87 constraints, 87-9 I potentials, 91 rainwater utilization efficiency, see Rain, utilization efficiency yield and asphalt barrier, 106-107 Roots, wheat, 245-247 Runoff, phosphorus losses, 45-51 tillage and manure application effects, 46-47

S Salinity discharge wetlands, 160- 161 flowthrough wetlands, 159- 160 as indicator of climate and hydrology, 147-148 soil and water, development, 144- 145 in wetlands, 135, 137, 148-149 Sedimentation, wetlands, 139- 141 Sediments, hydraulic conductivity, effects on wetlands, 132

Seeding, rice, puddling versus dry, 92-97 Soil cation exchange capacity, 180 compaction, for efficient rainwater utilization by lowland rice, 97-100 subsurface, 99- 100 discharge wetlands, 160-161 fens, 150-155 flowthrough wetlands, 157- 160 landscape, salinity and, 142- 148 chemistry of dilute solutions, 143- 144 discharge conditions, 146- 147 flowthrough conditions, 145- 146 hydrology effects, 142- 143 as indicator of climate and hydrology, 147- 148 recharge conditions, 144- 145 salinity development, 144 nitrogen transformations, 25-3 1 phosphorus adsorption profile. 44 concentration and form in, 36-40 retention and movement, 40-51 potassium, see Potassium, soil prairie pothole edges, 161-163 recharge wetlands, 154- 157 texture, deep marsh, 159 water-holding capacity, 101- 105 wetness indicators, 126- 127 Solar radiation mediterranean environments, 233-234 use by wheat, 258-261 Stewart and Kantrud system, wetland classification, 135-137 Storage, nitrogen from poultry waste, associated transformations, 24 Sulfur, biogeochemistry. wetlands, 164

T Temperature high, related stress, effect on wheat yield, 26 1- 262 mediterranean environments, 234-235 Texture, wetlands, 150 Thermodynamic exchange constant, 177 Till, calcareous clay-loam, 138 Tillage, effects on leaching losses, 95 rice grain yield, 94-96

INDEX Tillers, wheat, 244-245 Trace elements, in poultry waste, 51-56 Transportation, poultry waste, 63

U United States Fish and Wildlife Service, wetland classification system, 134- 135

V Vanselow exchange selectivity coefficients, 190, 199 Volatilization, ammonia, 25-27

W Waste management, see Poultry waste Water, see also Rain contamination with ammonia, 4-5, 7-9, 33-35 with phosphorus, 48-49 harvesting, 108- 109 quality land-use patterns and, 141- 142 nutrient management and, 4- 11 Arkansas, 8- 11 Delmarva Peninsula, 7-8 salinity, development, 144- 145 soil storage, 104-105 use by wheat, 251-258 Water-holding capacity, soils, 101- 105 Wetlands, 121 - 165, see also Prairie pothole region classification, 133- 138 Fish and Wildlife service system, 134-135 hydrologic, 135, 137- 138 Stewart and Kantrud system, 135-137 climate, 124- 126 discharge, 129, 146-147 erosion and sedimentation, 139- 141 evapotranspiration, 125 flowthrough, 145- 146 groundwater interactions, hydraulic aspects, 126-133

281

hydrologic factors, 123 interdisciplinary research, 163- 164 landform-oriented hydrogeologic studies, 164-165 sedge, 150 soil properties, 148- 150 calcium carbonate occurrence and formation, 150 organic matter, 145- 150 salinity, 148- 149 texture, 150 soil sequences, 150- 161 discharge wetlands, 160- 161 fens, 150-154 flowthrough wetlands, 154- 160 recharge wetlands, 154-157 Wheat biomass production and partitioning, 236-25 1 anthesis, 241-243 ear initiation, 240-241 floret initiation and stem elongation, 241 grain development, 243 grain growth, 247-249 growth and morphology, 243-25 I harvest index and ear growth, 249-251 leaves and tillers, 244-245 phenology, 239-243 roots, 245-247 vegetative development, 239-240 breeding, 230-231 high-temperature stress, 261 -262 morphological and physiological attributes, use for breeders, 262-265 radiation use, 259-261 efficiency, 259-261 interception, 259 transpiration patterns, 252-253 water use, 25 1-258 abscissic acid accumulation, 255-256 carbon isotope discrimination, 257-258 glaucousness, 255 osmoregulation, 256-257 pattern and early vigor, 252-254 xylem diameter effects, 254-255 yield improvements, 230-231