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Soil and Water Quality: An Agenda for Agriculture (1993)

Chapter: 2 Opportunities to Improve Soil and Water Quality

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Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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2
Opportunities to Improve Soil and Water Quality

The list of soil and water resource problems on the agricultural agenda has increased enormously over the past 15 years. Long-standing concerns about soil erosion and sedimentation have been supplemented with renewed concerns about soil compaction, salinization, acidification, and loss of soil organic matter. The loss of nitrates, phosphorus, pesticides, and salts from farming systems to surface water and groundwater has, in some ways, supplanted traditional concerns about soil degradation.

Efforts to address this larger complex of resource problems have been hampered by concerns about trade-offs. Management practices that have been designed to reduce soil erosion are now scrutinized for their role in increasing leaching of nitrates or pesticides to groundwater. Practices designed to reduce the amounts of sediment-borne pollutants delivered to waterways are sometimes thought to increase the amounts of the soluble forms of those pollutants delivered in runoff water. Such findings have raised doubts about society's ability to manage what appears to be unavoidable environmental trade-offs. Efforts to improve agriculture's environmental performance must be weighed against efforts to reduce costs of production, increase production, and maintain U.S. agriculture's share of world markets.

Uncertainty about trade-offs makes policy development difficult because it is hard to determine the best approaches for improving the effects of farming systems on soil and water resources. This difficulty is further complicated by the inherent regional and local variabilities in farm enterprises and soil and water resources. Soil and water quality

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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problems would be easier to solve if the most promising opportunities for improving farming systems were more clearly defined. Policies could then be developed to take advantage of those opportunities.

The committee analyzed the processes that cause soil degradation and that result in the delivery of sediments, salts, and agricultural chemicals from croplands to surface water and groundwater. The committee also analyzed the ways that farming systems affect these processes. Through its analysis, the committee attempted to develop general solutions to soil and water resource degradation that can lead to productive and environmentally sound farming systems and to provide practical guidance for implementing such solutions.

The committee defined four broad opportunities that hold the most promise for improving the environmental performance of farming systems while maintaining profitability. Current soil and water resource policies should

  1. conserve and enhance soil quality as a fundamental first step to environmental improvement;

  2. increase nutrient, pesticide, and irrigation use efficiencies in farming systems;

  3. increase the resistance of farming systems to erosion and runoff; and

  4. make greater use of field and landscape buffer zones.

These four opportunities are related to the fundamental processes that determine how farming systems affect the environment. The soil is the mediator between farming practices, agricultural chemicals, and the environment. Soil degradation directly and indirectly affects agricultural productivity and water quality. Increasing nutrient, pesticide, and irrigation use efficiencies addresses the input side of the equation. The goal of increased efficiency is to reduce the total mass of residuals from inputs, thus making less mass available for loss to the environment. Increasing the soil's resistance to erosion and runoff addresses the output side. Erosion and runoff are the major pathways by which sediment, nutrients, pesticides, and other pollutants reach surface water, and erosion remains the greatest threat to soil quality. Finally, farming systems exist in a landscape, and landscape processes determine the ultimate effects of farming systems on soil and water quality. The creation of field and landscape buffer zones is a way to manipulate those landscape processes to gain further improvements in soil and water quality by intercepting pollutants and reducing the erosive force of runoff water.

Since agriculture and its associated soil and water resources vary

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Whitman County, Washington, is one of the nation's primary producers of winter wheat. It also has some of the region's most erodible soil. The Food Security Act of 1985 provides incentives to protect soil on the cropland most likely to erode. Credit: U.S. Department of Agriculture.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

dramatically across the United States, it is difficult to make specific recommendations for preventing soil degradation and water pollution that apply equally well to all farming systems. The four opportunities recommended here as goals for national policy, however, can be applied to policies and farming systems generally. The opportunity emphasized may change from one farming system or region to another, but the realization of only one of the four opportunities will not address the full complement of soil and water quality issues confronting the U.S. agricultural system. A combination of policies and policy instruments will be needed to pursue all four opportunities. This chapter examines each of the four opportunities in some depth to reveal their implications for soil and water quality policy.

CONSERVING AND ENHANCING SOIL QUALITY

The 1990 Clean Air Act (PL 101-549) and the 1987 Federal Water Pollution Control Act (PL 100-4) give national recognition to the fundamental importance of air and water resources. The fundamental importance of soil resources, however, is usually overlooked, even though the soil is the interface between human activities and the environment. The quality of the soil and its management in large part determine whether

THREE FUNCTIONS OF SOIL

Soils are living systems that are vital for producing the food and fiber humans need and for maintaining the ecosystems on which all life ultimately depends. Soil directly and indirectly affects agricultural productivity, water quality, and the global climate through its function as a medium for plant growth, a regulator and partitioner of water flow, and an environmental buffer.

Soils make it possible for plants to grow. Soils mediate the biological, chemical, and physical processes that supply nutrients, water, and other elements to growing plants. The microorganisms in soils transform nutrients into forms that can be used by growing plants. Soils are the water and nutrient storehouses on which plants draw when they need nutrients to produce roots, stems, and leaves. Eventually, these become food and fiber for human consumption. Soils—and the biological, chemical, and physical processes that they make possible—are a fundamental resource on which the productivities of agricultural and natural ecosystems depend.

Soils regulate and partition water flow through the environment. Rainfall in terrestrial ecosystems falls on the soil surface where it either infiltrates the soil or moves across the soil surface into streams or lakes.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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agriculture or other land uses will cause or prevent water pollution. The increasingly urban U.S. population is quick to recognize when air quality is degraded. Indeed, newspapers and radio and television weather forecasts regularly report air quality indicators. People are aware of water quality every time they turn on a water tap, and the presence of contaminants in water is widely reported. Soil quality degradation, however, often goes unnoticed because most people rarely encounter soil in their daily lives and because soil quality degradation is often difficult to see and measure. Soil quality is, nevertheless, as fundamental as air and water quality to future environmental quality and ecological integrity. The threat posed by soil degradation needs to receive the same kind of attention given to air and water quality degradation by national, state, and local policymakers.

Society needs to change the way it thinks about soils. Society generally views soils simply as the rooting medium for plants. Society often fails to recognize that soils also regulate and partition water flow and buffer environmental changes. The way society thinks about soils affects the kinds of policies, programs, and research that investigators devise to manage soil resources. This narrow conception of soil is no longer adequate to address the linked problems of soil degradation and water pollution that agriculture faces.

The condition of the soil surface determines whether rainfall infiltrates or runs off. If it infiltrates the soil, it may be stored and later taken up by plants, move into groundwaters, or move laterally through the earth, appearing later in springs or seeps. This partitioning of rainfall between infiltration and runoff determines whether a storm results in a replenishing rain or a damaging flood. The movement of water through soils to streams, lakes, and groundwater is an essential component of recharge and base flow in the hydrological cycle.

Soils buffer environmental change. The biological, chemical, and physical processes that occur in soils buffer environmental changes in air quality, water quality, and global climate. The soil matrix is the major incubation chamber for the decomposition of organic wastes including pesticides, sewage, solid wastes, and a variety of other wastes. The accumulation of pesticide residues, heavy metals, pathogens, or other potentially toxic materials in the soil may effect the safety and quality of food produced on those soils. Depending on how they are managed, soils can be important sources or sinks for carbon dioxide and other gases that contribute to the greenhouse effect (greenhouse gases). Soils store, degrade, or immobilize nitrates, phosphorus, pesticides, and other substances that can become pollutants in air or water.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Defining Soil Quality

Soil quality is best defined in relation to the functions that soils perform in natural and agroecosystems. The quality of soil resources has historically been closely related to soil productivity (Bennett and Chapline, 1928; Hillel, 1991; Lowdermilk, 1953). Indeed, in many cases the terms soil quality and soil productivity have been nearly synonymous (Soil Science Society of America, 1984). More recently, however, there is growing recognition that the functions soils perform in natural and agroecosystems go well beyond promoting the growth of plants. The need to broaden the concept of soil quality beyond traditional concerns for soil productivity have been highlighted at a series of recent conferences and symposia.

Johnson and colleagues, in a paper presented at a Symposium on Soil Quality Standards hosted by the Soil Science Society of America in October 1990, suggested that soil quality should be defined in terms of the functions of soils in the environment and defined soil function as ''the potential utility of soils in landscapes resulting from the natural combination of soil chemical, physical, and biological attributes" (Johnson et al., 1992:77). They recommended that policies to protect soil resources should protect the soil's capacity to perform several functions simultaneously including the production of food, fiber, and fuel; nutrient and carbon storage; water filtration, purification, and storage; waste storage and degradation; and the maintenance of ecosystem stability and resiliency.

Larson and Pierce defined soil quality as "the capacity of a soil to function, both within its ecosystem boundaries (e.g., soil map unit boundaries) and with the environment external to that ecosystem (particularly relative to air and water quality)" (Larson and Pierce, 1991:176). They proposed "fitness for use" as a simple operational definition of soil quality and stressed the need to explicitly address the function of soils as a medium for plant growth, as a means to partition and regulate the flow of water in the environment, and as an environmental buffer. Parr and colleagues, in a paper presented at a Workshop on Assessment and Monitoring of Soil Quality hosted by the Rodale Institute Research Center in July 1991, defined soil quality as "the capability of a soil to produce safe and nutritious crops in a sustained manner over the long-term, and to enhance human and animal health, without impairing the natural resource base or harming the environment" (Parr et al., 1992:6). Parr and colleagues (1992) stressed the need to expand the notion of soil quality beyond soil productivity to include the role of the soil as an environmental filter affecting both air and water

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

quality. They suggested that soil quality has important effects on the nutritional quality of the food produced in those soils but noted that these linkages are not well understood and that research is needed to clarify the relationship between soil quality and the nutritional quality of food.

The growing recognition of the importance of the functions of soils in the environment requires that scientists, policymakers, and producers adopt a broader definition of soil quality. Soil quality is best defined as the capacity of a soil to promote the growth of plants, protect watersheds by regulating the infiltration and partitioning of precipitation, and prevent water and air pollution by buffering potential pollutants such as agricultural chemicals, organic wastes, and industrial chemicals. The quality of a soil is determined by a combination of physical, chemical, and biological properties such as texture, water-holding capacity, porosity, organic matter content, and depth. Since these attributes differ among soils, soils differ in their quality. Some soils, because of their texture or depth, for example, are inherently more productive because they can store and make available larger amounts of water and nutrients to plants. Similarly, some soils, because of their organic matter content, are able to immobilize or degrade larger amounts of potential pollutants.

Soil quality can be improved or degraded by management. Erosion, compaction, salinization, sodification, acidification, and pollution by toxic chemicals can and do degrade soil quality. Increasing the protection the soil is afforded by crop residues and plants; adding organic matter to the soil through crop rotations, manures, or crop residues; and carefully managing fertilizers, pesticides, tillage equipment, and other elements of the farming system can improve soil quality.

Management of soil resources should be based on a broader concept of the fundamental roles that soils play in natural and agroecosystems. The implications of this broader concept of soil on policy development become clearer if one examines in more detail the ways that soils affect agricultural productivity, water quality, and the global climate.

Importance of Soil Quality

Changes in agricultural productivity, water quality, and global climate are linked to soil quality through the chemical, physical, and biological processes that occur in soils.

Agricultural Productivity

Damage to agricultural productivity from soil degradation has historically been the major concern about soil resources. Agricultural technologies

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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have, in some cases, improved the quality of soils or have masked much of the yield loss that could be attributed to declining soil quality, except on soils vulnerable to rapid and irreversible degradation. Studies have predicted that losses in crop yields because of soil erosion will be less than 10 percent over the next 100 years, assuming high levels of inputs (Crosson and Stout, 1983; Hagen and Dyke, 1980; Pierce et al., 1984; Putnam et al., 1988). Those studies have begun to shift the emphasis of federal policy to the off-site damages caused by erosion.

Conservation of soil productivity should remain an important long-term goal of national soil resource policy.

The effect of soil degradation on agricultural productivity has been underestimated. Estimates of the agricultural productivity lost because of soil erosion have not accounted for the damages caused by gully and ephemeral erosion, sedimentation (Pierce, 1991), or reduced water availability as a result of decreased infiltration of precipitation. Those studies also assume that the optimum nutrient status is maintained by using fertilizers to replace the nitrogen, phosphorus, and potassium lost from eroding lands. Larson and colleagues (1983) estimated that the value of the nitrogen, phosphorus, and potassium lost through erosion in 1982 was $677 million, $17 million, and $381 million, respectively. The total mass of nitrogen and phosphorus estimated to be lost in eroded sediments in 1982 was equal to 95 and 39 percent, respectively, of the total nitrogen and phosphorus applied to all U.S. croplands in that same year (Larson et al. [1983] estimates of nitrogen and phosphorus applications were from Vroomen [1989]). In addition to erosion, compaction, salinization, and acidification, other deleterious forces can also cause yield losses and increase costs. More important, erosion, compaction, salinization, and acidification may interact synergistically to accelerate soil degradation. Losses in yields and increases in costs will be greater than those projected if investigators consider all degradation processes and their interactions. In addition, reductions in yield can be severe where soil degradation is serious but are obscured in estimates of U.S. average soil erosion or yield reductions. Other analysts have suggested that losses in potential productivity will occur sooner and be of a larger magnitude than absolute losses in yields from soil degradation.

Crosson (1985) pointed out that it is the cost of erosion, not the predicted yield losses, that is of interest. Similarly, the cost of compensating for reduced soil quality because of degradation by compaction, acidification, salinization, and loss of biological activity, as well as erosion, is of the most importance when assessing the effects of soil degradation on soil productivity.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Evidence of gully and ephemeral erosion is a clear indication that soil quality is threatened. Credit: U.S. Department of Agriculture.

To date, improvements in agricultural technology have kept the costs of compensation for losses in soil quality low enough or increases in yields high enough to offset the costs of soil degradation on most croplands. Given the multiple processes of soil degradation, however, and the probable underestimation of the full cost of soil degradation on the cost of production, on-site changes in soil quality may have significant effects on society's ability to sustain a productive agricultural system. The increased amounts of fertilizers, pesticides, and other

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

inputs used to compensate for declining soil quality have themselves become a problem when they pollute surface water and groundwater.

Water Quality

Policies to prevent water pollution caused by agricultural production should seek to enhance and conserve soil quality as a fundamental step to improving water quality.

Soil and water quality are inherently linked; conserving or enhancing

SOIL QUALITY AFFECTS AGRICULTURAL PRODUCTIVITY

The potential of a soil to produce crops is largely determined by the environment that the soil provides for root growth. Roots need air, water, nutrients, and adequate space in which to develop. Soil attributes such as the capacity to store water, acidity, depth, and density determine how well roots develop. Changes in these soil attributes directly affect the health and productivity of the crop plant.

The figure illustrates how changes in one soil attribute, bulk density (bulk density is a measure of the compactness of a soil), affects agricultural productivity. When the bulk density of a soil increases to a critical level, it becomes more difficult for roots to penetrate the soil and root growth is impeded. As bulk density increases beyond this critical level, root growth is more and more restricted. At some point, the soil becomes so dense that roots cannot penetrate the soil and root growth is prevented. Heavy farm equipment, erosion, and the loss of soil organic matter can lead to increases in bulk density. Similarly, these processes of soil degradation can lead to reduced soil depth, reduced water-holding capacity, and increased acidity. At some critical point, these changes in soil quality affect the health and productivity of the crop plant, leading to lower yields and/or higher costs of production.

Source: Derived from F. J. Pierce, W. E. Larson, R. H. Dowdy, and W. A. P. Graham. 1983. Productivity of soils: Assessing long-term changes due to erosion. Journal of Soil and Water Conservation 38:39–44.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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soil quality is a fundamental step toward improving water quality. Reducing losses of nutrients, pesticides, salts, or other pollutants will be impossible or difficult if soil degradation is not controlled. Indeed, use of nutrients, pesticides, and irrigation water to compensate for declining soil quality may be an important cause of water pollution.

Soil quality degradation causes both direct and indirect degradation of water quality. Soil degradation from erosion leads directly to water quality degradation through the delivery of sediments and attached agricultural chemicals to surface waters. Clark and colleagues (1985) estimated that the cost of sedimentation from eroding croplands on recreation, water storage facilities, navigation, flooding, water conveyance facilities, and water treatment facilities, among other damages, was $2.2 billion annually (in 1980 dollars based on 1977 erosion rates).

The indirect effects of soil quality degradation may be as important as the direct damages from sediment delivery, but they are often overlooked. Soil erosion and compaction degrade the capacities of watersheds to capture and store precipitation, altering stream flow regimes by exaggerating seasonal patterns of flow; increasing the frequency, severity, and unpredictability of high-level flows; and extending the duration of low-flow periods. The increased energy of runoff water causes stream channels to erode, adding to sediment loads and degrading aquatic habitat for fish and other wildlife.

Erosion, compaction, acidification, and loss of biological activity reduce the nutrient and water storage capacities of soils, increase the mobilities of agricultural chemicals, slow the rate of waste or chemical degradation, and reduce the efficiencies of root systems. All of these factors can increase the likelihood of loss of nutrients, pesticides, and salts from farming systems to both surface water and groundwater (Figure 2-1).

Improvements in soil quality alone, however, will not be sufficient to address all water quality problems unless other elements of the farming system are addressed. Improving soil quality, for example, will not reduce nitrate damages to surface water and groundwater if producers apply excessive amounts of nitrogen to the cropping system. If nitrogen applications are too high, changes in soil quality may change the proportion of nitrates delivered to surface water rather than to groundwater, but total nitrate losses may remain excessive. In this example, improved soil quality must be linked to improved nitrogen management.

Even if soil quality is very high, producers who mismanage inputs may still have unacceptable losses of nutrients, pesticides, and other

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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FIGURE 2-1 Changes in soil quality affect water quality.

pollutants from their farming systems. High soil quality is not a substitute for careful management of all components of the farming system.

Global Climate

The effect of soil management on global climate change should receive more attention in environmental policy.

Soils can serve as a source or sink of carbon, depending on how they are managed. Lal and Pierce (1991), for example, estimated that if 1

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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percent of the organic carbon stored in predominantly tropical soils is mineralized per year, 128 billion metric tons (141 billion tons) of carbon will be released into the atmosphere. This figure compares with an estimated 0.325 billion metric tons (0.358 billion tons) of carbon emitted each year from fossil fuels and 1.659 billion metric tons (1.828 billion tons) emitted from deforestation (Brown et al., [1990] as cited by Lal and Pierce [1991]). Little is known, however, about the contribution of soil-related processes to greenhouse gas emissions under different soil and crop management systems. What is known suggests that soil may play an important role in regulating greenhouse gas concentrations.

Soil Policy Goals

The long-term goal for soil management should be the conservation and enhancement of soil quality.

To date, the national debate over the appropriate goals and objectives of soil management has been driven by estimates of the effect of erosion on soil productivity. Conserving soil productivity alone, however, is not a sufficient objective for national soil resource policies and programs. The cost of soil degradation is greater than simply the effect on agricultural productivity. The direct and indirect effects of soil degradation on water quality and global climate change, may, in many circumstances, be more important than the effect of soil degradation on agricultural productivity.

Erosion Control Alone is Not Sufficient

Soil management policies should explicity address compaction, salinization, acidification, loss of biological activity, and soil pollution as well as erosion.

In the same way that soil productivity has framed the debate about soil management policies, efforts to control erosion have dominated programs and policies for protecting soil resources. Erosion control is an important means of conserving and enhancing soil quality, but it is not the only means. Other processes of soil degradation may, in some circumstances and regions, be more important threats to soil quality than erosion.

Not all forms of soil degradation are equally damaging. Erosion, salinization, and compaction by, for example, wheeled traffic, are most worrisome because their effects are often not easily reversible. Acidification can have important effects, but in most cases it is reversible through proper management. Biological degradation is difficult to define, but it is closely related to organic matter content. The soil's biological activity has important effects on all other soil quality attributes

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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and on the capacities of soil to function as an environmental buffer and water regulator. More important, processes of degradation interact to accelerate soil degradation. Soil compaction, for example, reduces the soil's water-holding capacity, in turn increasing surface runoff and thereby accelerating erosion, which reduces the soil's biological activity by stripping away organically enriched topsoil.

In the United States, slightly more than 20 million ha (49 million acres) (13 percent) of cropland were estimated to be eroding at greater than the soil loss tolerance level in 1987 because of sheet and rill erosion (U.S. Department of Agriculture, Soil Conservation Service, 1989a), and wind-caused erosion greater than the soil loss tolerance level was estimated to be occurring on about 15 million ha (37 million acres) (9 percent) of U.S. cropland in 1982 (U.S. Department of Agriculture, Soil Conservation Service, 1989b). Salinization or sodification affected nearly 20 million ha (49 million acres) (9 percent) of cropland and pastureland; about 5.6 million ha (13.8 million acres), or 21 percent of irrigated cropland, was slightly saline or sodic in 1982 (U.S. Department of Agriculture, Soil Conservation Service, 1989b). Little information is available for other forms of soil degradation. No national estimates of the extent or severity of soil compaction or acidification exist, and researchers have made little attempt to estimate the loss of biological activity.

Soil Degradation as an Environmental Problem

Protecting soil quality, like protecting air and water quality, should be a fundamental goal of national environmental policy.

Currently available data underestimate the severity and extent of soil degradation and overlook many of the costs that soil degradation impose on the environment. The Science Advisory Board of the U.S. Environmental Protection Agency (EPA) (1990) recently recommended that the ecological risk imposed by human activities, including soil degradation, should receive much greater attention by the agency and the nation because they pose relatively high-risk problems to the natural ecology and human welfare.

A new effort is needed to reevaluate the relative importance soil degradation should receive in national environmental policy. This effort should have three components. First, new criteria are needed to quantify soil quality. Second, national soil and water resource assessments need to be redirected to provide the information needed to determine the extent and seriousness of soil degradation. Third, soil management at the farm level needs to receive greater attention as a fundamental component of efforts to improve soil and water quality.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Measurement of Soil Quality

If the conservation and enhancement of soil quality are to be the primary objectives of soil resource policies, methods for measuring changes in soil quality and predicting the effects of farming systems on soil quality are needed. Key indicators of soil quality need to be identified and used as the basis for monitoring and predicting changes in soil quality. A great deal is known about the general relationship of specific soil attributes to soil quality, and several authors have recently recommended various soil attributes as indicators of soil quality (Alexander and McLaughlin, 1992; Arshad and Coen, 1992; Granatstein and Bezdick, 1992; Griffith et al., 1992; Hornsby and Brown, 1992; Larson and Pierce, 1991; Larson and Pierce, in press; Olson, 1992; Pierce and Larson, 1993; Reagnold et al., 1993; Stork and Eggleton, 1992; Visser and Parkinson, 1992; Young, 1991). More work needs to be done, however, to develop more quantitative methods of estimating change in soil quality. Over time, changes in these soil quality indicators will provide the information needed to assess the effects of current farming systems and land use on soil quality, to develop new farming systems that improve soil quality, and to guide the development of national policies to protect soil and water quality.

The Secretary of the U.S. Department of Agriculture (USDA) and the Administrator of the EPA should initiate a coordinated research program to develop a minimum data set of soil quality indicators, standardized methods for their measurement, and standardized methods to quantify changes in soil quality.

The development of methods to quantify changes in soil quality will require measurable indicators that are relatively easy to sample and not subject to extreme variation in time or space. Models that can integrate measurements of multiple soil attributes into quantitative estimates of change in soil quality with reasonable confidence given the spatial and temporal variability of soils will also be needed. This task will require integrating research from many scientific disciplines and scientists from universities, industries and government agencies. This research effort should include

  • identification of the soil attributes that can serve as indicators of change in all three soil functions (promotion of plant growth, regulation and partitioning of infiltration and runoff, and environmental buffering) and development of simplified models that relate changes in the selected attributes to changes in soil quality;

  • standard field and laboratory methodologies to measure changes in indicators of soil quality;

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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  • a coordinated monitoring program that can quantify changes in the indicators of soil quality; and

  • a coordinated research program designed to support, test, and confirm the models used to predict the impact of management practices on soil quality.

Such a coordinated research effort would be comparable to efforts that have been undertaken to improve erosion simulation models for use in resource assessments, conservation planning, and program implementation. A comparable effort will be needed to develop the data and models required to estimate changes in soil quality.

National-Level Assessments of Soil Quality

The National Resources Inventory should include quantifiable measures of changes in selected soil quality indicators and should be broadened to produce estimates of compaction, salinization, sodification, acidification, and biological degradation in addition to erosion.

The 1977, 1982, and 1987 National Resources inventories (U.S. Department of Agriculture, Soil Conservation Service, 1989a,b,c), were by far the most extensive and quantitative inventories of soil resources in the United States. These inventories and assessments, however, are limited by their focus on quantifying rates of erosion and related processes of soil degradation rather than a focus on assembling and assessing the information needed to monitor changes in soil attributes that can be related to changes in soil quality.

Measures of changes in soil quality indicators should be included as part of national-level resource inventories such as the National Resources Inventory. A system that enables more direct quantification of actual changes in soil attributes will allow policymakers to direct policies and programs more specifically toward monitoring actual damages to soil quality. Quantifiable estimates of soil degradation processes in addition to erosion are also needed to direct national soil management efforts comprehensively and to set priorities for soil management and conservation programs.

Assess Currently Available Data

The Resource Conservation Act appraisal process should assemble all currently available information to assess the current state of and trends in soil quality.

Currently available data on rates of erosion, salinization, sodification,

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

acidification, and compaction should be assembled and interpreted to make preliminary estimates of the full extent and severity of soil degradation. Much information on the quality of U.S. soils could be assembled from state and private soil testing laboratories; local, state, and regional soil conservation programs; and other sources to supplement the data already collected as part of the National Resources Inventory.

Currently available models should be used to predict the effects of erosion, compaction, and other forces of degradation on those soil attributes related to soil quality. Information on crop yields and cropping patterns, for example, could be used to predict trends in soil organic matter content, an important and integrating indicator of soil quality (see Table 5-1 in Chapter 5). Other models could be used to predict the effects of current tillage, harvest, and other machinery on compaction to make preliminary estimates of the extent and severity of compaction. Data in SOILS-5 (a data base maintained by USDA that contains information on the attributes of different soils) on surface soil horizons could be used with existing models to estimate the locations and geographic extent of soils particularly vulnerable to different forms of soil degradation.

Such an appraisal would identify the utility of current data and models for soil quality assessment and would clearly identify gaps in the data and understanding needed to complete comprehensive assessments of the quality of U.S. soil resources.

Soil Management at the Farm Level

Public policies for soil management at the farm level have, to date, focused primarily on erosion control. The major thrust of programs such as Conservation Compliance, Sodbuster, and the Conservation Reserve Program has been to reduce erosion rates to the soil loss tolerance level or to adopt farming practices that result in a specified reduction in erosion rates. The measures used to evaluate management of soils at the farm level need to be refined to reflect a broader concern for the protection of soil quality.

Soil Quality Thresholds

Soil quality indicators and models should be used to set threshold levels of soil quality that can be used as quantitative guides to soil management.

Once in place, these threshold values should be used as the basis for conservation planning and programs such as Conservation Compliance.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Shown is a farmer using the stubble mulch tillage system. The stubble that remains after harvest reduces erosion and enriches the soil when it is incorporated into the soil as mulch during tillage. Credit: U.S. Department of Agriculture.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

Trend in soil attributes toward or away from threshold values will indicate whether current management is improving, degrading, or maintaining soil quality. In the long-term, changes in soil quality should replace the soil loss tolerance level as the standard used to determine acceptable rates of erosion. In the short-term, however, the soil loss tolerance level is the best standard available. Quantitative standards to quantify the effect of erosion, compaction, salinization, biological degradation, and other processes of soil degradation on the minimum data set of soil quality indicators are needed to enable comprehensive and cost-effective management of soil resources.

A set of soil quality indicators should be added to Soil Conservation Service field office technical guides, and the effects of recommended practices on soil quality should become an integral part of the development of integrated farming system plans.

The Soil Conservation Service should add to their field office technical guides a simple checklist of soil attributes that can serve as soil quality indicators. This checklist should be developed and used while a more rigorous effort goes forward to develop soil quality indicators suitable for national-level monitoring and quantification of changes in soil quality. The checklist of indicators could serve as an important integrating concept while a more holistic approach to resource management systems is implemented. The effect of alternative management systems on soil quality indicators could be depicted in simple matrices, as could the significance of these effects on soil and water quality.

Measures of soil attributes in the checklist of soil quality indicators should be added to routine soil test reports, and the significance of the measured levels of indicators should become part of the routine interpretations issued with soil test reports. For example, estimates of organic matter, leaching potential, crop yield potential, erosion potential, and other interpretive information should be added. A rapidly increasing number of counties throughout the United States have computerized soil survey information. Given the legal description of the farm, soil test reports could be enhanced by also reporting the soil mapping units and interpretive information from the soil survey (see Table 5-1 in Chapter 5).

Soil-Specific Management

Tailoring the ways that farming systems are managed to differences in soil quality is a way to improve soil quality, water quality, and profitability simultaneously. Soils vary greatly over the landscape and

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

often within the same field. Soils in one part of the field may be much more vulnerable to erosion or compaction. Similarly, soils within the same field or farm differ in their capacity to hold water, nutrients, and pesticides. Because the irregular soil quality distribution over the landscape does not match up well with the regular geometric pattern of crop fields, most of these differences in soil quality are ignored during farming operations (Larson and Robert, 1991). Ignoring the differences in soil quality between and within-fields leads to soil degradation and water pollution by agricultural activities.

Promise of New Technology

New technology can link digitized soil maps with the exact position of equipment in the field to tailor applications of agricultural chemicals to differences in soil quality (see, for example, Reichenberger and Russnogle [1989] and Robert and Anderson [1986]). This new technology holds the promise of allowing producers to make on-the-go changes in rates of nutrient and pesticide applications as soil quality changes over the landscape.

A similar technology can be used to vary tillage and residue cover for controlling erosion and compaction, the planting rate, the crop variety selection, and many other facets of the crop production system. Robert and colleagues (1992) recently recounted the advances and the research needed in guidance systems, field equipment, soil and terrain mapping, environmental protection, and the economic consequences of this promising and rapidly developing technology.

Better Use of Available Information

The soil maps and soil information available in county Soil Conservation Service and extension offices can, if used properly, help tailor management practices to gross differences in soils between and within-fields. When linked with soil test results for fields that are appropriately sampled, this information can lead to better management, even in the absence of new technology.

Computers offer great opportunities for combining all of the information available for a field or a farm. Soil surveys can be digitized and made available along with interpretive information such as crop yield potential, erosion potential, nutrient status, and leaching potential. This interpretive information—when combined with soil test data; records of actual crop yields and pest problems; tillage practices; nutrient, pesticide, and irrigation water applications; and crop rotations over a period

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

of years—builds the information base needed to greatly refine management of the farming system. Crop-soil consultants and some crop producers have already developed and implemented these computer-based systems. Expert system software can be used to make farming system information management easier and more useful. Such software is becoming more commonly available. A great deal of information about soils is currently collected as part of soil surveys, research projects, or from routine soil tests that could be used to improve soil management. Current data collection protocols and systems of storing and processing these data may need to be updated to facilitate their use.

Tailoring farming system management practices to differences in soil quality can reduce the potential for runoff or leaching of nutrients and pesticides and can reduce the potential for erosion and other soil degradation processes. In some cases, increases in profitability are also possible when the increased costs required to obtain the information are offset by reduced spending on nutrients, pesticides, or other inputs.

Development and implementation of these technologies could lead to increases in the total efficiency and effectiveness of agricultural production. Widespread use of such technologies could also lead to the development of extremely accurate data bases that link actual production practices on croplands to their effects on productivity, soil degradation, and water pollution. These data bases would be extremely useful in developing and implementing new farming systems.

INCREASING INPUT USE EFFICIENCIES

Nitrogen, phosphorus, and pesticides are important inputs to agricultural production systems. They are also important pollutants when they are delivered to air and water. The drainage from irrigated fields transports salts, pesticides, and nutrients to both surface water and groundwater, and the management of irrigation water has important effects on the kinds and amounts of pollutants carried in drainage water.

Mass Balance between Inputs and Outputs

The nitrogen, phosphorus, and pesticides introduced into the environment during crop production follow various pathways that determine their eventual fates in the environment. Figures 2-2 through 2-4 show simplified pathways of the nutrients (nitrates and phosphorus), pesticides, and irrigation water, respectively, used as inputs to agricultural production systems. The nutrients introduced in fertilizers or manures or fixed by legumes become part of the nutrient cycle in the

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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FIGURE 2-2 Nutrient cycle and pathways in agroecosystems.

FIGURE 2-3 Pesticide pathways in agroecosystems.

soil-crop system. Nitrogen and phosphorus can leave the soil-crop system in the harvested crop, or they can be lost through erosion, runoff, or leaching (Figure 2-2).

The fate and transport processes for pesticides are more complicated. There is no natural pesticide cycle comparable to nutrient cycles. The pesticides added to the soil-crop system can be immobilized or degraded in the soil or can be taken up by plants or animals. Pesticides taken up

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

FIGURE 2-4 Irrigation pathways of water in agroecosystems.

by plants and animals can be removed with the harvested crop, passed on to other animals through the food chain, or detoxified by biological processes within the crop or pest organism. Pesticides can be lost from the soil-crop system through runoff, erosion, leaching, and volatilization (Figure 2-3).

Irrigation water flows through the soil-crop system, and irrigation drainage water carries salts, nutrients, pesticides, and trace elements. Much of the applied irrigation water that is taken up by plants is transpired back into the atmosphere. A smaller portion is incorporated into plant tissues, and the remainder leaves the soil-crop system by leaching, runoff, or subsurface drainage (Figure 2-4).

In farming systems, mass balances can be used to review the balance between inputs and outputs to assess where the opportunities lie for preventing pollution. Although the nature of the inputs and outputs vary among farming systems, regions, and fields, mass balances provide a conceptual framework that can be applied across a diversity of farming systems and geographic scales.

Increased Input Efficiency

Increasing the efficiency with which nutrients, pesticides, and irrigation water are used in farming systems should be a fundamental objective of policies to improve water quality.

Despite the complexity and regional diversity of the fate and transport processes that determine how agricultural production affects soil and water quality, two general approaches can control loadings of nutrients,

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

pesticides, salts, and trace elements. One approach is to reduce the total residual masses of nutrients, pesticides, salts, and trace elements in the soil-crop system by increasing the efficiency with which nutrients, pesticides, and irrigation water are used. The second approach is to keep these residuals in the soil-crop system by curtailing the transport processes (leaching, runoff, erosion, volatilization) that carry pollutants out of the soil-crop system or by increasing the mass of inputs immobilized or degraded in the soil-crop system.

To date, the effects of most agricultural conservation programs on water quality have been to reduce erosion and runoff, thereby reducing the transport of nutrients, pesticides, salts, and trace elements to surface water. The nutrients and pesticides retained in the upper portion of the soil profile are subject to additional management efforts that can biologically or chemically reduce their concentrations through microbial degradation or chemical reactions or through subsequent uptake by crops. Once pesticides, nutrients, and salts have moved out of the upper soil profile to deeper levels in the soil or to surface runoff, the probability that they will be delivered to surface water or groundwater increases. Keeping the applied inputs in the soil-crop system by increasing the resistance of farming system soils to erosion and runoff should continue to be an important element of programs to improve soil and water quality. (This approach is discussed in the next section.)

Reducing the total residual mass of nutrients, pesticides, salts, and trace elements in the soil-crop system by increasing the efficiency with which nutrients, pesticides, and irrigation water are used, however, is essential to preventing surface water or groundwater pollution. This approach is also promising because of the potential for financial gains for the producer by reducing the input costs per unit of production.

There are two ways to improve input efficiency. The first and most direct way is to bring the amount of nitrogen, phosphorus, pesticides, and irrigation water applied into better balance with crop needs. The second more indirect but not less promising way is to alter cropping systems through rotations, cover crops, intercrops, and other cropping patterns that both reduce the need for inputs and help to keep those inputs that are applied in the soil-crop system.

Improved management can greatly increase the efficiency with which inputs are used in crop production systems. Input management can be improved by using information and technologies that already exist. The diversity of production systems, soils, and landscapes that characterize U.S. agriculture, however, makes generalizations difficult. Part Two of this volume provides more in-depth discussions of ways to improve input management. The most important opportunities

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

that are generally applicable to crop production systems are discussed here.

Improving Nitrogen Management

Economically viable food and fiber production often requires additions of large amounts of nitrogen. Nitrogen cycling in the soil-crop system is complex (see, e.g., Figure 6-1 in Chapter 6). Nitrogen in the form of nitrate is very mobile in the environment, making some losses of nitrogen into the environment inevitable. Many facets of nitrogen management can be improved, however, to reduce those losses.

Reduction of Residual Nitrogen in the Farming System

Reducing the amount of residual nitrogen in the soil-crop system by bringing the nitrogen entering the system from all sources into closer balance with the nitrogen leaving the system in harvested crops should be the objective of nitrogen management to reduce losses of nitrogen to the environment.

The nitrogen supplied in excess of that needed for crop requirements leaves a pool of residual nitrogen in the soil. Over time, the size of the residual nitrogen pool directly influences the magnitude of losses of available or mobile forms of nitrogen to surface water, groundwater, and the atmosphere. Nitrogen applications beyond the amount required for crop growth lead to increases in the mass of residual nitrogen that is vulnerable to loss to the environment through leaching or subsurface drainage. Nitrate losses tend to be greatest in agricultural watersheds in which nitrogen inputs from synthetic fertilizer, manure, or legumes greatly exceed the amount of nitrogen taken up by the crop (Meisinger and Randall, 1991).

The nitrogen delivered in fertilizers, manures, rainfall, and irrigation water; the nitrogen mineralized from soil organic matter and crop residues; and the nitrogen fixed by legumes all contribute to the nitrogen budget of a particular agricultural field. Nitrogen in the form of ammonium ions and nitrate are of particular concern because they are very mobile forms of nitrogen and are most likely to be lost to the environment. All forms of nitrogen, however, are subject to transformation to ammonium ions and nitrate as part of the nitrogen cycle in agroecosystems and all can contribute to residual nitrogen and nitrogen losses to the environment. The importance of any particular source depends on the type of agricultural enterprise, soil properties, geographic location, and climate. (See Chapter 6 for a discussion of the nitrogen cycle.)

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×
Nitrogen Mass Balances

National-, regional-, and farm level nitrogen mass balances suggest that current nitrogen inputs from all sources usually exceed the nitrogen harvested and removed with crops. Table 2-1 presents estimates of national and regional nitrogen inputs and outputs. (See Chapter 6 for a discussion of nitrogen mass balance estimates and the Appendix for a complete discussion of how these estimates were made.) In most regions and for the United States as a whole, the nitrogen applied in synthetic fertilizers is less than that harvested in crops. The nitrogen in synthetic fertilizers is not the only nitrogen source; the nitrogen in manures (manure-N) and the nitrogen fixed by legumes (legume-N) provide nearly as much nitrogen as is applied in commercial fertilizers in these estimates (the nitrogen in crop residues is assumed to be present in inputs and outputs in equal amounts). If all sources of nitrogen are accounted for, the estimated nitrogen inputs are nearly 1.5 times as great as the nitrogen removed in harvested crops or crop residues. Very little nitrogen is applied to legumes such as alfalfa and soybeans, yet those crops account for more than 35 percent of the nitrogen harvested by all crops. If only nitrogen inputs and outputs for major commodities such as corn, wheat, and cotton are considered, only about 35 percent of total nitrogen inputs are accounted for in the harvested crop.

Peterson and Frye (1989) estimated the amount by which the nitrogen in synthetic fertilizers applied to corn fields (aggregated at the national level) replaced the amount of nitrogen removed in the grain of that year's crop. The nitrogen applied to corn in synthetic fertilizer exceeded that removed in the grain by 50 percent or more for every year since 1968. Peterson and Frye's (1989) estimates of the amount of nitrogen applied included that in synthetic fertilizers only and did not include estimates of the nitrogen provided in manures or by legume fixation.

Similarly, Peterson and Russelle (1991) estimated the amount of nitrogen applied to corn in synthetic fertilizers and that supplied by alfalfa in the Corn Belt (see Table 6-7 in Chapter 6). Their estimate of the amount of nitrogen supplied by alfalfa included estimates of the amount of nitrogen that might be applied as manure from alfalfa fed to cattle as well as nitrogen fixed by alfalfa. Depending on whether they used a low or a high estimate of how much nitrogen was supplied by alfalfa, Peterson and Russelle estimated that nitrogen applications could be reduced by between 8 and 14 percent for the region as a whole. For states such as Michigan, Minnesota, and Wisconsin, which grow more alfalfa than other states, the estimated nitrogen reductions were much

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

TABLE 2-1 Regional and National Estimates of Nitrogen Inputs, Outputs, and Balances on Croplands, Medium Legume-N Fixation Scenario (metric tons)

 

Input

Output

Region

Nitrogen Fertilizer

Recoverable Manure-N

Legume-N

Crop Residues

Total

Harvested Crop

Crop Residues

Total

Balance

Northeast

252,000

224,000

284,000

70,200

831,000

412,000

70,200

482,000

349,000

Appalachia

564,000

109,000

395,000

102,000

1,170,000

491,000

102,000

593,000

577,000

Southeast

609,000

78,100

173,000

43,800

904,000

236,000

43,800

280,000

624,000

Lake States

988,000

278,000

984,000

368,000

2,620,000

1,420,000

368,000

1,780,000

834,000

Corn Belt

2,720,000

240,000

2,750,000

1,220,000

6,940,000

3,860,000

1,220,000

5,080,000

1,850,000

Delta

468,000

59,400

552,000

105,000

1,180,000

444,000

105,000

548,000

636,000

Northern Plains

1,510,000

256,000

1,020,000

602,000

3,390,000

1,930,000

602,000

2,530,000

859,000

Southern Plains

920,000

183,000

82,000

129,000

1,310,000

503,000

129,000

632,000

682,000

Mountain

554,000

160,000

451,000

158,000

1,320,000

786,000

158,000

944,000

379,000

Pacific

798,000

146,000

177,000

88,700

1,210,000

498,000

88,800

587,000

623,000

United States

9,390,000

1,730,000

6,870,000

2,890,000

20,900,000

10,600,000

2,890,000

13,500,000

7,420,000

NOTE: See the Appendix for a complete discussion of the methods used to estimate the nitrogen inputs, outputs, and balances given here.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

TABLE 2-2 Nitrogen Budgets for Four Farms (A, B, C, and D) in Southeastern Minnesota

 

kg of Nitrogen/ha Used

Nitrogen Budget Item

A

B

C

D

Sources

Commercial fertilizer

174

146

162

155

Soybean credits

29

NA

NA

NA

Alfalfa credits

11

30

19

NA

Manure

37

146

32

NA

Total sources

251

321

214

155

Nitrogen needed to meet yield goals

184

172

184

169

Excess nitrogen per hectare

67

149

30

-15

NOTE: NA, not applicable.

SOURCE: Adapted from T. D. Legg, J. J. Fletcher, and K. W. Easter. 1989. Nitrogen budgets and economic efficiency: A case study of southeastern Minnesota. J. Prod. Agriculture 2:110-116.

larger—20 to 36 percent for Michigan, 13 to 23 percent for Minnesota, and 37 to 66 percent for Wisconsin.

Such national- or regional-level mass balances are crude generalizations of the real situation in particular crop fields. The actual balance between the nitrogen applied and that required for crop growth varies from region to region, from farm to farm, and even from field to field. Nitrogen must be applied in excess of the amount actually harvested in grain and residues because the efficiency of nitrogen uptake by the crop is less than 100 percent and because precise crop needs vary with time and weather. The magnitude of the unaccounted for nitrogen in estimated mass balances, however, indicates the underlying reason for the loss of nitrogen from crop production and illustrates the potential for improvements in nitrogen management.

Farm level nitrogen mass balances in many instances reinforce the picture that emerges from national- and regional-level estimates of nitrogen mass balances. Legg and colleagues (1989), for example, estimated nitrogen budgets for four farms in southeastern Minnesota (Table 2-2). Generally, the amount of nitrogen applied in synthetic fertilizer was about the same as the amount of nitrogen removed in the crop. The nitrogen from all sources, however, was far in excess of the nitrogen removed in the crop on those farms where multiple sources of nitrogen were available, suggesting that the use of supplemental applications of nitrogen in synthetic fertilizer could have been reduced. Similar budgets at the farm level have been reported by Lanyon and

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

Beegle (1989), Duffy and Thompson (1991), Schepers and colleagues (1986), Bouldin and colleagues (1984), and Pratt (1984).

Refining Fertilizer Recommendations

The most important immediate opportunity for improving nitrogen management is to refine recommendations for application of synthetic fertilizers containing nitrogen.

Although nitrogen is supplied to cropping systems from many sources, including legumes and manures, most adjustments to the total nitrogen applied to cropping systems come by refining the quantity, location, and time of year that producers apply synthetic fertilizers containing nitrogen. Applications of synthetic fertilizers containing nitrogen are much easier to manage because the amount of nitrogen applied is known with accuracy. More important, when livestock or legumes are an important part of the farm enterprise, nitrogen additions from these sources are a fixed part of the nitrogen budget for the enterprise, and adjustments in the total amount of nitrogen applied will likely be made by adjusting the amounts of synthetic fertilizers containing nitrogen that producers apply. Use of legumes and applications of manure may be needed to improve soil quality in addition to their value as sources of nitrogen. Improving the management of synthetic fertilizers containing nitrogen presents the greatest opportunity for improved nitrogen use efficiency.

Recommendations for application of synthetic fertilizers containing nitrogen can be improved by setting realistic yield goals and accounting for all sources of nitrogen when making fertilizer recommendations.

Realistic Yield Goals

As a crop's yield increases, the crop's need for nitrogen increases, at least initially. The dilemma for producers is that nitrogen must be applied before the crop yield is known. Nitrogen recommendations, therefore, must be based on some expectation of crop yield. For many crops, nitrogen requirements and recommendations are based on yield goals, that is, the yield expected by the producer under optimum growing conditions. The importance of setting realistic yield goals as the basis for making both economically and environmentally sound recommendations has often been highlighted (see, for example, Bock and Hergert [1991]; Peterson and Frye [1989]; University of and Wisconsin Department of Agriculture, Trade and Consumer Protection [1989]; U.S. Congress, Office of Technology Assessment

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

[1990]). An unrealistically high yield goal will result in nitrogen applications in excess of what is needed for the yield that is actually achieved and will contribute to the mass of residual nitrogen in the soil-crop system.

Yield goals should be established on the basis of the historical yields achieved on a field-by-field basis.

The actual yield in any year depends on the weather and the inherent soil quality. The importance of weather in determining yields means that yields will vary from year to year, even under the best-management conditions. If the producers apply nitrogen after planting, there is more opportunity to adjust applications to weather during the growing season. If most or all of the nitrogen is applied before planting, the yield goal set by the producer has important effects on the potential for water pollution. In such cases, the best way to establish yield goals is to obtain an average yield for the field during the previous 5 years.

If a producer uses the yield from a bumper crop as the goal, the result will be overapplication of nitrogen during most years. This practice increases production costs and the amount of residual nitrogen. In addition, many soils, though not those with very low levels of organic matter, typically supply the additional nitrogen needed during a bumper crop year because optimal weather and soil conditions that lead to a bumper crop also increase the amount of nitrogen mineralized from the organic matter in the soil (Schepers and Mosier, 1991).

No national data are available to estimate how realistically producers now set their yield goals. A number of local studies (Schepers and Mosier, 1991; Schepers et al., 1986) suggest that the yield goals set by producers are not often achieved. The data reported by Schepers and colleagues (1986) showed that producers in Hall County, Nebraska, generally set their yield goals at 2,688 kg/ha (40 bu/acre) more than the yield that they actually achieved. These unrealistically high yield goals meant that nitrogen was applied at rates averaging more than 45 kg/ha (40 lbs/acre) over the rate recommended by the University of Nebraska soil testing laboratory.

Accounting for All Sources of Nitrogen

The amount of nitrogen that needs to be applied to cropland depends on the amount already available in the soil from all sources. Producers must account for the nitrogen available from manure applications, legumes, soil organic matter, and other sources before recommendations for supplemental nitrogen applications can be made. The importance

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

of carefully accounting for all nitrogen sources has repeatedly been stressed as a way to improve nitrogen management (see, for example, Bock and Hergert [1991]; Peterson and Frye [1989]; Schepers and Mosier [1991]; University of Wisconsin-Extension and Wisconsin Department of Agriculture, Trade and Consumer Protection [1989]; U.S. Congress, Office of Technology Assessment [1990]).

The nitrogen balances estimated in Table 2-1 reinforce the importance of accounting for all sources of nitrogen when making decisions about rates of nitrogen fertilization. The nitrogen that is recoverable in manure and legumes supplies roughly the same amount of nitrogen applied to crop fields in synthetic fertilizers containing nitrogen. In some regions where producers grow large amounts of soybeans or alfalfa, the nitrogen credits from legumes alone may exceed the amount supplied by synthetic fertilizers.

Although no comprehensive data are available on how well producers credit the nitrogen available from manures, legumes, and other sources when making fertilizer application decisions, those data that are available suggest real opportunities for improvement. Peterson and Russelle (1991), for example, estimated that fertilizer applications to corn in the Corn Belt could be reduced by between 237,000 and 435,000 metric tons (261,000 and 479,000 tons) by properly accounting for the nitrogen supplied by alfalfa (see Table 6-7 in Chapter 6). If the nitrogen supplied from soybeans had also been included in their analyses, the possible nitrogen application reductions would be greater. Two statewide surveys of producers in Iowa found that although 80 and 76 percent of producers took some credits for the nitrogen value of soybean and 92 percent of producers took some credits for the nitrogen value of alfalfa, the credits taken were inadequate and only 50 percent of the producers took credit for the nitrogen in manures (Duffy and Thompson, 1991; Kross et al., 1990). Nitrogen balances for individual farms also indicate the importance of accounting for the nitrogen in manures and fixed by legumes (Bouldin et al., 1984; Lanyon and Beegle, 1989; Legg et al., 1989; Padgitt, 1989).

El-Hout and Blackmer (1990) evaluated corn fields that followed alfalfa in rotation in northeast Iowa. Fertilizer application rates ranged from 6 to 227 kg/ha of nitrogen (5 to 203 lb/acre) and 59 percent of the fields sampled also received manure applications. Of the fields sampled, 86 percent had greater soil-nitrate concentrations than needed for optimal yields, 56 percent had at least twice the optimal amount, and 21 percent had at least three times the amount of soil-nitrate needed. Similarly, farm assessments completed as part of a nutrient and pesticide management program in Wisconsin showed that more than half of the farms

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

were applying 50 percent more nitrogen than recommended for optimal crop production (Nowak and Shepard, 1991).

Recommendations for applications of fertilizers containing nitrogen should be made by taking a full accounting of and by considering all internal sources of nitrogen, including the nitrogen fixed by legumes and in manures.

The single most important way to improve nitrogen management is to reduce supplemental applications of nitrogen to account for nitrogen supplied by legumes and manures. Most states publish estimates for the nitrogen replacement value of alfalfa, soybeans, and other legumes. Widespread use of even these estimates when making fertilization application decisions would result in immediate improvements in nitrogen management.

Uncertainty in estimating the nitrogen contents of manures is an important constraint to an accurate accounting of the nitrogen contributed by manures (Schepers and Mosier, 1991). (See the section on manures later in this chapter and in Chapter 11.) Published estimates and ranges of the nitrogen contents of manures are available, however, and could be used as rules of thumb for improving manure management. If results of statewide surveys in Iowa are representative, more than one-half of the producers would benefit from taking even a conservative manure credit, since they now take no credits (Duffy and Thompson, 1991; Hallberg et al., 1991; Padgitt, 1989).

Synchronizing Fertilizer Applications with Crop Needs

Supplying the nitrogen needed for crop growth during the period when it is most needed can be an important way to improve nitrogen management. Nitrogen is needed most during the period when the crop is actively growing. The nitrogen applied before that time is vulnerable to loss through leaching or lateral subsurface flow because of the mobility of nitrates in the soil system. Larger nitrogen applications are required if the nitrogen is applied in the fall or early spring before planting to make up for the nitrogen lost or that becomes unavailable in the soil during the period between application and crop growth. Use of a nitrification inhibitor that slows down the rate at which nitrogen is converted to mobile nitrates produces an intermediate effect but may not reduce losses to the environment over the long-term.

Fertilizers containing nitrogen should, whenever possible, be applied during and/or after planting.

The opportunity to increase the efficiency with which nitrogen is used by synchronizing applications with periods of crop growth has often

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

TABLE 2-3 Crops Receiving Fertilizer Nitrogen Before, During, and After Seeding

 

 

 

 

Percent of Fertilized Area Receiving Nitrogen

 

 

 

 

Before Seeding

Crop

Area Planted (103 ha)

Percent of Planted Area Fertilized

Average Application Rate (kg/ha)

Fall

Spring

At Seeding

After Seeding

Corn

23,800

97

148

28

57

43

26

Cotton

3,940

80

96

35

44

8

51

Rice

729

97

128

10

16

4

92

Soybeans

19,500

17

27

24

47

23

8

Winter Wheat

16,300

84

69

73

NA

22

42

Spring Wheat

6,400

69

59

38

34

63

2

 

SOURCE: H. H. Taylor. 1991. Fertilizer application timing. Pp. 30-38 in Agricultural Resources: Inputs Situation and Outlook. Report No. AR-24. Washington, D.C.: U.S. Department of Agriculture, Economic Research Service, Resources and Technology Division.

been highlighted (Ferguson et al., 1991; Jokela and Randall, 1989; Peterson and Frye, 1989; Randall, 1984; Russelle and Hargrove, 1989; University of Wisconsin-Extension and Wisconsin Department of Agriculture, Trade and Consumer Protection, 1989; U.S. Congress, Office of Technology Assessment, 1990). Similarly, increased losses of nitrogen from nitrogen applications in the fall or early spring have also been noted. Applications of manures without incorporation in fall, winter, and spring can be a particularly important source of surface water pollution (University of Wisconsin-Extension and Wisconsin Department of Agriculture, Trade and Consumer Protection, 1989). Data on the timing of nitrogen applications, however, suggest that preplant applications are the rule, not the exception, for most major commodity crops (Table 2-3). Only 26 percent of corn, 51 percent of cotton, and 8 percent of soybeans, received fertilizer containing nitrogen following seeding.

Some improvement in nitrogen management could be achieved by increasing the percentage of crops treated with nitrogen postplanting rather than preplanting except for small applications at planting that may be needed as starter fertilizer. Corn production alone consumes more than 40 percent of the nitrogen applied to commodity crops (Vroomen, 1989). Opportunities to improve nitrogen management by adjusting the timing of nitrogen applications appear to be particularly great for corn production, for which fall and spring applications are both common and application rates are high (Table 2-3). The advantages of

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

changing application times may be specific to the climate and site conditions (Killorn and Zourarakis, 1992). When coupled with efforts to account for all sources of nitrogen and to set realistic yield goals, significant financial and environmental benefits may be achieved.

New and Improved Tools

Current nitrogen management data suggest that substantial improvements in nitrogen management could be achieved through the more widespread use of current technology for setting yield goals, making and implementing fertilizer recommendations, and increasing postplanting nitrogen applications. Weather and crop yield variabilities, however, create uncertainties about a crop's nitrogen requirements and the amount of residual nitrogen available from the soil that makes refining nitrogen management difficult.

The development, testing, and implementation of improved methods of estimating crop nitrogen requirements following planting should be the highest research priority for improving nitrogen management.

Typical, currently used methods of testing soils are not suitable for supplying the information needed to reduce the uncertainty in estimating a crop's nitrogen needs. The initial results from testing new methods of estimating the nitrogen content of soils or crop tissues appear promising. Practical and accurate testing methods that would allow nitrogen fertilizer recommendations to be made following planting is the single most important technical innovation needed to improve nitrogen management. The inadequacy of current methods for reducing this uncertainty is a serious impediment to improving nitrogen management.

Various plant and tissue tests have proved to be valuable tools for more efficient nitrogen management in vegetable and citrus crops, but such methods must be refined and implemented for the major row crops such as corn, to which most of the nitrogen used in the United States is applied. Many methods are being tested across the Corn Belt (Binford et al., 1992; Blackmer et al., 1989; Cerrato and Blackmer, 1991; Fox et al., 1989; Magdoff, 1991a; Motavalli et al., 1992; Piekielek and Fox, 1992; Tennesse Valley Authority, National Fertilizer Development Center, 1989). The presidedress soil nitrate test developed in Iowa, for example, measures the amount of nitrogen available in the upper 0.3 to 0.6 m (1 to 2 feet) of the soil profile and has been used to refine recommendations for supplemental fertilizer applications. In a project in which fertilizer dealers used the test to refine fertilizer recommendations, nitrogen applications were reduced an average of 42 percent while maintaining crop yields (Blackmer and Morris, 1992; Hallberg et al., 1991).

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Development of monitoring and modeling systems to help estimate the nitrogen available to the crop from the soil and from carryover of nitrogen applied the previous year are also needed. Models that integrate climatic, soil, and crop conditions to predict the nitrogen available from the previous year could help producers refine their annual plans for nitrogen application.

Research to refine crop yield response models for use in estimating optimal nitrogen application rates should be undertaken.

Fertilizer recommendations are made on the basis of models of crop responses to various rates of nitrogen application. These models are normally developed by each state for the crops grown in that state and for the soils and climatic regions of that state. The accuracies of these models in large part determine the level of refinement possible in fertilizer recommendations.

Various studies have noted that different models that use the same data from crop response field tests predict very different optimal rates of nitrogen application. Cerrato and Blackmer (1990) evaluated the five most widely used response models. All of the models predicted very similar maximum obtainable yields, but the optimal nitrogen rates differed by 250 kg/ha (223 lbs/acre). In addition, the crop response models in use have often been developed on the basis of studies of plots with only two to four different rates of nitrogen application. Blackmer (1986) found that much greater refinements in the estimation of crop requirements were obtained from crop response models developed from a greater variety of application rates.

Refining current crop response models to allow greater precision in estimates of optimal nitrogen application rates is an important way to improve nitrogen management.

Improving Phosphorus Management

Phosphorus, like nitrogen, is both an important plant nutrient and a serious pollutant when delivered to surface water. Most forms of phosphorus compounds are bound more tightly to most soils than nitrogen compounds, creating important differences in the approaches taken to control phosphorus losses from farming systems.

Phosphorus Cycle

Like nitrogen and other plant nutrients, the phosphorus added to the soil-crop system goes through a series of transformations as it cycles

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

through plants, animals, microbes, soil organic matter, and the soil mineral fraction. Unlike nitrogen, however, phosphorus is tightly bound in most soils and only a small fraction of the total phosphorus found in the soil is available to crop plants.

Most of the phosphorus in soil is found as a complex mixture of mineral and organic materials. Organic phosphorus compounds in plant residues, manures, and other organic materials are broken down through the action of soil microbes. Some of the organic phosphorus can be released into the soil solution as phosphate ions that are immediately available to plants. Much of the organic phosphorus is taken up by the microbes themselves. As microbes die, the phosphorus held in their cells is released into the soil. A considerable amount of organic phosphorus is held in the humic materials that make up soil organic matter. A portion of this organic phosphorus is released each year as these humic materials decay. The phosphate ions released from the decomposition of organic phosphorus compounds or added directly in fertilizers containing inorganic phosphorus readily react with soil minerals and are immobilized in forms that are unavailable for plant growth. (Figure 7-1 in Chapter 7 provides an illustration of the phosphorus cycle in the soil-crop system.)

Transport Processes

Phosphorus can be lost from the soil-crop system in soluble form through leaching, subsurface flow, and surface runoff. Particulate phosphorus is lost when soil erodes. Phosphorus loss by leaching to groundwater, in most regions of the United States, is not a problem (Gilliam et al., 1985). The majority of phosphorus lost from agricultural lands is with surface flow, both in solution (soluble phosphorus) and bound to eroded sediment particles (particulate phosphorus). Most of the total phosphorus loss from cropped land is in the sediment-bound form (Gilliam et al., 1985; Sharpley and Menzel, 1987; Viets, 1975). Soluble phosphorus is more readily available to stimulate eutrophication, but particulate phosphorus can be a long-term source of phosphorus once it is delivered to surface water (Gilliam et al., 1985; Sharpley and Menzel, 1987).

Phosphorus Mass Balance

Phosphorus is added to agricultural lands in crop residues and manures, in synthetic fertilizers, and from phosphorus-bearing minerals in the soil. Part of the phosphorus entering the soil-crop system is

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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removed with the harvested crop; the balance is immobilized in the soil, incorporated into soil organic matter, or lost in surface or shallow subsurface flow, primarily to surface water.

Table 2-4 provides estimates of regional and national phosphorus inputs and outputs in 1987. (See Chapter 7 for a discussion of phosphorus mass balance estimates and the Appendix for a complete discussion of how these mass balance estimates were made.) The difference between phosphorus inputs and outputs in crops and crop residues is reported as the phosphorus mass balance.

The phosphorus in synthetic fertilizers is the single most important source of phosphorus added to croplands in the United States (Table 2-4). Approximately 3.6 million metric tons (4.0 million tons) of phosphorus was added to croplands in 1987. The amount of synthetic fertilizer applied represents 79 percent of phosphorus inputs. The amount of recoverable phosphorus voided in manures is small compared with that supplied in synthetic fertilizers at the national level. Locally, the proportion of phosphorus supplied by manures can be large. The recoverable phosphorus in manure (manure-P), for example, supplies 65 percent of total phosphorus inputs in Vermont (see Table 7-3 in Chapter 7).

Approximately 1.3 million metric tons (1.4 million tons) of phosphorus—or 29 percent of total phosphorus inputs—was harvested along with crops in the United States in 1987 (Table 2-4). Another 272,000 metric tons (300,000 tons) of phosphorus—or 6 percent of total phosphorus inputs—was contained in crop residues. About 1.6 million metric tons (1.8 million tons)—or 36 percent of total phosphorus inputs—can be accounted for in harvested crops and crop residues, leaving an unaccounted for balance of 2.9 million metric tons (3.1 million tons)—or 63 percent of total phosphorus inputs.

The fraction of total phosphorus inputs lost in eroded soil and in surface runoff can be substantial, but it is difficult to estimate. Larson and colleagues (1983) estimated that 1.74 million metric tons (1.92 million tons) of phosphorus—or about 50 percent of the estimated total phosphorus balance in Table 2-4—was lost in eroded sediments in 1982. Additional phosphorus can be lost in solution.

The majority of the unaccounted for phosphorus balance on croplands is immobilized in the soil's mineral or organic fractions. The actual magnitude of the unaccounted for balance of phosphorus added to farming systems varies from region to region, soil to soil, and farm to farm. The national mass balance in Table 2-4, however, suggests that the potential for buildup of phosphorus levels in cropland soils over time is large. The buildup of phosphorus in soil increases the

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

TABLE 2-4 Regional and National Estimates of Phosphorus Inputs, Outputs, and Balances on Croplands, 1987 (metric tons)

 

Input

Output

Region

Fertilizer-P

Recoverable Manure-P

Crop Residues

Total

Harvested Crop

Crop Residues

Total

Balance

Northeast

155,000

76,700

6,660

239,000

48,300

6,660

55,000

184,000

Appalachia

334,000

42,300

9,130

385,000

60,500

9,130

69,600

315,000

Southeast

264,000

32,400

3,960

300,000

27,800

3,960

31,800

269,000

Lake States

442,000

98,500

34,100

575,000

168,000

34,100

202,000

373,000

Corn Belt

1,130,000

115,000

112,000

1,350,000

481,000

112,000

593,000

761,000

Delta

119,000

22,200

10,600

151,000

54,600

10,600

65,200

86,200

Northern Plains

446,000

100,000

58,800

605,000

261,000

58,800

319,000

286,000

Southern Plains

274,000

59,900

14,300

348,000

72,400

14,300

86,700

262,000

Mountain

186,000

53,300

14,500

254,000

93,500

14,500

108,000

146,000

Pacific

224,000

54,300

8,450

287,000

57,700

8,450

66,200

221,000

United States

3,570,000

655,000

272,000

4,500,000

1,320,000

272,000

1,600,000

2,900,000

NOTE: See the Appendix for a complete discussion of the methods used to estimate the phosphorus inputs, outputs, and balances given here.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

amount of phosphorus lost in runoff water and sediments from croplands.

Control Phosphorus Buildup in Soil

Because phosphorus is tightly bound to the soil, most efforts to reduce the amount of phosphorus lost from farming systems have focused on reducing erosion. Reducing erosion alone, although essential, will not be sufficient to control phosphorus losses if the phosphorus levels in soil buildup to high levels. Excessive levels of phosphorus in the soil increases the amount of soluble phosphorus lost in surface runoff and the concentration of phosphorus in sediments, thus counteracting some of the reductions in phosphorus pollution gained by controlling erosion.

Efficient management of phosphorus inputs to prevent the buildup of excess phosphorus levels in soil while providing adequate phosphorus for crop growth should be a fundamental part of programs to reduce phosphorus loadings to surface water.

The level of phosphorus in surface soil is a critical factor that determines the phosphorus loads in runoff water and the relative proportions of phosphorus lost in solution and attached to soil particles. Increased residual phosphorus levels in the soil lead to increased phosphorus loadings to surface water, both in solution and attached to soil particles. Policies and programs to reduce phosphorus losses from farming systems should pay much more attention to improving the management of phosphorus inputs to reduce the buildup of phosphorus in soil. Erosion control should remain an important objective for reducing the amount of phosphorus lost from farming systems, but it should be coupled with efforts to reduce the buildup of phosphorus in soil.

Because of a history of phosphorus applications in excess of that harvested or naturally high phosphorus levels in soil, or both, phosphorus levels have increased in many U.S. soils (Thomas, 1989) and many now have high phosphorus levels. The results of tests for the levels of phosphorus in the soil are reported as being very low, low, medium, high, or very high. These results are based on the probability that crops grown on that soil will respond to an application of phosphorus fertilizer rather than on the absolute amounts of extractable phosphorus that were detected in the soil. A crop grown on a soil testing very low for phosphorus, for example, has a high probability (90 to 100 percent) of responding to supplemental applications of phosphorus fertilizer. Conversely, a crop grown on a soil testing very high for phosphorus has a low probability (0 to 10 percent) of responding to supplemental applications

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

TABLE 2-5 Percentage of Soil Tests Reporting High to Very High Levels of Soil Phosphorus

State

Percent

State

Percent

Alabama

35

Nebraska

31

Arizona

51

Nevada

48

Arkansas

14

New Hampshire

California

41

New Jersey

Colorado

43

New Mexico

Connecticut

51

New York

38

Delaware

65

North Carolina

67

Florida

45

North Dakota

30

Georgia

38

Ohio

68

Idaho

60

Oklahoma

48

Illinois

63

Oregon

49

Indiana

78

Pennsylvania

44

Iowa

56

Rhode Island

Kansas

39

South Carolina

40

Kentucky

42

South Dakota

56

Louisiana

37

Tennessee

49

Maine

51

Texas

37

Maryland

74

Utah

60

Massachusetts

Vermont

25

Michigan

73

Virginia

58

Minnesota

76

Washington

54

Mississippi

34

West Virginia

Missouri

35

Wisconsin

66

Montana

41

Wyoming

38

NOTE: Dashes indicate no data were reported.

SOURCE: Adapted from Potash and Phosphate Institute. 1990. Soil test summaries: Phosphorus, potassium, and pH. Better Crops with Plant Food 74(2):16-18.

of phosphorus fertilizer. Table 2-5 reports the percentage of soil tests in each state that reported soils testing high to very high for phosphorus.

The phosphorus level in many U.S. soils is high enough that applications of additional phosphorus would not increase crop yields (McCollum, 1991; Novais and Kamprath, 1978; Yerokun and Christenson, 1990). Mallarino and colleagues (1991) cited several studies reporting that increases in soybean or corn yields are small or nonexistent when phosphorus levels in soil are in the medium category (Grove et al., 1987; Hanway et al., 1962; Million et al., 1989; Obreza and Rhoads, 1988; Olson et al., 1962; Rehm, 1986; Rehm et al., 1981). Phosphorus additions to soils that test high for phosphorus typically do not increase corn or soybean yields in the Corn Belt (Bharati et al., 1986; Hanway et al., 1962; Olson et al., 1962; Rehm, 1986). This suggests that applications of

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

additional phosphorus to 56, 63, 78, 68, and 35 percent of the soils tested in Iowa, Illinois, Indiana, Ohio, and Missouri, respectively, would not be expected to increase yields.

Similar situations exist in the southeastern United States. Kamprath (1967, 1989) and McCollum (1991) have shown that corn and soybeans grown on Piedmont and Coastal Plain soils testing high in available phosphorus do not respond to phosphorus fertilizer additions. On the basis of the soil test data presented in Table 2-5, no response to phosphorus would be expected on approximately half of the soils that were tested in this region. In North Carolina, the amount of phosphorus recommended for use on soybeans grown in soils that tested medium for phosphorus is higher than the amount of phosphorus removed in the grain (Kamprath, 1989). Thus, current recommendations will lead to phosphorus levels in soil higher than those needed for corn or soybean production. The magnitude of the potential reduction in application of phosphorus, however, depends on the soil, climate, and crop planted.

Thresholds for Phosphorus Levels in Soil

Threshold levels of phosphorus in soil—beyond which no crop response from added phosphorus except for small starter applications would be expected—should be established.

Most states have soil testing procedures and facilities that could be used to establish threshold levels of phosphorus in soil beyond which no crop response would be expected. Application of phosphorus to soils that contain phosphorus in excess of threshold levels should be discouraged or disallowed in extreme cases in which phosphorus loadings are causing severe damage. Once established, such threshold levels should be routinely reported as part of soil test results and fertilizer recommendations made by public and private organizations.

Reducing or suspending phosphorus applications to soils already testing high or very high for phosphorus is an important way to improve both the economic and environmental performance of farming systems. Mallarino and colleagues (1991), for example, studied the effect on yields of phosphorus additions to a soil testing high for phosphorus. They reported occasional positive yield responses to fertilization, but these positive responses were not, in most cases, sufficient to pay for the cost of the added phosphorus. In the 11 years of the study, phosphorus applications to soils testing high for phosphorus provided appreciable positive economic returns in only 1 years for corn. Added phosphorus provided no economic benefits for soybeans. The addition of phosphorus resulted in negative returns in most years for both corn and

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

TABLE 2-6 Proportion of Cropland Soils Tested for Nutrient Levels, Major Field Crops, 1989

 

 

Percent Soil Tested

 

 

Phosphate or Potash

Nitrogen

Crop

Area Planted (ha)

1987

1988–1989

1987

1988–1989

Corn

23,431

26

33

13

20

Cotton

3,417

20

29

16

23

Winter wheat

14,047

16

16

10

10

Spring wheat

6,710

28

32

26

30

Durum wheat

1,214

14

26

14

26

Northern soybeans

15,277

25

27

NR

NR

Southern soybeans

20,692

23

26

NR

NR

Rice

844

15

20

9

14

NOTE: NR, no data reported.

SOURCE: M. Spiker, S. Dabrekow, and H. Taylor. 1990. Soil Tests and 1989 Fertilizer Application Rates. Pp. 46-49 in Agricultural Resources: Inputs Situation and Outlook. Report No. AR-17. Washington, D.C.: U.S. Department of Agriculture, Economic Research Service, Resources and Technology Division.

soybeans, with losses in some years being greater than $49/ha ($20/acre) for corn.

Several studies have investigated the buildup of soil phosphorus under continuous phosphorus fertilization (McCallister et al., 1987; Schwab and Kulyingyong, 1989); another study documented the loss of soil under a continuous cropping system in which only residual phosphorus was available for crop uptake (Novais and Kamprath, 1978). Both the buildup and the decline of soil phosphorus phases have been studied as well (Cope, 1981; McCollum, 1991; Meek et al., 1982), but relatively few (e.g., Cope, 1981; McCollum, 1991) have been conducted over long time spans (several decades). These few studies may provide some of the best information that can be used to aid in the prediction of residual phosphorus effects and actual phosphorus fertilization needs. These long-term studies suggest that soils with high phosphorus levels can be cropped for a decade or more without the amount of phosphorus in soil reaching a level at which fertilizer additions would result in a crop yield increase.

Few comprehensive data are available on how often and how many producers currently use soil tests when deciding how much phosphorus to apply. Data assembled by the Economic Research Service of USDA (Table 2-6) suggest that immediate improvements in phosphorus management and pollution prevention could be realized simply by expanding

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

the use of currently available soil tests by producers coupled with setting thresholds for phosphorus levels in soil.

Improving Manure Management

Manure supplies nitrogen, phosphorus, and other nutrients for crop growth; adds organic matter and improves soil structure and tilth; and increases the soil's ability to hold water and nutrients and to resist compaction and crusting. Disposal of manure as a waste often leads to both surface water and groundwater degradation. Improved manure management can effectively capture the benefits of manure as an input to crop production and can reduce the environmental problems associated with manure disposal.

Nutrient Value of Manures

About 124 million metric tons (137 million tons) of manure was produced by cattle, sheep, swine, and poultry in the United States in 1987 (see the appendix for a discussion of how these estimates were made). These manures contained more than 5.0 million metric tons (5.5 million tons) of nitrogen and nearly 1.4 million metric tons (1.5 million tons) of phosphorus. At 1987 prices for nitrogen in anhydrous ammonia and phosphorus in superphosphate (Vroomen, 1989), the values of these nutrients in manures were about $1.2 billion and $450 million, respectively. Only part of the total nitrogen and phosphorus voided in manures is economically recoverable for use on croplands. some of the manure is voided on pastures and rangelands where recovery is not feasible. Nitrogen is lost from manures by volatilization, and both nitrogen and phosphorus can be lost from barnyards and feedlots in runoff water. Tables 2-1 and 2-4 estimate that nearly 1.8 million metric tons (2 million tons) of nitrogen (34 percent of the total nitrogen voided) and about 726,000 metric tons (800,000 tons) of phosphorus (49 percent of the total phosphorus voided) were available from manures in 1987. The importance of manures as a source of nitrogen and phosphorus in crop production systems varies from region to region (Tables 2-1 and 2-4). Nationally, nitrogen from manures supplies about 8 percent of the total nitrogen applied to croplands in synthetic fertilizers, legumes, crop residues, and manures but ranges from 3 to 26 percent among farm production regions. In the Northeast region, the amount of nitrogen applied to croplands in manures is nearly equal to the amount in synthetic fertilizers. Phosphorus in manures supplies about 15 percent of total phosphorus inputs nationally, ranging from 8 percent in the Corn Belt to 32 percent in the Northeast.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×
Manure Is an Important Source of Water Pollution

Improvements in manure management should be a high priority in programs to improve water quality.

Manures pose particularly difficult environmental problems. Manures reaching surface water disrupt aquatic ecosystems by depleting dissolved oxygen. Human health can be endangered by contamination of drinking water supplies with fecal bacteria and viruses carried in manures. Losses of nitrogen and phosphorus from feedlots and barnyards can be great, and they can be an important source of water quality problems (Bouldin et al., 1984; Brown et al., 1989; Daniel et al., 1982; Pinkowski et al., 1985; University of Wisconsin-Extension and Wisconsin Department of Agriculture, Trade and Consumer Protection, 1989; U.S. Congress, Office of Technology Assessment, 1990). Losses of nitrogen and phosphorus in runoff from croplands on which manures have been applied to the soil surface can also be great (Brown et al., 1989; Moore et al., 1978; U.S. Congress, Office of Technology Assessment, 1990). In places where animal agriculture is important and manures are not well managed, manures can be a particularly important source of nitrogen and phosphorus pollution.

Obstacles to Improving Manure Management

Several important obstacles will make improvements in manure management difficult and costly. These include the concentration of livestock, which leads to a shortage of available cropland to which manure can be applied, the buildup of nitrogen and phosphorus in soils after repeated manure applications, and high capital costs.

Livestock Concentration

The concentration of livestock production in areas with insufficient cropland for effective utilization of the nitrogen and phosphorus in manure is perhaps the single greatest challenge to improving manure management. The concentration of livestock production in large confinement feeding operations or the development of regional concentrations of dairy, poultry, or other animal agricultural systems has created situations in which more manure is being produced than can be used on available cropland (see Figure 11-2 in Chapter 11). In Lancaster County, Pennsylvania, for example, the number of beef cattle increased 55 percent, dairy cattle increased 61 percent, hogs increased 677 percent, poultry layers and pullets increased 193 percent, and broilers increased

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

540 percent between 1960 and 1986 (Lanyon and Beegle, 1989). The resulting oversupply of manure leads directly and indirectly to increased pollution of surface water and groundwater with nitrogen, phosphorus, bacteria, and other organic pollutants.

Reducing supplemental nitrogen and phosphorus inputs to account for the nitrogen and phosphorus in manures will lead to improvements even in areas where animal agriculture is concentrated. In some watersheds, however, reducing nitrogen and phosphorus loadings to surface water or groundwater will be difficult unless livestock concentrations are reduced or unless the means of processing, transporting, or marketing manures or products derived from manures are developed.

Nitrogen and Phosphorus Buildup after Repeated Applications

The problem of manure oversupply is exacerbated by the buildup of phosphorus and nitrogen in soil after repeated manure applications. When manure is applied to the same field year after year, each succeeding year requires less manure to maintain the same amount of nitrogen available to the crop. For example, when manure containing only 1 percent nitrogen on a dry weight basis is added to cropland, it requires about 20 metric tons (22 tons) to supply 112 kg of available nitrogen per ha (100 lbs/acre) the first year but only about 5.1 metric tons (5.6 tons) after 15 years of repeated applications (see Table 11-3 in Chapter 11).

This problem is even more acute for phosphorus. The ratio of nitrogen to phosphorus in manure applied to the land is often between 2 to 1 and 3 to 1. Therefore, when manure is applied to supply adequate nitrogen for most cropping conditions, excess amounts of phosphorus are added, leading to phosphorus buildup in the soil. Application of manures at rates that prevent the buildup of phosphorus in soil, however, dramatically increases the amount of surplus manure that needs to be used. In regions where phosphorus pollution of surface water and groundwater is not a problem, the benefits derived from using manures to enhance overall soil quality and as a primary source of nitrogen for plant growth may outweigh any potential negative effects associated with increased phosphorus levels in the soils.

High Capital Costs

Effective use of the nutrients in manures requires equipment for collection and application and facilities for storage. Manures must be collected and stored until they can be applied to croplands. Storing

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

manures where they are exposed to runoff and leaching leads to large losses of nitrogen and phosphorus. Equipment is needed to inject or incorporate manures into the soil to reduce runoff losses. The rate at which manures are applied to croplands is often imprecise and better application equipment is needed. Schepers and Fox (1989), for example, reported that actual manure application rates ranged from 29 to 101 metric tons/ha (13 to 45 tons/acre) in numerous manure calibration demonstrations in Lancaster County, Pennsylvania, even though most producers thought they were applying 45 metric tons/ha (20 tons/acre). Capital, equipment, and labor costs can be an important constraint to improving the efficiency of manure management.

Special Emphasis on Manure Management

Manure management can be improved by improving manure collection and storage facilities, using better application equipment, and testing manures for nutrient content. In many cases, good manure management with accompanying reductions in outlays for synthetic fertilizers can lead to improved profits (Bouldin et al., 1984; Hallberg et al., 1991; Lanyon and Beegle, 1989). In regions or watersheds where manures supply a significant proportion of nitrogen and phosphorus to crop production, improved manure management to protect water quality should be emphasized.

There are few comprehensive national data that can be used to judge how well producers currently manage manures. Those studies that are available for particular farms and regions suggest that there is a substantial opportunity to improve manure management by taking appropriate credits for the manures that have been applied and by improving applications and storage practices (Bouldin et al., 1984; Duffy and Thompson, 1991; Hallberg et al. 1991; Lanyon and Beegle, 1989; Padgitt, 1989).

It may be necessary to provide subsidies or impose penalties. The capital cost of improving manure collection, storage, and application equipment may be large enough to constrain adoption without providing subsidies or imposing penalties. When the amount of manure produced is greater than the amount that can be efficiently applied to the available cropland, even the best manure management may still lead to large losses of nitrogen and phosphorus, increasing the potential for water pollution. There are no easy solutions to the obstacles to improving manure management created by large concentrations of livestock (see Chapter 11 for a more complete discussion of this problem). Restricting the number of animals or the amount of manure that a

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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producer can produce may have a substantial effect on profitability and viability of the enterprise, and the capital cost of manure management facilities can be high.

There are technologies such as composting, anaerobic digestion, and gasification that hold some promise of producing products such as soil amendments, fertilizers, feedstuffs, and fuels from manures that can be transported out of regions with concentrations of livestock. None of these alternatives are problem free, however. The cost of transportation and alternatives are problem free, however. The cost of transportation and application of fertilizers and soil amendments derived from manures may make these products unattractive to other agricultural producers. Special markets such as homeowners, landscapers, or greenhouses willing to pay higher prices for soil amendments or fertilizer may not be large enough to absorb the supply of such products. Use of manure as fuel for power generation on a scale large enough to be profitable may require transport of manures over distances that increase costs well beyond alternative feedstocks. Preliminary evaluations of such alternatives in Lancaster County, Pennsylvania, for example, indicated that the alternatives would be expensive to implement on a scale sufficient to solve excess nutrient problems in the county (Young et al., 1985).

Refining the composition of feeds to minimize the nitrogen and phosphorus in manures may hold promise. Van Horn (1991), for example, using data from Morse (1989) and National Research Council (1989c), concluded that the amounts of both phosphorus and nitrogen voided in manure can be controlled by composition of the feed. They recommended that management of the diet of dairy cattle, with this result in mind, should become an important component of nutrient management on dairy farms.

Solutions to the problem of manure management in regions with large concentrations of livestock will require efforts on multiple fronts. Livestock producers will have to be encouraged or required to reduce supplemental applications of nitrogen and phosphorus and improve the collection, storage, and application of manures. Regulation of feedlots and confined animal feeding facilities are needed to ensure that adequate manure handling, storage, and disposal systems are in place. Research to explore the feasibility of manure processing and to enable refined management of animal diets to reduce nitrogen and phosphorus in manure is also needed.

Improving Pesticide Management

Natural biological processes play an important role in controlling damages caused by pests and pathogens. In highly managed ecosystems

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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such as modern agricultural systems, these natural processes are often disrupted, increasing the risk of damage to crops and animals caused by insects, weeds, pathogens, or nematodes. The advent of synthetic chemical pesticides after World War II revolutionized pest control in agriculture. Pesticides have now become an important and nearly universally used input in agricultural production systems.

Constraints to Making General Recommendations

Pesticides create particularly difficult problems for policymakers because general recommendations are difficult to make. Nearly 50,000 pesticide products are now registered for use with EPA, although the number of pesticides used extensively is much smaller. The ways that these pesticides behave in the environment depend on the interactions among their chemical properties, the soil at the site, the cropping system in which they are used, and the way in which they are applied. Improved pesticide management is therefore a chemical- and site-specific process. This specificity makes broad generalizations on the most promising ways to improve pesticide management more difficult to establish. The list of specific best-management practices to improve the ways in which pesticides are used in agricultural production systems is extensive (University of Wisconsin-Extension and Wisconsin Department of Agriculture, Trade and Consumer Protection, 1989; U.S. Congress, Office of Technology Assessment, 1990). Some general approaches, however, can form the basis of a national policy to improve pesticide management.

Reducing the Total Mass of Pesticides Used

Source control to reduce the total mass of pesticides applied to cropping systems should be the fundamental approach to reducing pesticide losses from farming systems.

Unlike nitrogen and phosphorus, there is no inherent, natural pesticide cycle comparable to the nutrient cycles in agroecosystems. (Figure 8-1 in Chapter 8 describes the fates of pesticides applied to farming systems.) Pesticides applied to cropping systems are volatilized and lost to the atmosphere, lost to surface water bodies in solution or attached to sediments, leached to groundwater, exported with harvested crops, immobilized in the soil, or degraded in soil or by plants. Pesticide properties, soil properties, site conditions, and management practices interact to determine the fate of a pesticide. These interactions are complex and often site and chemical- specific. The environmental effect

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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of pesticides is also chemical-specific. A small mass of a highly toxic or active pesticide may be more damaging than a larger mass of a less active or toxic pesticide. In some cases there is uncertainty about the fate, transport, and effect of pesticide degradation products as well.

In general, however, pesticides that are not either degraded or immobilized are eventually lost to the air or water. Site-specific processes determine how that mass of pesticides that is not degraded or immobilized is partitioned among surface water, groundwater, and the atmosphere. The first step toward reducing the amount of pesticides eventually delivered to surface water, groundwater, or the atmosphere is to reduce the total mass of pesticides introduced into the environment in farming systems.

Pesticide Mass Balance

Because of the complexity of the processes that determine the fate and transport of pesticides used in agricultural production systems, construction of pesticide mass balances is difficult. Despite the vast knowledge base available on pesticide reactivity and transport, a complete mass balance on the fate of any pesticide applied to a field does not exist. Some studies (see below) have measured the pathways followed by pesticides applied to crop fields, and those studies provide some perspective on ways to improve pesticide efficiency.

Losses through volatilization and spray drift during pesticide application can be substantial. Spray drift accounts for 3 to 5 percent of loss of insecticides applied under low-speed-wind conditions, but under normal conditions spray drift loss is typically 40 to 60 percent for many insecticides. Loss by volatilization from spray application ranges from 3 to 25 percent for most insecticides, but it may be as great as 20 to 90 percent for the insecticide methylparathion, for example. The delivery loss to soil and peripheral nontarget foliage may be as high as 60 to 80 percent for most sprays.

The percent of pesticide losses from soil-incorporated application are much lower. Volatilization from soil-incorporated application normally ranges from 2 to 13 percent, but it can be much higher for particularly volatile pesticides. Seasonal surface runoff of pesticides is less than 1 to 5 percent (Wauchope, 1978). Losses of pesticides through leaching are more difficult to estimate. Using a simulation model, Tanji (1991a) found that only very small fractions of the dibromochloropropane (DBCP) applied to the soil surface made its way through the soil to groundwater; concentrations of 1,500 mg/L (1,500 ppm) at the soil surface translated to concentrations of 0.009 mg/L (0.009 ppm) in groundwater. Data on

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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DBCP concentrations from 240 wells confirmed the simulation model results. For DBCP, however, even these minuscule leaching losses were a cause for concern, since the maximum contaminant level of DBCP considered acceptable in drinking water is 0.00002 mg/L (0.00002 ppm).

It is difficult to obtain mass balances for pesticides and, therefore, to predict their fate and transport in site-specific locations. Even very small losses of pesticides to particular parts of the environment can be a cause for concern. A concerted effort at source control through increased efficiency in the use of pesticides offers the best assurance that losses of pesticides from agricultural production systems will be reduced.

Improved Pesticide Use Efficiency

Aggressive efforts to adopt currently available technologies, systems, and practices to reduce the total mass of pesticides used should be pursued.

Immediate gains in reducing the total mass of pesticides used in agricultural production systems can be achieved by using currently available improved pest management practices. The opportunities to increase pesticide use efficiency can be grouped into (1) use of integrated pest management practices, (2) improvements in pesticide formulations, (3) improvements in application practices and, (4) matching pesticide characteristics to site-specific conditions. If producers integrate currently available technologies and practices into their farming systems, many will be able to reduce the amounts of pesticides they use and sustain the profitabilities of their operations. The magnitude of financially feasible reductions will vary from region to region, crop to crop, and farming system to farming system.

Integrated Pest Management

Integrated pest management (IPM) is an ecologically based pest control strategy that integrates all available pest control tactics—including crop rotations, tillage practices, water management, residue management, biological controls, and pesticides—to achieve an optimal level of pest control. The concept of a treatment threshold is central to IPM systems. Pest control is designed to keep pest populations below a given threshold level at which damage is expected to cause losses in yields, profits, or some other measure of damage (Zalom et al., 1992).

IPM has a proven track record of reducing the need for pesticide applications while maintaining adequate levels of pest control. The application of IPM to cotton production beginning in 1971, for example, has led to dramatic declines in insecticide use. In 1971, an estimated 6.5

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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TABLE 2-7 Use of Integrated Pest Management for 12 Major Crops in the United States, 1986

Crop

Hectares Planted (103)

Hectares under IPM (103)

Percent Total Hectares under IPM

Alfalfa

10,833

515

5

Applesa

187

121

65

Citrusb

428

284

70

Corn

31,053

6,075

20

Cotton

4,068

1,963

48

Peanuts

637

279

44

Potatoes

492

79

16

Rice

972

379

39

Sorghum

6,205

1,606

26

Soybeans

24,899

3,603

14

Tomatoes

153

126b

83

Wheat

29,173

4,328

15

NOTE: Integrated pest management (IPM) is defined broadly to include all lands where basic scouting and economic threshold techniques reportedly are used.

a Includes the area under IPM by USDA's Cooperative Extension Service, grower organizations, producer industries, or consultants.

b Data are based in part on conversations with IPM entomologists in major growing regions for citrus and tomatoes.

SOURCE: Adapted from National Research Council. 1989. Alternative Agriculture. Washington, D.C.: National Academy Press.

kg of insecticide per ha (5.8 lbs/acre) was applied to cotton; in 1982, insecticide applications were 1.7 kg/ha (1.5 lbs/acre). In 1976, cotton crops received 49 percent of the total mass of insecticides applied to major field crops. By 1982, this was reduced to 24 percent. Some of the reduction was due to the adoption of new pesticides that were effective at lower rates, but use of IPM was also responsible (Zalom et al., 1992).

Zalom and colleagues (1992) also cited other examples of the success of IPM in increasing the efficiency of pesticide use. Use of insecticides on peanut crops declined from 4.4 to 0.9 kg/ha (3.9 to 0.8 lbs/acre) as producers adopted IPM practices. Use of IPM techniques in California almond production reduced crop damage, increased total production, and reduced total pesticide use by 31 percent.

IPM programs should be accelerated.

For some crops, particularly high-value crops, the use of IPM has become common (Table 2-7). The use of IPM for major field crops such as corn, soybeans, and wheat on which the largest masses of pesticides, particularly herbicides, are used is much less common. Yet, application

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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of IPM techniques may have great potential for increasing pesticide use efficiency in the production of major field crops. Use of crop rotations can effectively reduce the need for soil insecticides to control corn rootworms in the Corn Belt. Banding of herbicides—a practice by which an herbicide is applied only to the crop row rather than broadcast over the entire field—can reduce herbicide applications by nearly half, for example. The development and adoption of IPM techniques suitable for use in the production of major field crops is needed.

Research to develop IPM practices for weeds should be accelerated.

IPM has had its major successes in the control of insects. Weed control is also important, particularly in the production of major field crops; and producers use more herbicides than any other class of pesticide in their agricultural production systems. Crop rotations and herbicide banding currently have the most promise for weed management in the corn-soybean rotation system (Edwards and Ford, 1992). Improved diagnostic tools to determine when weed control is needed could also increase pesticide use efficiency (Edwards and Ford, 1992). Much more research is needed to develop, test, and implement IPM strategies that are widely adaptable to weed management, particularly in major field crops.

Design Better Pesticides

The chemical and physical properties of pesticides have important effects on their ultimate fates when they are applied to farming systems. It is possible to design new pesticides that pose lower risks because they are, for example, less toxic, less likely to be lost to surface water or groundwater, or more effective at lower application rates. Mechanisms to encourage and facilitate the registration of more environmentally benign pesticides could help increase the options available to producers.

Improve Pesticide Application Practices

Simple improvements in pesticide application practices—such as following the directions on the pesticide label, carefully measuring the pesticides added to spray mixtures, and calibrating and maintaining spray equipment—can reduce the amounts of applied pesticide that are lost as well as increase application efficiencies. Technologies that apply pesticides only to the target site or pest (for example, banding) can also reduce pesticide losses. Improved application technologies such as

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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controlled droplet applicators, drift-shielded applicators, ultra-low-volume equipment, electrostatic sprayers, and computer-controlled equipment or formulations that thicken the spray can also reduce pesticide losses. Aerial application methods generally result in higher drift losses than ground application and should be done when wind speeds are low and temperatures are cooler but not when rain is likely to occur.

Match the Pesticide to Site Conditions

The ability to predict the behavior and transport of pesticides under field conditions appears to be weak, in part because of the spatial and temporal variabilities of pesticides in field soils. On the basis of chemical-specific properties and vulnerable site conditions, however, it should be possible to assess whether or not a given pesticide will contaminate surface water or groundwater.

Existing knowledge should be used more fully to match pesticide selections to site conditions.

Wauchope and colleagues (1992) have recently developed a hierarchy of pesticide properties; it lists pesticides according to their surface loss and leaching potentials. These pesticide properties can be matched with soil ratings information available in soil surveys so that producers can select those pesticides with the lowest potential for loss to surface water or groundwater. The relative toxicities of pesticides can be used as collateral criteria to refine pesticide selections (Hornsby, 1992). These data should be widely used by producers, crop-soil consultants, pesticide dealers, extension agents, and others who make pesticide recommendations.

Increased resources should be devoted to the development of sampling, monitoring, analysis, and modeling protocols for pesticides in the environment.

Sampling, monitoring, and improved modeling of the efficacies and fates of pesticides in the environment require substantial additional resources, facilities, and time. In the long-term, these investments in efficient pesticide use will provide the models and data needed to refine the management of pesticides with greater precision. In the short-term, however, current understanding should be used to reduce the total mass of pesticides used, reduce runoff and erosion from cropping systems, improve the efficacy of pesticide applications, and match pesticide selection to site conditions. These efforts should go forward at the same time that understanding of pesticide behavior in the environment is improved.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Alternative Pest Control Technologies

Research required to develop alternative pest control strategies and to develop farming systems based on alternative pest control practices should be accelerated.

New methods of pest control that rely on biological or natural processes may provide alternatives to current pesticides, with less potential for water pollution. A large number of new biologically based methods of pest control appear to hold promise. Such methods range from crop rotations, cover crops, intercrops, development of resistant plant varieties, the use of pheromones and other plant or animal compounds to disrupt reproduction of pests, the use of highly specific toxins produced by bacteria or other organisms, and the introduction or management of living biological control agents as diverse as insects, nematodes, fungi, viruses, and bacteria (Charudattan, 1991; DeBach and Rosen, 1991; Luna and House, 1990; McManus, 1989; University of California, Study Group on Biological Approaches to Pest Management, 1992; U.S. Congress, Office of Technology Assessment, 1992; Watson, 1991). The development of such biologically based systems of pest and disease control, if developed, implemented, and adopted, could solve many of the environmental problems currently associated with pesticide use while assuring effective pest and disease control. Long-term gains in reducing the total mass of pesticides used in farming systems can be achieved only by continued efforts in research and development of alternative pest control management and farming systems.

Improving Irrigation Management

Irrigation can cause soil salinization and waterlogging of soils, particularly in arid environments. These problems have plagued irrigated agriculture for centuries (Tanji, 1990). Today, irrigated agriculture faces the same problems. The irrigated agricultural system in California's San Joaquin Valley, for example, is facing an economic and ecological crisis. About 38 percent of the irrigated cropland is waterlogged, and 59 percent is affected by the accumulation of salts (San Joaquin Valley Drainage Program, 1990) (see Chapter 10 for a more complete discussion).

Irrigation also inevitably requires the disposal of drainage water that carries salts, trace elements, pesticides, and nutrients. In the San Joaquin Valley, disposal of irrigation water has become a critical problem and the recent discoveries of selenium and other toxic trace elements in irrigation drainage water has increased the difficulty in managing irrigation-induced soil and water quality problems (National Research Council, 1989b).

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Disposal of Drainage Water

Irrigation requires natural or constructed systems that dispose of drainage water. When producers apply irrigation water to crops, much of the water is taken up by crops and is evaporated. As the water evaporates, the salts and other minerals left behind buildup in the soil. Unless it is flushed out of the soil in irrigation drainage water, the resulting buildup in soil leads to yield losses and eventual soil destruction. Irrigation, then, inevitably leads to the need for disposal of drainage waters that contain salts, nitrates, pesticides, trace elements, and other pollutants.

Reduction of the Volume of Drainage Water

Reducing the volume of irrigation drainage water by increasing the efficiency with which irrigation water is used should be the major objective of programs to reduce salt and trace-element loadings to surface water and groundwater.

A combination of carefully controlled, efficient irrigation with an appropriate match between the crop grown and water quality will minimize the amount of drainage water requiring disposal and, thereby, reduce the potential for water pollution and soil degradation (National Research Council, 1989b). Efforts to reduce the damage to soil and water quality caused by irrigated agriculture should have as their primary objective a reduction of the volume of drainage water needing disposal. Reducing the total mass of applied irrigation water through increased efficiency of water use is the most promising means of achieving this objective.

Currently available technology, if used, could result in immediate improvements in the efficiency with which irrigation water is used. Improved irrigation scheduling can greatly increase the efficiency of water use by ensuring that irrigation water is applied only when and in the amounts needed for crop growth. Reusing drainage water or tailwater, blending or using alternate sources of irrigation water to match the crops' salt tolerance, and changes in the type and sequence of crops grown can also improve the management of irrigation water.

New Cropping Systems

The preceding sections of this chapter focused on improved management of inputs—nitrogen, phosphorus, manures, pesticides, and irrigation—as ways to improve soil and water quality. In many cases,

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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improving input management promises to improve both the financial and the environmental performances of cropping systems. The goal of improving input management is to bring input applications closer to optimal rates for crop growth, thus minimizing the total mass of residual nitrogen, phosphorus, pesticides, salts, and trace elements lost from farming systems. It is impossible to be certain, however, that improved input management alone will be enough to meet water quality standards in all regions. Both technical and economic constraints have an impact on the degree to which input management, particularly for nitrogen, phosphorus, and pesticides, can be refined.

Technical Constraints to Input Management

The studies available are not sufficient for making comprehensive predictions of how seriously technical constraints will reduce the effectiveness of refined input management. These studies do suggest, particularly for mobile pollutants like nitrates, that in some regions changes in land use or cropping systems will be needed.

Hall (1992), for example, monitored changes in groundwater nitrate concentrations beneath heavily fertilized and manured fields in Lancaster County, Pennsylvania, following the implementation of nitrogen management practices to reduce nitrogen inputs. Fertilizer and manure inputs were decreased between 39 and 67 percent (222 to 423 kg/ha, or 198 to 378 lbs/acre), and nitrate concentrations in the groundwater decreased by 12 to 50 percent. By the end of the study, however, all wells still exceeded federal drinking water standards for nitrate. The decreases in groundwater nitrate concentrations were much greater where the initial nitrate concentration was highest, suggesting that while reductions in nitrate will be significant in cases of dramatic overfertilization, achieving reductions in more conventional situations will be more difficult.

Clausen and Meals (1989) reported that during 7 years of monitoring water quality in a dairying region of Vermont, the levels of dissolved oxygen, phosphorus, turbidity, and fecal coliform bacteria in runoff and stream water frequently exceeded water quality standards, despite implementation of best-management practices in the watershed. Addiscott and Powlson (1989) and Addiscott and Darby (1991) have argued that reducing nitrogen inputs will not result in large reductions in groundwater nitrate concentrations as long as there are extended periods when soil nitrate levels are high without the presence of actively growing plants; a common occurrence in modern cropping systems. Cartwright and colleagues (1991), for example, reported that

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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in Baden-Württemburg, Germany, 3 years of reductions in nitrogen inputs brought no improvement in groundwater nitrate levels, while in North Rhine/Westphalia, a more aggressive program that included purchase of sensitive areas and mixing 15,000 metric tons (16,500 tons) of clay into sandy soils brought rapid improvements in groundwater quality. Studies of subsurface drainage water quality from continuous corn production in the fertile soils of Iowa suggest that only corn with zero nitrogen applied would consistently keep nitrate concentrations in drainage water below the 10 mg/L level set for drinking water. Use of no added nitrogen significantly reduced yields and is clearly not sustainable for the producer (Baker and Melvin, 1992).

These studies confirm that impressive improvements in input management, particularly for nitrogen, are possible and, in many cases, will significantly improve water quality. These studies also indicate that even dramatic improvements in input management may not result in meeting water quality standards in certain regions or quickly enough to meet legislated deadlines. There may be significant lag times between improvements in input management and changes in water quality. Phosphorus and pesticides in stream sediments, nitrates in surficial aquifers, and phosphorus or salts that have built up in soils, for example, will continue to contribute to water pollution for a period of time after input management is improved. The length of the lag time is difficult to predict. In cases where technical constraints to improving input management are large or lag times unacceptably long, new cropping systems or changes in land use will be required.

Economic Constraints to Input Management

Producers try to apply inputs at economically optimum rates if they want to maximize their profits. Economically optimum rates of application are closely related to the rates that are optimal for crop growth, but they are not necessarily the same. Economically optimum rates can be greater than optimum rates for crop growth because of uncertainties about outcomes and the prices of inputs and the crop.

This problem is best illustrated by an example developed by Bock and Hergert (1991) for nitrogen management (Figure 2-5). Producers are often thought to apply nitrogen at rates greater than those required for optimal crop growth as insurance against making a wrong decision that leads to lower yields. Figure 2-5 shows average losses caused by the underapplication of nitrogen and the gains from the overapplication of nitrogen as insurance. Bock and Hergert concluded that economic

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Figure 2-5 Economic return from insurance nitrogen (N) and deficit N applications. Source: B. R. Bock and G. W. Hergert. 1991. Fertilizer nitrogen management. Pp. 140-164 in Managing Nitrogen for Groundwater Quality and Farm Profitability, R. F. Follet, D. R. Keeney, and R. M. Cruse, eds. Madison, Wis.: Soil Science Society of America. Reprinted with permission from © American Society for Agronomy, Crop Science Society of America, and Soil Science Society of America.

incentives to nitrogen application at the optimum rate are not great, particularly when the yield response to nitrogen is highly variable and nitrogen/crop price ratios are low.

This example illustrates the general case—that the economically optimal rate of nutrients, pesticides, or irrigation can be larger than the rate that is technically optimal for crop growth. In addition, the risk of making a mistake increases as input use approaches the technical optimum application rate. The economically optimal rate, therefore, may exceed the environmentally optimal rate under current management. Technologies that afford greater precision in managing inputs can help solve this problem, but input management alone may not be sufficient to prevent water pollution. Managing cropping systems through rotations, cover crops, and multiple crops may be needed to augment efforts to improve input management.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Managing Cropping Systems

The use of cover crops should receive much greater attention as an integral part of soil and water quality programs.

Much recent research has shown that the use of cover crops planted to cover the soil following harvest, or in some cases while the crop is growing, shows promise for reducing nitrogen and phosphorus losses from cropping systems, reducing the need for pesticides, and reducing soil erosion and runoff.

Cover crops have demonstrated the ability to reduce erosion, surface runoff, and leaching of nitrates to groundwater. Sharpley and Smith (1991) reported that the addition of a cover crop to farming systems that produce corn, wheat, cotton, and soybeans consistently reduced runoff, soil erosion, and the amounts of nitrogen and phosphorus transported in erosion and runoff. The use of cover crops dramatically reduced erosion and runoff when they were used in corn, cotton, and soybean cropping systems in Georgia, Iowa, Kentucky, Louisiana, Mississippi, Missouri, New York, Oklahoma, South Carolina, Tennessee, Texas, and Wisconsin (Langdale et al., 1991). Meisinger and colleagues (1991) showed that the use of nonleguminous cover crops to capture and hold residual nitrogen reduced the amount of nitrogen leached from farming systems by between 31 and 77 percent.

Currently, cover crops are widely used only in the southeastern United States (Power and Biederbeck, 1991). Langdale and colleagues (1991) reported that cover cropping systems are more well developed in the Southeast than in other parts of the United States. The drawbacks and concerns associated with cover crop use include depletion of the water in soil by cover crops, the slow release of nutrients contained in cover crops biomass, added costs of production, and difficulties in establishing and then killing cover crops, especially in northern areas of the United States (Frye et al., 1988; Lal et al., 1991; Wagger and Mengel, 1988).

Research to develop cover cropping systems that can be used in colder and drier regions should be accelerated.

The use of cover crops in colder and drier regions of the United States is limited by the lack of available cultivars and the lack of techniques to establish and manage cover crops where soil moisture is limiting (Power and Biederbeck, 1991). The potential benefits of cover crop use in these regions, if technical obstacles can be overcome, are great.

Research to develop innovative cropping systems to meet long-term soil and water quality goals is needed.

Cover crops are likely to become more important components of

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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environmentally sound farming systems. More dramatic changes in the structure and function of farming systems, however, will be required to achieve soil and water quality goals in many areas because of the problems discussed above. In the long-term, cropping systems very different from those that are most commonly used now may be needed if farming is to continue in areas with severe soil and water quality problems.

Kirschenmann (1991) suggested that there are currently two basic conceptions of what these new farming systems may entail. Some observers anticipate the development of multiple cropping systems in which companion plants are used to produce nitrogen through biological fixation and to provide weed control by acting as a living mulch; insect and disease control will rely on sophisticated webs of biological control agents. Other observers anticipate improved information-gathering devices in the crop field that will interface with farm machinery capable of varying the application of inputs to achieve higher efficiency. These two approaches are not mutually exclusive and both are important for guiding efforts to improve the environmental performance

POTENTIAL BENEFITS OF COVER CROPS

Cover crops are legumes, grasses, cereals, or other crops that are added to crop rotations to protect the soil, reduce pest infestations, and improve water quality. Unlike other crops, cover crops are not normally harvested but are killed or plowed under when the cash crop is planted. There are drawbacks to cover crops in some regions and situations including soil moisture depletion, competition with the cash crop, and increased cost of production. When these problems can be solved, cover crops can reduce erosion and runoff, protect soil quality, suppress pests, and prevent water pollution.

Reduced erosion and runoff. Cover crops provide a protective vegetative layer that shields the soil from the impacts of raindrops, reduces the velocity of runoff, and increases the portion of runoff that is absorbed by the soil. Together these protective effects of cover crops can dramatically reduce erosion and runoff from farming systems. Langdale and colleagues (1991), for example, reported erosion reductions of up to 91 percent after adding a cover crop to soybean fields.

Improved soil quality. In addition to protecting soil from erosion, cover crops also improve soil structure, enhance soil fertility, and sustain or increase soil organic matter and soil biological activity. Cover crops may be particularly effective in restoring the quality of degraded soils. Langdale and colleagues (1992b) reported that the combination of a cover crop and conservation tillage significantly improved the structure, organic matter content, and infiltration rate of severely eroded soils in Georgia. After 5 years, crop yields from the soils that had been severely eroded were the same as those from only slightly eroded soils.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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of agriculture. Farming systems that integrate both approaches will likely be more profitable and environmentally sound.

Guiding the research to develop new farming systems requires a long-term perspective and a vigorous imagination. The cropping systems and management practices currently used have little resemblance to the systems that were common 50 to 75 years ago. It is reasonable to expect that future systems will be equally different from current systems. Long-term, imaginative direction to current research programs is important and should result in better farming systems in the future.

INCREASING RESISTANCE TO EROSION AND RUNOFF

Reducing erosion and runoff should be fundamental to efforts to improve soil and water quality.

Erosion is the single greatest threat to soil quality. Some of the most direct and serious water pollution problems result from the delivery of sediments to surface water; and the cost of dredging several million

Pest suppression. Use of cover crops may help control weeds through nutrient competition, allelopathy (suppression of growth of one plant species by another by the release of toxic substances), and physical effects. Although some living mulches also compete with row crops, compatible cover crops can provide an alternative to herbicide use without significantly decreasing productivity. A 3-year study in New Jersey showed that corn planted into growing subterranean clover (Trifolium subterraneum), a winter legume, produced the same or better yields than corn grown with conventional herbicides and no mulch, regardless of the type of tillage used (Enache and Ilnicki, 1990). Cover crops can also increase the diversity of insects, including species that prey on crop pests.

Prevent water pollution. Cover crops prevent water pollution by a combination of effects that have already been discussed. Reducing erosion and runoff also reduces the amount of sediment and agricultural chemicals that reach surface water. Improving soil quality improves the effectiveness of a the soil as a filter to capture and degrade potential pollutants, and the need to use less pesticides reduces the chance that pesticides will pollute surface water or groundwater. In addition to these effects, cover crops may capture, recycle, or immobilize residual nitrogen, phosphorus, and pesticides from crop production. The potential for cover crops to trap these potential pollutants is promising but not yet well understood.

SOURCE: Adapted from R. Lal, E. Regnier, D. J. Eckert, W. M. Edwards, and R. Hammond. 1991. Expectations of cover crops for sustainable agriculture. Pp. 1–10 in Cover Crop for Clean Water, W. L. Hargrove, ed. Ankeny, Iowa: Soil and Water Conservation Society.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

cubic meters of sediments from U.S. rivers, harbors, and reservoirs is quite high. Erosion and runoff also deliver nutrients, pesticides, and salts to surface waters. Increased runoff volume and energy from croplands disrupt water flow regimes, increasing discharge peaks and stream channel erosion. Degradation of watersheds and disruption of stream channels has caused erosion runoff of stream banks and streambeds to become an important contributor to sediment loads in streams and rivers.

Controlling erosion and runoff has been the emphasis of traditional conservation programs and there is a vast body of information available on-farming practices that effectively control erosion. These farming practices are well understood and a great deal of effort has been and should be expended by federal, state, and nonprofit organizations to increase the use of these practices by producers. There are, however, opportunities to make national erosion control efforts more effective by understanding the time lag between erosion control and sediment reduction, by addressing stream channel degradation, and by recognizing the importance of episodic events that suddenly and dramatically increase erosion and runoff.

Time Lag of Sediment Load Reductions

Current sediment loads are more an indication of past rather than current erosion rates because there are long time lags between the time when soil is eroded from a field and when the sediment is finally delivered to a stream or river. Reducing erosion will not, therefore, often result in immediate reductions in sediment loads. The time lag between reduced erosion and reduced sediment loads depends on the length of time that sediments spend in storage in the watershed before they are delivered to larger streams and water bodies.

Most sediment spends most of its life in storage. In the Piedmont region, between southern Virginia and eastern Alabama, for example, an estimated 25 km3 (6 miles3) of soil was eroded from the uplands in the last 200 years (Meade, 1982; Trimble, 1975). Estimates indicate, however, that 90 percent of that soil is still stored on the hill slopes and the valley floors of the region. Similarly, Trimble (1983) estimated that only 7 percent of the human-induced eroded soil has actually left the immediate watershed where the erosion occurred in Wisconsin.

In some areas, stream sediment loads are increasing, despite changes that have resulted in reduced levels of erosion on croplands in the watershed (Meade, 1982). The activities of a stream can be viewed simply as a struggle for the stream to balance its sediment load with its

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

sediment transport capacity. Decreased soil erosion does not immediately translate into less suspended sediment in a stream. As the sediment load in runoff water from croplands decreases because of erosion control, the capacity of the cleaner water to pick up sediment in streambeds or stream banks increases. This process continues until the stream channel and the runoff water develop a new equilibrium between the sediment delivered in runoff water and the sediment stored in the stream channel.

Many streams and rivers are still adjusting to the erosion and watershed disruption that occurred in the past. Large amounts of sediments are still in storage in streambeds, stream banks, and floodplains. Sediment loads will continue to be large in these watersheds, even if current erosion rates are reduced.

Protecting Stream Channels

High volumes and energy of runoff water disrupt stream channels, increasing the erosion of sediments from streambeds and stream banks. In many watersheds, erosion of sediments from the streambed and stream bank contributes a large share of the sediment load. The volume and energy of runoff water is also closely related to the amount of pesticides, phosphorus, nitrates, and other pollutants delivered to surface water.

Programs to protect water quality should seek to reduce the total volume and energy of runoff from croplands in addition to reducing total erosion.

Erosion and runoff are closely related, but they are not the same process. Erosion reductions without comparable reductions in runoff energy and volume can cause trade-offs between the delivery of pollutants to surface water, attached to sediments, or dissolved in runoff water. Conservation tillage systems, for example, reduce total erosion, but reductions in total runoff are not as great. The decreased erosion reduces the amount of sediment-attached phosphorus that is lost, but it may increase the amount of phosphorus lost in soluble form in the runoff water (Alberts and Spomer, 1985; Angle et al., 1984; Barisas et al., 1978; Langdale et al., 1985; McDowell and McGregor, 1984; Romkens and Nelson, 1974; Romkens et al., 1973).

Erosion reduction should be linked to efforts to improve agricultural watersheds by protecting riparian zones, stream channels, and wetlands and by using other measures to manage the volume and energy of surface runoff reaching surface water bodies. Reducing erosion from croplands is a fundamental first step toward improving soil and water quality. However, managing the volume and energy of runoff water is

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

just as important. Efforts to reduce erosion and runoff from croplands through the use of residue management systems should be coupled with efforts to protect agricultural watersheds by protecting or restoring riparian vegetation, wetlands, and grassed waterways and by using other measures that reduce the damage caused by excessive volumes and energy of runoff water. Such field and landscape buffer zones hold great promise for improving water quality in agricultural watersheds. (See the section on field and landscape buffer zones later in this chapter and in Chapter 12.)

Resistance to Episodic Damage

Agricultural ecosystems are vulnerable to erosion and runoff during storm events because plant diversity is intentionally low in such

CHANNEL INSTABILITY

Many streams experience serious instability as a result of land use changes and the adverse impacts of river management on drainage and flood control. The primary response in the fluvial system is degradation, which leads to damage to in stream and riparian ecosystems, damage to infrastructure (bridges, dams, and roads), and the generation of heavy sediment loads, which cause aggradation problems downstream. Lowering of the water level in a stream or riverbed also increases the chance that the stream bank or riverbank may collapse. Thorne (1991) reported that when banks become unstable, the thrust of the channel instability switches from degradation to rapid stream widening. Widening involves destruction of valuable valley bottomlands, damage to infrastructure, and the prolongation of the heavy sediment supply to the system downstream. Rapid stream widening associated with bank instability results from bed scour and lateral toe (bank-bed contact) erosion in the degrading channel. The precise timing of failure and the mode of bank collapse are controlled by bank geometry, bank stratigraphy, bank material properties, as well as the hydrology above the failure areas, which in turn affect the flow hydraulics.

Thorne (1991) investigated these instability problems in the loess hills of the Yazoo Basin in Mississippi. He showed that a grade control structure could be used successfully to halt bed degradation and induce aggradation as a mechanism to produce width stabilization. The bank stabilization provided by a grade control structure was determined to be a cost-effective solution to chronic problems of retreating banks and widening channels in this highly erodible area.

SOURCE: Thorne, C. R. 1991. Analysis of channel instability due to catchment land use change. Pp. 111–122 in Sediment and Stream Water Quality in a Changing Environment-Trends and Explanations. IAHS Publication No. 203.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

ecosystems and the period during which plants are actively growing and the soil is covered is often short. Much of the soil and water quality degradation can occur during episodic climatic events, such as heavy rainfalls, droughts, or windstorms. Farming system resistance to episodic damages can be increased by lengthening the period during which growing plants or residues are present and by increasing the amount of soil cover provided by plants and residues.

Conservation Tillage and Residue Management

Efforts to increase the use of conservation tillage and other forms of residue management should continue to be an important component of programs to protect soil and water quality.

Immediate gains in soil and water quality can be attained if producers adopt currently available conservation tillage and residue management systems. If producers incorporate such conservation systems into current farming systems, resistance to episodic events will increase and runoff energy and soil erosion will be reduced. A great diversity of tillage and residue management systems are available to producers (see Table 9-1 in Chapter 9), although the use of these systems may be less attractive in some situations because of unfavorable physical or economic factors. Use of these systems results in dramatic decreases in erosion and runoff from farming systems (see Table 9-2 in Chapter 9) and from agricultural watersheds (see Table 9-3 in Chapter 9).

The use of conservation tillage practices has increased over time, and considerable time and effort have been devoted to increasing the use of these systems by producers. The percentage of croplands on which various forms of conservation tillage are used varies by crop and region (see Table 9-4 in Chapter 9). In 1985, the proportion of cropland on which producers practiced some form of conservation tillage varied from about 12 to 48 percent, depending on the farm production region. Table 2-8 indicates that the use of conservation tillage also varies by crop.

The most important way to increase the soil and water quality benefits that could be realized through wider use of conservation tillage and residue management systems is to increase adoption of those systems on those lands that are most vulnerable to soil quality degradation or that contribute the most to water quality degradation. The data in Table 2-8 suggest that large percentages of highly erodible land were not farmed by conservation tillage techniques in 1990. Full enforcement of the conservation compliance provisions of the 1985

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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TABLE 2-8 Highly Erodible, Not Highly Erodible, and Nondesignated Lands on which Conservation Tillage Systems Are Used for Various Crops, 1990

 

Highly Erodible

Not Highly Erodible

Nondesignated

Crop

Total Area (1000 ha)

Percent Conservation Tilled

Total Area (1000 ha)

Percent Conservation Tilled

Total Area (1000 ha)

Percent Conservation Tilled

Corn

5,140

31

17,510

26

1,160

21

Cotton

850

ID

2,810

2

284

ID

Winter wheat

4,702

23

10,370

17

1,190

18

Spring wheat

968

38

4,860

23

567

25

Durum wheat

38

NR

1,015

35

203

38

Northern soybeans

2,908

34

11,120

25

717

20

Southern soybeans

470

46

3,710

17

620

9

Rice

16

NR

644

4

3,170

3

NOTE: Conservation tillage is any tillage system resulting in 30 percent or greater surface residue cover after planting. ID, insufficient data; NR, no data reported. Totals of percentages may not add to 100 because of rounding.

SOURCE: Adapted from U.S. Department of Agriculture, Economic Research Service. 1991. Agricultural Resources: Inputs Situation and Outlook. Report No. AR-21. Washington, D.C.: U.S. Department of Agriculture.

Food Security Act is expected to dramatically increase the amount of highly erodible land on which conservation tillage or some other form of residue management is used. A concentrated effort to increase the use of conservation tillage in watersheds where water quality degradation is greatest would lead to even greater benefits.

Develop New Cropping Systems

Most natural ecosystems resist erosion through biotic control over the abiotic environment (Bormann and Likens, 1979). The plant canopy intercepts incoming precipitation, which greatly reduces the energy and erosive potential of rainfall. The litter layer, the forest floor, and organic soil horizons further reduce the erosive potential of rainfall, allowing gradual infiltration of rainfall water into the surface soil horizons. The presence of a litter layer also helps to maintain soil structure and prevent soil crusting. Because of these multiple layers of soil protection, most natural ecosystems have very low rates of soil erosion and are resistant to most storm events.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

Research and development of economically viable cropping systems that incorporate cover crops, multiple crops, and other innovations should be accelerated.

As long as there are extended periods when the soil is inadequately covered, farming systems will be vulnerable to erosion and runoff events. To achieve higher resistance to the erosive power of large and small precipitation events, researchers and producers could develop farming systems that mimic the multilayer soil protection that occurs in many natural ecosystems. Increasing the number of layers of vegetative cover over the soil will further increase the resistance of farming systems to severe episodic damages. Some cropping systems already achieve this multilayer soil protection. Relay or double-cropping systems in which producers use no-tillage techniques, such as the wheat-soybean system used in the southeastern United States, have nearly continuous vegetative cover and a well-developed litter layer of crop residues. Rates of erosion in these systems are very low, even under the highly erosive conditions that exist in the southeastern United States (Langdale et al., 1992a). Further development of cropping systems that are more resistant to erosion and runoff should be a high priority. In the long-term, incorporation of cover crops, multiple crops, and other changes in cropping systems hold great promise for increasing resistance to erosion and runoff.

Probability Analysis

Conservation systems should be designed to increase soil cover during periods when the probability of episodic damages is highest.

Immediate gains in preventing soil degradation and water pollution can be achieved by incorporating the probability of episodic events into the design of farming systems. Current computer simulation capacities and available climatic data could be used to analyze the probability of episodic events that would lead to damaging erosion or runoff events.

Average annual soil loss is normally used for conservation planning programs by USDA. For example, cropping systems designed to meet the soil loss tolerance concept based on universal soil loss equation evaluations may not be optimum. Hjelmfelt and colleagues (1988) measured the distribution of erosion events over a 37-year period in Missouri and found that soil loss was greater than average during only 9 of the 37 years (Figure 2-6). On an individual-event basis, 4 percent of the events accounted for 50 percent of the total soil loss. Similar data from Iowa indicate that sediment yields were greater than the average for 4 years of an 18-year record. Three percent of the individual storm

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

FIGURE 2-6 Distribution of erosion events over 38 years on a field in Missouri. Source: A. T. Hjelmfelt, Jr., and L. A. Kramer. 1988. Unit hydrograph variability for a small agricultural watershed. Pp. 357–366 in Modeling Agricultural, Forest, and Rangeland Hydrology. Proceedings of the 1988 International Symposium, December 12–13, 1988, Chicago, Illinois. St. Joseph, Mich.: American Society of Agricultural Engineers.

events accounted for more than 50 percent of the total erosion. Thus, in most years, conservation systems designed for average annual soil loss would be overdesigned, yet during years with severe storms, the damage might be catastrophic. These data indicate that conservation planning to increase the resistance of farming systems to erosion and runoff cannot be done only on the basis of an average year or an average event but should also be done on the basis of extreme events.

Erosion control techniques should be based on practices that result in a certain probability of controlling erosion from a storm event of a specified duration and intensity. Another possibility would be to apply conservation treatments that control soil erosion during specific types of storm events during specific periods of the year. Such approaches might assist in more precise placement of conservation practices on landscapes.

Current sedimentation estimation technology should be improved to (1) adequately address the transport of chemicals adsorbed to soil particles, (2)

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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produce probabilistic models that can be used to control erosion risks in a manner similar to that of flood control systems, and (3) develop wind erosion models that can predict the effects of wind erosion on water quality.

The technology currently used to estimate and predict erosion and sedimentation suffers from weaknesses that constrain researchers' and producers' abilities to target erosion control efforts. Currently available erosion and sedimentation data are also often incompatible with modern computer simulation technology.

Fundamental understanding of the physical processes of water and wind erosion, however, is increasing as a result of recent research. USDA's Agricultural Research Service is currently developing process-based erosion prediction technology called the Water Erosion Prediction Project and the Wind Erosion Prediction System; this technology will offer agencies such as the Soil Conservation Service an opportunity to evaluate new approaches for conservation planning.

CREATING FIELD AND LANDSCAPE BUFFER ZONES

The preceding sections argued for the need to change farming systems in ways that conserve and enhance soil quality, increase input use efficiency, and increase resistance to erosion and runoff. Farming systems, however, exist within a landscape described by the patterns of soils and slopes; the patterns of streams, lakes, and wetlands; and the adjacent ecosystems such as forests and wetlands. The ultimate effects of farming systems on soil and water quality are affected by the interaction between cropland and livestock production systems and the landscape in which production takes place.

The preceding sections have described various opportunities to improve the management of farming systems at the farm level. These improvements will reduce the losses of sediments, nutrients, pesticides, or salts from each individual farm. There are limits, however, to the soil and water quality protection that can be achieved by working only at the farm level. In some watersheds, even small losses from individual farms may, in the aggregate, result in water pollution or soil degradation; the cost of reducing losses from those individual farms further, however, may be great. In addition, large storms or other episodic events, as discussed in the preceding section, may result in soil degradation and water pollution even from well-managed farming systems. Managing the landscape by creating or restoring buffer zones is a promising way to increase the effectiveness and lower the cost of programs to protect soil and water quality.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

A 2-acre pothole (area of low wetland) in a soybean field is being studied to determine water movement, chemical transport, and the fate of agricultural chemicals in saturated soils. Credit: Agricultural Research Service, USDA.

Creating Managed Buffer Zones

Buffer zones can be divided into two different types: (1) field-scale buffer or filter strips, usually containing managed grasses, and (2) landscape- or watershed-scale riparian or wetland buffer zones. Buffer zones can range from a few to many meters in width. The kind of vegetation and location of the buffer zone, in addition to its size, have important effects on its pollutant-trapping capacity. Both field and landscape zones can be useful in protecting soil and water quality, but uncertainties exist in how best to design and manage buffer zones.

The creation, protection, and management of field and landscape buffer zones should be an important objective of programs to protect soil and water quality.

Field-by-field efforts to conserve soil quality, improve input use efficiency, and increase resistance to erosion and runoff will not be adequate where overland and subsurface movements of nutrients, pesticides, salts, and sediment are pervasive. The use of buffer zones—ranging from natural riparian corridor vegetation (vegetation along waterways) to simple, strategically placed grass strips, to sophisticated artificial wetlands—to intercept or immobilize pollutants before they

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

reach surface water or groundwater holds promise for increasing the effectiveness of efforts to protect soil and water quality.

The purpose of creating field and landscape buffer zones is to create landscape sinks that trap or immobilize sediments, nutrients, pesticides, and other pollutants before they reach surface water or groundwater. The importance of field and landscape buffer zones in reducing the delivery of pollutants has received increasing attention (Dillaha et al., 1988, 1989b; Ehrenfield, 1987; Hayes and Hairson, 1983; Jacobs and Gilliam, 1985; Karr and Schlosser, 1978; Kovacic et al., 1991; Phillips, 1989). Grass waterways and vegetative strips have been used for erosion and runoff control in croplands for some time, but the use of these areas as sinks for nutrients and pesticides is more recent.

Research to develop design and management standards for field and landscape buffer zones should be accelerated.

The use of field-scale buffer or filter strips can be useful for reducing runoff, and for trapping the sediments, nutrients, and pesticides that move in surface runoff from specific fields. Unless the filter strips are relatively large, however, they can become clogged with sediments and their trapping efficiencies can decrease. Filter strips can fail during large storm events because of lateral water flow along the field-filter interface, leading to strip channelization at a low point along the interface. Moreover, although the sediment trapping abilities of well-maintained filter strips are well established, nutrient and pesticide uptake by filter strips is poorly understood.

Forested riparian buffer zones have been demonstrated to be effective at trapping the sediments and nutrients that move in both surface and subsurface flows from crop fields (Groffman et al., 1992; Jacobs and Gilliam, 1985; Karr and Schlosser, 1978; Lowrance et al., 1984a; Peterjohn and Correll, 1984; Simmons et al., 1992). The remaining uncertainty about riparian buffer zones relates to their long-term effectiveness: will these areas be able to absorb sediments and nutrients indefinitely, or will they become saturated over time? The fates of trapped sediments, nutrients, and pesticides following fire, logging, or flooding are also unclear.

Protection of Existing Natural Vegetation

Federal, state, and local government programs to protect existing riparian vegetation, whether bordering major streams or small tributaries, lakes, or wetlands, should be promoted.

Existing riparian vegetation, particularly the vegetation bordering smaller streams and tributaries, is an important resource that should be

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
×

protected to serve as sinks for sediments, nutrients, and pesticides, to protect the stream bank from erosion, and to reduce excessive runoff into stream channels. Loss of these areas will increase existing water quality problems or create new ones. The cost of replacing the water quality benefits of existing vegetation with efforts on the farm level to increase soil quality, input use efficiency, and resistance to erosion may be high.

Balance Needed

The creation of field or landscape buffer zones should augment efforts to improve farming systems. They should not be substitutes for such efforts.

The creation of field or landscape buffer zones cannot be seen as an alternative to efforts to improve farming systems. The capacities of field and landscape buffer zones to trap and immobilize sediments, nutrients, and pesticides are limited by the size of the buffers, the plants growing in the buffers, and the manner in which the buffers are managed. Both field and landscape buffer zones can be overwhelmed by large flows of runoff water, sediment, nutrients, and pesticides. Efforts to improve farming systems and to create field or landscape buffer zones are complementary. Emphasis on one effort to the exclusion of the other will achieve much less improvement in soil and water quality than is possible by striking a balance between the two efforts.

Suggested Citation:"2 Opportunities to Improve Soil and Water Quality." National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. Washington, DC: The National Academies Press. doi: 10.17226/2132.
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Soil and Water Quality: An Agenda for Agriculture Get This Book
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How can the United States meet demands for agricultural production while solving the broader range of environmental problems attributed to farming practices? National policymakers who try to answer this question confront difficult trade-offs.

This book offers four specific strategies that can serve as the basis for a national policy to protect soil and water quality while maintaining U.S. agricultural productivity and competitiveness. Timely and comprehensive, the volume has important implications for the Clean Air Act and the 1995 farm bill.

Advocating a systems approach, the committee recommends specific farm practices and new approaches to prevention of soil degradation and water pollution for environmental agencies.

The volume details methods of evaluating soil management systems and offers a wealth of information on improved management of nitrogen, phosphorus, manure, pesticides, sediments, salt, and trace elements. Landscape analysis of nonpoint source pollution is also detailed.

Drawing together research findings, survey results, and case examples, the volume will be of interest to federal, state, and local policymakers; state and local environmental and agricultural officials and other environmental and agricultural specialists; scientists involved in soil and water issues; researchers; and agricultural producers.

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