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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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 conserve and enhance soil quality as a fundamental first step to environmental improvement; increase nutrient, pesticide, and irrigation use efficiencies in farming systems; increase the resistance of farming systems to erosion and runoff; and 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
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Soil and Water Quality: An Agenda for Agriculture 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.
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Soil and Water Quality: An Agenda for Agriculture 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.
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Soil and Water Quality: An Agenda for Agriculture 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.
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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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.  estimates of nitrogen and phosphorus applications were from Vroomen ). 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.
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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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.
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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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.
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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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.
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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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)
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Soil and Water Quality: An Agenda for Agriculture 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.
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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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
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Soil and Water Quality: An Agenda for Agriculture 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.
Representative terms from entire chapter: