5
Monitoring and Managing Soil Quality

Soil, water, air, and plants are vital natural resources that help to produce food and fiber for humans. They also maintain the ecosystems on which all life on Earth ultimately depends. Soil serves as a medium for plant growth; a sink for heat, water, and chemicals; a filter for water; and a biological medium for the breakdown of wastes. Soil interacts intimately with water, air, and plants and acts as a damper to fluctuations in the environment. Soil mediates many of the ecological processes that control water and air quality and that promote plant growth.

Concern about the soil resource base needs to expand beyond soil productivity to include a broader concept of soil quality that encompasses all of the functions soils perform in natural and agricultural ecosystems. In the past, soil productivity and loss of soil productivity resulting from soil degradation have been the bases for concern about the world's soils. Equally important, however, are the functions soils perform in the regulation of water flow in watersheds, global emissions of greenhouse gases, attenuation of natural and artificial wastes, and regulation of air and water quality. These functions are impaired by soil degradation.

The ability of modern agricultural management systems to sustain the quality of soil, water, and air is being questioned. This chapter suggests methods that can be used to evaluate whether soil quality is being degraded, improved, or maintained under given management systems and methods of evaluating whether alternative management systems will sustain the quality of soil resources.



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Soil and Water Quality: An Agenda for Agriculture 5 Monitoring and Managing Soil Quality Soil, water, air, and plants are vital natural resources that help to produce food and fiber for humans. They also maintain the ecosystems on which all life on Earth ultimately depends. Soil serves as a medium for plant growth; a sink for heat, water, and chemicals; a filter for water; and a biological medium for the breakdown of wastes. Soil interacts intimately with water, air, and plants and acts as a damper to fluctuations in the environment. Soil mediates many of the ecological processes that control water and air quality and that promote plant growth. Concern about the soil resource base needs to expand beyond soil productivity to include a broader concept of soil quality that encompasses all of the functions soils perform in natural and agricultural ecosystems. In the past, soil productivity and loss of soil productivity resulting from soil degradation have been the bases for concern about the world's soils. Equally important, however, are the functions soils perform in the regulation of water flow in watersheds, global emissions of greenhouse gases, attenuation of natural and artificial wastes, and regulation of air and water quality. These functions are impaired by soil degradation. The ability of modern agricultural management systems to sustain the quality of soil, water, and air is being questioned. This chapter suggests methods that can be used to evaluate whether soil quality is being degraded, improved, or maintained under given management systems and methods of evaluating whether alternative management systems will sustain the quality of soil resources.

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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; Lowdermilk, 1953; Hillel, 1991). 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 carry out 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 (1992), 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 function soils play 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" (page 77). They recommended that policies to protect soil resources should protect the soil's capacity to serve 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 (1991) 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)" (page 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, in partitioning and regulating the flow of water in the environment, and as an environmental buffer. Parr and colleagues (1992), 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" (page 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 quality. They suggested that soil quality has important effects on the nutritional quality of the food

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Soil and Water Quality: An Agenda for Agriculture produced in those soils but noted that these linkages are not well understood and research is needed to clarify the relationship between soil quality and the nutritional quality of food. There is a growing recognition of the importance of the functions soils perform in the environment. The importance of those functions 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 management can either improve or degrade soil quality. Erosion, compaction, salinization, sodification, acidification, and pollution with toxic chemicals can and do degrade soil quality. Increasing soil protection by crop residues and plants; adding organic matter to the soil through crop rotations, manures, or crop residues; and careful management of fertilizers, pesticides, tillage equipment, and other elements of the farming system can improve soil quality. IMPORTANCE OF SOIL QUALITY Soils have important direct and indirect impacts on agricultural productivity, water quality, and the global climate. Soils make it possible for plants to grow by mediating the biological, chemical, and physical processes that supply plants with nutrients, water, and other elements. Microorganisms in soils transform nutrients into forms that can be used by growing plants. Soils are the storehouses for water and nutrients. Plants draw on these stores as needed to produce roots, stems, leaves, and, eventually, food and fiber for human consumption. Soils—and the biological, chemical, and physical processes they make possible—are a fundamental resource on which the productivities of agricultural and natural ecosystems depend. The soil, which interacts with landscape features and plant cover, is a key element in regulating and partitioning water flow through the

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Soil and Water Quality: An Agenda for Agriculture environment (Jury et al., 1991). 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. The condition of the soil surface determines whether rainfall infiltrates or runs off. If it enters the soil it may be stored and later taken up by plants, it may move into groundwaters or move laterally through the earth, appearing later in springs. This partitioning of rainfall determines whether a rainstorm 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 the hydrological cycle. The biological, chemical, and physical processes that occur in soils buffer environmental changes in air quality, water quality, and global climate (Lal and Pierce, 1991). The soil matrix is the major incubation chamber for the decomposition of organic wastes, for example, pesticides, sewage, and solid wastes. Depending on how they are managed, soils can be important sources or sinks of carbon dioxide and other gases, also known as greenhouse gases, that contribute to the so-called greenhouse effect. Soils store, degrade, or immobilize nitrates, phosphorus, pesticides, and other substances that can become air or water pollutants. Soil degradation through erosion, compaction. loss of biological activity, acidification, salinization, or other processes can reduce soil quality. These processes reduce soil quality by changing the soil attributes, such as nutrient status, organic and labile carbon content (organic carbon is the total amount of carbon held in the organic matter in the soil; labile carbon is that fraction of organic carbon that is most readily decomposable by soil microorganisms), texture, available water-holding capacity (the amount of water that can be held in the soil and made available to plants), structure, maximum rooting depth, and pH (a measure of the acidity or alkalinity). Some changes in these soil attributes can be reversed by external inputs. Nutrient losses, for example, can be replaced by adding fertilizers. Other changes such as loss of the soil depth available for rooting because of soil erosion or degradation of soil structure because of subsoil compaction are much more difficult to reverse. Soil Quality and Agricultural Productivity Damage to agricultural productivity has historically been the major concern regarding soil degradation. Agricultural technology has, in some cases, improved the quality of soils. In other cases, improved technology has masked much of the yield loss that could be attributed to

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Soil and Water Quality: An Agenda for Agriculture declining soil quality, except on those soils that are vulnerable to rapid and irreversible degradation. Effect of Soil Degradation on Productivity Four major studies predicted that yield losses resulting from soil erosion would be less than 10 percent over the next 100 years (Crosson and Stout, 1983; Hagen and Dyke, 1980; Pierce et al., 1984; Putnam et al., 1988). Such projections of low-yield losses, coupled with increasing concern over off-site water quality damages from agricultural production, have begun to shift the emphasis of federal policy to the off-site damages caused by erosion. On-site losses of soil productivity from current degradative forces, however, have been underestimated. The projections for low levels of erosion-induced losses in agricultural productivity largely result from the hypothesis that almost two-thirds of U.S. croplands will suffer little or no yield loss over the next 100 years (Pierce, 1991). Productivity losses on the remaining one-third of the lands may be serious (Pierce et al., 1984), but the losses are masked by the larger area of soils that are less vulnerable to erosion (Pierce, 1991). More important, estimates of productivity losses resulting from erosion have not accounted for damages caused by gully and ephemeral erosion, sedimentation (Pierce, 1991), or reduced water availability because of decreased infiltration of precipitation. Those studies also assumed that the optimum nutrient status is maintained on the eroding lands through application of fertilizers, manures, or other sources of plant nutrients. Replacing these nutrients comes at a cost. Larson and colleagues (1983) estimated that in 1982 the amount of nitrogen, phosphorus, and potassium from U.S. croplands lost in eroded sediments was 9,494, 1,704, and 57,920 metric tons, respectively (10,465, 1,878, and 63,846 tons, respectively). The value of the nitrogen, phosphorus, and potassium lost was estimated at $677 million, $17 million, and $381 million, respectively. In addition, estimates of the effects of soil degradation on productivity have focused on the yield losses expected from erosion-induced damage to croplands. The nation's croplands are also being damaged by compaction, salinization, acidification, and other forces. These damages will add to the yield losses resulting from erosion. More important, erosion accelerates the processes of compaction, salinization, and acidification. The reverse is also true. Yield losses will be greater than those projected in the past if all degradation processes and their interactions are considered. Walker and Young (1986) have suggested that the use of absolute crop yield reductions as the measure of productivity losses masks more

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Soil and Water Quality: An Agenda for Agriculture Even though this cropland has been tilled, ephemeral rills are still evident. During heavy rains, water will collect in these small channels and increase the severity of runoff. Credit: U.S. Department of Agriculture. subtle but important productivity losses. The analyses concluded that losses in potential yields will occur sooner and will be of greater magnitude than losses in absolute yields resulting from reduced soil quality. New, high-yielding crop varieties often require increased inputs of nutrients and more stable water regimes in order to produce maximum yield. Loss of soils' ability to hold and store nutrients and water can significantly restrain achievement of the full yield potentials of new agricultural technologies. New technologies may allow yields to increase or stay the same, even in the face of soil degradation, but these yields may mask important losses in the productive potential that could have been realized if soil quality had not been reduced. The true loss of productivity because of soil mismanagement or degradation is this loss in productive potential (Walker and Young, 1986).

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Soil and Water Quality: An Agenda for Agriculture Effect of Soil Degradation on Costs of Production Crosson and colleagues (1985) indicated that it is the cost of erosion, not predicted yield losses, that is really of interest. They suggested that farmers can substitute fertilizers, tillage, and other inputs for losses in soil productivity caused by soil erosion and that, from a production standpoint, increases in costs to reduce erosion are no different than higher input costs to compensate for erosion. Similarly, it is the cost of compensating for reduced soil quality resulting from degradation by compaction, acidification, salinization, loss of biological activity, and erosion that is most important when assessing the effects of soil degradation on soil productivity. Estimating the effect of soil degradation from erosion on the costs of production has proved difficult. Larson and colleagues (1983) suggested that soil degradation results in both replaceable and irreplaceable losses in soil productivity. A replaceable loss, for example, may be nutrients lost in eroded soil; an irreplaceable loss may be the loss in water-holding capacity resulting from decreased soil depth. Similarly, Walker and Young (1986) and Young (1984) distinguished between reparable and residual loss of yields resulting from soil erosion. Reparable yield losses were those that could be compensated for by substitution of other inputs such as fertilizer. Residual yield losses were those that remain even after substitution of other inputs and represent the cost to the yield of losing irreplaceable elements of soil quality such as soil depth. A total assessment of the costs of erosion would have to account for the costs of both the substituted inputs and the residual yield losses. Few data are available to estimate the effects of soil degradation from compaction, salinization, acidification, loss of biological activity, and other processes of soil degradation on production costs. Estimates of the extent or cost of compaction nationwide are not available. Eradat Oskoui and Voorhees (1990) extrapolated data from studies on yield losses resulting from subsoil compaction in Minnesota. They suggested that the value of the lost corn yield (based on a corn price of $0.06/kg [$2/bushel]) in Minnesota, Wisconsin, Iowa, Illinois, Indians, and Ohio could be $100 million annually. In years with high levels of water stress, when root growth is limited because of too much or too little water, yield losses would be higher. The U.S. Department of Agriculture (USDA), Soil Conservation Service (1989a) estimated that the productivity of 9 percent of the nation's croplands and pasturelands, including more than one-fifth of the irrigated lands, was being lowered by salinization or sodification. No data are available to suggest the extent or the cost of soil degradation resulting from the loss of biological activity or acidification.

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Soil and Water Quality: An Agenda for Agriculture Sustaining Soil Quality Is Essential to Improving Agricultural Productivity Given the multiple processes of soil degradation and the probable underestimation of the full cost of erosion on the cost of production, it can be concluded that soil degradation may have significant effects on the ability of the United States to sustain a productive agricultural system. The costs of reversing multiple causes of soil degradation to maintain yields may be large enough to affect the costs of production, even if absolute yields are not affected. To date, improvements in agricultural technologies have kept the costs of compensation for losses in soil quality low enough or increases in yields large enough to offset the costs of soil degradation on most croplands. Soil Management Finally, although attention has understandably been focused on soil degradation, soil management to improve soil quality holds the promise of producing gains in productivity. Current research suggests that soil management to improve infiltration, aeration, and biological activity can lead to significant gains in crop yields (Allmaras et al., 1991; Edwards, 1991). Yield gains from improved soil quality can be large on croplands that have suffered historic degradation from erosion. Soil management to improve soil quality is an opportunity to simultaneously improve profitability and environmental performance. Soil Quality and Water Quality Soil quality losses increase environmental as well as production costs. Indeed, investigators have argued that the costs of off-site damages from soil erosion are greater than the costs imposed by decreased productivity (Clark et al., 1985; Crosson and Stout, 1983). Soil degradation causes both direct and indirect degradation of water quality. Direct Effects Soil degradation from erosion leads directly to water quality degradation through the delivery of sediments and agricultural chemicals to surface water. Clark and colleagues (1985), using admittedly imperfect methods, estimated that the cost of sediment delivery on recreation, water storage facilities, navigation, flooding, water conveyance facilities, and water treatment facilities, among other damages, at $2.2 billion (1980 dollars) annually. Soil degradation resulting from compaction, salinization, acidification,

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Soil and Water Quality: An Agenda for Agriculture Soil degradation leads directly to water pollution by sediments and attached agricultural chemicals from eroded fields. Soil degradation indirectly causes water pollution by increasing the erosive power of runoff and by reducing the soil's ability to hold or immobilize nutrients and pesticides. Credit: U.S. Department of Agriculture.

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Soil and Water Quality: An Agenda for Agriculture or loss of biological activity can increase the vulnerability of soils to erosion and exacerbate the water quality problems associated with sedimentation. Indirect Effects The indirect effects of soil quality degradation may be as important as the direct damages resulting from sediment delivery, but they are often overlooked. Soil degradation impairs the capacity of soils to regulate water flow through watersheds. The physical structure, texture, and condition of the soil surface determine the portion of precipitation that runs off or infiltrates soils. In the process, the volume, energy, and timing of seasonal stream flows and recharge to groundwater are determined. Soil erosion and compaction degrade the capacities of watersheds to capture and store precipitation. Stream flow regimes are altered: seasonal patterns of flow are exaggerated, increasing the frequency, severity, and unpredictability of high-flow periods 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. Channel erosion was estimated to contribute from 25 to 60 percent of the sediment load in rivers in Iowa, Illinois, and Mississippi (see Chapter 6). Soil degradation that leads to the loss of a soil's capacity to buffer nutrients, pesticides, and other inputs accelerates the degradation of surface water or groundwater quality. Erosion not only results in the direct transport of sediment, nutrients, and pesticides to surface waters but also reduces the nutrient storage capacity of soils. A reduced nutrient storage capacity may lead to less efficient use of applied nutrients by crop plants and a greater potential for loss of nutrients to surface water and groundwater (Power, 1990). The pesticides held by soil organic matter or clay may become more mobile in the soil environment as erosion reduces organic matter levels and changes the soil's texture (Wagenet and Rao, 1990). Reduced biological activity can slow the rate at which pesticides are degraded, increasing the likelihood that the pesticides will be transported out of the soil to surface water or groundwater (Sims, 1990). Compaction in combination with other soil degradation processes can reduce the health of crop root systems, leading to less efficient nutrient use and increasing the pool of residual nutrients that can be lost to surface water or groundwater (Dolan et al., 1992; Parish, 1971). Soil Quality and Water Quality Are Linked Soil degradation results in both direct and indirect degradation of surface water and groundwater quality. Protecting or improving soil

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Soil and Water Quality: An Agenda for Agriculture quality is a fundamental step toward improving the environmental performance of agricultural ecosystems. Changes in farming systems that attempt to address the loss of nutrients, pesticides, salts, or other pollutants will not be as effective unless soil quality is also protected or improved. Soil quality improvement alone, however, will not be sufficient to address all water quality problems unless other elements of the agricultural system are addressed. Soil quality improvement alone, for example, will not solve the problem of nitrate contamination of surface water and groundwater if excessive nitrogen is applied to the cropping system. If nitrogen applications are excessive, changes in soil quality may change the proportion of nitrates delivered to surface waters rather than to groundwaters, but total nitrate losses may remain the same. Soil Quality and the Global Climate Recently, the role of the soil resource as a global climate regulator has received more attention as a result of heightened concern over human-induced climate changes. Depending on how it is managed, soil is a source (or sink) of carbon and nitrogen. Lal and Pierce (1991), for example, estimated that if 1 percent of the organic carbon stored in the most widely occurring types of tropical soils is mineralized annually, 128 billion metric tons (130 billion tons) of carbon will be released into the atmosphere. Lal and Pierce (1991) point out that this quantity compares with annual carbon emissions of an estimated 325 million metric tons (330 million tons) from burning of fossil fuels and 1,659 million metric tons (1,686 million tons) from deforestation (Brown et al., 1990). Little is known, however, about the contribution of soil-related processes to greenhouse gas emissions under different systems of soil and crop management. What is known suggests that the soil resource may play an important role in regulating greenhouse gas concentrations. Soil Quality as a Long-Term Goal of Soil Management The ways that humans use soils affect soil quality. Soil erosion can strip away fertile topsoils and leave the soil less hospitable to plants. Heavy farm machinery can compact the soil and impede its capacity to accept and store water. Loss of organic matter because of erosion or poor cropping practices can seriously impede the soil's ability to filter out potential pollutants. In the past, soil erosion was used as a convenient proxy for all of the processes of soil degradation, and efforts to control erosion have

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Soil and Water Quality: An Agenda for Agriculture TABLE 5-5 Amounts of Organic Carbon Needed Annually in Residues to Maintain Soil Organic Carbon on Lands with Different Slopes and Erosion Levelsa       Organic Carbon (kg/ha) Area (1,000 hectares) Slope (percent) Average Erosion (metric tons/ha/year)b In sedimentc Needed in Residue 853 0-2 5 135 1,900 1,157 2-6 18 486 6,840 819 6-12 61 1,647 23,180 376 12-20 114 3,078 43,320 a Major land resource area 107 (Iowa and Missouri Deep Loess hills). b From U.S. Department of Agriculture, Soil Conservation Service. 1982. Basic Statistics 1977 National Resources Inventory. Statistical Bulletin No. 686. Washington, D.C.: U.S. Department of Agriculture. c Enrichment ratio of 1.5; organic carbon in soil = 1.8 percent. SOURCE Adapted from W. E. Larson and B. A. Stewart. 1992. Thresholds for soil removal for maintaining cropland productivity. Pp. 6–14 in Proceedings of the Soil Quality Standards Symposium. Watershed and Air Management Report No. WO-WSA-2. Washington, D.C.: U.S. Department of Agriculture, Soil Conservation Service. the productivity index model of Pierce and colleagues (1983), Larson and colleagues (1985) calculated which of the four soil attributes in the subsoil—available water-holding capacity, bulk density, pH, or rooting depth—would cause the greatest decline in soil productivity on 75 major soils of the Corn Belt, assuming erosion removed 50 cm (20 inches) of soil from the surface. Of the 75 soils, the productivity index decreased significantly (for example, the productivity index was less than 0.1) in 37 of the soils. Thirteen of the soils showed a significant degradation in the available water-holding capacity in the subsoil, 4 were degraded because of increased bulk density, 7 were degraded because of decreased rooting depth, and 13 were degraded because of both bulk density and decreased rooting depth. Rijsberman and Wolman (1985) reported that nutrients and total organic carbon, in addition to available water-holding capacity, pH, and bulk density, were attributes readily degraded by erosion and were important in maintaining soil productivity. Maintenance of total organic carbon was important in preventing the formation of soil surface crusts. The specific soil quality attributes degraded by erosion depend on the soil characteristics, climate, and amount of erosion. In most cases, erosion reduces the quality of more than one attribute.

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Soil and Water Quality: An Agenda for Agriculture Compaction One aspect of soil degradation that is of increasing concern is soil compaction caused by the wheeled traffic involved in normal farming operations. Surface Soil Compaction Most of the farm machinery in use today is sufficiently heavy to cause compaction in the surface 10 to 20 cm (4 to 8 inches) of soil. Although tillage operations after the use of heavy machinery are often sufficient to alleviate compaction, the increasing use of no-till and ridge-till farming systems can result in areas within a field that remain relatively dense in the surface 10 cm (4 inches), despite annual freezing (Voorhees, 1983). Under these conditions, the soil water runoff control benefits associated with reduced-tillage can be lost (Lindstrom and Voorhees, 1980; Lindstrom et al., 1981). These studies showed that interrow wheeled traffic during spring planting operations may negate the beneficial tillage management effects and compact the soil to the point of significantly decreasing infiltration rates in those interrows. Young and Voorhees (1982) reported that about 34 percent of the total runoff and 49 percent of total soil loss from a bare field can originate from the 22 percent of the field surface that is used as wheel tracks during planting operations. A concentration of plant residues in the wheel-trafficked interrows may be a practical solution. Plant growth response is another aspect of surface layer soil compaction (less than 30 cm [12 inches] deep) that relates to soil degradation. Several researchers have reported a wide range of growth responses to surface layer compaction by a variety of crops (Draycott et al., 1970; Fausey and Dylla, 1984; Johnson et al., 1990; Van-Loon et al., 1985; Voorhees et al., 1990). The theory is that the crop yield response to surface compaction should follow a parabolic relationship (Soane et al., 1982), inferring that there is an optimum degree of compactness for maximum crop yield. There is evidence for this inference (Voorhees, 1987), and efforts are under way to develop the technology needed to assess the economic extent of soil compaction in the Corn Belt (Eradat Oskoui and Voorhees, 1990). Even though surface soil compaction may be economically important because it decreases crop yields, it is potentially manageable because surface layer compaction can be alleviated by normal tillage equipment. In systems that do not require annual tillage, the detrimental effects of surface compaction on either runoff and erosion or crop yield

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Soil and Water Quality: An Agenda for Agriculture can be minimized with the application of interrow plant residues and proper management of wheeled traffic. Subsoil Compaction Soil compaction deeper than the normal tillage depth (subsoil compaction) is a much more serious consequence of modern agricultural production and should be considered an important factor in soil degradation. The reasons are threefold: (1) subsoil compaction persists longer than surface soil compaction, (2) the trends of increasing farm and farm machinery sizes tend to worsen the potential difficulties with subsoil compaction, and (3) it is difficult and costly to remedy subsoil compaction by mechanical means. The previously cited research reporting the persistence of surface soil compaction, despite annual freezing and thawing cycles, forewarns of even more persistence of compaction at deeper depths. More than one freeze-thaw cycle may be required to ameliorate compacted soil. As the soil depth increases, the number of freeze-thaw cycles decreases. In northern latitudes, the front extends to a deeper depth than in warmer climates, but the volume of soil subjected to more than one freeze-thaw cycle is greatly diminished. Lowery and Schuler (1991) reported that a 11.3-metric ton (12.5-ton) axle load causes a significant and persistent increase in penetrometer (an instrument used to measure soil compaction) resistance in the subsoil of silt loam and silty clay loam soils in Wisconsin for 4 years. In Sweden, increased vane shear resistance was measured in the subsoil 7 years after application of a 9-metric ton (10-ton) axle load (Hakansson, 1985). Higher bulk density was still measurable 9 years after compacting the subsoil of a clay loam in Minnesota (Blake et al., 1976). In all of those studies, the subsoil went through at least one freeze-thaw cycle each winter. Current harvest equipment in the Corn Belt ranges from about 9 metric tons per axle (10 tons per axle) for an empty six-row combine to 36 metric tons per axle (40 tons per axle) for a loaded grain cart. Many hard-surfaced public highways have axle load limits ranging from 5 to 8 metric tons (6 to 9 tons). The subsoil compacting effect of the increased axle weights of current farm machinery can be partially offset by increasing the surface areas of the tires that carry the load. However, with current machinery design, there are practical limits to this approach. Prototype models of new tire track designs that reduce subsoil compaction have not been proven in the field. Meanwhile, the mechanical forces applied to soils will likely continue to increase, as will the potential for increasing subsoil compaction.

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Soil and Water Quality: An Agenda for Agriculture Alleviation of Subsoil Compaction Since tillage is often shown to be an effective way of alleviating surface compaction, subsoil tillage should alleviate subsoil compaction. However, deep tillage experiments in Iowa and Illinois (Larson et al., 1960) and more recently in Minnesota (Johnson et al., 1992) show that while deep tillage effectively loosens the soil, it does not automatically lead to increased crop yields. Data from the research in Minnesota showed that deep tillage (about 55 cm [22 inches]) followed by a relatively dry growing season results in a 1,176-kg/ha (15-bushel/acre) decrease in corn yield. Deep tillage is an expensive operation, so a considerable increase in crop yield is needed to pay for the operation. The inconsistent effects of deep tillage on crop yield, coupled with the slow rate at which natural forces ameliorate a compacted subsoil, emphasize the potential degrading effect that wheeled traffic-induced soil compaction can have on productivity. Corn Yield Response to Subsoil Compaction A series of field experiments recently conducted across the Corn Belt of the United States and southern Canada showed that wheeled traffic with axle loads typical for harvest operations can cause soil to be excessively compacted to depths of 60 cm (24 inches). The subsoil compaction can persist for a number of years, despite annual freezing and thawing, and crop yields may be decreased for a number of years after a one-time application of heavy wheeled traffic. Data from the Minnesota site illustrate the response. In the fall of 1981 a Webster clay loam in southern Minnesota was trafficked with a load of 18 metric tons per axle (20 ton/axle). The surface 20 cm (8 inches) was intensively tilled to alleviate surface compaction. All subsequent wheeled traffic on the plots was limited to an axle load of less than 4.5 metric tons (5 tons). Corn yields were then measured for the next 9 years. Corn yields were significantly reduced by 30 and 13 percent in the first and second years, respectively, after heavy wheeled traffic. The yields were reduced by 7 and 3 percent in the third and fourth years, respectively, but the reductions were not statistically significant. There were no significant yield responses the fifth, sixth, or eighth years, but yields were significantly decreased by 15 percent in year 7 (a relatively dry year) and year 9 (a relatively wet year). Ignoring the yield data for the first year, which may have been a combination of surface soil and subsoil compaction effects, the average yield reduction over the 8 years was 6 percent. Yield responses at other sites across the Corn Belt were similar or even more negatively affected by subsoil compaction.

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Soil and Water Quality: An Agenda for Agriculture There are two facts that must be considered in extrapolating these data to whole field situations. First, the entire plot surface was tracked with wheeled traffic four times at the beginning of the experiment. Producers do not do this during normal farming operations; thus it could be argued that the experimental yield responses overestimate the real farm situation. However, the wheel tracks from a six-row combine alone cover about 27 percent of the field. A typical grain cart for a combine of that size also tracks about 27 percent of the field if it is pulled beside the combine for on-the-go unloading. Then, the tractor pulling the grain cart must be considered. When these three types of wheeled traffic are considered in total, a major portion of a field may be covered with heavy wheeled traffic or a significant portion of the field may be covered with wheeled tracks more than once. Thus, the actual situation in the field may not be so different from the experimental conditions outlined above. The second difference between plots and whole fields is that the producer puts heavy wheeled traffic on the field every harvest season, whereas it was applied only once on the experimental plots. Since natural forces are relatively ineffective in ameliorating the subsoil compaction in 1 year, it can be argued that subsoil compaction in a real farming situation may be long lasting, if not permanent. If one is willing to accept the assumption that the experimental data are somewhat typical for a real farming situation, and conservatively extrapolating the long-term 6 percent average plot yield reduction in Minnesota to 30 percent of a given field and 50 percent of the corn acreage in the states of Minnesota, Wisconsin, Iowa, Illinois, Indiana, and Ohio, the annual monetary loss caused by subsoil compaction is estimated at about $100 million (assuming corn prices at $63/metric ton [$2/bushel]). It could be more in high-stress years, when root growth is limited because of either too much or too little water. Chemical Degradation Chemical degradation processes can lead to a rapid decline in soil quality. Nutrient depletion, acidification, and salinization are common soil degradation processes in the United States that have had a serious impact on crop production. Chemical degradation is also caused by the buildup of toxic chemicals resulting from human activities. Salinization Investigators normally distinguish between saline and sodic soils. Saline soils suffer from an excess of salinity caused by a range of ions. When

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Soil and Water Quality: An Agenda for Agriculture TABLE 5-6 Extent of Salinity and Associated Problems by Land Use in California   Millions of Hectares Primary Land Use Nonfederal Land Area Saline or Sodic Soilsa High Water Tableb Water Quality Irrigated cropland 4.01 1.18 1.09 1.38 Dry cropland 0.73 0.00 0.04 0.04 Grazed land 7.94 0.32 0.16 0.16 Timberland 3.60 0.00 0.00 0.04 Wildlife land 0.49 0.08 0.04 0.08 Urban 2.02 0.04 0.04 0.08 Other 3.64 0.08 0.04 0.12 Total 22.51 1.70 1.41 2.02 a Areas with electrical conductivity of 4 dS/m (about 2.500 mg/L) or greater and/or exchangeable sodium values of more than 15 percent. b High water table indicates a depth of 1.5 m or less or at a depth that affects the growth of commonly grown crops. Includes parameters such as salinity or boron toxicity. SOURCE: U.S. Department of Agriculture, Soil Conservation Service. 1983. California's Soil Salinity. Davis, Calif.: U.S. Department of Agriculture, Soil Conservation Service. sodium is the prevalent cation, the soils are generally classified as sodic. Salinity problems are not restricted to irrigated areas. In fact, huge dryland areas suffer from salinity and/or sodicity problems. In reverse, all irrigated areas in arid (and semiarid) regions are subject to salinization if adequate drainage is not provided. Investigators have attempted to inventory the extent of salinity problems and to establish trends. The data base for the United States is weak at best; many of the figures are only estimates. Large-scale mapping projects in Europe and other continents provide a more reliable data base. According to Szabolcs (1989), the total area of salt-affected soils in the world approaches 1 billion ha (2.5 billion acres). Postel (1989) has made a different estimate, but a large part of the difference is that Postel's estimate included only irrigated lands, while Szabolcs' estimate included nonirrigated lands. Another estimate comes from the Soil Conservation Service for California (Backlund and Hoppes, 1984). Backlund and Hoppes reported that the area of the San Joaquin Valley with salinity problems would increase to 1.46 million ha (3.6 million acres) by the year 2000 (Tables 5-6 and 5-7). Although good statistics are hard to find, it is the consensus of specialists that, worldwide, the salinity problem continues to increase substantially. In the United States, contamination of irrigation drainage

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Soil and Water Quality: An Agenda for Agriculture TABLE 5-7 Salinity and Drainage Problems by Major Irrigated Areas (approximate area)   Millions of Hectares Location Irrigated Area Saline or Sodic Soila High Water Tableb Water Quality San Joaquin Valley 2.27 0.89 0.61 0.93 Sacramento Valley 0.85 0.08 0.16 0.12 Imperial Valley 0.20 0.08 0.20 0.20 Other areas 0.77 0.12 0.12 0.12 Total 4.09 1.17 1.09 1.37 a Areas with electrical conductivity of 4 dS/m (about 2.500 mg/L) or greater and/or exchangeable sodium values of more than 15 percent. b High water table indicates a depth of 1.5 m or less or at a depth that affects the growth of commonly grown crops. Includes parameters such as salinity or boron toxicity. SOURCE: U.S. Department of Agriculture, Soil Conservation Service. 1983. California's Soil Salinity. Davis, Calif.: U.S. Department of Agriculture, Soil Conservation Service. water with toxic trace elements has been added to concerns about salinity. This newly identified component has led to greater emphasis on the off-site effects of irrigation as opposed to the on-site effects of salinization. Thus, sustainability and evaluation of the impacts of salinity caused by irrigation have taken on an entirely new aspect (van Schilfgaarde, 1990). It is too early to give reliable estimates of the areas affected, but it is not too early to recognize this potentially serious problem (National Research Council, 1989b). Acidification Acidity is an important attribute of a soil because it influences many of the chemical and biological reactions that occur in the soil. Through these reactions, the pH influences plant growth. Soil pH influences microbial populations and activities and thus is important in buffering environmental reactions. Soils become more acidic when bases (for example, calcium, magnesium, potassium, and sodium) are leached from the soil and replaced by hydrogen ions on the exchange complex. In humid regions soils usually become more acidic with time, even if they are uncultivated. Many cultivated soils in the eastern, southeastern, and midwestern United States were too acidic for optimum plant growth when they were first cultivated from the native forests and prairies. Without the addition of lime, they have become more acidic under cultivation as a result of leaching and the addition of nitrogenous fertilizers.

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Soil and Water Quality: An Agenda for Agriculture The pH of a soil is a reflection of the nature of the cations on the exchange complex. Soils with pHs of less than 7.0 (acidic soils) have hydrogen in exchangeable form, whereas those with pHs of greater than 7.0 (alkaline soils) have exchange complexes that are dominated by bases, usually calcium and magnesium. Soils with pHs of less than about 5.5 may have significant amounts of exchangeable aluminum. Soils with free calcium and magnesium carbonates usually have pHs of greater than 8.2. In the soil's native state in the humid regions of the United States, the lowest pH is often below the tilled layer, whereas in arid areas, the highest pH is below the tilled layer. Corrective management practices such as application of lime to raise the pHs of the acid soils or lower the pHs of sodic soils is common. It is more difficult to alter the pH below the tilled layer. Some poorly drained soils contain significant concentrations of pyrite (iron disulfide), which oxidizes to sulfuric acid when it is drained of water, creating unusually low soil pHs. These soils usually have pHs below 3.5. Acidic soils may limit plant growth because they have insufficient calcium or magnesium or toxic concentrations of exchangeable aluminum or because they decrease the availability of certain essential nutrients. As soils become more acidic, the microbial populations tend to shift from bacteria to fungi, changing the decomposition rates of soil organic matter and organic residues. Acidic soil conditions often cause a reduction in the amount of nitrogen fixed by legumes. Sodic soils may limit plant growth by having toxic concentrations of exchangeable sodium or sodium concentrations that keep the soil dispersed and that maintain a poor soil structure. Application of nitrogenous fertilizers can lower pHs in both surface soils and subsoils (Pierre et al., 1970). The use of large amounts of nitrogenous fertilizers has accentuated the lowering of pHs on croplands in humid regions. Movement of acidity to depths below the tilled layer is of particular concern because of the difficulty in modifying the acidity in lower soil layers. A pH near neutrality (pH 6.5 to 7.5) is usually considered best for plant growth. Many soils in the humid regions of the United States are acidic, with pHs ranging from 7.0 to 5.0 or lower. Soils in arid regions may have pHs greater than 8.5, which usually indicates excessive amounts of exchangeable sodium. The optimum soil pH for plants varies. The optimum pH ranges for selected field crops, for example, are: corn, 5.5 to 7.5; soybeans, 6.0 to 7.0; wheat, 5.5 to 7.5; oats, 5.0 to 7.5; sorghum, 5.5 to 7.5; alfalfa, 6.2 to 7.8; and sweet clover, 6.5 to 7.5.

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Soil and Water Quality: An Agenda for Agriculture The acidity of a soil may be reduced (pH increased) by the addition of basic materials. Application of ground limestone (calcium and magnesium carbonates) is most common. The amount of ground limestone needed to raise the pH to acceptable levels depends on the initial pH of the soil, the desired pH for crop growth, the texture of the soil, and the soil's clay properties. The amount of lime required to raise an 18-cm (7-inch) layer of a silt loam soil from pH 5.5 to 6.5 in the northern and central United States is about 5.2 metric tons/ha (2.3 tons/acre); the value is 3.6 metric tons/ha (1.6 tons/acre) in the southern United States (Kellogg, 1957). Approximately 30 million metric tons (33 million tons) of pulverized limestone was applied to soils in the United States in 1980 at a cost of $180 million. Assuming that the lime was applied at 1.8 metric tons/ha (2 tons/acre), it would be applied to 7 million ha (16.5 million acres) of the approximately 162 million ha (400 million acres) of cropland. This is probably a small fraction of what is needed for maximum crop production. Pierre and colleagues (1970) concluded that it requires about 300 kg (660 lb) of calcium carbonate to neutralize the acidity produced in about 1 metric ton (1 ton) of ammonium nitrate fertilizer. Figure 5-3 shows the percentage of U.S. soils with pHs of 6 or less. Soils of the northeast, southeast, and northwest have higher percentages of acidic soils. The proportion of states east of the Mississippi River with soils that have pHs of 6.0 or less range from 13 percent in Wisconsin to 75 percent in New Hampshire and Vermont. Management of acidic subsoils with high amounts of toxic exchangeable aluminum, which restricts root growth, is a major problem in the Southeast. Relatively few soils in the Great Plains have pHs of less than 6.0. The soils west of the Cascade Mountains in the Pacific Northwest are usually acidic. Biological Degradation Biological degradation includes reductions in organic matter content, declines in the amount of carbon from biomass, and decreases in the activity and diversity of soil fauna. Biological degradation is perhaps the most serious form of soil degradation because it affects the life of the soil and because organic matter significantly affects the physical and chemical properties of soils. Biological degradation can also be caused by indiscriminate and excessive use of chemicals and soil pollutants. Biological degradation is generally more serious in the tropics and subtropics than it is in temperate zones because of the prevailing high soil and air temperatures. Tillage also stimulates biological degradation because it increases the exposure of organic matter to decomposition processes.

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Soil and Water Quality: An Agenda for Agriculture FIGURE 5-3 U.S. pH soil test summary as percentage of soils testing 6.0 or less in 1989. Source: Potash and Phosphate Institute. 1990. Soil test summaries: Phosphorus, potassium, and pH. Better Crops with Plant Food 74(2):16–18. Reprinted with permission from © Potash & Phosphate Institute. Organic Matter Content Although losses of mineral and organic soil particles through erosion are relatively well studied and documented, changes in the biological properties of soils induced by agricultural activities are less well known. Biological degradation of soil can be analyzed by looking at changes in total living soil biomass or by quantifying changes in specific biological populations or functions. Biological activities are associated with organic matter decomposition, nutrient cycling, the genesis of soil structure, degradation of pollutants, and disease suppression (Sims, 1990). Degradation of these activities through erosion, compaction, organic matter depletion, or toxic inputs results in subtle but significant changes in cropping system performance. Carbon from Biomass Cultivation has long been known to cause marked reductions in the total organic carbon content of between 20 and 50 percent (Paul and Clark, 1989). More recent work has shown even more dramatic reductions

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Soil and Water Quality: An Agenda for Agriculture in labile or microbial carbon associated with cultivation (Bowman et al., 1990; Follett and Schimel, 1989; Schimel et al., 1985). These reductions should lead to decreases in various soil biological activities such as nitrogen mineralization, genesis of soil structure, and specific soil enzyme activities (Sims, 1990). A study at Pendleton, Oregon, found that intensive cultivation and fallow decreased the total carbon content and microbial biomass in the soil, whereas increasing returns of crop residues, manure, and grass increased the organic carbon content and biomass (Granatstein, 1991). Soil Fauna Activity and Diversity The effects of toxic compounds such as pesticides and other organic compounds on soil biology are not as well studied as cultivation effects. The effects of a wide range of pesticides on microbial growth, biomass, and activity have been tested, but mostly in short-term laboratory studies (Sims, 1990). Most of these studies have found that, at the pesticide concentrations found under field conditions, pesticides have little effect on microbial parameters. Certain compounds (in particular, fungicides) have been found to reduce total microbial biomass, soil fauna populations, or both. Organic compounds other than pesticides, such as petroleum products, have been found to have much more marked effects than pesticides on soil biology (Sims, 1990), but these compounds are rarely encountered in agricultural soils. Effects of Biological Degradation The effects of biological degradation should be more important in cropping systems that rely heavily on biological nutrient cycling processes than systems that rely on chemical fertilizers for fertility. Similarly, systems that rely on natural biological pest suppression rather than pesticides for pest control are more sensitive to biological degradation. Since understanding of specific nutrient cycling and biological pest suppression mechanisms is limited for conventional, chemical-based cropping systems and low-input systems, the extent of the effects of biological degradation on cropping system performance are not known.