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MONITORING AND MANAGING SOIL QUALITY 189 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.
MONITORING AND MANAGING SOIL QUALITY 190 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
MONITORING AND MANAGING SOIL QUALITY 191 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. Soilsâand the biological, chemical, and physical processes they make possibleâare 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
MONITORING AND MANAGING SOIL QUALITY 192 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
MONITORING AND MANAGING SOIL QUALITY 193 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
MONITORING AND MANAGING SOIL QUALITY 194 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). 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.
MONITORING AND MANAGING SOIL QUALITY 195 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.
MONITORING AND MANAGING SOIL QUALITY 196 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,
MONITORING AND MANAGING SOIL QUALITY 197 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.
MONITORING AND MANAGING SOIL QUALITY 198 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
MONITORING AND MANAGING SOIL QUALITY 199 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
MONITORING AND MANAGING SOIL QUALITY 200 dominated programs and policies to protect soil resources. Soil erosion has been and continues to be the single most important process that degrades soil quality. In the long-term, however, all processes of soil degradationâcompaction, salinization, acidification, loss of biological activity, pollution, and erosionâ need to be considered when making soil management decisions. In the past, soil productivity was the primary value attached to soils and crop yield reductions were the primary measure used to assess the significance of soil degradation. In the long-term, however, soil management goals need to be broadened to include the roles that soils play in regulating water flow through watersheds and buffering environmental changes. Conservation of soil quality should become the goal of long-term soil management policies and programs. The relative importance of the three components of soil quality and the relative importance of the processes of soil degradation vary from area to area. Soil quality varies dramatically from soil to soil. Some soils are shallow and restrict plant growth. Others have impermeable layers beneath the surface that limit the soil's capacity to store water and restrict plant root growth. Still others are so acidic or basic that the biological activity needed to recycle wastes is seriously impaired. Certain soils are more vulnerable to the loss of one or the other components of soil quality and vary in their resistance to different soil degradation processes. The value that society places on the three components of soil quality also vary from place to place. In some cases, management to protect a soil's capacity to accept and degrade wastes may take precedence over conservation of soil's productivity. Similarly, on some soils compaction is a more important problem than erosion. Setting soil quality as the long-term goal of soil management has implications for national-level assessments of soil resources, for the design of programs to conserve soil resources, and for analyses of sustainable farming systems. National Assessments of Soil Resources Since the 1930s, investigators have periodically made national-level assessments of the amounts of erosion and its consequences. The 1938 yearbook of agriculture (U.S. Department of Agriculture, 1938) reported, based on a minimum of quantitative data, that of the total U.S. land area (770 billion ha [1,903 million acres]), 114 million ha (282 million acres) was ruined or severely damaged and 314 million ha (775 million acres) was moderately damaged. In the 1950s and 1960s, investigators made periodic estimates of the amounts of erosion on the basis of
MONITORING AND MANAGING SOIL QUALITY 201 reconnaissance surveys. In 1977, the U.S. Congress passed the Soil, Water and Related Resources Conservation Act, which called for an assessment every 5 years of the status of U.S. natural resources and their ability to provide the long- term resource needs of the United States. In response to the Soil, Water and Related Resources Conservation Act, the Soil Conservation Service of the USDA established, on croplands throughout the United States, primary sampling units where information was gathered. Although investigators gathered many kinds of information, the emphasis was on soil erosion. The 1977, 1982, and 1987 National Resources Inventories were by far the most extensive and quantitative inventories of soil resources in the United States. These inventories and assessments, however, were limited by their focus on quantifying rates of erosion and other processes of soil degradation rather than assembling and assessing the information needed to monitor the changes in soil attributes that can be related to changes in soil quality. The 1977, 1982, and 1987 National Resources Inventories, for example, did not include direct measurements of the changes in soil attributes caused by erosion. Rather, the inventories concentrated on comparing estimated erosion amounts with values such as T, usually defined as the maximum amount of erosion that can be tolerated; below this level of erosion, crop yields can be maintained economically and indefinitely (Wischmeier and Smith, 1978). The estimated erosion was calculated by the universal soil loss equation and the wind erosion equation. Although widely used, the accuracies of the equations need improvement and the scientific validity of T has been questioned (Johnson, 1987). National- level assessments need to be redirected to include quantifiable measures of soil attributes if soil quality changes are to be estimated. In addition, soil resource assessments need to be broadened to include all soil degradation processes. Monitoring of the processes of soil degradation such as erosion is an important component of such assessments, but monitoring must be strengthened by the collection and interpretation of data that can be related quantitatively to changes in the soil resource itself. A system that enables more direct quantification of actual changes in soil attributes will allow policies and programs to be directed more closely to alleviating actual degradation of soil quality. Soil Quality and Soil Conservation The concept of soil quality should be the principle guiding the recommendations for use of conservation practices and the targeting of programs and resources. Soil quality can be defined as the ability of a
MONITORING AND MANAGING SOIL QUALITY 202 soil to perform its three primary functions: to function as a primary input to crop production, to partition and regulate water flow, and to act as an environmental filter. Many attributes (or properties) of a soil contribute to soil quality, and the attributes are highly interrelated. Thus, no single attribute can be used as an index of soil quality. However, a few key attributes can be selected as indicators. Because many soil attributes are interrelated, the indicator attributes can often be used to estimate other attributes. The indicator attributes can then be used in simple models to predict a soil's ability to perform its three primary functions. TABLE 5-1 Reference and Measured Values of Minimum Data Set for a Hypothetical Typic Hapludoll from North-Central United States Horizon and Characteristic Reference Value Measured Value Surface horizon Phosphorus (mg/kg) 30 15 Potassium (mg/kg) 300 300 Organic carbon (percent) Total 2 1.5 Labile 0.2 0.15 Bulk density (mg/m3) 1.3 1.5 pH 6.0 5.5 Electrical conductivity (S/m) 0.10 1.0 Texture (percent clay) 30 32 Subsoil horizon Texture (percent clay) 35 35 Depth of root zone (m) 1.0 0.95 Bulk density (mg/m3) 1.5 1.5 pH 5.5 5.5 Electrical conductivity (S/m) 0.10 0.10 The utility of the concept of soil quality in guiding soil management can be seen in a simple example. Table 5-1 lists the changes in specific soil attributes for a Typic Hapludoll soil from the north-central United States. The changes in these soil attributes suggest the process of degradation that needs to be addressed, the corrective management practices that are needed, and whether further investigation is needed to improve soil quality. In this example, the available phosphorus is low, suggesting the need for improved nutrient management. The organic carbon level in the soil has declined, indicating that additional organic matter is needed. Management of residues needs to be improved or the crop sequence needs to be changed to include more closely grown crops, cover crops, legumes, or other sources of organic matter. Alternatively, tillage intensity could be reduced to slow the rate of organic matter decline.
MONITORING AND MANAGING SOIL QUALITY 203 The bulk density of the surface soil has increased, perhaps as a consequence of the lowered organic carbon content, the use of intensive tillage practices, or the use of heavy harvest machinery (compaction). The increase in the surface clay content (enriched from the subsoil) and the decrease in the rooting depth suggest that erosion may be serious. The decrease in surface soil pH may have resulted from the use of acid-forming fertilizer or natural leaching and may signal the need for lime. On this soil, an analysis of changes in soil attributes suggests that conservation practices need to focus on the maintenance of organic carbon, phosphorus, and pH and on erosion control. Reductions in erosion rates alone may not be sufficient to reduce evident compaction or the declining organic matter levels. Changes in these soil attributes, hover, are directly linked not only to the maintenance of soil productivity but also to the regulation of water flow through the environment and the capacity of the soil to buffer environmental changes. Such analyses of changes in soil attributes would be enriched if data on management variables such as cropping sequence, residue levels, tillage practices, and nutrient management were available. The combination of data on changes in soil attributes with management variables would allow for a more useful analysis of the kinds of conservation practices needed to protect soil quality. Soil Quality and Sustainability The concept of sustainable agricultural systems, whether for croplands, rangelands, or forestlands, is gaining acceptance as a framework that can be used to guide research and bridge the apparent conflicts between agricultural production and environmental goals. Investigators have provided several definitions and descriptions of sustainability, but the critical analysis of sustainable production systems has been constrained by the lack of systematic and scientifically sound criteria against which to compare alternative production systems. The soil quality criteria proposed for inclusion in a minimum data set can serve as the first step toward the development of systematic criteria of sustainability. These criteria will be discussed in the next section. Current research and historical data should allow researchers and managers to predict the impact of a particular farming system on soil quality. An accepted set of criteria for soil quality would permit comparative analyses of farming systems and would help to systematize the debate and research that attempt to define and implement the concept of sustainability. The comparison of alternative farming systems with a set of soil quality criteria would be only the first step in a more systematic
MONITORING AND MANAGING SOIL QUALITY 204 investigation of sustainability. The impacts of farming systems on air and water quality would also need to be evaluated, and economic analyses of those alternative farming systems would be required to complete the picture. Because of the soil's role in integrated impacts on both air and water quality, development of systematic soil quality criteria is an important first step. IMPORTANCE OF MONITORING CHANGES IN SOIL QUALITY A system that measures changes in soil quality is needed if conservation of soil quality is to become the long-term goal for management of the soil resource. A system that monitors changes in soil quality could be used for three major purposes. First, such a system can be used to track national trends in soil quality by incorporating measures of soil quality indicators into national resource surveys and assessments. Second, such a system can improve the management of soil conservation programs by aiding in setting tolerable soil erosion standards, targeting lands that need conservation measures, and identifying lands most suitable for inclusion in long-term easement programs. Finally, a system of soil quality indicators can aid in the analysis of the sustainability of farming systems by providing a set of criteria against which farming systems can be compared. The need for systems to monitor changes in soil quality has received increasing attention recently. Several authors have called for soil quality monitoring as a basic component of national policies to protect soil resources (Haberen, 1992; Hortensius and Nortcliff, 1991; Johnson et al., 1992; Larson and Pierce, 1991, 1994; Parr et al., 1992; Pierce and Larson, 1993; Young, 1991). Larson and Pierce (1991) compared a system that measures the quality of a soil to a medical clinic that assesses human health. A routine health assessment includes measurements of key indicators of health such as temperature, blood pressure, heart beat, and a few simple blood characteristics. These are considered indicators of possible problems. If the assessment finds abnormalities in any of the key indicators, more detailed information will be requested. Likewise, key soil quality indicators are needed so that investigators can monitor changes in soil quality. Over time, changes in soil quality indicators will provide the information needed to assess the effects of current farming systems and land use on soil quality, develop new farming systems that improve soil quality, and guide the development of national policies to protect soil and water quality. Soils, however, are difficult to inventory and assess. Soils vary greatly, with variations often occurring at distances of only a few
MONITORING AND MANAGING SOIL QUALITY 205 meters. Gross differences in soil surfaces can be seen or felt and usually reflect differences in organic matter content, mineralogy, or texture. The soil characteristics below the usual depth of cultivation, however, are often not carefully observed and characterized except by soil specialists. It is often difficult and laborious to obtain samples from the deeper horizons of the soil. A system to monitor changes in soil quality will require the following: â¢ identification of the soil attributes that can serve as indicators of change in soil quality, â¢ standard field and laboratory methodologies that can be used to measure changes in indicators of soil quality, â¢ a coordinated monitoring program that can quantify changes in soil quality indicators, and â¢ a coordinated research program designed to support, test, and confirm models that can be used to predict the impact of management practices on soil quality. Indicators of Soil Quality The quality of a soil is a composite of its physical, chemical, and biological properties. Indicators of soil quality are needed that relate to all three functions soils perform in natural and agroecosystems: (1) promote plant growth, (2) protect watersheds by partitioning and regulating precipitation, and (3) prevent air and water pollution by buffering agricultural chemicals, organic wastes, industrial chemicals, and other potential pollutants. It will be impossible and unnecessary to monitor changes in all of the soil attributes that relate to these three soil functions. Monitoring of a select set of soil attributes that can serve as indicators of change in soil quality is possible and can yield useful information on trends in soil quality. Many soil attributes could serve as indicators of soil quality. Soil attributes are often highly correlated, which makes interpretation of the significance of changes in selected indicators of soil quality difficult (Larson and Pierce, 1991). A change in soil organic matter, for example, has a direct effect on soil quality, but it also changes other measurable indicators of soil quality such as structure or bulk density. A system made up of soil quality indicators that are independent of one another would be ideal, but such a system is not possible because of the interrelated nature of the soil system. In addition, the measured value of a selected soil quality indicator will have a different interpretation depending on the soil or region from
MONITORING AND MANAGING SOIL QUALITY 206 which the sample was obtained. Critical bulk density values, for example, vary with the texture of the soil. The correlation between soil quality indicators and their soil or region specificity means that change in any set of soil quality indicators must be evaluated as a group. Interpretation of changes in one indicator without relating the change to other indicators may lead to misleading results. Interpretation of changes in soil quality indicators will also vary with soil taxa (classification group), probably at the suborder level. Minimum Data Set A great deal is known about the relationship of specific soil attributes to soil quality, and several authors have recently recommended various soil attributes as indicators of soil quality. Larson and Pierce (1991) recommended a combination of physical, chemical, and biological attributes as a minimum data set of soil quality indicators including nutrient availability, organic carbon, texture, water-holding capacity, structure, rooting depth, and pH. Griffith et al. (1992) reported that the Forest Service of USDA was using soil quality standards including amount of soil cover, soil porosity, and organic matter content to protect long-term soil productivity on National Forest System lands. Olson (1992) suggested that surface soil properties such as erosion phase, aggregation, organic carbon content, texture, and amount of coarse fragments coupled with subsoil properties including mechanical strength, aeration porosity, residual porosity, bulk density, permeability and rooting depth could be used to quantify and monitor changes in soil quality. They further suggested that soil quality thresholds could be set for each indicator depending on the effect of a change in that indicator on soil productivity. Hornsby and Brown (1992) reviewed the soil properties most important for determining the fate and transport of pesticides and suggested organic matter content, ion exchange capacity, type and amount of clay minerals, metal oxide content, pore size distribution, soil water content, temperature, pH, and bioactivity as important parameters. Alexander and McLaughlin (1992) suggested that changes in soil structure were particularly important indicators of change in soil quality on forests and rangelands and suggested the use of bulk density and cone penetrometer reading to monitor changes in structure. Granatstein and Bezdick (1992) stressed the need to integrate a combination of soil tests into a meaningful index that correlates with productivity, environmental, and health goals. They suggested that indicators of improved soil quality included increases in infiltration, macropores, aeration, biological activity, water-holding capacity, aggregate stability, and soil
MONITORING AND MANAGING SOIL QUALITY 207 organic matter. Decreases in bulk density, runoff, erosion, nutrient losses, soil resistance, diseases, and production costs were also suggested as indicators of improving soil quality. Physical and chemical indicators of soil quality were suggested by Arshad and Coen (1992) including soil depth to a restricting layer, available water- holding capacity, bulk density, penetration resistance, hydraulic conductivity, aggregate stability, organic matter, nutrient availability, pH, electrical conductivity, and exchangeable sodium. Visser and Parkinson (1992) noted that indicators of biological activity are less well developed than physical and chemical properties, and suggested that ecosystem processes such as carbon cycling, nitrogen cycling, nutrient leaching from soils, and soil enzymes may be the most useful indicators of soil microbial activity. Stork and Eggleton (1992) suggested that measures of changes in soil invertebrate populations including the abundance, biomass, and density of keystone species or of selected orders and classes of invertebrates along with species richness of dominant groups of soil invertebrates could serve as useful indicators of soil biological activity. Finally, Reagnold and colleagues (1993) compared the effect of different farming systems on soil quality by measuring differences in texture, structure, bulk density, penetration resistance, percent carbon, respiration rates, mineralizable nitrogen, ratio of mineralizable nitrogen to carbon, topsoil thickness, cation-exchange capacity, total nitrogen and phosphorus, exchangeable phosphorus, sulfur, calcium, magnesium, potassium, and pH. There are many soil properties that may serve as indicators of soil quality, as shown by the diverse list of indicators suggested by the authors cited above. The minimum data set, however, need only include those indicators that are most generally applicable to soils in varying climates and landscapes. Additional indicators could be added to the minimum data set to address properties that are particularly important in certain types of soils or regions. Table 5-2 presents a list of indicators that may be most useful for a minimum data set. The indicators suggested in the table are those that have been commonly recommended or used by several authors and should serve as a useful starting point for the development of a system to monitor changes in soil quality. A brief discussion of each suggested indicator follows. Nutrient Availability Nutrient availability is an important soil attribute for plant productivity and water quality and is significantly altered by soil management practices. Nutrient availability can be estimated by extracting nutrients
MONITORING AND MANAGING SOIL QUALITY 208 from the different components in the soil with chemicals and measuring the nutrient content in the extract. Nitrogen, phosphorus, and potassium are the major nutrients in the soil that are measured by extraction. TABLE 5-2 Indicators of Change in Soil Quality and Their Relationship to Soil Functions Soil Functions Soil Quality Promote Plant Regulate Water Buffer Environmental Indicator Growth Flow Changes Nutrient availability Direct Indirect Direct Organic carbon Indirect Indirect Direct Labile carbon Indirect Direct Direct Texture Direct Direct Direct Water-holding Direct Direct Indirect capacity Soil structure Direct Direct Indirect Maximum rooting Direct Indirect Indirect depth Salinity Direct Direct Indirect Acidity/alkalinity Direct Direct Indirect Organic Carbon Soil organic carbon or soil organic matter is perhaps the single most important indicator of soil quality and productivity. Depletion of soil organic carbon is followed by depletion of plant nutrients, deterioration of soil structure, diminished soil workability (Frye, 1987), and lower water-holding capacity of the soil. The amount of organic carbon in the soil affects permeability, water retention, and hydraulic conductivity, which all determine the way rainfall is portioned and potential pollutants transported. It also alters the efficacies and fates of applied pesticides. Depletion of soil organic carbon and erosion are interrelated, since a decrease in organic carbon increases the susceptibility of a soil to erosion, thereby increasing the rate of depletion of soil organic carbon. Because of its importance and its susceptibility to change by soil erosion, organic carbon should be included in the minimum data set and monitored periodically. Total organic carbon in the soil can be affected by management and has been shown to be directly related to the amount of organic matter added to the soil in crop residues, manures, or other sources (Larson and Stewart, 1992). The total organic matter in the soil may change slowly, however; even changes restricted to the few millimeters of surface soil can have substantial effects on infiltration,
MONITORING AND MANAGING SOIL QUALITY 209 aeration, and erosion (Bruce et al., 1988). Organic residues and soil organic matter at the interface between the soil and atmosphere are extremely influential in partitioning water and influencing surface soil structure. Tillage and residue management can stratify the organic carbon content at various levels on or within the soil. Labile Carbon Although total organic carbon provides important information, it is the labile carbon fraction that is most active in the soil. The amount of labile carbon is most directly related to important biological processes in the soil including rates of mineralization of nutrients, the generation of soil structure, and the attenuation of potential pollutants. Simple chemical procedures to assess the labile carbon fraction are available. Texture The soil's texture (particle-size distribution) in the surface soil layer may be altered as a result of the selective removal of fine particles during the erosion process, as a result of mixing subsoil into the surface layer during tillage (as cumulative erosion reduces the thickness of the surface soil layer) and as a result of deposition of eroded sediments on the soil surface. Changing the surface soil texture can have important effects on crop productivity (Frye, 1987; Lal, 1987), for example, by reducing the amount of nutrients or water the soil can hold or by restricting the growth of plant roots. Texture also influences the partitioning of rainfall and the flow of water and potential pollutants through the soil. Water-Holding Capacity An important attribute of a soil is its ability to store and release available water to plants. The importance of water available to plants and its measurement were discussed by Ritchie (1981). Plant-available water capacities are a required input for nearly all crop simulation models. The plant-available water capacities should be determined to the depth of rooting, and temporal changes in plant-available water capacitiesâthose that are either natural or induced by management or erosionâshould be determined in the surface layers. Water-holding capacity is also directly related to the effect of changes in soil quality on water quality. Water-holding capacity is related to the rate at which water enters and leaves the soil. The rate and direction of water flow through soils is an important factor determining the effect of
MONITORING AND MANAGING SOIL QUALITY 210 farming practices on water quality. Models for estimating water retention curves from particle-size distributions, organic matter, and bulk density are available and were recently reviewed by Rawls et al. (1992). Management of the soil can have significant effects on water-holding capacity by changing the depth and texture of surface layers, the structure and compactness of surface and subsurface layers, and by affecting the rate of infiltration of precipitation. Structure The term soil structure, as broadly defined by Kay (1989), has three components. The first is structural form, which refers to the geometry of the soil pore space (porosity, pore size distribution, and pore continuity). The second is aggregate stability, which refers to the size distribution and resistance of aggregates to degradation. The third is structural resiliency, which refers to the ability of the soil structure to re-form once it has been degraded. Measurements of structural form include bulk density, macroporosity (porosity (pores >60 Âµm), and saturated hydraulic conductivity. Determination of compact soil layers that impede root growth are important for determination of effective soil rooting volume. Either bulk density or penetration resistance measurements (interpreted with respect to water contents) can be used to identify root-impeding layers. Critical limits of bulk density for soils of different textures were given by Pierce and colleagues (1983). Compaction by wheeled traffic has direct and sometimes irreversible effects on soil structure. Texture, organic matter, and labile carbon are also related to structure, and soil management that results in changes in these soil attributes will also affect soil structure. Rooting Depth Soil thickness has been related to crop productivity, particularly in mine reclamation studies (Power et al., 1981). Soils in which the rooting depth is limited by the presence of a physical or chemical constraint are generally less productive. As limiting layers are moved closer to the soil surface as erosion removes the topsoil, crop productivity generally declines. Rooting depth varies by crop species, and the limits for various species were given by Taylor and Terrell (1982). Maximum rooting depth should be determined at the time of physiological maturity of the crop species under study. Management of the soil can have important effects on rooting depth. Erosion reduces rooting depth by removing layers of surface soil, and compaction reduces rooting depth by creating layers in the soil that are impenetrable by crop roots.
MONITORING AND MANAGING SOIL QUALITY 211 Acidity and Alkalinity Soil pH is a measure from which many general interpretations about the chemical properties of a soil can be made. The acidity, neutrality, or alkalinity of a soil suggests the solubilities of various compounds in the soil, the relative bonding of ions to exchange sites, and the activities of microorganisms (McLean, 1982). A pH of less than 4 indicates the presence of free acids, generally from oxidation of sulfides; a pH of less than 5.5 indicates the likely occurrence of exchangeable aluminum; and a pH from 7.8 to 8.2 indicates the presence of calcium carbonate (Thomas, 1967). Acidity and alkalinity can be readily managed, in many soils, by careful management of fertilizer and lime applications. Pedotransfer Functions Prediction of the direction in which soil attributes are changing can often be made without direct measurement of the specific attribute. For example, Larson and Stewart (1992) showed that a simple regression equation could predict organic matter changes in several U.S. soils on the basis of the amount of crop residue that was added to the soil. Important hydraulic properties of soils including water retention, hydraulic conductivity, and water-holding capacity can be estimated from data on texture and organic matter content (Gupta and Larson, 1979a; Rawls et al., 1992). Larson and Pierce (1991) have suggested the development of pedotransfer functions that can be used to evaluate changes in soil quality from a minimum data set of indicators. Pedotransfer functions could dramatically increase the utility of a minimum data set of soil quality indicators. Estimations of changes in many important soil attributes could be simulated from measures of relatively few indicators of soil quality. Larson and Pierce (1991) suggest that a review of the literature would uncover many already developed pedotransfer functions that could be used to simulate changes in soil quality. Table 5-3 provides some of the pedotransfer functions described by Larson and Pierce (1991). The development and validation of pedotransfer functions for use in simulation of soil quality should be an urgent research priority. Quantifying Soil Quality The preceding section demonstrates that a great deal of information and understanding is available to select soil attributes. The selection of soil attributes for use in a minimum data set, however, depends not only
MONITORING AND MANAGING SOIL QUALITY 212 TABLE 5-3 Some Pedotransfer Functions Estimate Relationship Reference Chemical Phosphate sorption capacity PSC = 0.4 (Alox + Feox) Breeusma et al., 1986 Cation-exchange capacity CEC = a OC + b C Breeusma et al., 1986 Change in organic matter C = a + b OR Larson and Stewart, 1992 Physical Bulk density Db = b0 + b1 OC + b2 %si + Bouma, 1989 b3 M Bulk density Random packing model Gupta and Larson, 1979 using particle size distribution Bulk density Db = f(OC, cl) Manrique and Jones, 1991 Water retention WR = b0 %sa + b2 %si + b3 Gupta and Larson, 1979 %cl + b4 %OC Water retention WR = b0 + b1 C + b2 Sy Bouma, 1989 Random roughness from RR = f(soil morphology) Allmaras et al., 1967 moldboard plowing Porosity increase P = f(Mr, IP, cl, si, OC) Allmaras et al., 1967 Hydraulic Hydraulic conductivity Ks = f(texture) Childs and Collis- George, 1950; Marshall, 1958; Millington and Quirk, 1961 Seal conductivity SC = f(texture) Gupta et al., 1991 Saturated hydraulic Ds = f(soil morphology) McKeague et al., 1982 conductivity Productivity Soil productivity PI = f(Db, AWHC, pH, EC, Pierce et al., 1983; Kiniry ARE) et al., 1983 Rooting depth RD = f(Db, WHC, pH) Pierce et al., 1983 NOTE: Variables other than italicized coefficients are defined as follows: PSC, phosphate sorption capacity; Alox, oxalate extractable-aluminum; Feox, oxalate extractable iron; CEC, cation-exchange capacity; OC, organic carbon; C, change in organic carbon; OR, organic residue; Db, bulk density; si, silt; M, median sand fraction; WR, water retention; sa, sand; cl, clay; Sy, 1/Db; RR, random roughness; P, porosity; Mr, moisture ratio; IP, initial porosity; Ks, hydraulic conductivity; SC, seal conductivity; Ds, saturated hydraulic conductivity; PI, productivity index; AWHC, available water- holding capacity; EC, electrical conductivity; ARE, available rooting environment; RD, rooting depth; WHC, water-holding capacity. SOURCE: W. E. Larson and F. J. Pierce. 1991. Conservation and enhancement of soil quality. Pp. 175-203 in Evaluation for Sustainable Land Management in the Developing World. Volume 2: Technical Papers. Bangkok, Thailand: International Board for Soil Research and Management. Reprinted with permission from Â© International Board for Soil Research and Management.
MONITORING AND MANAGING SOIL QUALITY 213 on our understanding of the relation of those attributes to soil quality, but also on the utility of the attributes for use in sampling programs and in pedotransfer functions to provide more quantitative estimates of change in soil quality. Sampling can be expensive. It is important that the attributes selected for a minimum data set be as few as possible. Testing and empirical evaluation of proposed indicators will be required to identify those most suited for use based on the ease and accuracy with which they can be sampled, the degree of spatial and temporal variability of the attribute, their utility as parameters in pedotransfer functions, and their applicability to a wide range of soils, climates, and landscapes. The following discussion summarizes some of the experience with soil attributes in quantitative assessments of soil quality. Indicators of Productivity Pierce and colleagues (1983) used a simple model to estimate the potential soil productivity loss over time on the soils in the Corn Belt. The model was expressed as follows: where PI is the productivity index, Ai is sufficiency of available water- holding capacity, Ci is sufficiency of bulk density adjusted for permeability, Di is sufficiency of pH, WF is a weighing factor based on root distribution, and r is the number of horizons in the maximum rooting depth. In the study by Pierce and colleagues (1983), Ai, Ci, and Di and were taken from the Soil Conservation Service's (U.S. Department of Agriculture) SOILS-5 data base for the soil mapping unit at each of the primary sampling units in the 1982 National Resources Inventory. The sufficiency curves for the soil attributes as used by Pierce and colleagues (1983) were modified from those presented by Kiniry and colleagues (1983). Erosion was simulated by removing the erosion amount (depth) given in the National Resources Inventory from the surface for a soil mapping unit and then adding an equal depth to the base of the 100-cm (40- inch) profile by using the attributes and values for that horizon. In this way the productivity index was computed initially and after 25, 50, and 100 years of erosion. The study by Pierce and colleagues (1983) illustrates the usefulness of making predictions with simple models on the basis of soil quality attributes or indicators. Although the study by Pierce and colleagues
MONITORING AND MANAGING SOIL QUALITY 214 (1983) used estimated values for soil attributes given in the SOILS-5 data base, the estimations could have been enhanced if actual measured values at the primary sampling unit had been available. Indicators of Water Regulation Soil quality and the changes in soil quality that occur with soil management can be expected to affect natural resource models. For example, consider the Water Erosion Prediction Project (WEPP) model (Laflen et al., 1991a,b), in which soil quality is assumed to affect both the water infiltration and the soil erosion portions of the model. Infiltration in the WEPP model is quantified with the Green-Ampt infiltration model (a simple mathematical equation used to estimate how much and how quickly water soaks into the soil) (Laflen et al., 1991a,b). The soil capillary potential is assumed to be proportional to bulk density and soil texture (sand, clay, and porosity). The saturated hydraulic conductivity is determined by the amount of coarse fragments, the amount of soil cover, whether the soil is frozen, and whether crusting occurs. Thus, for example, an increase in the amount of clay decreases hydraulic conductivity and a decrease in bulk density increases water infiltration. A baseline bulk density is assumed to be proportional to the amount of sand, clay, and organic matter plus the cation- exchange capacity. Erosion in the WEPP model is predicted by interrill and rill soil erodibility terms. For croplands, the interrill erodibility term is assumed to be a function of the soil's texture and the magnesium and aluminum concentrations. Rill erodibility is estimated from the soil organic matter, cation-exchange capacity, sodium absorption ratio, aluminum concentration, and soil texture (very fine sand, clay, and sand). The critical shear (the amount of water-induced shear that initiates sediment movement) is also quantified from soil properties such as the amounts of clay and sand, the specific surface area of the soil, and a sodium absorption ratio. On rangelands, interrill erodibility is estimated from the amount of sand, silt, and organic matter and estimates of the soil's water-holding capacity. Rill erodibility is estimated from the amount of clay, organic matter, bulk density, and root mass in the soil. Critical shear is assumed to be proportional to the amount of sand, organic matter, and bulk density. In general, improvements in such soil quality attributes as organic matter and bulk density can be expected to increase infiltration, reduce runoff, and decrease soil erosion (with the exception of interrill erodibility).
MONITORING AND MANAGING SOIL QUALITY 215 This unprotected soil is cut by numerous channels caused by water runoff. The more shallow channels, rills, will be filled in by tillage; however, the deeper channels, ephemeral rills, will remain and enhance the processes of soil erosion. Credit: U.S. Department of Agriculture. Indicators of Buffering Capacity Soil quality attributes are needed to make such estimations as the desirability of soils for use in waste management. For example, in Minnesota soils, texture, pH, total organic carbon content, and cation-exchange capacity are used as indicators of the suitability of applying processed sewage sludge to the land and the application amount. Reliable measures or estimates are required for each mapping unit in the proposed area of application. In this example, texture is used as an indicator of the susceptibility of soils to leaching of
MONITORING AND MANAGING SOIL QUALITY 216 chemicals, pH is an indicator of the solubility of heavy metals and their ease of uptake by plants, and total organic carbon content and cation-exchange capacity are indicators of the capacity of the soil to absorb chemicals. Temporal and Spatial Variabilities The desired frequency of measurement of the minimum data set depends on the particular use of the data as well as on the particular attribute and soil and climatic conditions. For national-level or area assessments, the frequency of measurement may be less than that if the minimum data set is to be used to guide management systems. Temporal Variability The frequency of measurement also varies between indicators, since the rates of change of the various indicators differ. Depending on the indicator selected, rates of change can vary from less than 1 year to more than 1,000 years. Some indicators selected for inclusion in the minimum data set may need to be measured more frequently than others. The frequency of measurement may also depend on both the climate and the management system used. In temperate regions considerable periods of time may be needed to measure differences in organic matter content. On the other hand, under a slash-and-burn system in the tropics, significant changes in organic matter can occur rapidly. Alarming changes in bulk density may occur during one pass of a heavy vehicle. Arnold and colleagues (1990) categorized the fluctuations and trends apparent in soils into three groups: (1) nonsystematic or random changes; (2) regular, periodic, or cyclical changes; and (3) trend changes. Nonsystematic changes are short-term changes brought about by daily weather fluctuations or episodic human or natural disturbances. These kinds of changes are difficult to predict. Periodic or cyclical changes may be brought about by annual fluctuations in weather, crop growth periods, or soil moisture content. Monitoring of soil quality should be primarily directed toward the detection of trend changes. Trend changes show a definite tendency in a general direction over time. Such changes may include an increase or decrease in soil organic matter content, for example, and an increase or decrease in soil nutrient status. Longer-term changes might be brought about by the slow processes of soil development. Monitoring changes in soil quality for assessment of the sustainability
MONITORING AND MANAGING SOIL QUALITY 217 of current management systems and land uses requires a focus on trend changes that are measurable over a 1- to 10-year period, for example, soil water content at the permanent wilting point of plants, soil acidity, soil cation-exchange capacity, exchangeable cation content, and the ion composition of soil extracts (Larson and Pierce, 1991). The detected changes must be real, but they must change rapidly enough so that human intervention can correct problems before serious and perhaps irreversible loss of soil quality occurs. The separation of trend changes in soil quality from periodic or random changes will be a major challenge. Spatial Variability Soils and landscapes vary spatially, sometimes dramatically. This means that the choice of the basic unit for soil quality assessments is important. Three unit sizes can be considered (Larson and Pierce, 1991): (1) the local landscape unit (Van Diepen et al., 1991), (2) the soil mapping unit (Lamp, 1986), or (3) the pedon (the smallest unit or volume of soil that represents all the horizons of a soil profile) (Lamp, 1986). The local landscape unit represents a combination of topographic, climatic, and management units that typify a particular region. The soil mapping unit covers a number of hectares with soil variability that is recognized as part of the mapping unit description. The pedon may represent only a few square meters. To date most land evaluation efforts have focused on the landscape unit. The Food and Agriculture Organization of the United Nations, for example, has suggested 25 land qualitiesâsuch as radiation, temperature, nutrient availability, rooting conditions, flood hazard, soil workability, and soil degradation hazardâas factors that can be used in the evaluation of rainfed agricultural systems (Food and Agriculture Organization of the United Nations, 1983). In the United States, much effort has gone into the delineation and description of soil mapping units, which appear to be useful units of study. In many cases, mapping of these units has been completed, and soil surveys represent a wealth of information about the soil attributes characteristic of different mapping units. Spatial variation, however, can be substantial at the mapping unit scale because of natural soil development processes or human- induced variability. Bulk density, for example, can vary over short distances because of alternating rows and wheel tracks in a row crop field. Because of spatial variability, sampling at the pedon level, that is, within a few square meters may be needed in some cases to estimate variability of soil quality indicators.
MONITORING AND MANAGING SOIL QUALITY 218 EXTENT OF DEGRADATION OF U.S. SOILS A decline in soil quality results from soil degradation. Soil degradation is an outcome of human activities that deplete soil and the interaction of these activities with natural environments. The three principal types of soil degradation are physical, chemical, and biological. Each type is made up of different processes, as illustrated in Figure 5-1. Physical Degradation Physical degradation leads to a deterioration of soil properties that can have a serious impact on water infiltration and plant growth. Wind and water erosion are generally the dominant physical degradation processes, but compaction is also a widespread concern in places where heavy machinery is commonly used. Hardsetting of a cultivated soil is also a process of compaction, but it results from wetting structurally weak or unstable soil rather than from the application of an external load. Laterization is the desiccation and hardening of exposed plinthitic material (material consisting of clay and quartz with other diluents; it is rich in sesquioxides, poor in humus, and highly weathered), but lateritic soils are rare in the United States. Figure 5-1 Processes of soil degradation. Source: R. Lal and B. A. Stewart. 1990. Soil degradation: A global threat. Advances in Soil Science 11:13â17. Reprinted with permission from Â© Springer-Verlag New York.
MONITORING AND MANAGING SOIL QUALITY 219 FIGURE 5-2 Interactions of factors that cause soil degradation. Source: R. Lal and B. A. Stewart. 1990a. Need for action: Research and development priorities. Advances in Soil Science 11:331â336. Reprinted with permission from Â© Springer-Verlag New York. Soil degradation is a complex phenomenon driven by strong interactions among socioeconomic and biophysical factors (Figure 5-2). Soil degradation is fueled worldwide by increasing human populations, fragile economies, and misguided farm policies. There is also often a conflict between short-term benefits and long-term consequences. An example in the United States is the difficulty of developing sustainable agricultural systems. The physical and chemical attributes of a soil can be protected from degradation by a number of general types of management practices.
MONITORING AND MANAGING SOIL QUALITY 220 The Badlands of South Dakota is an example of the more extreme long-term effects of erosion by water and wind. The area is primarily rocky spires, buttes, and gorges embedded with the petrified remains of prehistoric camels, three- toed horses, and saber-toothed tigers. Credit: The South Dakota Department of Tourism. These include crop rotation, fertilizer and lime applications, tillage, residue management, strip-cropping, and mechanical practices. One or several of these practices used in various combinations is usually needed. The choice of practices will depend on the soil, landscape, crop sequence, climate, and social conditions. Erosion Soil erosion is a natural phenomenon that has occurred since Earth was formed. Erosion by water and wind has helped shape the landscapes that people know today. Quantitative studies of the amount of erosion that occurred during periods of geologic and historic time show that the rate is highly variable in space and time. This variability can be
MONITORING AND MANAGING SOIL QUALITY 221 caused by external factors, such as changes in climate and vegetation, or by internal factors that result in episodic erosion. Usually, however, erosion rates under most current farming systems are much greater than they were before farming began and are greater than those in uncultivated areas. According to a review by Franzmeier (1990), erosion rates in the central United States before European settlement varied from 0.02 to 11 metric tons/ha/year (0.009 to 5 tons/ acre/year), but postsettlement rates have varied from 7 to 86 metric tons/ha/year (3 to 38 tons/acre/year). Erosion represents the major agent of soil degradation worldwide, although the amounts of erosion and the damage that it causes are difficult to quantitate (Dudal, 1982; Lal, 1990). Although damage from erosion was recognized as a serious threat to agriculture in U.S. colonial times, rapid expansion of agriculture to new lands west of the Allegheny Mountains lessened the need for careful husbandry of soil resources. Sporadic reports of the effects of soil erosion in the United States occurred in the literature in the early part of the twentieth century (McDonald, 1941), but no concerted national effort for control occurred until the 1930s when the Soil Erosion Service of the USDA was founded. The Soil Erosion Service was quickly reorganized into the Soil Conservation Service and was then expanded. Damage from water erosion on croplands, although widespread, generally increases as the slope (steepness and length) of the land increases. Other factors that affect the rate of erosion are rainfall amount and intensity, the nature of the soil, the cropping and tillage practices used, and mechanical farming practices such as terracing and contouring. Erosion Estimates Many estimates of the amounts of erosion and the consequences of erosion have been made since the formation of the Soil Conservation Service. The early estimates were made on the basis of a minimum of quantitative data (U.S. Department of Agriculture, 1938). The most comprehensive analysis of the amounts of wind and water erosion was contained in the 1982 National Resources Inventory published in the Second Resource Conservation Act Appraisal (U.S. Department of Agriculture, Soil Conservation Service, 1989a), which was in response to the Resource Conservation Act passed by Congress in 1977. The 1977 National Resources Inventory concentrated on the amounts of erosion from water and comparison of the amounts of erosion with the established soil loss tolerance values. Erosion was estimated by statistically establishing primary sampling units, identifying the appropriate
MONITORING AND MANAGING SOIL QUALITY 222 coefficients for the universal soil loss equation, and solving the equation to estimate the amount of soil eroded. The calculated erosion amounts represent movement of soil material from the sampled point. The computed amounts of erosion tell nothing about the eventual deposition of the eroded sediment. The sediment may be moved a few meters to other places in the field or to nearby riparian areas or wetlands or it may be deposited in streams. The National Resources Inventory estimates of 1982 were made for sheet and rill erosion by water as well as wind erosion and did not include ephemeral, gully, or stream bank erosion, all of which are significant sources of sediment. In the Soil Conservation Service's second appraisal of the Resource Conservation Act (U.S. Department of Agriculture, Soil Conservation Service, 1989a), the average amount of soil lost through sheet and rill erosion on all cropland (170 million ha, or 421 million acres) was 9.8 metric tons/ha/year (4.4 tons/acre/year) and the average amount lost through wind erosion was 6.7 metric tons/ha/year (3.0 tons/acre/year). Areas within the United States with particularly serious erosion include the Palouse (Washington, Oregon, Idaho), southeastern Idaho, the Texas Blackland Prairie, the southern Mississippi Valley, the Corn Belt, and Aroostock County, Maine. Nationwide in 1982, 40 percent of cropland was eroding at rates higher than the soil loss tolerance level, and 20 percent was eroding at rates higher than twice the soil loss tolerance level. Nearly 9 percent of pasturelands and 18 percent of rangelands were eroding excessively. About 43 million ha (106 million acres) of cropland was considered highly erodible on the basis of the results of the 1982 National Resources Inventory. Wind erosion is a severe problem in the western half of the United States and in certain sandy areas in the eastern part of the country. The National Resources Inventory estimated that wind erosion for all croplands was 6.7 metric tons/ha/year (3.0 tons/acre/year). However, in Texas, average wind erosion amounts were 29.3 metric tons/ha/year (13.1 tons/acre/year); average wind erosion rates in Colorado, Nevada, and Montana were 20.8 (9.3), 20.6 (9.2), and 18.6 metric tons/ha/year (8.3 tons/acre/year), respectively. Wind erosion is greatest in the Great Plains and Mountain States. Effect of Erosion on Soil Quality Sheet and rill erosion significantly reduced the productivity indices of Pierce and colleagues (1983) on most of the 75 soils selected from the north- central United States after 25 and 50 cm (10 and 20 inches) of soil
MONITORING AND MANAGING SOIL QUALITY 223 was removed from the surface by erosion. Fifty percent of the soils exhibited a reduction in the productivity index of more than 0.1; 32 percent of the soil exhibited a reduction of more than 0.2; and 16 percent exhibited a reduction of more than 0.3 when 50 cm (20 inches) of the soil was eroded (Larson et al., 1985). In the productivity index model, the productivity indices range from 0 to 1.0, with 1.0 being the most productive soil. Unprotected soil is vulnerable to wind erosion. Here sandy soil is blown over and between rows of crops. Credit: U.S. Department of Agriculture. The lack of direct measurements of soil attributes that can be linked to changes in soil quality make assessments of soil quality degradation caused by erosion difficult. Some analyses are available for some sections of the United States and are suggestive of the effects of current erosion rates on soil quality. Larson and colleagues (1972) added a variety of organic residues at various rates to a Typic Hapludoll in Iowa for 11 years while cropping the soil to corn (Zea mays L.) by moldboard plowing. At the end of the 11 years, the organic carbon content varied linearly with the amount of residues added. Rasmussen and Collins (1991) have shown a similar
MONITORING AND MANAGING SOIL QUALITY 224 The effects of sheet erosion are subtle and difficult to recognize. The sheeting action of the runoff has deposited thin layers of eroded soil in the foreground and in the background (note the darker bands across the rows of crops). Even though difficult to recognize, sheet erosion can be quite damaging. Credit: U.S. Department of Agriculture. linear relationship for the Palouse region of Oregon and Washington. From these linear equations, they calculated the annual amount of residue that was required to be returned to the soil to maintain the organic content at the level present at the start of the experiment. The linear relationships developed by Larson and colleagues (1972) and Rasmussen and Collins (1991) were used to estimate the changes in organic matter content presented in Table 5-4. They also calculated the average amount of organic carbon in the residues produced from corn (Zea mays L.) in major land resource area 107 (Iowa) and wheat (Triticum aestivum L.) in major land resource area 9 (Oregon, Washington), assuming a harvest ratio of 1.0 for corn and 1.5 for wheat (Iridium aestivum L.). Because changes in the amount of organic carbon in the soil are difficult to measure, the ratio between the amount of organic carbon in crop residues and the amount needed to maintain organic matter in the soil is a useful index of whether organic carbon is increasing or decreasing in a given major land resource area. For the four areas described in Table 5-4, the amount of residues returned to
MONITORING AND MANAGING SOIL QUALITY 225 the soil either exceeded or nearly equaled the amount required to maintain organic matter levels in the soil, a key indicator of trends in soil quality. TABLE 5-4 Organic Carbon Additions Necessary to Maintain Soil Organic Carbon at Present Levels at Several Locations Amount of Organic Carbon (kg/ha/year) Location Crop To Maintain Produced Harvest Rotationa Soil Organic from Crops Ratio Carbon Shenandoah, Iowa C-C 3,272 3,020 0.9 Pendleton, W-F 2,288 1,764 0.8 Oregon Pullman, W-F 1,933 1,764 0.9 Washington Pullman, W-W 780 3,528 3.9 Washington a C, corn; W, wheat; F, fallow. SOURCE: Adapted from W.E. Larson and B.A. Stewart. 1992. Thresholds for soil removal for maintaining cropland productivity. Pp. 6â14 in Proceedings of the Soil Quality Standards Symposium, San Antonio, Texas, October 23, 1990. Washington, D.C.: U.S. Department of Agriculture, Forest Service. Erosion, however, removes organic carbon along with sediments. Table 5-5 provides estimates of the organic carbon needed in crop residues to replace organic carbon lost from soil as a result of different average erosion rates and different slope classes. The data in Table 5-5 indicate that at any erosion rate exceeding 5 metric tons/ha/year (2.2 tons/acre/year), the amount of residues returned to the soil from corn production is not enough to prevent declines in organic carbon content. On lands with steeper slopes, the amount of residuesrequired far exceeds that produced by corn or other grain crops. Hence, on lands with steeper slopes where erosion is severe, the organic carbon content will decline to low levels. Since the organic carbon content is an important indicator of soil quality, the analyses presented above suggest that current rates of erosion may have significant effects on long-term soil quality. Soil erosion influences most of the soil attributes that determine soil quality. Eroded sediments usually contain higher amounts of plant nutrients than do bulk soils, thus degrading the soil of the important attributes of nitrogen, phosphorus, potassium, and total organic carbon (Barrows and Kilmer, 1963; Young et al., 1985). Erosion can also bring soil horizons closer to the surface of the soil profile. These horizons usually have low pHs, low available water-holding capacities, and high bulk densities and can thus influence soil quality. Using
MONITORING AND MANAGING SOIL QUALITY 226 the productivity index model of Pierce and colleagues (1983), Larson and colleagues (1985) calculated which of the four soil attributes in the subsoilâ available water-holding capacity, bulk density, pH, or rooting depthâwould 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. 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 Slope (percent) Average In sedimentc Needed in hectares) Erosion Residue (metric tons/ ha/year)b 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. 6â14 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. 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.
MONITORING AND MANAGING SOIL QUALITY 227 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
MONITORING AND MANAGING SOIL QUALITY 228 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.
MONITORING AND MANAGING SOIL QUALITY 229 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.
MONITORING AND MANAGING SOIL QUALITY 230 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
MONITORING AND MANAGING SOIL QUALITY 231 sodium is the prevalent cation, the soils are generally classified as sodic. TABLE 5-6 Extent of Salinity and Associated Problems by Land Use in California Millions of Hectares Primary Land Use Nonfederal Land Saline or High Water Water Area Sodic Soilsa Tableb 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. 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
MONITORING AND MANAGING SOIL QUALITY 232 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). TABLE 5-7 Salinity and Drainage Problems by Major Irrigated Areas (approximate area) Millions of Hectares Location Irrigated Area Saline or High Water Water Quality Sodic Soila Tableb 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. 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.
MONITORING AND MANAGING SOIL QUALITY 233 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.
MONITORING AND MANAGING SOIL QUALITY 234 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.
MONITORING AND MANAGING SOIL QUALITY 235 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):16â18. Reprinted with permission from Â© 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
MONITORING AND MANAGING SOIL QUALITY 236 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.