Soil and Water Quality An Agenda for Agriculture (1993) / Chapter Skim
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5 Monitoring and Managing Soil Quality
5 Monitoring and Managing Soil Quality
Pages 189-236
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From page 189... ...
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.
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From page 190... ...
stressed the need to expand the notion of soil quality beyond soil productivity to include the role of the soil as an environmental filter affecting both air and water quality. They suggested that soil quality has important effects on the nutritional quality of the food
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From page 191... ...
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.
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From page 192... ...
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)
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From page 193... ...
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.
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From page 195... ...
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.
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From page 196... ...
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.
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From page 197... ...
..x :s. :s, s,<; Soil degradation leads directly to water pollution by sediments and attached agricultural chemicals from eroded fields.
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From page 198... ...
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.
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From page 199... ...
Loss of organic matter because of erosion or poor cropping practices can seriously impede the soil's ability to filter out potential pollutants. In the past, soil erosion was used as a convenient proxy for all of the processes of soil degradation, and efforts to control erosion have
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From page 200... ...
In the long term, however, all processes of soil degradation-compaction, salinization, acidification, loss of biological activity, pollution, and erosion 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.
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From page 201... ...
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.
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From page 202... ...
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.
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From page 203... ...
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.
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From page 204... ...
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.
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From page 205... ...
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.
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From page 206... ...
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.
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From page 207... ...
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.
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From page 208... ...
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~.
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From page 209... ...
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.
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From page 210... ...
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~.
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From page 211... ...
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)
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From page 212... ...
RD = f(Db, WHC, pH) Breeusma et al., 1986 Breeusma et al., 1986 Larson and Stewart, 1992 Bouma, 1989 Gupta and Larson, 1979 Manrique and Jones, 1991 Gupta and Larson, 1979 Bouma, 1989 Allmaras et al., 1967 Allmaras et al., 1967 Childs and Collis-George, 1950; Marshall, 1958; Millington and Quirk, 1961 Gupta et al., 1991 McKeague et al., 1982 Pierce et al., 1983; Kinily et al., 1983 Pierce et al., 1983 NOTE: Vanables 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; se, sand; cl, day; Sy, 1/Db; RR, random roughness; P
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From page 213... ...
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)
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From page 214... ...
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)
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From page 215... ...
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.
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From page 216... ...
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.
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From page 217... ...
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 qualities-such as radiation, temperature, nutrient availability, rooting conditions, flood hazard, soil workability, and soil degradation hazard as factors that can be used in the evaluation of rain-fed agricultural systems (Food and Agriculture Organization of the United Nations, 1983~.
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From page 218... ...
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.
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From page 219... ...
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.
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From page 220... ...
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.
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From page 221... ...
. 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~.
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From page 222... ...
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From page 223... ...
At the end of the 11 years, the organic carbon content varied linearly with the amount of residues added. Rasmussen and Collins (1991)
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From page 224... ...
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From page 225... ...
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~.
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From page 226... ...
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.
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From page 227... ...
that relates to soil degradation. growth responses to surface layer compaction by a variety ot crops (Draycott et al., 1970; Fausey and Dylla, 1984; Johnson et al., 1990; Van-Loon et al., 1985; Voorhees et al., 1990~.
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From page 228... ...
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)
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From page 229... ...
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)
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From page 230... ...
Chemucal 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.
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From page 231... ...
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~.
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From page 232... ...
SOURCE: U.S. Department of Agriculture, Soil Conservation Service.
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From page 233... ...
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.
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From page 234... ...
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.
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From page 235... ...
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 reduc
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From page 236... ...
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.
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