USDA-ARS, National Soil Tilth Laboratory

2150 Pammel Dr., Ames, IA 50011-4420

1. What is Soil Quality?

Soil quality has recently been defined as the "capacity of a specific kind of soil to function within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and

habitation" (Karlen et al., 1997). This was a slight modification of the original which defined soil quality as "the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health"(Doran and Parkin, 1994). For the farmer, crop consultant, or agronomist working to develop and recommend management decisions for crop production, both definitions can be simplified to "how is the soil functioning" on a farm or within a particular field. Differences among these three definitions primarily reflect the scale or type of problems which may be of concern. For farmers, the primary issue is usually how to produce an economically viable yield in an environmentally safe manner. Others may be evaluating soil quality with regard to its ability to support forests, recycle municipal biosolids, or provide a stable and resilient rangeland for cattle or sheep production.

It is also important to note that "soil quality" and "quality soil" are not two ways of stating the same idea. The difference between the two terms is that the quality of a soil primarily reflects the natural or inherent characteristics of a soil or the properties determined

by the parent material, climate, time, topography, and vegetation under which it developed. Soil quality, however, describes the status or condition of a soil as a result of current and past land use or management decisions. Thus, a high quality soil that has been poorly managed could be degraded to where it has poor quality (Fig. 1). On the other hand, a poor soil (Fig.2. Soil B) will never have the same quality as a good soil (Fig. 2, Soil A). By using the best possible soil management practices, either soil could be aggraded until it has the highest possible quality that local climate and original parent material will support.

Figure 1. Potential changes in soil quality as a result of management decisions.

Figure 2. Differences in natural or inherent quality for two soils.

 

2. How Did the Concept Evolve?

1. How Do I Measure Soil Quality?

The capacity to support the various function is determined by complex interactions among biological, chemical, and physical properties of every soil. Therefore, soil quality cannot be measured directly, but must be inferred by measuring chances in various soil

attributes or attributes of the ecosystem (Seybold et al., 1997). These "indicators" of soil quality should (1) encompass ecosystem processes and relate to process oriented modeling, (2) integrate soil physical, chemical, and biological properties and processes, (3) be accessible to many users and applicable to field conditions, (4) be sensitive to variations in management and climate, and where possible, (5) be components of existing soil data bases (Doran and Parkin, 1994). They should also be easily measured and reproducible (Gregorich et al., 1994) and sensitive enough to detect changes in the soil resource as a result of anthropogenic (human induced) degradation (Arshad and Coen, 1992).

It would be impossible to use all ecosystem or soil attributes as indicators. Therefore, a minimum data set (MDS) consisting of a core set of attributes encompassing chemical, physical, and biological soil properties has been suggested for soil quality assessment (Larson and Pierce, 1991). Several slightly different MDS have been proposed (Arshad and Coen, 1992; Doran and Parkin, 1994; Gregorich et al., 1994; Larson and Pierce, 1994; Papendick et al., 1995). One example (Table 1) is that suggested by Seybold et al. (1997). It is also important to note that the specific suite of indicators used for assessing soil quality can vary from location to location depending on the kind of land that is being evaluated (e.g. rangeland, wetland, agricultural land). Land use, soil function, and the soil forming factors that may be involved are also important considerations (Arshad and Coen, 1992; Hellkamp et al., 1995).

 

 

 

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Table 1. Physical, chemical, and biological indicators for screening the quality or health of soils ˜

Indicators Relationship to soil condition and function: rationale as a priority measurement

Physical

Texture Retention and transport of water and chemicals; model use, soil erosion and variability estimate

Depth of soil and Estimate of productivity potential and erosion; normalizes landscape and geographic variability

Rooting

Infiltration and Potential for leaching, productivity, and erosivity; bulk density (SBD) needed to adjust analyses to

Bulk density volumetric basis

Water holding Related to water retention, transport, and erosivity; available water; calculate from SBD, texture, and OM

Capacity

Chemical

Soil organic matter Defines soil fertility, stability, and erosion extent; use in process models and for site normalization

(OM)

pH Defines biological and chemical activity thresholds; essential for process modeling

Electrical Defines plant and microbial activity thresholds; presently lacking in conductivity and in most process Conductivity models

Extractable N, P, Plant available nutrients and potential for N loss; productivity and environmental quality indicators

And K

Biological

Microbial biomass Microbial catalytic potential and repository for C and N, modeling; early warning of poor management

C and N effects on soil organic matter

Potentially Soil productivity and N supplying potential, mineralizable N; process modeling (surrogate indicator for mineralizable N. biomass)

Soil respiration Microbial activity measure (in some cases plants); process modeling; estimate of biomass activity

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˜ After Seybold et al. (1997)

One indicator which has been included in every MDS suggested for evaluating soil quality (Gregorich et al., 1994) is soil organic matter (SOM). Several different fractions, such as microbial biomass, water-soluable organic matter, particulate organic matter, and humas or stabilized organic matter, have been included in the various MDS. SOM is one of the more useful indicators of soil quality, because it interacts with numerous soil components. It affects water retention (Hudson, 1994), aggregate formation, bulk density, pH, buffer capacity, cation exchange, nutrient mineralization, pesticide and agricultural chemical sorption, color (warming effect and spectral environment for plant seedlings (Kasperbauer and Hunt, 1987)], infiltration, aeration, and activity of soil organisms (Schnitzer, 1991).

Stevenson (1994) stressed that it was not SOM acting alone, but rather the interaction of various components which determined soil quality. In addition to the amount of SOM, its quality may also be an important indicator of soil quality. For example, after 90 years of cropping, Schjonning et al. (1994) found that organic matter present in manured plots differed in quality from that in plots which had been fertilized with inorganic materials.

Descriptive indicators, which are inherently qualitative, have also been used for assessing soil quality. Arshad and Coen (1992) listed soil crusting or surface sealing, rills, gullies, ripple marks, sand dunes, salt crusts, and standing or ponded water as observational or morphological indicators of soil quality. Romig (1995) reported that farmers often describe the quality of their soil using descriptive terms such as loose, soft, crumbly, loamy, earthy smelling, darkly colored, massive, lumpy, dense, and so on. They also found that farmers tend to rely more heavily on what their senses tell them about soils quality and how that relates to ease of tillage and crop productivity than any specific soil measurement or plant response.

Crop yield, plant vigor, and rooting patterns are among the plant responses that have been used to assess soil quality (Parr et al., 1992, Acton and Gregorich, 1995). Yield is often an important indicator because it gives information about the interacting soil properties of the systems as a whole. However, it is important to have good weather records in addition to the information about soil properties when assessing soil quality based primarily on yield. With regard to ecosystem assessment of soil quality, processes involving cycling of carbon and nitrogen from the SOM, nutrient retention, organic matter decomposition, soil respiration, and microbial biomass carbon are among the indicators most frequently included in MDS used at that scale.

 

2. How Does Scale Affect Soil Quality?

Scale influences soil quality assessment in both time and space. Areas of consideration can be as small as a point on the landscape, research plot, field, farm, watershed, region, or as large as a nation or the world. Time frames are important because of the effects that climate, soil moisture, human actions, plant growth, and other factors can have on the temporal variability of various indicators (Seybold et al., 1997).

Two approaches are being utilized to deal with spatial scale. These are: (1) selecting indicators to match the geographic scale for which a soil quality assessment is being made; or (2) expanding point scale indicator information to larger areas of consideration. Karlen et al. (1997a) discussed selecting general soil quality indicators tailored to the scale of assessment. Their approach is similar to that of Hole and Campbell (1985) for scaling maps based on taxonomic generalization. Both authors caution, however, that evaluations at the farm, watershed, and regional scales will become progressively less precise because of fewer actual measurements and a greater dependence on simulation models, remote sensing, and existing databases to estimate measurements. Loss of precision in assessment is one of the major disadvantages of this approach. For example, if productivity becomes unstable, assessments of the reasons for instability are difficult, if not impossible, to make without additional data collection. The main advantage of this approach is thin economy of resources needed to collect the data (Seybold et al., 1997). Table 2 presents a list of possible indicators for soilquality assessments at various scales.

Point scale indicators provide the most detail, but are also the most expensive and time consuming to gather and interpret. Therefore, it becomes necessary to develop methods to generalize or relate information collected at the point scale to the level for which soil quality assessments are being made. At field or farm scales, his usually involves geostatistical methods (Smith et al., 1993, 1994, Burrough, 1989; Bauma, 1988) or landscape models (Pennock et al., 1994). At the regional and national scales, a longitudinal survey of soil, water, and related resources to assess conditions and trends at various time intervals within the U.S. has been employed by the National Resource Inventory (NRI) of the Natural Resources Conservation Service. The Environmental Protection Agency used a similar sampling approach in the Environmental Monitoring and Assessment Program (Hellkamp et al., 1995).

Table 2. Potential indicators of soil quality at different scales'

Biological	Chemical	Physical
Field, Farm, or Watershed Indicators
Crop Yield	Changing quality/quantity of organic matter	Topsoil thickness
Crop appearance	pH changes	Soil color
Weed pressure	Available P & K changes	Subsoil exposure
Nutrient deficiencies	Change in cation levels	Compaction
Earthworms	Change in N availability	Crusting
Root growth patterns	Change in heavy metals	Infiltration
Biomass production	Change in salinity	Runoff
Canopy cover	Nutrient loss to streams and groundwater	Sediment fans
	Pesticide loss to streams and groundwater	Surface cover
		Rill and gully erosion
		Plant emergence
		Ease of tillage
		Soil structure
Regional or National Indicators
Productivity (yield stability)	Organic matter trends	Desertification
Taxonomic diversity at the group level	Acidification	Vegetation cover
Species richness/diversity	Salinization	Water erosion
Keystone species and ecosystem engineers	Change in water quality	Wind erosion
Biomass density and abundance	Changes in air quality	Siltation of rivers and lakes
		Sediment load in rivers

3. *after Karlen et al., (1998)
5. How Can Soil Quality Be Evaluated?

One method for using the information collected as part of any MDS is to compute a soil quality index. Such an index could be used to monitor and predict the effects of farming systems and management practices on soil quality or to provide an early warning of soil degradation (Parr et al., 1992). Several types of indexing procedures are currently being evaluated throughout the U.S. and around the world. Romig et al. (1995, 1997) developed a soil quality scorecard for Wisconsin that is farmer-based and uses subjective ratings to place 43 different indicators into rating scales of healthy, impaired, or unhealthy. Currently, the scorecard gives equal priority to all indicators which are based almost exclusively on sensory observations (e.g. look, feel, smell). The approach is qualitative and rather subjective, but it is easy to use, and therefore may be more informative to the land manager than some more sophisticated approaches. Efforts to modify the scorecard for other regions and farming systems are being coordinated by members of the NRS Soil Quality Institute.

Several quantitative approaches for assessing soil quality have also been proposed. Larson and Pierce (1991) suggested quantifying soil quality, by expressing soil quality (Q) as a mathematical function of measurable soil attributes which they referred to as soil qualities (q₁). Their overall assessment for attributes l to n can be described by equation 1,

Q = f(q₁ ..... q_n) [l]

with changes in soil quality, over time described by the relationship (dQ/dt). Singh et al. (1992) developed a tilth index based on five soil physical properties or indicators - bulk density, cone index, aggregate size uniformity coefficient, organic matter, and plastic index. A normalized tilth coefficient is computed for each indicator and represented by a second order polynomial. The index is currently a multiplicative combination of the tilth coefficients, but it could be expanded to include other soil chemical and physical attributes as part of an overall soil quality index. Smith et al. (1993) used multiple indicator kriging to integrate an unlimited number of spatially measured soil quality indicators into an overall soil quality index. They then used their procedure to evaluate soil quality across landscapes. Karlen and Stott (1994) developed a framework for quantifying soil quality using multi-objective analysis principles of systems engineering. They defined critical soil functions and potential chemical and physical indicators for assessing those functions. For each indicator, a scoring function and realistic baseline and threshold values are established. The scoring function uses similar mathematical principles as assigning membership in a fuzzy set (Burrough, 1989). All indicators affecting a particular soil function are grouped together and assigned a relative weight based on importance. After scoring each indicator with appropriate scoring functions (Fig. 3), the unitless value is multiplied by the appropriate weight, and an overall soil quality rating is calculated by summing the weighted score for each soil function. This indexing approach was tested for two long -term studies which focused on crop residue and tillage effects. The results showed that no-till management had an improved soil quality rating over moldboard plow and chisel plow treatments (Karlen et al., 1994b), and that doubling crop residue improved soil quality ratings compared to leaving the crop residue or removing it (Karlen et al., 1994a). The same approach was also used to evaluate the effects of various post-CRP (Conservation Reserve Program) management strategies (Karlen et al., 1998).

Figure 3. typical scoring functions used to normalize soil quality indicator data prior to combining the information into an overall soil quality index.

 

6. How Can I Improve Soil Quality?

For a group dedicated to no-till agriculture, the simplest and easiest answer to this question would be to say "adopt no-till technology and soil quality will improve". Although generally correct, no single soil and crop management strategy can be recommended to automatically sustain or improve soil quality (Karlen et al., 1992). This qualification is important because soils and the five basic factors which create them [parent material, climate, topography, vegetation, and time (Jenny, 1941)] are always different. However, management practices which preserve and/or increase soil organic matter (such as no-till) should be encouraged, tailored to local conditions, and adopted whenever possible. As previously stated, SOM is an important indicator of soil quality, because it influences water retention, aggregation, bulk density, pH, buffer capacity, cation exchange, nutrient mineralization, pesticide and agricultural chemical sorption, infiltration, aeration, and activity of soil organisms. SOM is the component that showed the greatest decline when virgin prairie was broken for cultivation (Melsted, 1954; Bauer and Black, 1981). It also declines more rapidly with cropping systems that include fallow periods than with continuous cropping (Unger,1982). Boyle et al. (1989) summarized the importance of SOM by concluding that returning carbon to the soil is "a necessary expense that insures a sustainable harvest".

Reduced or conservation tillage practices that are tailored to local soil and climatic conditions may be one of the best strategies for improving soil quality though increased soil biological activity and organic matter content (Karlen et al., 1992). Conservation tillage can significantly reduce soil erosion (Moldenhauer and Wiscchmeirer, 1960, Taylor et al., 1964) and improve water use efficiency by increasing infiltration and decreasing evaporation (Smika and Wicks, 1968; Unger and Phillips, 1973, Smika md Unger, 1986). A major limitation to conservation tillage in semiarid regions is low production of crop residue by non-irrigated crops. The lack of surface cover becomes even worse when crop residues are removed for feed or other purposes (Unger et al., 1990) and are not available to protect the soil against erosion, to improve water conservation, or to provide the carbon necessary for microbiological activity.

Kanwar et al. (1997) concluded that ridge-till and no-till practices preserved a better macropore network than chisel plow or moldboard plow practices. Since environmental impact is also an indicator of how a soil is functioning, this suggests that chemical management practices need to be evaluated in conjunction with tillage practices. In other words, for research to be most useful, a systems approach should always be encouraged.

Increasing temporal and spatial diversity by using different crop rotations may improve soil quality by mimicking natural ecosystems. For example, Peterson et al. (1996) demonstrated that by using more intensive cropping systems instead of the traditional wheat-fallow rotation, precipitation use efficiency could be increased in the Great Plains. They concluded that decreased tillage and maintaining crop residues on the soil surface were the prerequisites for improving water use efficiency. Use of a narrow strip-crop rotation was also demonstrated to be an effective management practice for decreasing nitrate-N concentrations in tile drainage water to below the 10 mg/L drinking water standard (Kanwar et al., 1997). Finally, crop rotation generally increases yield (Karlen et al., 1994c), thus providing an even more direct incentive for adoption than improving soil quality.

 

7. SUMMARY AND CONCLUSIONS

This paper discusses the concept of. soil quality, which can be defined simply as "how a soil is functioning". The concept evolved from efforts focused on sustainable agriculture and sustainable land management. It also reflects an increased awareness that the condition of our soil resources has a direct effect on our air, water, wildlife, and human resources.

Quantitative and qualitative methods for assessing soil quality are discussed. These include the use of various indicators as tools for making those assessments and techniques for combining the information into soil quality indexes. Use of no-till or reduced tillage and crop rotation are recommended as management practices that can be used to sustain or improve soil quality. The specific soil and crop management practices that are needed to sustain or improve soil quality will differ from region to region, but in general, practices that preserve and increase soil organic matter will be the most effective for sustaining or improving soil quality.

 

8. REFERENCES