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Soil Quality





What is soil quality?
Soil quality is how well soil does what we want it to do. More specifically, soil quality is 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.

What does soil do?
Healthy soil gives us clean air and water, bountiful crops and forests, productive rangeland, diverse wildlife, and beautiful landscapes. Soil does all this by performing five essential functions:
  • Regulating water. Soil helps control where rain, snowmelt, and irrigation water goes. Water and dissolved solutes flow over the land or into and through the soil.
  • Sustaining plant and animal life. The diversity and productivity of living things depend on soil.
  • Filtering potential pollutants. The minerals and microbes in soil are responsible for filtering, buffering, degrading, immobilizing, and detoxifying organic and inorganic materials, including industrial and municipal by-products and atmospheric deposits.
  • Cycling nutrients. Carbon, nitrogen, phosphorus, and many other nutrients are stored, transformed, and cycled through soil.
  • Supporting structures. Buildings need stable soil for support, and archeological treasures associated with human habitation are protected in soils.
Soil has both inherent and dynamic qualities.
Inherent soil quality is a soil's natural ability to function. For example, sandy soil drains faster than clayey soil. Deep soil has more room for roots than soils with bedrock near the surface. These characteristics do not change easily.

Dynamic soil quality is how soil changes depending on how it is managed. Management choices affect the amount of soil organic matter, soil structure, soil depth, and water- and nutrient-holding capacity. One goal of soil quality research is to learn how to manage soil in a way that improves soil function. Soils respond differently to management depending on the inherent properties of the soil and the surrounding landscape.

Soil quality is linked to sustainability.
Understanding soil quality means assessing and managing soil so that it functions optimally now and is not degraded for future use. By monitoring changes in soil quality, a land manager can determine if a set of practices is sustainable.

Assessing soil quality.
Soil quality is an assessment of how well soil performs all of its functions. It cannot be determined by measuring only crop yield, water quality, or any other single outcome. The quality of a soil is an assessment of how it performs all of its functions now and how those functions are being preserved for future use.

Soil quality cannot be measured directly, so we evaluate indicators. Indicators are measurable properties of soil or plants that provide clues about how well the soil can function. Indicators can be physical, chemical, and biological characteristics. Useful indicators:
  • are easy to measure
  • measure changes in soil functions
  • encompass chemical, biological, and physical properties
  • are accessible to many users and applicable to field conditions
  • are sensitive to variations in climate and management.
Indicators can be assessed by qualitative or quantitative techniques. After measurements are collected, they can be evaluated by looking for patterns and comparing results to measurements taken at a different time or field. Here are some examples of indicators of soil quality:

Indicator Relationship to Soil Health
Soil organic matter (SOM) Soil fertility, structure, stability, nutrient retention; soil erosion.
PHYSICAL: Soil structure, Depth of soil, Infiltration and bulk density; Water-holding capacity Retention and transport of water and nutrients; habitat for microbes; estimate of crop productivity potential; compaction, plow pan, water movement; porosity; workability.
CHEMICAL: ph; Electrical conductivity; extractable N-P-K Biological and chemical activity thresholds; Plant and microbial activity thresholds; Plant available nutrients and potential for N and P loss.
BIOLOGICAL: Microbial biomass C and N; Potentially mineralizable N; Soil respiration. Biological and chemical activity thresholds; Plant and microbial activity thresholds; Plant available nutrients and potential for N and P loss


For more information on soil quality, review the following Web pages: http://urbanext.illinois.edu/soil/sq_info/sq_intro.pdf or http://soils.usda.gov/sqi
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