Soil Quality

RCA Issue Brief #5
November 1995

What are the functions of soils?
What is soil quality?
What factors reduce soil quality?
How can we improve soil quality?
Future approaches to maintaining soil quality?
How does soil quality benefit us?

Did you know ...

...that a spoonful of soil contains more micro-organisms than there are people on earth? And that many of these micro-organisms are key indicators of soil quality?

...that virtually all fresh water falls on soils and travels over them, percolates through them, evaporates from them, is stored in them, or interacts with them to drive several chemical, physical, and biological processes?

...that soil quality is a moving target, depending on soil use? But that in general, soil quality is an assessment of the fitness of a soil to perform certain functions such as supporting plant growth, partitioning and holding water, and acting as an environmental buffer?

...that soil quality directly affects water quality, biodiversity, wildlife habitat, plant growth, and crop production? And that the key determinant of whether soil quality is improved, restored, sustained, or degraded is management?

...that even though fewer acres of land are farmed today than in the 1930's, food and fiber production is far greater and more land is used for timber and wood products?

What are the functions of soils?

Soils are the loose, transformed mineral and organic materials at the earth's surface that are capable of supporting plant growth. Soils develop specific characteristics over long periods of time through complex interactions among climate, biological processes, parent materials (including rocks), and topographic (landscape) factors. These processes, over the course of thousands of years, give rise to unique kinds of soils called soil series. There are more than 17,000 soil series in the United States, each of which has specific qualities and reacts differently to use. Soils are living, dynamic systems that are altered by changes in water content, temperature, and human activities.

Some soils, because of their physical and chemical properties and the climate where they occur, have naturally high capacity for growing certain crops, while other soils are better suited to other crops, trees, or other uses. Soils sustain biological activity such as plant growth and microbial activity; regulate and partition the flow of water through the landscape; filter, transform, immobilize, buffer, and degrade organic and inorganic materials such as municipal and animal wastes; store and cycle nutrients and other elements such as carbon dioxide; and support buildings and protect archeological treasures.

These soil functions are universal; however, the importance of each function depends on the kind of ecosystem or land use. For example, nutrient cycling and storage for plant growth is extremely important everywhere, but is critical where the natural fertility of soils is low, as in humid tropical forest systems. The partitioning and storage of water is critical in ecosystems where rainfall is limited.

If a soil performs its functions well, air quality is higher because wind erosion is lower and water quality is protected as the soils filter or neutralize harmful chemicals or toxins.

What is soil quality?

Soil quality is the capacity of a soil to function for specific land uses or within ecosystem boundaries. This capacity is an inherent characteristic of a soil and varies from soil to soil. Such indicators as organic-matter content, salinity, tilth, compaction, available nutrients, and rooting depth help measure the health or condition of the soil-its quality-in any given place.

For example, organic-matter content, biological activity, acidity, and salinity are related to the ability of a soil to store and cycle nutrients for plant growth. Soil tilth, compaction, and available water capacity reflect the ability of a soil to regulate and partition the flow of water. Texture, such as loam or clay, is an important soil property in the support of buildings and roads. An enhancement of soil health or quality could be measured by an increase in organic matter content in cultivated soils over the years, which would reflect the soil's ability to cycle nutrients.

If, through human use, the ability of a soil to perform certain functions is maintained or enhanced, its quality contributes to sustain-ability. However, if the ability of a soil to perform beneficial functions has been impaired, the quality or condition of that soil has been degraded and that land use detracts from the sustainability of the soil resource.

What factors reduce soil quality?

Every year in every country, soil resources are impaired and in some cases lost for productive use because of misuse, application of toxic materials, or poor land management systems. Why does this happen?

• Lack of knowledge about individual soils and their properties as they relate to soil quality and land use.
• Lack of understanding of the impact of management activities on individual soils and the ability of the soil to maintain beneficial functions.
• Excessive human demands on available soil resources, thus forcing use of more fragile or erodible lands for production of food and fiber.

A major cause of reduced soil quality is soil erosion, the removal of the topsoil. Although soil erosion is a natural geologic process, it is often accelerated by cultivation and resource development to meet human needs. However, at least 40 percent of United States erosion losses result from nonagricultural activities and events, such as logging, construction, off-road vehicles, floods, droughts, and fires. Erosion degrades soil condition by lowering organic-matter content, decreasing rooting depth, and decreasing available water capacity. In addition, sediment (the product of erosion) can pollute streams, lakes, and other bodies of water with soil particles and associated chemicals and plant nutrients. Soil erosion by wind pollutes the air and can damage plants through a sandblasting effect.

Compaction, accumulation of salts, excess nutrients and chemicals, and toxic chemicals are also significant soil quality concerns. Overapplication of plant nutrients and pesticides can exceed the capacity of a soil to cycle nutrients efficiently. This increases the potential for those nutrients and chemicals to leach from a soil in percolating waters and eventually pollute ground and surface waters. Some crop management practices, such as excessive tillage, compact the soil, hampering the ability of the soil to receive rainwater; thus, more water runs off the soil surface instead of entering it and being stored for plant use. Although natural soil processes can transform substances such as sewage sludge and animal wastes into nutrients for plant growth, overapplication of these materials will impair the ability of a soil to transform them-again, leading to a hazard of ground or surface water pollution.

How can we improve soil quality?

The sodbuster and conservation compliance provisions in the Food Security Act of 1985 have led to increased application of conservation cropping systems, which-along with ongoing conservation efforts on America's farms and ranches and application of crop residue management technologies-have significantly cut soil erosion. Sheet and rill erosion (the removal of soil in thin layers [sheets] or tiny channels [rills]) and wind erosion on the Nation's cropland declined from 3.1 billion tons in 1982 to 2.1 billion tons in 1992. In addition, these conservation systems have increased the organic-matter content of soils and improved the tilth of their surface layers.

The Natural Resources Conservation Service has adopted a holistic approach to help land managers protect soil and related resources. This approach assesses the natural processes and functions of an area, such as a watershed, and determines how these functions affect the quality of the soil, water, air, plant, and animal resources. The holistic approach incorporates land managers' objectives and resource concerns and then provides a set of conservation practices that will maintain or enhance soil and environmental quality.

Soil scientists and others are developing tools to measure and monitor soil quality by developing key indicators to assess soil condition or health. While monitoring is important for assessing change, it alone will not improve soil quality. Monitoring does help provide feedback on how prescribed conservation management systems are performing and provides conservationists with data to design needed modifications in these systems.

Dominant land uses in the United States. People generally put their soils to uses that the soils can support; in this way land use reflects soil quality.

Future approaches to maintaining soil quality

For the long term, the best way to maintain or improve soil quality is to locate agricultural production on the soils that are best suited for the particular use, and use other soils according to their capacity. Beyond that, we should remediate soil quality problems and address areas of greatest concern not only for the soils, but also for related water and air resources. We should also develop technologies that maximize the ability of soils to function for specific land uses-such as soil-specific nutrient and pesticide management, no-till and other crop residue management technologies, appropriate use of urban and animal wastes, and conservation management systems. And finally, we should develop the public's awareness and concern for the quality of the soil resource and how it affects their daily lives.

How does soil quality benefit us?

High-quality soils ensure that the primary agricultural lands in the United States are sustained for future generations. Soils of high quality are essential for the production of a bountiful supply of safe food and fiber. Healthy food translates to a healthy people and a healthy nation. High-quality soils support:

• clean water by transforming harmful substances and chemicals to nontoxic forms, cycling nutrients, and partitioning rainfall to keep sediments and chemicals out of lakes and streams;
• clean and healthy air by keeping dust particles out of the air and cycling other gases;
• healthy plant growth by storing nutrients and water and providing structural support through a receptive rooting medium; and
• storage of greenhouse gases such as carbon dioxide in the form of organic matter in the soil.

The primary benefit of enhanced soil quality is the protection of a finite resource. Maintenance and enhancement of soil quality maintains maximum efficiency in crop productivity over time by enhancing nutrient cycling and encouraging site-specific application of nutrients and pesticides. It protects water and air quality and preserves the beneficial functions of the soil in specific ecosystems.

Soil and soil quality are closely linked with the other natural resources-water, air, plants, and animals. As soil quality improves, so does the quality of the other resources. The key is using management practices to care for the land in a way that improves soil quality-to better filter water, reduce airborne particles, maintain or improve productivity, use less chemicals, and increase plant, animal, and micro-organism diversity.

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