Soil quality is evaluated using indicators that measure specific physical, chemical, and biological properties. Although it is useful to examine these aspects of soil quality individually, soil should be viewed as an integrated system rather than a collection of separate parts or processes.
Physical and chemical properties are shaped by biological activity, and biological activity is enhanced or limited by chemical and physical conditions. For this reason, specifically categorizing some soil indicators is difficult. For example, cation exchange capacity (CEC) could be classified as either a physical or a chemical property, and organic matter as either a chemical or a biological property. The best and most useful indicators of soil quality integrate the combined effects of several properties or processes.
Important physical indicators of soil quality include those related to water storage and movement, soil structure, and soil or aggregate stability. Important chemical indicators include the presence and amounts of mineral elements and plant growth inhibiting substances. Biological indicators often refer to the amounts, types, and activities of soil organisms. A large, diverse, and active population of soil organisms may be the most important indicator of a healthy, high-quality soil. Yet, soil biological activity may be the most difficult indicator to satisfactorily measure and interpret.
Soil-quality indicators are limited to properties impacted by soil management. For this reason, some soil characteristics are not considered soil-quality indicators although they may influence soil use or productivity. Soil texture, topsoil depth, and slope or topography are examples of fixed soil properties that cannot be altered (except over long time periods or in extreme cases of erosion or sediment deposition). Although these characteristics clearly influence soil use or productivity, they are not used as soil-quality indicators.
Nevertheless, it is important to be knowledgeable about inherent soil properties because they often set limits on the maximum soil quality management practices can achieve. For example, soil texture refers to the relative amounts of primary mineral particles (sand, silt, clay) found in a soil. Coarse-textured sandy soils have proportionately more larger sand particles, while fine-textured clayey soils have proportionately more smaller clay particles. Soil texture is an unalterable soil property which strongly influences many soil-quality indicators, like drainage and water-holding capacity. But soil texture itself is not an indicator of soil-quality.
| Leading Indicators of Soil Quality | ||
| Physical | Chemical | Biological |
| structure | pH | respiration rate |
| bulk density | cation exchange capacity | earthworms |
| drainage | plant-available nutrients | microbial numbers |
| water infiltration rate | organic matter | microbial biomass |
| water-holding capacity | soluble salts | species diversity |
| soil strength | contaminants | pathogens |
| (penetration resistance) | (metals,toxins) | (plant and human) |