The concept of soil quality has been developed to help quantify factors that affect the ability of soil to function effectively in a variety of roles. Primary measures of this effectiveness are enhanced biological productivity, environmental quality, and human and animal health. Rapid population growth has demanded an increased emphasis on enhancing biological productivity, but if soil quality is to be improved, we must simultaneously achieve the other two goals as well.
The ongoing degradation of natural resources (erosion, salinization, contamination of ground and surface waters) is closely associated with a loss of soil quality. The concept of soil quality is 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 definition provides a focal point for assessing the intensity of soil degradation. See Soil Degradation and Soil Erosion.
Soils have various levels of quality that are defined by stable features related to soil forming factors and dynamic changes induced by soil management. See Soil Pedogenesis.
The terms "soil quality" and "soil health" are often used interchangeably in scientific literature and in the public press. Soil health is best used to refer to the condition of a soil as a result of its management, while soil quality may refer to both permanent soil properties and soil condition.
Development of Soil Quality Concept
NRCS employees examine a soil sample on a farm that has incorporated conservation practices to protect and enhance soil quality. (Photo credit: NRCS)
Soil quality concerns are not new; evaluations of soil characteristics for crop growth appeared in the first written literature and certainly predate those records. The soil quality concept per se was introduced by Warkentin and Fletcher (1977) as an approach to facilitate better land use planning for the various functions that soil must accommodate. Early efforts to define soil quality were followed by more formalized definitions (Larson and Pearce 1991, Karlen et al. 1997), selection of indicators (Doran and Parkin 1994), and specific strategies to enhance soil quality (Doran et al. 1996).
The concept is not without its critics. There have been several research editorials that recommend moving away from subjective efforts to develop an index of soil quality. Letey et al. (2003) suggest a move toward utilizing available technical information to motivate and educate farmers on management practices that optimize the combined goals of high agricultural production, low environmental degradation, and a sustained soil resource. Sojka and Upchurch (1999) reported that "quality of soil management rather than soil quality management" should be the goal of soil science.
Research efforts to monitor and index soil quality indicators need to be balanced with efforts to better define relationships between the status of soil quality indicators and soil functions. In addition, consideration must be given to the simultaneity of the diverse and sometimes conflicting nature of soil functions. Greater relevance to research efforts may be achieved under a broader context that first defines strategies to enhance agricultural and natural resource sustainability, and then uses indicators encompassing all aspects of agroecosystem performance.
Tools for Assessing Soil Quality
Our ability to assess soil quality is complicated by many physical, chemical, and biological processes and their interactions in time, space, and intensity. It is not usually possible to directly measure the rate of soil processes; instead, they can be inferred by measuring specific soil properties that are indicative of these rates. These measurements then can be used in simulation models to predict future changes in process rates and, in turn, soil quality. The properties measured are termed indicators of soil quality. The best soil quality indicators are those that integrate the combined effects of several properties and processes. Sets of basic soil quality indicators should:
- Encompass ecosystem processes and relate to process-oriented modeling.
- Integrate soil physical, chemical and biological properties and processes.
- Be accessible to many users and applicable to field conditions.
- Be sensitive to variations in management and climate.
- Be included in existing soil data bases (where possible).
Efforts to assess soil quality have resulted in the development of tools to evaluate the impact of management practices on the soil and environment. The soil quality kit (reviewed by Cramer 1994) and soil quality scorecard (USDA-NRCS-SQI 1999) are examples of two tools used to quickly evaluate soil properties and processes with simple equipment and relatively limited experience.
Image credit: USDA Natural Resources Conservation Services (Soils)
An important role for soil scientists is to determine appropriate indicators for various management goals or land uses. Doing so will ensure that assessments of soil quality will be useful and understandable to farmers and other land managers who are the stewards of soil quality.
NOTE: To learn more about soil quality concepts, take the UBC course APBI 402/SOIL 502 - Sustainable Soil Management. (The syllabus for this course can be found here (Course:APBI402-SOIL502/Syllabus).