Soil Quality

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. The 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. Soils have various levels of quality that are defined by stable features related to soil forming factors and dynamic changes induced by soil management.

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 & Pearce 1991, Karlen et al. 1997), selection of indicators (Doran & Parkin 1994), and specific strategies to enhance soil quality (Doran et al. 1996).

Soil quality can be viewed in two ways:

1. as inherent soil quality, which is regulated by the soil’s inherent properties as determined by the five soil-forming factors, and
2. as dynamic soil quality, which involves changes in soil properties influenced by human use and management.

In both, the first consideration deals with origins of soil and processes of soil development. The inherent soil quality (together with theories of soil genesis) provides the basis for soil classification. The dynamic soil quality, is the focus of this course and will be discussed in great detail.

The soil quality concept is not without its critics. Recently, 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. They, however, used the inherent soil quality incorrectly as a criticism toward soil quality. As Norfleet et al. (2003) pointed out, the important point to keep in mind is that the assessment of soil quality should not be done across contrasting soil types but across contrasting management practices on the same soil type.

The inherent and dynamic soil qualities follow one another (rather than mirror one another as some critics thought). A land use is chosen for a soil because of the inherent properties and/or socioeconomic needs. Subsequent management practices alter the original properties, creating change toward a new steady state for the dynamic properties. The degree of change of dynamic properties and the effect that this change has on the landscape are addressed by soil quality.

Research efforts to monitor soil quality 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 diverse and sometimes conflicting nature of soil functions.