Soil quality or health can be defined as the capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to (1) sustain plant and animal productivity, (2) maintain or enhance water and air quality, and (3) support human health and habitation (Karlen et al. 1997). Soil quality encompasses the interrelated physical, chemical, and biological aspects of soil. For example, soil organisms decompose crop residues to release nutrients and drive the nitrogen cycle (mineralization, immobilization, denitrification). Soil fungi play a large role in formation of soil aggregates and structure, a physical property. Soil biota are in turn affected by soil pH (chemical property) where there is generally lower biological activity in more acidic soils, and waterlogging or compaction of soil (physical properties) often favor anaerobic organisms, some of which cause disease and others that cause nitrogen loss via gaseous forms (Granatstein 2003). Some soil properties, such as soil respiration, change quickly and are highly variable while others, like soil carbon, can take years or decades to change. Soil texture (sand, silt, clay) is generally considered fixed, but organic matter levels can ameliorate some of the negatives. For example, sandy soils have good aeration and drainage but relatively poor water-holding capacity and nutrient retention. Organic matter can increase the latter two. On the other end of the spectrum, clay soils have high water and nutrient retention but poor aeration and physical structure, and organic matter can address these limitations.
The soil properties are influenced by the natural environment (e.g., climate, geology, vegetation) as well as by human activity (e.g., erosion, fertilization, irrigation, plants). However, soil quality itself is not a soil property but rather a human judgment about how well a given soil can perform desired functions (Sojka and Upchurch 1999). Soil quality is important to growers since it plays a large role in crop production as well as on the environmental performance of a farm, affecting soil erosion, air and water quality, and greenhouse gas relations.
One factor in evaluating soil quality is your reference point. Often it has been the native soil in your location. So the prairie or grassland soils are a reasonable reference point for soil quality in a wheat field in Kansas or the Palouse. However, many soils had very different properties in their native ecosystems compared with their status when farmed, as is the case for the irrigated Columbia Basin. What should be the reference point for an irrigated potato field in Washington
Evaluations of soil quality rely on choosing a set of indicator properties that can be quantitatively measured and related to a baseline or reference point for comparison. Indicators should reflect a problem to be solved or a desired state to be achieved. For example, if poor water infiltration is a problem, then indicators related to this property such as infiltration rate should be used to monitor whether management changes have the desired effect. Various studies have sought to find an ideal suite of soil measurements for evaluating soil quality (Hefner et al. 2009; Moebius-Clune et al. 2016). Of these, one of the better developed and practical is the Cornell Soil Health Assessment which measures 10 properties, normalizes them, rates them according to specific criteria, and then calculates an overall soil health rating. However, this assessment was developed for soils in the northeastern US, which differ greatly from western US soils in organic matter levels (higher) and chemical properties (more highly weathered, in general). Therefore, this test can be useful in comparing different management but may not reflect optimal conditions for western US soils. Often the crop itself can be used as an indicator of soil quality change, as it integrates the effects of the different soil properties. Improved crop performance is an outcome desired by growers and one they can usually measure quantitatively.
Soils and Columbia Basin Agriculture
Irrigated growers in the Columbia Basin of Washington State have expressed increased interest in improving soil quality and in learning about the benefits versus the costs of implementing soil improvement practices. In addition, producers have been under increasing public scrutiny concerning efforts to maintain and improve soil resources, especially for off-farm impacts such as wind erosion and water quality. A 2012 survey of attendees at the WSU Building Soils for Better Crops Workshop in Moses Lake, Washington, showed that 73% had increased their use of soil improvement practices in the last five years, with “improved soil tilth” as the most recognized benefit (Granatstein and McGuire 2012).