Linda F. Benedict, Gaston, Lewis A., Jeong, Changyoon, Wang, Jim Jian
Lewis Gaston, Jim J. Wang and Changyoon Jeong
Although the term soil quality is modern, the concept is not. Ancient farmers measured the productivity of a soil and knew fundamental relationships between soil properties, such as enrichment with organic matter and crop production. What is new is that scientists now understand that soil has many functions in addition to helping crops grow. The quality of a soil affects hydrology, water quality, nutrient cycling, carbon storage, biodiversity and other ecological functions.
Besides long-recognized indicators of soil quality – such as levels of organic matter, acidity and salinity – additional chemical indicators and several biological and physical indicators are considered in measuring soil quality. The measured value of each indictor is assigned a rating, and these ratings are averaged to give the overall soil quality for a particular soil function. The utility of this approach lies in being able to compare the overall value for soil at one location to values at locations considered to be functioning well. If the overall soil quality value is relatively low, changes in management practices may be needed to improve the soil.
If overall soil quality value is low, the soil is considered degraded. The cause of such degradation is some disturbance that has negatively affected one or more indicators of soil quality. Two important points are that soils vary in their susceptibility to degradation and in their capacity to recover from it. A soil that tends to withstand a disturbance is said to have resistance, and a soil that can recover well is said to have resilience.
Measured values for soil quality indicators depend on properties inherent to a soil, such as texture, and on how the soil has been managed. Inherent soil properties are more important than soil management in imparting resistance to a disturbance. How a soil has been managed, however, may strengthen or weaken the capacity of a soil to resist degradation. The dynamic effects of soil management largely control resiliency, but inherent properties also affect how quickly soil quality is restored.
The terms resistance and resilience are not exclusive. A resistant soil is minimally degraded by a disturbance. Even so, it is not functioning at the level it was before the disturbance, and some changes in soil management may be needed to restore lost quality. If the damage is undone quickly, this resistant soil is also resilient. The same level of disturbance more severely degrades nonresistant soils. In the case of severely degraded soils, major changes in management are needed to restore soil quality.
Regardless of the level of resistance that inherent properties give a soil, if it has been managed for optimal quality, the effect of a disturbance will be minimized. The cost and effort needed to restore soil quality will be comparatively low, and the likelihood of success will be comparatively high. The opposite is true as well. Thus, environmental resilience depends on soil quality.
Examples from studies in Louisiana will illustrate some of the points. In one study involving long-term cotton tillage (conventional and no-till) and cover crops (hairy vetch, wheat or winter annual weeds) on a silt loam soil, the organic carbon was measured. The results showed that the organic carbon was increased with no-tillage. Thus, soil quality was improved by no-till management, indicating resiliency of this soil.
Other studies by LSU AgCenter scientists are examining the benefits of various soil amendments on improving soil quality. For instance, biochar made from the burning of crop residues such as sugarcane harvest trash has been shown to improve different aspects of soil quality. Biochar contains long-term stable carbon, which increases soil carbon sequestration. Biochar benefits plant growth through positive interaction with plant-available nutrients and increased soil water-holding capacity. The latter improves resistance to drought and imparts resiliency to the system because the crop is more likely to recover. Recent AgCenter research has showed that soils treated with biochar produced from sugarcane harvest trash increase water-holding capacity by 15 percent to 100 percent (Figure 1). Additional soil quality parameters are being examined with biochar-amended soils.
Lewis Gaston is an associate professor, Jim J. Wang is a professor, and Changyoon Jeong is a post-doctoral researcher in the School of Plant, Environmental & Soil Sciences.
(This article was published in the spring 2013 issue of Louisiana Agriculture magazine.)