By Louise E. Jackson, Irenee Ramirez, Ron Yokota, Steven A. Fennimore, Steven T. Koike, Diane M. Henderson, William E. Chaney, and Karen M. Klonsky

Soil quality research focuses on soil organic matter (SOM), its activity and function, and related chemical and physical properties. It also considers the larger picture, including impacts of soil management on production, pests, and economics. Use of cover crops, compost, and reduced tillage may increase SOM, especially the active fraction of SOM that is largely composed of decaying plant material and microbial cells, and plays an important role in nutrient cycling and retention. These methods may also increase nutrient availability, leading to less reliance on fertilizers that are derived from non-renewable fossil fuel, and improve soil physical properties such as aeration and water infiltration. They can improve soil quality by minimizing nitrate leaching below the root zone and reducing groundwater contamination.

Adoption of these practices will be enhanced if: (1) farmers face few economic disadvantages due to the new procedures; (2) the start-up costs and effort are feasible; and (3) new practices do not result in onset of new problems. In this on-farm research, farmers participated in the design and implementation of experiments to assess the multiple trade-offs involved in transitioning to new management practices.

In an on-farm experiment in a vegetable production field in the Salinas Valley, a multi-disciplinary approach was used to evaluate responses to tillage and organic matter (OM) management. Conventional tillage (sub-soiling, disking, and surface mulching to a depth of 20 inches) was compared to minimum tillage where only the surface layer of semi-permanent beds was tilled to a depth of 8 inches. Also, organic amendment (+OM) and no organic amendment (-OM) treatments were compared. Organic matter was added to the soil in the form of both cover crops and compost, to incorporate carbon (C) and nitrogen (N) sources, like plant residues, that are readily available to plants, and more resistant sources of C and N that remain after the composting of manure, municipal yard waste, and other plant materials.

Many conventional and organic vegetable growers use both inputs in their crop rotations, and so these materials were used together to study their effects on SOM and other variables. Soil microbial biomass is one measure of the readily available, active fraction of SOM, and it usually represents < 5% of the total C and N in the SOM. Total soil C and N contain many diverse compounds that are resistant to breakdown in soil, and typically change more slowly in response to management than the active fraction of SOM.

The objectives of the 2-year experiment were to determine the effects of alternative tillage and OM management practices by: (1) monitoring changes in crop yield, nutrient uptake, and soil parameters throughout a 2-year period; (2) documenting effects on weeds, pathogens, and insect pests; and (3) assessing the total economic costs and net returns incurred while adopting practices conducive to increasing soil quality.

Use of cover crops and compost increased soil quality in irrigated, intensive production of lettuce and broccoli in the Salinas Valley. These methods had the beneficial impacts of increasing soil microbial biomass, increasing total soil carbon and nitrogen, reducing surface bulk density and decreasing the potential for groundwater pollution as a result of nitrate leaching below the root zone. These soil benefits did not lead to lower yields and occasionally resulted in fewer weeds and lower lettuce corky-root disease. Although surface minimum tillage reduced yields, it led to reduced potential for nitrate leaching below the root zone. Use of conventional tillage, cover crops, and compost produced high vegetable yields and acceptable net economic returns over a 2-year period, but broccoli was more profitable than lettuce under this regime. Understanding the trade-offs of various costs and benefits will help growers choose management practices that optimize economic and environmental benefits.