Non-Leguminous Cover Crops In Cool-Season Vegetable Crop Systems

Louise Jackson and Lisa Wyland
Department of Vegetable Crops, University of California, Davis, CA 95616

For several years, we have conducted research on the use of non-leguminous cover crops to decrease leaching of nitrate in vegetable crop systems in the Salinas Valley, where nitrate contamination of groundwater is a serious problem. In order to minimize the time and energy for cover cropping, cover crops are planted on beds in the fall, then flail-mowed and incorporated with minimum-tillage equipment directly on the beds, and vegetables are planted in the same beds in the spring. Time and energy is saved by the use of semi-permanent beds because the field is not disked and rebedded before planting.

During the past year, our efforts have been focused on the completion of two projects: 1) a "systems-level", multidisciplinary analysis of cover crops using the minimum-tillage methods in an on-farm trial; and 2) short-term nitrogen dynamics immediately after cover crop incorporation, using the stable isotope, ¹⁵N.

On-farm Multidisciplinary Study

Impacts of cover crops on yield, nitrate leaching, pests and management costs were determined for a broccoli field in the Salinas Valley. The treatments used were: winter-bare, Phacelia tanacetifolia, and Secale cereale, cv. Merced. The cover crops reduced nitrate leaching by 65-70%. Decomposition and mineralization of N from the cover crop residues were delayed due to low soil moisture at incorporation, but after moisture increased, there were large surges of inorganic N, net mineralizable N and soil microbial biomass, which subsided within six weeks. Plots with phacelia during the winter had increased broccoli yield. No insect or disease problems occurred. The economic costs of cover cropping were low compared to the cost of producing broccoli.

Fates of ¹⁵N in Cover Crop Residues

In the spring of 1994, ¹⁵N-labelled cover crop residues were incorporated into large, deep cylinders at a research station site, which was farmed similarly to growers' fields, using "best management practices" for water and fertilizer application. The fates of the labeled N in soil, soil microbial biomass, and plants were followed for one year, beginning a few days after incorporation. Soil community structure (bacteria, fungi, protozoa, and nematodes) were also measured during the first few weeks after incorporation of cover crop residues. Data analysis is still in progress.

Future Plans

During the past year, field work on cover crops has been minimal, but we are continuing to work on minimum-tillage options for cool-season vegetables.

Publications

Wyland, L.J., L.E. Jackson, W.E. Chaney, K. Klonsky, S.T. Koike, and B. Kimple. Altering surface soil dynamics with winter cover crops in a vegetable cropping system: impacts on yield, nitrate leaching, pests and management costs. In press, Agriculture, Ecosystems and Environment.

Wyland, L.J., L.E. Jackson, and K.F. Schulbach. 1995. Soil-plant nitrogen dynamics following incorporation of a mature rye cover crop in a lettuce production system. Journal of Agricultural Science, Cambridge 124:17-25.

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