Winter, 1990 (v2n2)
	Cover Crops: The Sequel Editor's note: This article is a continued summary of the July 1989 cover crops meeting at UC Davis, sponsored by UC SAREP with funding from the USDA's Alternative Agricultural Opportunities program. The purpose of the meeting of 50 researchers, educators and growers was to stimulate interaction among people with both research and practical experience. Information presented at the meeting is being compiled by SAREP for publication. Similar meetings in other states within the western US are planned as part of a two-year grant from the USDA's Low-Input Sustainable Agriculture Program (LISA). Organic Field Crops Dan Cohen, director of the California Organic Field Crops Association, stressed the importance of fieldscale trials to demonstrate soil-improving properties of cover crops. Farmers are seldom persuaded by trials that rely on small experimental plots, he said. Nitrogen cost is a significant concern to his growers, who farm a total of 500,000 acres. They are also interested in the development of rotational schemes that take into account nematodes and other pests, Cohen said. High vs. Low-Input Vegetables Laurie Drinkwater, a UC Davis vegetable crops department researcher, gave a preliminary report on an interdisciplinary study funded by LISA. Drinkwater, UC Davis Vegetable Crops Assistant Professor Carol Shennan, UC Davis Plant Pathology Assistant Professor Ariena van Bruggen, UC Santa Cruz ecologist Deborah Letourneau, and Phil LeVeen, an independent economist, are assessing soil, water, insect, cost, and yield parameters for fresh-market tomato production. The study is being conducted at 14 sites with one tomato variety and horticultural practices varying from organic to conventional. Three of the farms used cover crops in the past year. Ecological Roots Rob Klusen, a UC Santa Cruz agroecology graduate student, discussed the ecology of the root-zone and how it relates to nutrient cycling. When a root tip penetrates and fractures a soil aggregate (cluster of soil grains bound by organic matter), organic matter is exposed, fueling an explosion of soil bacteria, KIusen said. Amoebae in turn attack the bacteria and release nitrogen compounds that plants absorb, he said. As roots continue to grow, reaggregation of soil particles occurs due to polysaccharides coming from both the plant root and associated microbes, Klusen said. Organic matter encased by beneficial fungi (mycorrhizae) may be protected from decomposition until penetration by another root, he noted. KIusen is collaborating with Rick Knoll, an organic farmer from Brentwood. The two are particularly interested in promoting root growth by having weeder geese graze cover crops. When shoot material is removed, plants invest more of their resources in root growth, Knoll said. The plan is to contrast soil dynamics in a grazed no-till cover crop versus a cover crop that is simply disked under, he said. Klusen and Knoll are also exploring the use of cover crops to encourage the growth of mycorrhizae which will colonize the roots of vegetable crops. The fungi will help improve nutrient-uptake efficiency, and reduce transplant shock. Potassium Deficiency Ken Cassman, an assistant professor in the UC Davis agronomy department, has been using barley and wheat as winter cover crops to enhance potassium (K) availability to cotton. Extensive areas of the San Joaquin Valley contain vermiculite in the soil, a mineral that absorbs great amounts of K added as fertilizer, making the K unavailable to cotton, Cassman said. There is some evidence that added organic matter is gradually improving the K-response of cotton at the field site, he noted. Humic acid, the largest component of stable humus in soil also appears to be important in maintaining K in a form available to plants, he said. Work in progress is attempting to define this mechanism, Cassman said. The limited window for cover-crop use (December to early March) means that cool-season grasses cannot contribute much biomass, Cassman said. Because of this, he plans to use composted animal manure to add organic matter, and contrast the cotton yields to those obtained with fertilizers having the same nutrient value. He expects plots with added manure to have higher yields of cotton. Orchards Under Cover Cover crops can reduce compaction, and improve infiltration and retention of water, but some covers may increase total water use in almond orchards, according to Terry Prichard, a UC Davis water management extension specialist. Prichard is evaluating different cover crops and management practices in one mature and one young almond orchard. The treatments were: 1) chemical mowing (partial suppression of cover crop by herbicide); 2) solid coverage residual herbicide; 3) uncontrolled resident weedy vegetation; 4) Blando brome grass planted as a cover crop; and 5) Salina strawberry clover planted as a cover crop. Chemically-mowed resident cover and Blando brome showed less water use and soil compaction, and with increased water infiltration. Both resident cover and Salina strawberry clover used more water, and solid coverage herbicide reduced water infiltration. Index for Sustainable Agriculture Winter, 1990