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Sustainable Agriculture

Cover Crop Adds Fertility, Boosts Desert Vegetable Yields

Growers producing most of the nation’s winter lettuce in the desert along the California-Arizona border enjoy a hot, dry climate but contend with soils low in organic matter. To help growers improve soil quality, SARE-funded University of California- Riverside researchers tested cowpea and sorghum-sudangrass cover crops, which they substituted for the typical summer fallow following a lettuce-cantaloupe rotation.

Cowpeas, which fix nitrogen, and sorghum-sudangrass, with its plentiful biomass, also minimize erosion and dust, a significant problem during the windy summer. Cowpea proved a clear winner, significantly increasing yields of fall-planted lettuce and the subsequent cantaloupe crop. Comparing bare ground to cowpea incorporated into the soil—as well as cowpea used as a mulch and two treatments of sorghum-sudangrass— researchers found the highest net returns for cantaloupe and lettuce following cowpea incorporation. This was primarily due to a reduced need for commercial nitrogen. Returns improved even more if the system was run organically.

The project found that lettuce, for example, could net as much as $2,417 per acre if grown organically, with price premiums—compared to $752 per acre grown conventionally in 2000. The economic good news interests both organic growers seeking alternatives to commercial fertilizer and farmers seeking to sidestep rising fertilizer prices.

Adding a cover crop to the rotation can bring many other benefits, from out-competing weeds to moderating the desert’s extreme soil temperatures. Growers were so impressed with the findings that about 10 of them in the Coachella Valley and more throughout the state have begun growing cowpeas each summer. “We have changed the way producers look at things and provided them with new tools,” said research leader Milt McGiffen, estimating that farmers now grow cowpeas on more than 3,000 acres.

Cover crops are not just for lettuce growers, either. Date and citrus orchard owners have added cowpeas as a direct result of the UC-Riverside research. Grimmway Farms, one of California’s largest organic carrot growers, now uses cowpea in its rotation. “It knocks down weed populations and provides nitrogen and organic matter, so they’re very happy with the system,” said José Aguiar, who collaborated on McGiffen’s project.

 

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