There's good news for consumers who like salad and farmers who grow most of the nation's winter lettuce crop in desert soils near the California-Arizona border. UC researchers have found a way to enrich the region's low soil quality with cover crops.

“Typically, soils are left fallow, or empty, after a lettuce-cantaloupe rotation,” said research leader Milt McGiffen, a Cooperative Extension specialist and associate plant physiologist at UC Riverside. “When we replaced the summer fallow with cowpea and sorghum-sudangrass cover crops, soil fertility and quality improved.”

The successful project is the cover story in a national federal agriculture publication, the USDA's SARE (Sustainable Agriculture Research and Education) review of 2004 projects “Practical new ideas in vegetable production/livestock systems.”

Cowpeas capture nitrogen from the air and change it into useable fertilizer. Sorghum-sudangrass enriches the soil by adding carbon and minimizes erosion and dust, a significant problem during the windy summer, McGiffen said.

Cowpeas significantly increased yields and net returns of fall-planted lettuce and the cantaloupe crop that followed, he said. Returns improved even more if the system was farmed organically.

Good grower news

The project found that lettuce could net as much as $2,417 per acre if grown organically, with price premiums, compared to $752 per acre grown conventionally.

“The economic good news interests both organic growers seeking alternatives to commercial fertilizers, and farmers wanting to sidestep rising fertilizer prices,” said David Chaney, education coordinator for the University of California's Sustainable Agriculture Research and Education Program and Western Region SARE representative. SARE funded the original research.

McGiffen said that adding a cover crop to the rotation brings other benefits, from out-competing weeds to moderating the desert's extreme soil temperatures.

“Growers were so impressed with the findings that 10 in California's Coachella Valley and more throughout the state have begun growing cowpeas during the summer,” he said. “We have changed the way producers look at things and provided them with new tools.” He estimated that farmers now grow cowpeas on more than 3,000 acres in California and Arizona.

The use of cover crops is not limited to lettuce growers. Date and citrus orchard owners are using cowpeas as a result of the UC Riverside research, which published a final report in 2001.

Jose Aguiar, UC vegetable crops and small farm advisor, collaborated on McGiffen's project. Aguiar is pictured in a field of carrots on the cover of SARE's 2004 review of research projects. The carrot field is now farmed with cowpeas in its rotation.

“It knocks down weed populations and provides nitrogen and organic matter, so growers are very happy with the system,” Aguiar said.