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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