Over the last decade a large and growing number of farmers have taken up the practice of growing cover crops--to improve soil quality and reduce erosion and runoff.

But now the value of the "green manure" in legume cover crops is making them even more attractive. The higher costs of fertilizer have increased interest by conventional growers in legume cover crops, which have the ability to fix nitrogen from the atmosphere and put it into the soil.

"Sharp increases in synthetic fertilizer prices have helped to fuel interest in planting legume cover crops by non-organic growers," said Jeff Ehlers, University of California, Riverside, grain legume plant breeding specialist. Ehlers is one of the UC researchers who are working feverishly to find or develop cowpea cover crop varieties that serve multiple uses.

Some of the more useful cowpeas are exceptional at controlling weeds in particular time slots. Others help control nematodes or crop diseases. And there is even hope that a cowpea variety from Africa will produce a strong enough dose of a biofumigant to reduce symphylans, the persistent garden centipede that feeds on the roots of tomatoes, asparagus, leafy vegetables and other crops.

But all of the cowpea cover crops increase the nitrogen and organic matter in the soil. "Cowpea cover crops are cost effective because they enrich the soil with organic matter and over 100 pounds per acre of nitrogen, and can reduce pest populations such as root-knot nematodes," Ehlers said. He has been testing a new, improved version of the standard summer season cowpea cover crop.

"The cowpea cultivar most widely used as a cover crop is Iron Clay," Ehlers explained. "Iron Clay produces the abundant amounts of organic matter and nitrogen needed to enrich the soil, resists common forms of root-knot nematode, and grows well in our desert summers with moderate irrigation."

But when Iron Clay is grown in California, the pods open up and seeds cannot be produced. UC Riverside is releasing a new and improved cowpea cover crop variety, labeled CC 36, that remedies the seed production problem. The new variety also has broader nematode and Fusarium wilt resistance than its predecessor.

CC 36 is intended to provide weed control, and to increase soil fertility for the subsequent cash crop. The seed for this variety must be produced in warmer areas of Southern California. But that means that CC 36 will give a larger biomass yield when it is used as a cover crop in other areas of the state.

UC Cooperative Extension legume specialist Steve Temple is continuing to work with an African cowpea cover crop variety that was very successful for years in tomato rotations in the Sustainable Agriculture Farming Systems program in Davis. "This was used after early harvest tomatoes in the sustainable program," Temple said. He is continuing his farm trials of this cover crop with both dry bean and tomato growers.

The major advantage of this variety is that it puts out a canopy very quickly and continues to produce biomass with little water from August until late November or even early December. This cover crop crowds out weeds and adds nitrogen and organic matter to the soil. It also has resistance to some root-knot nematodes. This African cowpea was the major cover crop in a mix that prevented weed germination in a tomato rotation all the way through February, according to Temple.

But the most intriguing cowpea cover crop trial could be the attempt to find a variety that produces a natural fumigant in intense enough concentrations to reduce populations of symphylans, a destructive and persistent soil pest. Symphylans are extremely difficult to control because they can hide in small spaces deep in the soil, where it is nearly impossible to reach them with insecticides.

The Mucuna is an African cowpea that has an extremely high content of L-Dopa, a natural biocide. "There's definitely high amounts of L-Dopa in the seed, and there may be in the root material as well," said Margaret Lloyd, the UC Davis graduate student in plant pathology who is conducting the trial at the Davis test fields.

The Mucuna or velvet bean is native to West Africa, according to Lloyd. But varieties of the Mucuna were commonly used in cotton rotations in the southern United States in the early decades of the 20th century. People do eat Mucuna in some parts of the world, according to Lloyd. But they are very careful to boil them thoroughly to get rid of the L-Dopa before they eat the beans. And it is highly unlikely that anyone in the U.S. would eat the Mucuna beans, she said.

But the bean could prove to be an important tool in controlling a range of soil pests. "There's been a good amount of research with Mucuna controlling nematodes," Lloyd said. This cover crop is extremely vigorous and has also been shown to control some weed varieties.

According to Temple varieties of Mucuna have been used successfully in many areas to increase soil nitrogen, control weeds, build soil quality and control pests. The first step in the attempt to learn the possibilities in California is to find out which varieties of Mucuna can be efficiently grown here.

"We're growing them out here to see which will flower and produce seeds," Lloyd said. Once the more promising varieties have been selected, they will be grown in Southern California locations to learn if seeds can be produced.