The right cover crops can optimize organic production


Issue Date: September 9, 2009
Ching Lee

Using cover crops to improve soil and manage pests has become an integral part of many organic farming systems, especially with the cost of approved organic fertilizers and other materials suitable for organic production agriculture.

Long-term success in organic agriculture requires optimizing the natural processes on the farm, including using cover crops, said Mark Van Horn, director of the Student Farm at the Agricultural Sustainability Institute at the University of California, Davis. He spoke about the issue at the Crop Advisers and Certified Organic Production seminar held in Modesto this summer.

"If you rely mainly on products, then the success of your organic program would be short-lived," he said.

Cover crops can offer a number of benefits. They can provide nitrogen, increase soil organic matter and improve nutrient availability by increasing soil biological activity, he noted.

They can also improve soil tilth and structure, prevent soil erosion and increase the rate of water infiltration and availability. Many cover crops effectively suppress weeds and provide mulch to conserve soil moisture. They may also be used to provide habitat and food sources for beneficial insects while reducing other soil-borne pests.

"But cover crops don't come free," he said.

Cost of seed, energy and labor to plant cover crops may make them less attractive to some farmers. Another potential disincentive is water use, especially for farms that are struggling with current water shortages.

Van Horn warned that if cover crops are not managed effectively, they could actually increase weed problems, insects and pathogens in the soil. Cover crops that decompose slowly may also initially tie up nutrients needed by the cash crop. Another challenge of using a cover crop is finding a window of time when a cash crop is not being grown.

Choosing the right cover crop is critical, Van Horn said. Before growing a cover crop, he advises farmers to ask why they're growing it, what to grow, when to grow it and how it will be managed.

Once they've identified why they are growing the cover crop, that is, what they want the cover crop to do, they will need to select a cover crop that works well for their farm and that fits in their crop rotation.

"Nitrogen fixation is often the primary reason farmers grow cover crops in organic systems," Van Horn said. "And cover crops are often the cheapest nitrogen source for organic farms."

Resident vegetation, or weeds, can be used as an actual cover crop, he said. For example, properly managed resident vegetation on the orchard floor can provide a stable surface for machinery to operate under wet conditions that otherwise would prevent access to the orchard. Resident vegetation can also reduce soil compaction and the potential for erosion.

Other cover crops that farmers can grow include a variety of annual and perennial legumes and grasses for cool and warm seasons. Cool season annual legumes such as vetches, peas, clovers and medics can add 100 to 200 pounds of nitrogen per acre, and that nitrogen is typically available quickly, said Van Horn.

Summer annual legumes such as bean-type plants, including blackeye beans and hyacinth beans, as well as uprights such as sesbania and crotalaria, are drought tolerant and can fix 100 to 150 pounds of nitrogen per acre.

Most cool season annual grasses and non-legumes don't fix nitrogen but can affect nitrogen availability, Van Horn said. Cereal grasses such as oats, barley and wheat, and forage/pasture grasses such as annual ryegrass, brome and fescue may scavenge nitrogen from the soil and reduce the potential for nitrate leaching.

Other cool season annuals such as mustards, phacelia and buckwheat also take up nitrogen readily. However, they have a lower carbon-to-nitrogen ratio, so they decompose and release their absorbed nitrogen into the soil more easily than grasses.

Warm season annual grasses such as Sudan grass, sorghum and sorghum-Sudan grass hybrids produce ample amounts of biomass and are often used as a cover crop in organic vegetable rotations. These tall, fast-growing grasses have low nitrogen content—about 1.5 percent—and their decomposing residue can tie up nitrogen.

Legumes such as strawberry and white clovers and birdsfoot trefoil can be grown as perennial cover crops. Perennial grasses used as cover crops include California native grasses, perennial ryegrass and Bermuda grass.

Since residue from cover crops can increase or decrease nitrogen availability to the following crop, Van Horn said nitrogen mineralization must take place to release nitrogen where and when the cash crop needs it. During mineralization, the nitrogen in the plant tissue is converted by soil microbes into a form that subsequent plants can use.

Winter legumes and juvenile mustards release nitrogen rapidly, while summer legumes and mustards have a moderate to rapid release rate, and juvenile grasses and heading mustards have a moderate to slow release rate. For grasses that are near heading, the nitrogen may be immobilized or tied up for days to weeks after the crop has been incorporated in the soil and microbial activity increases, at which time nitrogen is released slowly to moderately. Generally, the more mature the crop, the slower the decomposition, Van Horn said.

He also noted that depending on the cash crop being grown, using cover crops as the main source of nitrogen might not be enough if the cover crop releases the nitrogen early in the season and the young plants are not able to use it.

"So if you have a long-growing season, the cover crop is likely not going to last you and you may need additional nitrogen," he said, adding that additional nitrogen could come from compost and organic fertilizers.

To optimize the use of nitrogen by the subsequent crop, Van Horn said farmers should synchronize the nitrogen release with the crop's demand. That could mean planting the cash crop soon after the cover crop has been incorporated in the soil. Managing water carefully to minimize nitrogen leaching also is important.

Cover crops can also help farmers manage pests, Van Horn said. For example, some cover crops such as perennial strawberry clover and annual cereal grasses, vetches and cowpea suppress weeds by out-competing them.

Other crop problems such as disease, nematodes and insects may become less problematic with the use of cover crops, which can provide food and shelter to beneficial insects, mites and spiders, Van Horn added. And while it is difficult to show consistent, specific impacts of cover crops, he said a well-reasoned diversity of cover crops, rotations, crop mixes and hedgerows seems to enhance bio-control.

(Ching Lee is an assistant editor of Ag Alert. She may be contacted at clee@cfbf.com.)

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.