Cover Crops: Benefits and Challenges

Written by Debbie Roos, Agricultural Extension Agent.

What is a Cover Crop?
A cover crop is "any crop whose main purpose is to benefit the soil and/or other crops in one or more ways, but is not intended to be harvested for feed or sale". Cover crops have been part of agriculture for at least a few thousand years, but have recently received renewed attention as the result of environmental and economic concerns.

Benefits of Cover Crops
Conventional agricultural practices can result in environmental problems such as soil erosion, surface and groundwater pollution, and overdependence on fossil fuels and other inputs. There is increasing interest in developing sustainable agricultural systems that decrease reliance on chemical and fossil fuel inputs by enhancing biological processes. Cover crops are an important component of a sustainable system.

Improve soil fertility
There is increasing interest in the use of cover crops to improve soil fertility. Much of this interest stems from a heightened awareness of the negative environmental impacts of synthetic fertilizers, including ground- and surface water contamination, long-term soil productivity, and the energy-intensiveness of fertilizer production. Cover crops are capable of trapping residual nitrogen in the soil and, in the case of legumes, fixing atmospheric nitrogen.

A successfully established leguninous cover crop can replace some or all of the nitrogen fertilizer needed to produce crops. Both legumes and non-legumes can help recycle and increase the availability of phosphorous, potassium and micronutrients. The quantity and availability of nitrogen provided by the cover crop depends on many factors:

Visit SAN's chart on Cover Crop Nitrogen Values to view a summary of nation-wide research on the nitrogen values of a number of popular cover crops.

Improve soil structure and reduce soil erosion
Many farmers are now planting cover crops on land that they used to leave bare in fallow periods. Cover crops can reduce the impact of raindrops on the soil surface and minimize runoff. Cover crops increase the organic matter content of soils and improve infiltration. Cover crop biomass production depends on many factors, including soil type, environment, fertilization, planting date, and time of kill. Deep-rooted cover crops can break up plowpans and improve soil tilth and water-holding capacity.

Suppress weeds
Because so few herbicides are registered for vegetables, many farmers are interested in the effect of cover crops on weed populations. Increasing public concern about herbicides has also contributed to the interest in alternative weed management strategies. Many cover crops and their residues can suppress weed growth by altering light and temperature. Cover crops also present a barrier to emerging weed seedlings.

Another way that cover crops suppress weed emergence is through allelopathy, the release of toxic compounds by one plant to a neighboring plant. Many cover crops have exhibited allelopathic effects. Rye residues
produce allelopathic effects that can suppress weeds for 30-75 days after the cover crop has been killed. The use of rye cover crops in tomato production can eliminate the need for soil-applied herbicides.

Rapeseed, another brassica, provided similar weed control as a herbicide in one study, reducing weed density and biomass 73-85% and 50-96%, respectively. However, excessive rapeseed residues can also have a negative effect on cash crop growth and development. An integrated weed management approach that supplements the use of cover crops with timely cultivation and/or herbicide application is recommended for optimal results.

Reduce insect problems
Cover crops can be an important part of an integrated pest management program. Cover crops can attract beneficial insects. However, caution is needed with this approach, since they can also attract pests! Pest management is a complex issue that warrants further research.

Research has shown that berseem and subterranean clover attract beneficial bigeye bugs, and flowering buckwheat attracts hoverfly. Buckwheat and annual white sweetclover attract many species of predatory wasps, while faba beans attract parasitic wasps. Certain cover crop species are more attractive to pests than others. Tarnished plant bugs seem to prefer crimson clover, 'Cahaba' white vetch, and 'Vantage' vetch over subterranean clover.

Cover crop management affects their capacity to attract beneficial insects. Usually, cover crops are incorporated soon after flowering but this may have a negative effect on beneficial insect that inhabit the cover crop. Insects that can reproduce and mature quickly are less likely to be harmed. Method of incorporation can have an impact on insect populations. Mowing is the most damaging while no-till causes the least disturbance.

A California study addressed the use of cover crops to promote beneficial insect populations and control aphids in lettuce. The cover crop used was sweet alyssum. The cover crop alone did not provide adequate pest control. However, the use of cover crops as part of an integrated system of pest management should be explored further.

Reduce disease and nematode problems
Some disease problems can be reduced with an appropriate cover crop rotation. Incorporated alfafa and white sweetclover residues can reduce the fungus Sclerotium rolfsii, while rye has demonstrated a capacity to reduce the incidence of Phythium. Cover cropping can also suppress plant-parasitic nematode populations. Sorghum, hairy indigo, cowpea, and jointvetch have all reduced nematode populations.

Reduce groundwater contamination
Groundwater contamination caused by leaching nitrates (NO3-N) from residual nitrogen fertilizers and the mineralization of soil organic matter is becoming a serious problem in crop production in some areas, especially during the winter fallow. Non-leguminous cover crops can help ameliorate the problem because they are capable of immobilizing as much as 70% of the available NO3-N in the upper soil profile. To effectively reduce winter nitrate leaching, a cover crop must grow rapidly and produce an extensive root system under winter conditions without supplemental inputs. Cover crops such as annual ryegrass, grain rye, and brassicas have all reduced nitrate leaching. Generally, non-legumes are about three times more efficient at reducing nitrate leaching than legumes.

Save money
Cover crops can potentially reduce production costs by decreasing your fertilizer and pesticide bills. They may also cut fuel and irrigation costs since deep-rooted cover crops can break up plowpans and improve soil tilth and water-holding capacity, reducing fuel and irrigation costs.

Opportunities and Challenges of Using Cover Crops in Vegetable Production

Cover crops have some potentially negative effects:

Optimal results depend on wise species selection, proper management techniques, and an understanding of the agro-ecosystem. Vegetable production systems present unique opportunities and constraints for using cover crops.

Constraints

Opportunities

Return to Cover Crops Home Page.

This page last updated January 16, 2006.

 

URL: http://chatham.ces.ncsu.edu/growingsmallfarms

North Carolina Cooperative Extension is an educational partnership helping people put research-based
knowledge to work for economic prosperity, environmental stewardship and an improved quality of life.