Cover crops provide multiple potential benefits to soil health and the following crops, while also helping maintain cleaner surface and groundwater (figure 10.1). They prevent erosion, improve soil physical and biological properties, supply nutrients to the following crop, suppress weeds, improve soil water availability, and break pest cycles. Some cover crops are able to break into compacted soil layers, making it easier for the following crop’s roots to more fully develop. The actual benefits from a cover crop depend on the species and productivity of the crop you grow and how long it’s left to grow before the soil is prepared for the next crop.
Grass cover crops are more likely than legumes to increase soil organic matter. The more residue you return to the soil, the better the effect on soil organic matter. The amount of residue produced by the cover crop may be very small, as little as half a ton of dry matter per acre. This adds some active organic matter, but because most of it decomposes rapidly after the crop is killed, there is no measurable effect on the total amount of organic matter present. On the other hand, good production of hairy vetch or crimson clover cover crops may yield from 1 1/2 to more than 4 tons of dry weight per acre. If a crop like winter rye is grown to maturity, it can produce 3 to 5 tons of residue.
A five-year experiment with clover in California showed that cover crops increased organic matter in the top 2 inches from 1.3% to 2.6% and in the 2to 6-inch layer from 1% to 1.2%. Some researchers have found that cover crops do not seem to increase soil organic matter. Low-growing cover crops that don’t produce much organic matter may not be able to counter the depleting effects of some management practices, such as intensive tillage. Even if they don’t significantly increase organic matter levels, cover crops help prevent erosion and add at least some residues that are readily used by soil organisms.
Cover crops help maintain high populations of mycorrhizal fungi spores during the fallow period between main crops. The fungus also associates with almost all cover crops, which helps maintain or improve inoculation of the next crop. (As discussed in chapter 4, mycorrhizal fungi help promote the health of many crop plants in a variety of ways and also improve soil aggregation.) Cover crop pollen and nectar can be important food sources for predatory mites and parasitic wasps, both important for biological control of insect pests. A cover crop also provides a good habitat for spiders, and these general insect feeders help decrease pest populations. Use of cover crops in the Southeast has reduced the incidence of thrips, bollworm, budworm, aphids, fall armyworm, beet armyworm, and white flies. Living cover crop plants and their residues also increase water infiltration into soil, thus compensating for the water that cover crops use.