Chapter 6

Cover Crops for Organic Farms

Selecting Cover Crop Species

Proper choice and management of cover crops are important in maximizing the benefits and reducing potential problems. To select a species or mix, the organic grower should first first identify the purpose and objectives for its use. Subsequent factors to consider include seed availability, moisture requirements, crop rotation, and equipment for seeding and planting. Cover crop choice is important because different species provide different benefits. For example, small grains such as barley, oat, rye, triticale and winter wheat may provide biomass for building soil organic matter and weed suppression. Some crops may also have allelopathic effects, where chemicals produced by the cover crop inhibit growth of weeds. This can be beneficial for weed control, but may be detrimental to small seeded vegetable crops that are direct sown within two weeks after cover crop kill. Legumes such as crimson clover, ladino white clover, red clover, hairy vetch and winter pea do not generally produce as much biomass as small grains, but are able to fix nitrogen that may be available for the following cash crop. A mixture of two or more cover crop species—usually a grain and a legume—can optimize the benefits of both crop types.

Nitrogen Contribution

Growing legumes (e.g., peas, vetches, clovers, beans) is one of the most important tools to increase soil fertility. Legumes fix nitrogen from the atmosphere and make it available in the soil to other plants. They use a symbiotic relationship with soil-dwelling bacteria (rhizobia), which take gaseous nitrogen from the soil air and convert or “fix” it into a form the plant can use.

Carbon Nitrogen Ratio

When selecting our crop and cover crop rotation it is important that the grower takes into consideration the C:N ratio of cover crops to be included in the rotation. The C:N ratio affects cover crop decomposition because the bacteria using the carbon from the cover crop residue also need nitrogen. If the cover crop residue has enough nitrogen (a low C:N ratio) to supply the bacteria need, any excess nitrogen is released into the soil for plants to use. This process is called mineralization. However, if the cover crop residue does not have enough nitrogen (a high C:N ratio) to supply the bacteria need, the bacteria will take ammonium and nitrate from the soil to grow.

Improve Nutrient Availability in the Soil

Cover crop plant roots release most of the sugars the plant has produced into the soil to feed soil microorganisms that in turn mineralize, release, and recycle nutrients to the next crop. When incorporated into the soil, cover crop biomass is decomposed by soil bacteria and fungi, releasing nutrients in the process. High biomass-producing cover crops will be best for nutrient cycling, such as sorghum-sudan for a summer annual, or cereal rye, triticale, or annual ryegrass for a winter annual.

Scavenge Nutrients

For best nutrient scavenging, use crops with extensive root systems that develop quickly after planting. Non-legumes such as winter annual grasses and small grains (triticale, wheat, rye, ryegrass, etc.) are excellent nutrient scavengers. Grasses make efficient use of nitrogen that might leach down through the soil and eventually be lost out of the root zone, because they scavenge and take up nitrogen rather than fixing their own as legumes do. Their roots are adept at branching outward and downward to tap into plant-available soil nitrogen. Late summer or early fall planted oats can be grown as a nurse crop in a winter annual mix, and grow very quickly in the fall to take up nutrients before the other slower growing winter annual cover crops are fully established.

Soil Erosion Considerations

Erosion occurs most rapidly on cropland where there is no soil cover. Cover crops can provide protection during those periods when a primary crop is not present. For example, cereal grains provide temporary winter cover after low residue summer crops (e.g., soybeans, peanuts, cotton) are harvested.

Improve Soil Structure

Increasing soil organic matter improves soil physical properties through a complex process. As plant residues degrade, the soil microbes feeding on them release glue-like compounds into the soil (gums, waxes, and other substances and exudates), which cement soil particles together to form stable soil aggregates, resulting in improved soil structure and tilth. Both grasses and legumes have mychorrizal associations, but grasses are characterized by dense masses of fibrous roots that improve soil structure by exuding polysaccharides.

Improve Drainage and Alleviate Soil Compaction

Deep rooted cover crops can help break through compacted layers in the soil like plow pans or hard pans, improving drainage. The penetrating roots of the cover crop make channels through which soil water can move after the root system decomposes. Annual ryegrass has deep branching roots, and species with taproots such as many brassicas and legumes also penetrate deep into soil.

Conserve Soil Moisture

The higher the carbon to nitrogen ratio of the crop, the more slowly it will decompose and the longer the residue will serve as a moisture-conserving, weed-suppressing mat.  Small grain cover crops are generally better for this purpose.

Winter Factors

Winter hardiness, cold temperature tolerance, and frost tolerance are three factors to consider for a winter cover crop. All three are slightly related, but are different as to the plants’ mechanism for coping with the problem. Winter hardiness is often related to the ability of the plant to go dormant before the harsh winter conditions hit.

Summer Factors

For those summer cover crops, three factors are most important…drought tolerance, heat tolerance, and low water requirement. Summer cover crops should not require a large amount of moisture to grow, as this will rob future crops of stored moisture, and irrigation costs money.

Relative Vigor of Cover Crops

Cover crops can reduce plant vigor in the case of tree/vine crops, which can be either an asset or a liability, depending on available moisture, tree/vine size, and available nutrients. In cases where plant growth in orchards/vineyards is vigorous, dense sod-forming grasses such as turf-type tall fescue and perennial rye grass may be grown to reduce excessive tree/vine growth.

Growth Habit

The growth habit of a cover crop affects its ability to protect the soil, smother weeds, or act as a nurse crop, and can affect residue management as well. Physical stature may be classified as erect, semi-erect, or prostrate. Prostrate forms do the best job of covering the soil, while semi-erect or erect forms are best as nurse crops when planted in mixtures with legumes.

Insect and Disease Suppression

Care must be taken when selecting species as to the efficacy of its ability to break the insect and disease cycles. Cover crops provide excellent habitats for predator insects, such as spiders and ladybugs, which like to feed on harmful insects like aphids, leafhoppers, thrips, mites or caterpillars.

Weed Management Objectives

Cover crops help control weeds in spring and fall by out-competing them for resources, by not allowing a niche for them to germinate and through allelopathic compounds. Small seeded annual weeds are controlled more than other weeds by cover crops. Cover crops do not target a single weed or family of weeds for reduction, but instead reduce the overall density of plant species. A dense cover crop population reduces weeds more than does a sparse stand or open-type growth habit.

Seed Availability and Cost of Planting

Often the seed available for cover cropping is limited to varieties that are grown for other purposes (e.g., grain production). Availability may change from year to year as new varieties are developed.

Cover Crops for Specific Purposes

The following list provides some basic information on a number of cover crops, including hardiness, biomass, weed suppression, other characteristics, and management methods.

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