Chapter 6

Cover Crops for Organic Farms

Green Manures

Cover crops are often planted to improve soil structure, suppress growth of weeds, and protect the soil from erosion caused by wind and water. Green manures are a subset of cover crops that are grown specifically to provide nitrogen, increase the organic matter content and/or scavenge nutrients in the soil when turned into the soil. Green manures and cover crops are often used interchangeably. Green manures are frequently used in vegetable crop production. The benefits of organic matter to the overall health of the soil are significant. Organic matter keeps nutrients from leaching down beyond reach of crops, provides food for microbial soil life, helps legumes fix nitrogen in their root nodules, and helps the soil produce good structure and maintain the air-pore spaces essential to good crop health. Green manures can be one of the most sustainable ways to provide nitrogen and other nutrients. As opposed to manure or compost, green manures do not cause phosphorus loading and there is reduced leaching of nitrogen because nutrients are released slowly. Like a cover crop, green manures protect against soil erosion, suppress weeds, and cycle nutrients that might otherwise be unavailable to crops.

Green Manures Species Selection

Green manures can be legumes or non-legumes. Legumes are generally considered to be nitrogen fixing but this will only happen in the presence of correct strains of Rhizobium bacteria. Legumes used as green manures can provide a significant source of nitrogen for the next crop; this is referred to as a nitrogen credit. Legumes make excellent green manures because they have low carbon to nitrogen ratios, which results in a relatively quick release of nitrogen as the plants breakdown. Because of this they add nitrogen relatively quickly to the soil but the amount of organic matter contributed to the soil is limited over the long-term. Non-leguminous green manures will not fix nitrogen but can be very effective at adding organic matter and/or scavenge nutrients in the soil.

Nitrogen Management with Green Manures

A vigorous green manure growing for four to six months before incorporation typically adds between 100 and 200 lb/acre nitrogen to the soil for the succeeding crop. Several factors influence the release of nitrogen from green manure crops including soil temperature and moisture and placement of the green manure. In general, nutrient release will be slower at lower soil temperatures because the soil organisms that breakdown organic matter has lower biological activity or work slower at lower temperatures. Nutrient release is slower when soil is dry or waterlogged for the same reason.

Carbon:Nitrogen Ratio of Green Manures

Within the first year after green manure incorporation, only a small portion of the green manure nitrogen will typically be used by the next crop. The nitrogen recovery rates for a crop after legumes range from 10 to 50 percent of the green manure nitrogen. This is attributable in part to immobilization of nitrogen by soil microorganisms and also to the fact that the nitrogen is liberated from the residues very quickly, before the greatest need of the crop.

Potential Nitrogen Losses

Nitrate is also the most water-soluble form of nitrogen. Whenever there is more nitrate than plant roots can absorb, the excess may leach with heavy rain or irrigation water. Conventional plowing and aggressive disking can cause a rapid decomposition of green manures, which could provide too much nitrogen too soon in the cropping season. No-till systems will have a reduced and more gradual release of nitrogen, but some of that nitrogen may be vulnerable to gaseous loss, either by ammonia volatilization or by denitrification, which occurs when nitrate (NO3¯) converts to gases under low oxygen (flooded) conditions. Some possible solutions at minimizing nitrogen losses are as follows:

Organic Matter Management with Green Manures

When green manures are incorporated into the soil they break down to form soil organic matter. This is really important as green manures act as a store for nutrients which are released when they are returned back to the soil. The challenge for organic farmers is to time this release to coincide with the demands of the next crop. Of more importance is the carbon content of green manures which ultimately breaks down to form humus. Cereals and grasses are high in carbonaceous (carbon containing) material; however, young growth is only slightly carbonaceous. As the crop ages and becomes more fibrous, the carbon-to-nitrogen (C:N) ratio increases and the material becomes more carbonaceous.

Undersowing with Green Manures

Green manure can be introduced into a rotation by undersowing or interseeding the new plant seeds into an existing crop. For example, a legume can be overseeded into a standing small grain crop in late winter. The green manure should be sown into a spring cereal when it is around six inches (15 cm) high. The legume will germinate and grow beneath the small grain canopy. When undersowing a cereal it is best to use one of the less aggressive green manure species such as white clover or yellow trefoil.

Mowing Green Manure Crops

Mowing is an essential part of growing most green manures. Early mowing can make the difference between a well-established green manure and a stand, which is persistently weedy. Most species–including red clover, white clover, lucerne, Persian clover, and yellow trefoil–can tolerate being topped close to the ground to control weeds.

Terminating Green Manure Crops

The green manure should be turned into the soil when the combination of growth and succulence of the crop is at its maximum. This condition generally occurs when the crop is at its half-height. Knowledge of the growth habit of the crop is needed to recognize this stage of growth. Another and probably more useful guideline is to turn the crop under at the initiation of bloom.

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