Green manure crops are grown for their green matter, which is incorporated into the soil. Understanding the benefits of green manure crops will help farmers select green manures and choose ways of managing them that work best for them.
Many crops can be used as green manures: annual species such as pea, lentil, chickling vetch, buckwheat, mustard, oilseed radish, and faba bean; winter annual species like fall rye and winter wheat; biennial species like sweet clover and red clover; and perennial species such as alfalfa and white clover. Sweet clover is the most common green manure in Saskatchewan, though alfalfa, pea, lentil, and chickling vetch are also common.
Green manures offer many benefits when included in a crop rotation. Green manures can increase soil organic matter and improve soil structure. The breakdown of crop residues stimulates biological activity in the soil. Green manure crops also prevent erosion by providing a protective cover during fallow seasons. Green manure crops can be essential to nutrient cycling in organic cropping systems. They act as temporary nutrient storage units that limit nitrogen losses from leaching or from processes that make the nitrogen less available to plants during fallows. Green manure crops break disease cycles and compete with weeds. They may also provide habitat for beneficial insects, such as parasitic wasps, and some produce chemicals that interfere with weed growth.
Legume green manure crops are an essential part of many organic crop rotations. Legumes can add nitrogen through nitrogen fixation and deep rooted green manure crops such as sweet clover and alfalfa can retrieve nutrients such as nitrogen and phosphorous from deep in the soil profile. The crop residues of the green manure are incorporated near the surface, so that when they break down, these nutrients are made more available to subsequent crops. A 2002 survey of 5% of organic farmers in Saskatchewan found fields were less likely to be deficient in soil nitrogen if they had been in green manure crops in the last 5 years.
Despite these many advantages, producers may hesitate to use green manure crops. The green manure year is a year with no income, yet it still requires the input of seed. Green manures can use up limited water reserves, and small seeded crops can be difficult to establish. Weed control can be difficult.
Many of the problems associated with green manure crop management can be overcome with careful crop selection and crop management. Seed costs are often less for smaller seeded crops, and a producer can reduce the impact of seed costs by growing his or her own seed. Water depletion by green manure crops can be reduced by terminating the crop early, and by incorporating the green manure so that some material is left upright for snow trap. Weed number and size are generally less when the green manure seeding rate is higher. Weeds also add soil nitrogen and organic matter, though they may interfere with the growth of the green manure crop. It is generally recommended that green manures be terminated before weeds set seed.
Green manure crops, with their many biological roles, are useful in low-input cropping systems. Green manure selection and the management practices chosen will depend on the producer’s priorities in growing the green manure. Short-term difficulties such as taking a field out of production for a year can be balanced against long-term advantages such as reducing the depletion of soil organic matter, soil nitrogen, and soil phosphorous.
(Source – http://www.organicagcentre.ca/NewspaperArticles/na_green_manures.asp)