Green manures or cover crops are the organic cornerstone of an ecologically sensible gardening. They can provide outstanding benefits for the soil, crop and you by:
Increasing organic matter, earthworms and beneficial micro-organisms.
Increasing the soil's available nitrogen and moisture retention.
Stabilising the soil to prevent erosion.
Bringing deep minerals to the surface and breaking up hardpans.
Also by providing habitat, nectar and pollen for beneficial insects and reducing populations of pests. And of course improving water, root and air penetration in the soil
Green manures are plants grown specifically to benefit the soil - replacing soil nutrients, improving soil structure and increasing humus content. They tend to be quick growing, producing a mass of weed smothering foliage. Some are legumes which have the ability to take up nitrogen from the air, tapping a free source of soil fertility. Nutrients which would otherwise be washed away are taken into the plants and then released when the green manure is cut down and turned in to the top six inches of soil. A green manure crop should be considered whenever an area of ground is to be left free for six weeks or more, and is of particular value through the winter.
Planting green manure will help your soil in many ways. Perhaps most important, it boosts your plot's organic matter (O.M.) level. And a high O.M. level (2.5 to 4%) It keeps nutrients from leaching down beyond reach of crops, it provides food for microbial soil life, It helps legumes fix nitrogen in their root nodules. And it also helps the soil produce good structure and maintains the air-pores necessary for a good crop. In addition to nitrogen from legumes, cover crops help recycle other nutrients on the farm. Nitrogen (N), phosphorous (P), potassium (KB], calcium (Ca), magnesium (Mg), sulfur (S), and other micro nutrients are accumulated by green manure/cover crops during a growing season. When the green manure is incorporated, or laid down as no-till mulch, these plant-essential nutrients become slowly available during decomposition.
Green manures also provide living mulch that will protect soil from erosion and other weathering effects. Indeed, right now, during the late summer and early autumn, is an excellent time to put in a green manure crop. The plants will protect your garden from winter damage and will produce organic matter during the off-season, when much of. Your plot would otherwise lie fallow. Then next spring, your soil will have good tilth instead of being hard and compacted. Many fall-planted green manure crops will also pump excess water out of the soil, allowing you to prepare the soil and plant crops much earlier than usual. French beans, for instance, can pump soil dry in as little as five days of warm weather. (If, however on the other hand, you are trying to conserve soil moisture in early spring, you may want to harvest your green manure crop on the first warm day.)
Winter green manure crop
A winter green manure crop is planted in late summer or autumn to provide soil cover during the winter. Often a legume is chosen for the added benefit of nitrogen fixation, the plant selected needs to possess enough cold tolerance to survive hard winters. Hairy vetch and rye are among the few selections that meet this need. These cool-season legumes include clovers, vetches, medics, and field peas. They are sometimes planted in a mix with winter cereal grains such as oats, rye, or wheat. Winter cover crops can be established by aerial seeding into maturing cash crops in the autumn, as well as by drilling or broadcasting seed immediately following harvest.
A summer green manure occupies the land for a portion of the summer growing season. These warm-season cover crops can be used to fill a niche in crop rotations, to improve the conditions of poor soils, or to prepare land for a perennial crop. Legumes such as cowpeas, soybeans, annual sweet clover, sesbania, crotalaria, or velvet beans may be grown as summer green manure crops to add nitrogen along with organic matter. Non-legumes such as sorghum-sudangrass, millet, forage sorghum, or buckwheat are grown to provide biomass, smother weeds, and improve soil tilth.
The foremost benefit obtained from green manures is the increase of organic matter to the soil. During the breakdown of organic matter by microorganisms, compounds are formed that are resistant to decomposition—such as waxes, and resins. These compounds—and the mycelia, mucus, and slime produced by the microorganisms that help bind together soil particles as granules, or aggregates. A well-aerated soil tills easily and has a high water penetration rate. Increased levels of organic matter also influence soil humus. The material that is the result of the decay of plant and animal materials into the soil. This provides an extensive range of benefits to crop production.
Sod-forming grass or grass-legume mixtures are important in crop rotations because they help replenish organic matter lost during annual cultivation. However, several years of sod production are sometimes required before measurable changes in humus levels occur. In comparison, annual green manures have a negligible effect on humus levels, because tillage and cultivation are conducted each year. They help replenish the supply of active, rapidly decomposing organic matter.
There is also a very informative piece on green manures on Wikipedia: