A green manure is a crop planted where you want to add organic matter to the soil. You dig it into the soil and let the roots, stems, and leaves decay there. A cover crop is a green manure intended to keep soil from washing or blowing away before the main crop (vegetables, groundcovers, lawn grass) fills in.

Green manures add organic matter to the soil, choke out weeds, and protect the soil from the eroding effects of wind and rain. Green manures take up nutrients that might otherwise wash from bare soil, then slowly return them to the soil as the plants decay. The roots loosen the soil and provide additional organic matter. If you grow a legume as a green manure, it adds nitrogen to the soil.

You can plant a grass or legume as a green manure, or a mixture of both. Grasses are better at adding organic matter and stimulating earthworms, while legumes add more nitrogen. If you plant a legume, spread a commercially available inoculant on the soil the first time you plat the legume. The inoculant contains the nitrogen-fixing bacteria that live in legume roots.
When choosing a green manure, look for a plant that germinates easily in your soil and grows quickly, covering the ground. Some manures are planted in the fall and turned under in the spring, giving you organic matter quickly. Others grow from spring to spring, giving long-term erosion control.

Green manures have drawbacks if you don't choose the crop wisely and manage it. If you let a grass green manure grow until it becomes tough, it can take a long time for soil organisms to break it down. You can get around that problem by adding another source of nitrogen, such as bloodmeal, or by growing the grass with a legume. If you plant seeds that can lie dormant in the soil for a few years before germinating, or let a green manure go to seed, you can create a new weed problem.

Planting
To plant a green manure, rake the soil surface to break up clumps. You can wait until you've harvested the existing crop, or sow it a few weeks before the season ends, to protect the soil after harvest. Scatter the seed as evenly as you can, then tamp the soil down. If you don't get rain while the seeds are germinating, water lightly daily. As the seedlings get bigger, water more deeply and less often.

If you plan to leave the manure crop on through the summer, mow or cut it each time it begins to flower, letting the clippings fall to the ground. That way, it can't go to seed and create a weed problem.

When you turn the manure under the soil depends on the results you want. Grasses and other non-legumes add mostly nutrients if you turn them under when they are young and tender. When they are older, they become a better source of organic matter. Mature legumes add more nitrogen than young ones, but can be harder to cut and turn under the soil.
A few weeks before you plan to plant your garden, use a scythe or other tool to chop the leaves and stems, or mow with a mulching mower. It will be easier to work the manure into the soil if you first let it dry for a few days.

If you covered only a small area with a green manure, you can turn it under with a spade. For a larger area, use a rototiller. Tough plants such as old alfalfa may require a few passes.

If you want to speed up the breakdown of the green manure, add animal manure, compost, or another well-balanced organic fertilizer after turning it under.

GROWING GUIDE FOR COVER CROPS AND GREEN MANURES
Annual ryegrass (Lolium multiflorum Lam.) Annual ryegrass dies out after one year, although some of the seed planted the first year may come up for the next two or three. Use it when you need a green manure that germinates quickly, loosens the soil, and prevents soil erosion. Annual ryegrass won't tolerate flooded conditions for long. Annual ryegrass germinates well in cool soils. It does well in a wide range of soil pH and textures, but prefers loamy or sandy soils. Annual ryegrass releases substances toxic to other plants. That trait makes it good for weed control, but means you can't plant small-seeded crops such as lettuce and radishes just after turning annual ryegrass under. Transplants and large-seeded crops aren't affected.
Growing Guidelines: Plant in the fall. You can plant it along with a legume. Scatter 1.5-3 ounces of seed per 100 square feet, or 1-2 pounds per 1000 square feet. Cover with about half an inch of soil, then tamp down the soil.

​Alfalfa (Medicago sativa) Alfalfa is the best legume for fixing nitrogen, if the soil is adequately fertile. Because it's deep rooted, alfalfa draws nutrients from the subsoil, which are returned to the topsoil when you turn the plants under. It is high in protein, minerals, and vitamins. It's a perennial grown for one year. Alfalfa needs a deep, well-drained soil, a pH near neutral, and adequate amounts of phosphorus, potassium, calcium, and sulfur.
Growing Guidelines: Before planting, apply an inoculant. Sow in the spring, planting 1.5 ounces per 100 square feet or 1 pound per 1000 square feet.

Crimson clover (Trifolium incarnatum L.) Crimson clover is a adapted to shady areas and grows well at low temperatures. It's often left in place for a year. Crimson clover can grow in both sandy and well-drained clayey soils if they're not extremely acidic. The germinating seeds and young seedlings need to stay moist. Crimson clover won't tolerate calcareous soil (soil high in limestone), and needs a good supply of phosphorus and potassium.
Growing Guidelines: Apply an inoculant before planting. Sow in October. Scatter 0.5 ounces per 100 square feet or 1 pound per 1000 square feet. Cover with a half inch of soil and tamp down. Keep the soil moist while the plants are young.

Soybeans (Glycine max) Grow soybeans, a summer annual legume, to add nitrogen to poorly drained soils. Soybeans are often grown with a grass such as buckwheat. Soybeans do best on a well-drained soil, with a moderate amount of moisture, but will tolerate poor drainage.
Growing Guidelines: Apply an inoculant to the soil before planting. Plant 3-5 ounces per 100 square feet or 2-3 pounds per 1000 square feet in the spring or early- to mid-summer. Cover with an inch of soil.

Winter wheat (Triticum aestivum) Use winter wheat, an annual grass, as a short-term green manure that you sow in the fall and turn under in the spring. Winter wheat prefers a fertile, loamy soil and a neutral pH.
Growing Guidelines: In the late summer, plant 3-6 ounces per 100 square feet or 2-3 pounds per 1000 square feet.

Buckwheat (Fagopyrum esculentum) Use buckwheat, a broadleaved non-legume annual, to quickly smother summer weeds. It matures in two months; you can plant a second crop for full summer coverage. Buckwheat builds organic matter and adds phosphorus to the soil. Buckwheat grows in most soils, including infertile and acidic ones.
Growing Guidelines: Plant 5 ounces per 100 square feet or 3 pounds per 1000 square feet in the spring or summer.

Sweet clover (Melilotus spp.) Sweet clover is the best legume for improving poor or disturbed soils that aren't acidic. It adds phosphorus, nitrogen, and large amounts of organic matter to the soil. Its deep roots loosen the soil and draw nutrients from the subsoil, which return to the top soil when you turn the green manure under. Sweet clover requires a well-drained soil with a pH near neutral. It absolutely won't tolerate acid soils.
Growing Guidelines: Sow in the spring or summer at a rate of 0.75 to 1.5 ounces per 100 square feet or 0.5-1 pound per 1000 square feet.

Vetch (Vicia spp.) There are several vetch species, including common vetch, hairy vetch, and purple vetch. All are vining annual legumes that are often planted with grasses. Use them as a winter annual to add nitrogen and organic matter to any well-drained or droughty soil. Hairy vetch grows in most soil types, while common vetch needs a fertile, loamy soil. All prefer well-drained soils. The vetches are drought-tolerant.
Growing Guidelines: Apply an inoculant to the soil before planting. Plant in the late fall unless you're growing common or purple vetch in the North, in which case you should plant in the spring. Plant 3 ounces per 100 square feet or 2 pounds per acre. Cover with 0.75 inch of soil.

Ladino Clover Ladino clover is a tall variety of white clover. Because it dies during the winter, you can more easily turn it under in the spring than regular white clover. It produces more organic matter than white clover.
Growing Guidelines: Apply an inoculant to the soil before planting. Plant in the fall, sowing 0.75 ounces of seed per 100 square feet or 0.5 pound per 1000 square feet. Turn under in the spring.

Winter Rye (Secale cereale) Because winter rye grows early in the spring and produces toxins that kill seedlings, it's ideal for smothering weeds. Like other grasses, its fibrous root system and succulent leaves add organic matter. Winter rye prefers a well drained soil.
Growing Guidelines: Plant in the late summer or fall at a rate of 4 ounces per 100 square feet or 2.5 pounds per 1000 square feet. Grow transplants or large-seeded plants after turning it under; small seeded plants can't tolerate the toxic substances.

Sudangrass (Sorghum bicolor) Grow this tall grass during the summer to choke out weeds. It reduces the number of nematodes in the soil and adds a lot of organic matter. Sudangrass has a dense root system that makes it well suited to erosion control and adds organic matter. Sudangrass survives drought well but performs better with more water, especially in the Southwest. It adapts to poorly drained soils.
Growing Guidelines: When the soil is warm, plant 1.5-3 ounces per 100 square feet or 1-2 pounds per 1000 square feet.