Green manures that are in the legume family, such as peas, beans, and clovers, have an added bonus. Nitrogen-fixing bacteria living around their roots can take nitrogen form the air and convert it to a form the plant can absorb. Most seed dealers sell the inoculum which is actually bacterial spores. Apply by coating seeds, according to manufacturer's directions, with fresh inoculum before sowing. This nitrogen can become available to subsequent crops.
Legumes break down fairly quickly when turned into soil or composted.
Legumes work well when mixed with a grass crop. The nutrients held in the green manure plant tissue is available to future crops through decomposition. As the harvested green manure rots, the carbon captured from the air through photosynthesis becomes an important part of soil organic matter. The various nutrients absorbed into the plant tissues from the soil or from the nitrogen-fixing bacteria are broken down into forms that crops can readily absorb. More information about this topic is available through your local Cooperative Extension Office .