Green manure is a crop that is cultivated and plowed under to incorporate into the soil for the purpose of improving its physical structure and fertility. It is a type of cover crop that is grown specifically to improve the soil as green manure. Green manure crops are typically quick-growing and are incorporated into the soil while still green. The biomass of the green manure is incorporated with a plow or disk, as is done in rice farming. Green manure crops can be categorized into two main types: legumes and non-legumes. Legumes, such as beans, alfalfa, clover, and lupines, have root systems rich in rhizobium, which is a soil organism that interacts with green manure to retain atmospheric nitrogen in the soil.

Green manure is used to improve soil fertility and structure, and it can also have other functions. Farmers use green manure to build soil organic matter, supply nitrogen and other nutrients for a following crop, and as a major part of the weed control program. Green manure crops can also be used to decrease the alkalinity/pH of alkali soils by generating humic acid and acetic acid.

Some examples of green manure crops include sunn hemp (sanai) - Crotalaria juncea, beans, alfalfa, clover, and lupines. Green manure crops are usually planted in autumn or winter to cover the ground before spring or summer sowing. In a market garden, green manures should effectively account for at least 1/4 of the rotation when supplemented with manures and 1/3 to 1/2 of the rotation when manures are not available.