Green manures are mostly used as a source of

Green manures are mostly used as a source of Nitrogen.
Green manuring is often used with legume crops to add nitrogen to the soil for following crops, especially in organic farming, but is also used in conventional farming.
Depending on the species of cover crop grown, the amount of nitrogen released into the soil lies between 40 and 200 pounds per acre.
With green manure use, the amount of nitrogen that is available to the succeeding crop is usually in the range of 40 - 60% of the total amount of nitrogen that is contained within the green manure crop.