ement means that the root nodules are sources of nitrogen for legumes, making them relatively rich in nitrogenous amino acids and protein. Nitrogen is therefore a necessary ingredient in the production of proteins. When a legume plant dies in the field, for example following the harvest, all of its remaining nitrogen, incorporated into amino acids inside the remaining plant parts, is released back into the soil. In the soil, the amino acids are converted to nitrate (NO − 3 ), making the nitrogen available to other plants, thereby serving as fertilizer for future crops. Legumes play a key role in the nitrogen cycle, making nitrates available to other plants in the soil. In many traditional and organic farming practices, crop rotation or polyculture involving legumes is common. By alternating between legumes and non-legumes, or by growing both together for part of the growing season, the field can receive a sufficient amount of nitrogenous compounds to produce a good result without adding nitrogenous fertilizer. Legumes are often used as green manure.[citation needed] Sri Lanka developed the polyculture practice known as coconut-soybean intercropping. Grain legumes are grown in coconut (Cocos nuficera) groves in two ways: intercropping or as a cash crop. These are grown mainly for their protein, vegetable oil and ability to uphold soil fertility. However, continuous cropping after 3–4 years decrease grain yields significa