Farmers' bulletin (United States. Dept. of Agriculture); no. 1250

"Green manuring means turning under suitable crops to enrich the soil. Such crops may be turned under green or when ripe. Green manuring adds organic matter and, directly or indirectly, nitrogen to the soil. Leguminous crops are most desirable for green manuring, since they add to the soil nitrogen gathered from the air in addition to the organic matter which they carry. Besides the nitrogen in the legumes turned under, an additional supply of nitrogen is fixed in the soil by the action of bacteria, using the carbon in the organic matter as a source of energy. Turning under an entire crop is advised only when the soil is poor and for the purpose of starting a rotation. Turning under catch crops or winter-grown green crops is an economical and successful method of supplying nitrogen." -- p.