A cover crop, also called green manure, is a plant specifically grown for reincorporation into the soil. Regular use of using cover crops improves soil nutrients and structure, suppresses opportunistic plants, prevents erosion, and can attract beneficial insects. Unlike cash crops which are plants grown for money, cover crops are not grown for commercial sell.
Some plants are better at drawing key nutrients, such as nitrogen, out of the environment and storing it in the plant's cells. Once the plants are reincorporated into the soil and rot, these nutrients are gradually released into the soil in forms that are more readily-available for other plants.
Cover crops can break down hard soils and add organic matter to light soils.