Green manure is made up of plants that are grown to improve the quality of the soil in farms and gardens. The benefits of using green manure, also known as cover crops, include improvements in the fertility and structure of soil, reduced erosion, and an increase in the number of organisms present in the ground. Cover crops can be grown alongside agricultural crops and decorative plants, or they can be grown as part of a rotation process, when areas of land are left bare for long periods of time.
Use of green manure is often recommended as an alternative or complimentary process in areas where animal fertilizers are not available or of limited availability. Cover crops should be easy to grow and should require no extra fertilization or irrigation. Once grown, the green manure plants should be cut back, with cut material left on the surface of the soil as mulch. Another option is to cut up the plants and plow them into the soil just beneath the surface, which allows nutrients to pass quickly into the seeds and young plants grown later.
The roots of cover crops penetrate deeply into the soil structure, providing aeration and nutrients for the soil, thus improving the quality of the ground. Problems with erosion can also be reduced by the introduction of green manure; when green leaf and plant materials are mulched on the surface of the ground, protection is provided from the weather for both the soil surface and organisms living within the soil. Plants that are plowed into the soil help feed worms and other lifeforms that live there, helping to increase soil fertility. Weeds and pests often find it difficult to populate an area planted with cover crops.
Two forms of green manure are used throughout the world: short-term and long-term cover crops. Long-term manure options include trees, such as the Calliandra calothyrsus, that are planted on land that is to be left fallow for long periods of time; long-term crops can be planted in close proximity to other crops, with green material moved to agricultural land when needed. More short-term approaches include the planting of fast growing plants and shrubs between the harvesting of one crop and the sowing of the next batch of seeds. Undersowing of a cover crop is a technique often used, meaning the green manure seeds are planted for growth alongside the previous crop before it is harvested to reduce growing time.
Green manure plants are usually chosen because of their ability to provide large amounts of nitrogen for the soil. Nitrogen is used by plants to aid and increase growth. Popular cover crops include legumes, such as beans, that take nitrogen from the air and transport it through their plant structures to the soil through bacteria in their roots. In agricultural regions, legumes are also used to provide a food crop when an area of land is rested in a crop rotation process.