Green manure is a type of cover crop grown primarily to add nutrients and organic matter to the soil. Typically, a green manure crop is grown for a specific period, and then plowed under and incorporated into the soil as they decompose. Green manures usually perform multiple functions, that include soil improvement and soil protection. They may help to reduce the need for synthetic fertilisers.
- Leguminous green manures such as clover contain nitrogen-fixing symbiotic bacteria in root nodules that fix atmospheric nitrogen in a form that plants can use.
- Green manures increase the percentage of organic matter (biomass) in the soil, thereby improving water retention, aeration, and other soil characteristics.
- The root systems of some varieties of green manure grow deep in the soil and bring up nutrient resources unavailable to shallower-rooted crops.
- Common cover crop functions of weed suppression and prevention of soil erosion and compaction are often also taken into account when selecting and using green manures.
- Some green manure crops, when allowed to flower, provide forage for pollinating insects.
Plants used as green manure[edit | edit source]
Temperate climates[edit | edit source]
- Winter cover crops such as oats or rye have long been used as green manures.
- Winter tares
- Winter field beans
- Velvet Bean (Mucuna pruriens) Common in the southern US during the early part of the 20th century, before being replaced by soybeans. Popular today in most tropical countries, especially in Central America where it is the main green manure used in Slash/Mulch farming practices.
Tropical climates[edit | edit source]
- Ferns of the genus Azolla have been used as a green manure in southeast Asia.
- Sesbania rostrata has been introduced as green manure crop before rainy season rice in the rice soybean system. The legume is broadcasted in mid May and incorporated into soil after 55-60 days. Rice is then transplanted 7-10 days after Sesbania incorporation. The on-station studies and on-farm participatory experimentation show convincing results of increasing rice yield over 20%, which is equivalent to yield obtained by applying 54 kgN/ha. After two years of farmer participation, the area planted to Sesbania rostrata in the test site as green manure crop has been increased three folds.
Green manures in organic farming[edit | edit source]
Organic farming relies on soil health and cycling of nutrients through the soil using natural processes, such as the addition of animal manures. If animal manures are not available, in a stockless rotation, green manures perform a vital function of fertilization.