Green Manure Crops
Another important contribution of green manure to an agricultural field is the nitrogen fixing ability and consequent nitrogen accumulation in the soil, particularly of those leguminous crops used. Depending on the species of cover crop grown, the amount of nitrogen released into the soil lies between 40 and 200 pounds per acre. With green manure use, the amount of nitrogen that is available to the succeeding crop is usually in the range of 40-60% of the total amount of nitrogen that is contained within the green manure crop.
| Average biomass yields and nitrogen yields of several legumes by crop: | Biomass tons acre−1 | N lbs acre−1 |
|---|---|---|
| Sweet clover | 1.75 | 120 |
| Berseem clover | 1.10 | 70 |
| Crimson clover | 1.40 | 100 |
| Hairy vetch | 1.75 | 110 |
- Winter cover crops such as oats or rye have long been used as green manures.
- Fava beans
- Mustard
- Clover
- Vetch (Vicia sativa)
- Buckwheat in temperate regions
- Lupin
- Fenugreek
- Sunn hemp, a tropical legume
- Alfalfa, which sends roots deep to bring nutrients to the surface.
- 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
- Tyfon, a Brassica known for a strong tap root that breaks up heavy soils.
- Ferns of the genus Azolla have been used as a green manure in southeast Asia.
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