Green manure
Organic material left on an agricultural field to be used as a mulch or soil amendment From Wikipedia, the free encyclopedia
Organic material left on an agricultural field to be used as a mulch or soil amendment From Wikipedia, the free encyclopedia
In agriculture, a green manure is a crop specifically cultivated to be incorporated into the soil while still green.[1] Typically, the green manure's biomass is incorporated with a plow or disk, as is often done with (brown) manure. The primary goal is to add organic matter to the soil for its benefits. Green manuring is often used with legume crops to add nitrogen to the soil for following crops, especially in organic farming, but is also used in conventional farming.[2]
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A field of clover, a green manure crop |
Farmers apply green manure by blending available plant discards into the soil.[3] Farmers begin the process of green manuring by growing legumes or collecting tree/shrub clippings. Harvesters gather the green manure crops and mix the plant material into the soil. The un-decomposed plants prepare the ground for cash crops by slowly releasing nutrients like nitrogen into the soil.[3]
Farmers may decide to add the green manure into the soil before or after cash crop planting. This variety in planting schedules can be seen in rice farming.[4]
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Green manures usually perform multiple functions that include soil improvement and soil protection:
| 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 |
Incorporation of green manures into a farming system can drastically reduce the need for additional products such as supplemental fertilizers and pesticides.
Limitations to consider in the use of green manure are time, energy, and resources (monetary and natural) required to successfully grow and utilize these cover crops. Consequently, it is important to choose green manure crops based on the growing region and annual precipitation amounts to ensure efficient growth and use of the cover crop(s).
Green manure is broken down into plant nutrient components by heterotrophic bacteria that consumes organic matter. Warmth and moisture contribute to this process, similar to creating compost fertilizer. The plant matter releases large amounts of carbon dioxide and weak acids that react with insoluble soil minerals to release beneficial nutrients. Soils that are high in calcium minerals, for example, can be given green manure to generate a higher phosphate content in the soil, which in turn acts as a fertilizer.[6]
The ratio of carbon to nitrogen in a plant is a crucial factor to consider, since it will impact the nutrient content of the soil and may starve a crop of nitrogen, if the incorrect plants are used to make green manure. The ratio of carbon to nitrogen will differ from species to species, and depending upon the age of the plant. The ratio is referred to as C:N. The value of N is always one, whereas the value of carbon or carbohydrates is expressed in a value of about 10 up to 90; the ratio must be less than 30:1 to prevent the manure bacteria from depleting existing nitrogen in the soil. Rhizobium are soil organisms that interact with green manure to retain atmospheric nitrogen in the soil.[9] Legumes, such as beans, alfalfa, clover and lupines, have root systems rich in rhizobium, often making them the preferred source of green manure material.[citation needed]
Many green manures are planted in autumn or winter to cover the ground before spring or summer sowing.[10]
Green manures have been used since ancient times. Farmers could only use organic fertilizers before the invention of chemical nitrogen fertilizer. There is evidence for the Greeks plowing broad beans and faba beans into the soil around 300 B.C. The Romans also used green manures like faba beans and lupines to make their soil more fertile.[3] Chinese agricultural texts dating back hundreds of years refer to the importance of grasses and weeds in providing nutrients for farm soil. It was also known to early North American colonists arriving from Europe. Common colonial green manure crops were rye, buckwheat and oats.[6]
Traditionally, the incorporation of green manure into the soil is known as the fallow cycle of crop rotation, which was used to allow the soil to regain its fertility after the harvest.[citation needed]
Managing green manure improperly or without additional chemical inputs may limit crop production. Mixing green manures into the soil without enough time before crop planting could stop the flow of nitrogen (nitrogen immobilization). When nitrogen stops flowing there won't be enough nutrients for the next crop planting.[3] Farming systems with short growth spans for green manure are not usually efficient. Farmers must weigh the cost of green manures with their productivity to determine suitability.[4]
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