In agriculture, green manure refers to crops which have already been uprooted (and have often already been stuffed under the soil). The then dying plants are of a type of cover crop often grown primarily to add nutrients and organic matter to the soil (e.g. nitrogen-fixing crops). Typically, a green manure crop is grown for a specific period of time, and then plowed under and incorporated into the soil while green or shortly after flowering. Green manure crops are commonly associated with organic agriculture, and are considered essential for annual cropping systems that wish to be sustainable. Traditionally, the practice of green manuring can be traced back to the fallow cycle of crop rotation, which was used to allow soils to recover.
Green manure crops may include legumes such as cowpeas, soybeans, annual sweet clover, vetch, sesbania, and velvet beans, as well as non-leguminous crops such as sudangrass, millet, sorghum, and buckwheat. Legumes are often used as green manure crops for their nitrogen fixing abilities, while non-leguminous crops are used primarily for weed suppression and addition of biomass to the soil. Green manures usually perform multiple functions, that include soil improvement and soil protection:
- Leguminous green manures such as clover and vetch 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.
Incorporation of cover crops into the soil is immediately followed by an increase in abundance of soil microorganisms that aid in the decomposition of this fresh material. The degradation of plant material allows the nutrients held within the green manure to be released and made available to the succeeding crop. This additional decomposition also allows for the re-incorporation of nutrients that are found in the soil in a particular form such as nitrogen (N), potassium (K), phosphorus (P), calcium (Ca), magnesium (Mg), and sulfur (S). Microbial activity in the soil also leads to the formation of mycelium and viscous materials which benefit the health of the soil by increasing its soil structure (i.e. by aggregation). Soil that is well- aggregated has increased aeration and water infiltration rates, and is more easily turned or tilled than non- aggregated soil. Further aeration of the soil results from the ability of the root systems of many green manure crops to efficiently penetrate compact soils. The amount of humus found in the soil also increases with higher rates of decomposition, which is beneficial for the growth of the crop succeeding the green manure crop.
Green manure crops are also useful for weed control, erosion prevention, and reduction of insect pests and diseases. The deep rooting properties of many green manure crops make them efficient at suppressing weeds. Green manure crops often provide habitat for many native pollinators as well as predatory beneficial insects, which allow for a reduction in the input of insecticides where cover crops are planted. Some green manures are also successful at suppressing plant diseases, especially Verticillium wilt in potato. Incorporation of green manures into a farming system can drastically reduce, if not eliminate, the need for additional products such as supplemental fertilizers and pesticides.
Nutrient creation 
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. Soil 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.
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. Legumes, such as beans, alfalfa, clover and lupines, have root systems rich in rhizobium, often making them the preferred source of green manure material.
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|
- Winter cover crops such as oats or rye have long been used as green manures.
- Fava beans
- Vetch (Vicia sativa)
- Buckwheat in temperate regions
- 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.
Use in organic farming 
Organic farming relies on soil health and cycling of nutrients through the soil using natural processes. Green manures perform the vital function of fertilization, in concert with the addition of animal manures if those are used. While there are a variety of beneficial aspects of the use of green manure, there are also limitations to consider. Time, energy, and resources (monetary and natural) are 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 also brings other organic advantages with it depending upon the plant type used. Buckwheat, for example, prevents the spread of weeds, and Winter wheat and Winter rye can also be used for grazing.
The value of green manure was recognized by farmers in Ancient Greece, who used broad beans by ploughing them into the soil. 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.
See also 
- Sullivan, Preston. 2003. Overview of Cover Crops and Green Manures: Fundamentals of Sustainable Agriculture.
- Vasilakoglou, Ioannis, Dhima, Kico, Anastassopoulos, Elias, Lithourgidis, Anastasios, Gougoulias, Nikolaos, and Chouliaras, Nikolaos. 2011. Oregano green manure for weed suppression in sustainable cotton and corn fields. Weed Biology and Management 11:38-48.
- Larkin, Robert P., Honeycutt, Wayne, and Olanya Modesto, O. 2011. Management of Verticillium Wilt of Potato with Disease-Suppressive Green Manures and as Affected by Previous Cropping History. Plant Dis. 95:568-576.
- Lawrence, James (1980). The Harrowsmith Reader, Volume II. Camden House Publishing Ltd. p. 145. ISBN 0920656102.
- Lawrence, James (1980). The Harrowsmith Reader, Volume II. Camden House Publishing Ltd. p. 146. ISBN 0920656102.
- Piper, C.V.; Pieters A.J. In USDA Farmer's Bulletin. Green Manuring. USDA Farmer's Bulletin. pp. 1250–1295. Retrieved Feb 2, 2010.