Green manure

In agriculture, a 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 of time, and then ploughed 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.

Functions

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 on a particular farm 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.

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.

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).

See also

• Fertilizer
• Organic gardening
• Soil innoculant
• Cover crop

External links

• Overview of cover crops and green manures
• Methods of Green Manuring
• Using green manure to grow rice [1]
• University of California cover crops database