Green manure
Encyclopedia
In agriculture
, a green manure is a type of cover crop
grown primarily to add nutrient
s and organic matter
to the soil
. 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.
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.
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).
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
, a green manure is a type of cover crop
Cover crop
Cover crops are crops planted primarily to manage soil fertility, soil quality, water, weeds, pests, diseases, biodiversity and wildlife in agroecosystems , ecological systems managed and largely shaped by humans across a range of intensities to produce food, feed, or fiber.Cover crops are of...
grown primarily to add nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
s and organic matter
Organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
to the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
. 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
Crop rotation
Crop rotation is the practice of growing a series of dissimilar types of crops in the same area in sequential seasons.Crop rotation confers various benefits to the soil. A traditional element of crop rotation is the replenishment of nitrogen through the use of green manure in sequence with cereals...
, 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 cloverCloverClover , or trefoil, is a genus of about 300 species of plants in the leguminous pea family Fabaceae. The genus has a cosmopolitan distribution; the highest diversity is found in the temperate Northern Hemisphere, but many species also occur in South America and Africa, including at high altitudes...
and vetch contain nitrogen-fixingNitrogen fixationNitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
symbiotic bacteriaBacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
in root nodules that fix atmospheric nitrogen in a form that plants can use. - Green manures increase the percentage of organic matter (biomassBiomassBiomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
) in the soil, thereby improving waterWaterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
retention, aeration, and other soil characteristics. - The rootRootIn vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
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 weedWeedA weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...
suppression and prevention of soil erosionErosionErosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
and compaction are often also taken into account when selecting and using green manures. - Some green manure crops, when allowed to flowerFlowerA flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
, provide forageForage (honeybee)For bees, their forage or food supply consists of nectar and pollen from blooming plants within flight range. The forage sources for honey bees are an important consideration for beekeepers. In order to determine where to locate hives for maximum honey production and brood one must consider the...
for pollinating insectInsectInsects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s.
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
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
(N), potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
(K), phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
(P), calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
(Ca), magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
(Mg), and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
(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
Soil structure
Soil structure is determined by how individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them...
(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
Verticillium wilt
Verticillium Wilt is a wilt disease of over 300 species of eudicot plants caused by one of two species of Verticillium fungus, V. dahliae and V. albo-atrum. Many economically important plants are susceptible including cotton, tomatoes, potatoes, eggplants, peppers and ornamentals, as well as others...
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 Trifolium alexandrinum Trifolium alexandrinum is an annual clover cultivated mostly in irrigated sub-tropical regions, and used as fodder, mainly for cattle and milk buffalo. It was an important winter crop in ancient Egypt, and was introduced into northern India in the early nineteenth century... |
1.10 | 70 |
| Crimson clover Crimson clover Trifolium incarnatum, known as Crimson clover or Italian clover, is a species of clover in the family Fabaceae, native to most of Europe. The species name incarnatum means "blood red".... |
1.40 | 100 |
| Hairy vetch Hairy vetch Vicia villosa, known as the hairy vetch, fodder vetch or winter vetch, is a plant native to Europe and western Asia. It is a legume, grown as a forage crop.... |
1.75 | 110 |
- Winter cover crops such as oatOatThe common oat is a species of cereal grain grown for its seed, which is known by the same name . While oats are suitable for human consumption as oatmeal and rolled oats, one of the most common uses is as livestock feed...
s or ryeRyeRye is a grass grown extensively as a grain and as a forage crop. It is a member of the wheat tribe and is closely related to barley and wheat. Rye grain is used for flour, rye bread, rye beer, some whiskeys, some vodkas, and animal fodder...
have long been used as green manures. - Fava beans
- MustardMustard plantMustards are several plant species in the genera Brassica and Sinapis whose small mustard seeds are used as a spice and, by grinding and mixing them with water, vinegar or other liquids, are turned into the condiment known as mustard or prepared mustard...
- CloverCloverClover , or trefoil, is a genus of about 300 species of plants in the leguminous pea family Fabaceae. The genus has a cosmopolitan distribution; the highest diversity is found in the temperate Northern Hemisphere, but many species also occur in South America and Africa, including at high altitudes...
- Vetch (Vicia sativa)
- BuckwheatBuckwheatBuckwheat refers to a variety of plants in the dicot family Polygonaceae: the Eurasian genus Fagopyrum, the North American genus Eriogonum, and the Northern Hemisphere genus Fallopia. Either of the latter two may be referred to as "wild buckwheat"...
in temperate regions - LupinLupinLupinus, commonly known as Lupins or lupines , is a genus in the legume family . The genus comprises about 280 species , with major centers of diversity in South and western North America , and the Andes and secondary centers in the Mediterranean region and Africa Lupinus, commonly known as Lupins...
- FenugreekFenugreekFenugreek is a plant in the family Fabaceae. Fenugreek is used both as a herb and as a spice . The leaves and sprouts are also eaten as vegetables...
- Sunn hempSunn HempCrotalaria juncea, known as Sunn or sunn hemp, is a tropical Asian plant of the legume family .Grown as a source of green manure, fodder and the lignified fiber obtained from its stem, it bears yellow flowers and elongate, alternate leaves.Sunn Hemp is also being looked at as a possible...
, a tropical legume - AlfalfaAlfalfaAlfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
, 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 BrassicaBrassicaBrassica is a genus of plants in the mustard family . The members of the genus may be collectively known either as cabbages, or as mustards...
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 AsiaAsiaAsia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
.
Use in organic farming
Organic farmingOrganic farming
Organic farming is the form of agriculture that relies on techniques such as crop rotation, green manure, compost and biological pest control to maintain soil productivity and control pests on a farm...
relies on soil health
Health
Health is the level of functional or metabolic efficiency of a living being. In humans, it is the general condition of a person's mind, body and spirit, usually meaning to be free from illness, injury or pain...
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
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
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).