agriculture Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created ...

, a green manure is a crop specifically cultivated to be incorporated into the soil while still green. Typically, the green manure's

biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...

is incorporated with a plow or disk, as is often done with (brown)

manure Manure is organic matter that is used as organic fertilizer in agriculture. Most manure consists of animal feces; other sources include compost and green manure. Manures contribute to the fertility of soil by adding organic matter and nut ...

. The primary goal is to add

organic matter Organic matter, organic material or natural organic matter is the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come fro ...

to the soil for its benefits. Green manuring is often used with

legume Legumes are plants in the pea family Fabaceae (or Leguminosae), or the fruit or seeds of such plants. When used as a dry grain for human consumption, the seeds are also called pulses. Legumes are grown agriculturally, primarily for human consum ...

crops to add nitrogen to the soil for following crops, especially in

organic farming Organic farming, also known as organic agriculture or ecological farming or biological farming,Labelling, article 30 o''Regulation (EU) 2018/848 of the European Parliament and of the Council of 30 May 2024 on organic production and labelling of ...

, but is also used in

conventional farming Intensive agriculture, also known as intensive farming (as opposed to extensive farming), conventional, or industrial agriculture, is a type of agriculture, both of crop plants and of animals, with higher levels of input and output per unit of ...

Method of application

Farmers apply green manure by blending available plant discards into the soil. 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. 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.

Functions

Green manures usually perform multiple functions that include soil improvement and soil protection: Leguminous green manures such as

clover Clovers, also called trefoils, are plants of the genus ''Trifolium'' (), consisting of about 300 species of flowering plants in the legume family Fabaceae originating in Europe. The genus has a cosmopolitan distribution with the highest diversit ...

and vetch contain

nitrogen-fixing Nitrogen fixation is a chemical process by which molecular dinitrogen () is converted into ammonia (). It occurs both biologically and abiological nitrogen fixation, abiologically in chemical industry, chemical industries. Biological nitrogen ...

symbiotic Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

in root nodules that fix atmospheric nitrogen in a form that plants can use. This performs the vital function of

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

. Depending on the species of

cover crop In agriculture, cover crops are plants that are planted to ground cover, cover the soil rather than for the purpose of being harvested. Cover crops manage soil erosion, soil fertility, soil quality, water, weeds, Pest (organism), pests, diseases ...

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. Green manure acts mainly as soil-acidifying matter to decrease the alkalinity/pH of alkali soils by generating

humic acid Humic substances (HS) are colored relatively recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The color of humic substances varies from bright yellow to light or dark brown lead ...

and

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

. Incorporation of cover crops into the soil allows the

nutrients A nutrient is a substance used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

held within the green manure to be released and made available to the succeeding crops. This results immediately from an increase in abundance of soil microorganisms from the degradation of plant material that aid in the decomposition of this fresh material. This additional decomposition also allows for the re-incorporation of nutrients that are found in the soil in a particular form such as

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

(N),

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

(K),

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

(P),

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

(Ca),

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

(Mg), and

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

(S). Microbial activity from incorporation of cover crops into the soil leads to the formation of

mycelium Mycelium (: mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Its normal form is that of branched, slender, entangled, anastomosing, hyaline threads. Fungal colonies composed of mycelium are fo ...

and viscous materials which benefit the health of the soil by increasing its

soil structure In geotechnical engineering, soil structure describes the arrangement of the solid parts of the soil and of the Pore space in soil, pore space located between them. It is determined by how individual soil granules clump, bind together, and Soil a ...

(i.e. by aggregation). The increased percentage of organic matter (

) improves

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

infiltration and retention, aeration, and other soil characteristics. The soil 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 In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Lati ...

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. Non-leguminous crops are primarily used to increase biomass. The

root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...

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. Non-leguminous crops are primarily used (e.g.

buckwheat Buckwheat (''Fagopyrum esculentum'') or common buckwheat is a flowering plant in the knotweed family Polygonaceae cultivated for its grain-like seeds and as a cover crop. Buckwheat originated around the 6th millennium BCE in the region of what ...

). The deep rooting properties of many green manure crops make them efficient at suppressing

weed A weed is a plant considered undesirable in a particular situation, growing where it conflicts with human preferences, needs, or goals.Harlan, J. R., & deWet, J. M. (1965). Some thoughts about weeds. ''Economic botany'', ''19''(1), 16-24. Pla ...

s. Some green manure crops, when allowed to

flower Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants ( angiosperms). Typically, they are structured in four circular levels, called whorls, around the end of a stalk. These whorls include: calyx, m ...

, provide

forage Forage is a plant material (mainly plant leaves and stems) eaten by grazing livestock. Historically, the term ''forage'' has meant only plants eaten by the animals directly as pasture, crop residue, or immature cereal crops, but it is also used m ...

for pollinating

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

s. Green manure crops also often provide habitat for predatory beneficial insects, which allow for a reduction in the application of insecticides where cover crops are planted. Some green manure crops (e.g.

winter wheat Winter wheat (usually ''Common wheat, Triticum aestivum'') are strains of wheat that are planted in the autumn to germinate and develop into young plants that remain in the vegetative phase during the winter and resume growth in early spring. C ...

and winter rye) can also be used for grazing.

Erosion control Erosion control is the practice of preventing or controlling wind or water erosion in agriculture, land development, coast, coastal areas, Bank (geography), river banks and construction. Effective erosion controls handle surface runoff and are ...

is often also taken into account when selecting which green manure cover crop to plant. Some green crops reduce plant insect pests and

diseases A disease is a particular abnormal condition that adversely affects the structure or function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical conditions that are asso ...

. Verticillium wilt is especially reduced in potato plants. 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).

Nutrient release

Green manure is broken down into plant nutrient components by

heterotrophic A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

bacteria that consumes organic matter. Warmth and moisture contribute to this process, similar to creating

compost Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical, and biological properties. It is commonly prepared by Decomposition, decomposing plant and food waste, recycling organic materials, and man ...

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

minerals, for example, can be given green manure to generate a higher

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

content in the soil, which in turn acts as a fertilizer. The ratio of carbon to

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 ''Rhizobium'' is a genus of Gram-negative soil bacteria that fix nitrogen. ''Rhizobium'' species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants. The bacteria colonize plant ce ...

are soil organisms that interact with green manure to retain atmospheric nitrogen in the soil.

Legume Legumes are plants in the pea family Fabaceae (or Leguminosae), or the fruit or seeds of such plants. When used as a dry grain for human consumption, the seeds are also called pulses. Legumes are grown agriculturally, primarily for human consum ...

s, such as beans, alfalfa, clover and lupines, have root systems rich in rhizobium, often making them the preferred source of green manure material.

Crops

Many green manures are planted in autumn or winter to cover the ground before spring or summer sowing. *

Alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial plant, perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, ...

, which sends roots deep to bring nutrients to the surface. *

Buckwheat Buckwheat (''Fagopyrum esculentum'') or common buckwheat is a flowering plant in the knotweed family Polygonaceae cultivated for its grain-like seeds and as a cover crop. Buckwheat originated around the 6th millennium BCE in the region of what ...

in temperate regions *

Cowpea The cowpea (''Vigna unguiculata'') is an annual herbaceous legume from the genus '' Vigna''. Its tolerance for sandy soil and low rainfall have made it an important crop in the semiarid regions across Africa and Asia. It requires very few inpu ...

Clover Clovers, also called trefoils, are plants of the genus ''Trifolium'' (), consisting of about 300 species of flowering plants in the legume family Fabaceae originating in Europe. The genus has a cosmopolitan distribution with the highest diversit ...

(e.g. annual sweet clover) *

Fava beans ''Vicia faba'', commonly known as the broad bean, fava bean, or faba bean, is a species of vetch, a flowering plant in the pea and bean family Fabaceae. It is widely cultivated as a crop for human consumption, and also as a cover crop. Vari ...

Fenugreek Fenugreek (; ''Trigonella foenum-graecum'') is an annual plant in the family Fabaceae, with leaves consisting of three small Glossary_of_leaf_morphology#Leaf_and_leaflet_shapes, obovate to oblong leaflets. It is cultivated worldwide as a semiar ...

*Ferns of the genus '' Azolla'' have been used as a green manure in southeast

Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

. *

Lupin ''Lupinus'', commonly known as lupin, lupine, or regionally bluebonnet, is a genus of plants in the legume family Fabaceae. The genus includes over 199 species, with centres of diversity in North and South America. Smaller centres occur in No ...

Millet Millets () are a highly varied group of small-seeded grasses, widely grown around the world as cereal crops or grains for fodder and human food. Most millets belong to the tribe Paniceae. Millets are important crops in the Semi-arid climate, ...

* Mustard *

Peanut The peanut (''Arachis hypogaea''), also known as the groundnut, goober (US), goober pea, pindar (US) or monkey nut (UK), is a legume crop grown mainly for its edible seeds. It is widely grown in the tropics and subtropics by small and large ...

* '' Phacelia tanacetifolia'' *

Radish The radish (''Raphanus sativus'') is a flowering plant in the mustard family, Brassicaceae. Its large taproot is commonly used as a root vegetable, although the entire plant is edible and its leaves are sometimes used as a leaf vegetable. Origina ...

such as tillage radish or daikon radish. *

Sesbania ''Sesbania'' is a genus of flowering plants in the pea family, Fabaceae, and the only genus found in tribe Sesbanieae. Riverhemp is a common name for plants in this genus. Notable species include the rattlebox (''Sesbania punicea''), spiny sesba ...

Sorghum ''Sorghum bicolor'', commonly called sorghum () and also known as great millet, broomcorn, guinea corn, durra, imphee, jowar, or milo, is a species in the Poaceae, grass genus ''Sorghum (genus), Sorghum'' cultivated for its grain. The grain i ...

Soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean. Soy is a staple crop, the world's most grown legume, and an important animal feed. Soy is a key source o ...

* Sudangrass * Sunn hemp, a legume widely grown throughout the tropics and subtropics * 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 * Vetch (''

Vicia sativa ''Vicia sativa'', known as the common vetch, garden vetch, tare or simply vetch, is a Nitrogen fixation, nitrogen-fixing legume, leguminous plant in the family Fabaceae. It is now naturalised throughout the world occurring on every continent, exc ...

'', ''

Vicia villosa ''Vicia'' is a genus of over 240 species of flowering plants that are part of the legume family (Fabaceae), and which are commonly known as vetches. Member species are native to Europe, North America, South America, Asia and Africa. Some other g ...

'') * Winter field bean

History

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. 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. Traditionally, the incorporation of green manure into the soil is known as the

fallow Fallow is a farming technique in which arable land is left without sowing for one or more vegetative cycles. The goal of fallowing is to allow the land to recover and store Organic compound, organic matter while retaining moisture and disrupting ...

cycle of

crop rotation Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. This practice reduces the reliance of crops on one set of nutrients, pest and weed pressure, along with the pro ...

, which was used to allow the soil to regain its fertility after the harvest.

Limitations of green manure

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. 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.

References

External links

Overview of cover crops and green manures
*Using green manure to grow ric
University of California cover crops database
{{Agriculture footer Crops Sustainable technologies Soil improvers Manure Ecology