A green manure crop is one that is grown with the intention of being absorbed into the soil while still green. Generally, these plants are grown on fallow ground and then dug into the soil before crops (or ornamental plants) are planted, though in certain situations, plants are cultivated in one location and the foliage and roots dug into the soil or used as mulch material in another. Legumes are the most common green manure cover crops. While growing, the crops cover and preserve the soil, as well as adding nutrients to boost fertility. In a single year, cover crops can add over thirty tonnes of organic matter and 200 kilograms of nitrogen per hectare of land. At the location, a variety of green manure cover crops are being evaluated for applicability and impact. The primary purpose is to improve the soil by adding organic matter. Green manuring is a method of adding nitrogen to the soil for subsequent crops, particularly in organic farming, but it is also employed in conventional farming.

Green manures usually perform multiple functions that include soil improvement and soil protection.

• Nitrogen-fixing symbiotic bacteria in root nodules of legume green manures like clover and vetch fix atmospheric nitrogen in a form that plants can utilise. This is where the fertilization function is carried out.

• By creating humic acid and acetic acid, green manure serves primarily as a soil-acidifying substance, lowering the alkalinity/pH of alkali soils.

• By incorporating cover crops into the soil, nutrients stored in green manure can be liberated and made available to subsequent crops. This occurs as a result of an increase in soil microbe population caused by the degradation of plant material, which aids in the decomposition of this new material. This further decomposition also allows for the re-incorporation of nutrients such as nitrogen (N), potassium (K), phosphorus (P), calcium (Ca), magnesium (Mg), and sulfur (S) that are contained in the soil in a specific form (S).

• Microbial activity from the introduction of cover crops into the soil results in the creation of rhizome and stiff components, which improve the soil's structure and health (i.e. by aggregation). 

• Some green manure cultivars' root systems reach deep into the soil, bringing up nutrient resources unavailable to shallower-rooted crops.

• Weed control is a common function of cover crops. The majority of crops are non-leguminous (e.g. buckwheat)   Many green manure crops are effective at suppressing weeds due to their deep roots capabilities. 

• When allowed to flower, several green manure crops provide nourishment for pollinating insects. Green manure crops also serve as a habitat for predatory beneficial insects, reducing the need for insecticides in areas where cover crops are grown.

• Some green manure crops (for example, winter wheat and winter rye) can be grazed.

• When deciding which green manure cover crop to sow, erosion control is typically a factor.

• Plant insect pests and illnesses are also reduced by some green crops.

References

https://en.wikipedia.org/wiki/Green_manure

https://www.icimod.org/activities/green-manure-cover-crops-mulching/