In agriculture, a green manure is a crop specifically cultivated to be incorporated into the soil while still green. Typically, the green manure's biomass is incorporated with a plow or disk, as is often done with (brown) manure. The primary goal is to add organic matter to the soil for its benefits. Green manuring is often used with legume crops to add nitrogen to the soil for following crops, especially in organic farming, but is also used in conventional farming.
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.
Green manures are meant to be used alongside chemical fertilizers as a way to reduce our use of chemical fertilizers. When used alone, we cannot see all the advantages of green manures.
Functions
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. This performs the vital function of fertilization.
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.
{ | class="wikitable" |- |+Average biomass and nitrogen yields of several legumes ! 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 |} Green manure acts mainly as soil-acidifying matter to decrease the alkalinity/pH of alkali soils by generating humic acid and acetic acid. Incorporation of cover crops into the soil allows the nutrients 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 (N), potassium (K), phosphorus (P), calcium (Ca), magnesium (Mg), and sulfur (S). Microbial activity from incorporation of cover crops into the soil 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). The increased percentage of organic matter (biomass) improves water 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 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 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) The deep rooting properties of many green manure crops make them efficient at suppressing weeds. Some green manure crops, when allowed to flower, provide forage for pollinating insects. 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 and winter rye) can also be used for grazing. Erosion control is often also taken into account when selecting which green manure cover crop to plant. Some green crops reduce plant insect pests and diseases. 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.