"Green manuring is the practice of enriching the soil by turning under undecomposed plant material (except crop residues), either in place or brought from a distance". It is obtained in two ways: by growing green manure crops or by collecting green leaf (along with twigs) from plants grown in wastelands, field bunds and forest. Green manuring is growing in the field plants usually belonging to leguminous family and incorporating into the soil after sufficient growth. The plants that are grown for green manure known as green manure crops. The most important green manure crops are sun hemp, dhaincha, Pillipesara, clusterbeans and Sesbania rostrata. The green manure/cover crops are grown to achieve following objectives:
1. Providing soil cover for No-Tillage (reduces water evaporation and soil temperature, and increases water infiltration).
2. Protect soil and adds organic matter.
3. Add biomass to soil (in order to accumulate soil organic matter, add and recycle nutrients, feed soil life).
4. Promote biological soil preparation and reduce pest and disease infestation.
5. Reducing the use of hazardous chemical fertilizers, pesticides and herbicides.
Different green manures crops are grown to offer different advantages. Some, like alfalfa, are grown for their deep roots and are used to breakup and loosen compacted soil. The legumes, clover and vetch, have the ability to grab nitrogen from the air and eventually release it into the soil through their roots and some assist with creating good soil structure and food for the microbes, once they are tilled in and begin to decompose. Frequently used crops for green manure include: annual ryegrass, barley, buckwheat, clover, winter wheat and winter rye.
CRITERIA FOR THE SELECTION OF GREEN MANURE CROPS:
• Fast growing and produces abundant and succulent shoots (foliages).
• Well adapted to the local condition.
• Have a high rate of nitrogen fixation.
• Have high water-use efficiency when used in drier regions
• Have low establishment and management costs.
• Be easy to sow and manage.
• Achieve good shading and weed suppression.
• Produce a favourable residual effect on cash and subsistence crops.
• Be rustic and require few crop management practices.
• Present good conservation characteristics.
• Avoid proliferation of pests and diseases.
• Avoid competition for land, labour, time, and space with cash or subsistence crops.
Types of green manure:
i) Winter green manures: sown in the autumn and incorporated in the following spring eg legumes (eg vetch) but a major use for this class of crops (even in conventional agriculture) is to minimise nitrogen leaching
ii) Summer green manures they may be grown for a whole season (say April to September) or for a shorter period between two cash crops. These shorter-term green manures can include non-legumes such as mustard and phacelia.
iii) Protected cropping systems offer particular challenges and opportunities for green manuring.
iv) Long term green manures. Leys, usually established for two or three years, are a basic part of many organic arable rotations eg pure clover (when nitrogen fixation is a priority) or a grass/clover mixture (when organic matter build up is also important).
o Green manures helps in protecting soil erosion, reduces water evaporation, soil temperature, and increases water infiltration.
o Green manure crops are an effective tool for controlling weeds.
o Improving soil microbiology by increasing the beneficial soil micro-organisms.
o Green manuring helps in improving soil structure by adding Organic matter.
o In addition to nitrogen from legumes, covers crops help recycle other nutrients on the farm. Nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), and other nutrients are accumulated by cover crops during a growing season.
o Weeds and disease cycles can be disrupted by the use of non-vegetable green manure crops in the crop rotation program. Some green manure plants, including cowpea, have extra-floral nectaries that attract beneficial insects like parasitic wasps to the cropping area.
o Helps in controlling the Pests and insects eg mustards, release natural chemicals into the soil and provide some bio-fumigant action against soil-borne diseases and nematodes which helps to kill harmful insects.
• Direct costs of seed and extra cultivations.
• Lost opportunities for cash cropping.
• Extra work at busy times of the year.
• Exacerbated pest and disease problems (due to the 'green bridge' effect)
• Potential for the green manures to become weeds in their own right.
The most direct economic benefit is derived from the legume cover crops is nitrogen saving. These savings in turn can help in offset cover crop establishment costs. Indirect benefits include herbicide reduction in the case of an allopathic rye cover crop, reduction in insect and nematode control costs in some cases, protection of ground water by scavenging residual nitrate, and water conservation derived from no-till mulch. Longer-term benefits are derived from the build-up of organic matter resulting in increased soil health. Healthy soils cycle nutrients better, quickly absorb water after each rain, and produce healthy crops and bountiful yields.
About Author / Additional Info:
Myself Priti raj pandita and I have done my M.Sc Biotechnology and now pursuing Ph.D. from central university of Gujarat in Environment and Sustainable development. I am very found of writing the article hoping everybody will understand my articles.