The green manuring is defined as a practice of ploughing in the green plant parts or adding the plant parts (branches, leaves, etc.) from outside and incorporating them into soil for the purpose of improving soil fertility and plant used for this means is termed as green manures.
Green manure crop is cultivated either in that place where green manuring is going to be done or can be brought from a distance. Record shows that best result of green manuring obtained from those crops which mainly belong to leguminous particularly pod bearing beans because they have an ability to fix atmospheric nitrogen in soil. Some commonly cultivated green manure crops are Mung bean (Vigna radiate L.), Sunnhemp (Crotalaria juncea L.), Soybean (Glycine max L.), Egyptian clover (Trifolium alexandrinum L.), cluster bean (Guar) (Cyamopsis tetragonaloba L.), cowpea (Vigna sinesis L.) etc.
An ideal green manure crops should possess the following traits: it should be of short duration show and high seedling vigour; should have fibrous root system; ability to produce high green bio-mass and N in 5-6 weeks; site specific i.e. crop should be established easily in local conditions; it should be tolerant to shade, drought and adverse temperature; have ability to fix maximum nitrogen in soil; it should be easy to incorporate and quickly decomposable; seed easily available in market as well as affordable; not closely related to the following crop as they can attract pests and disease.
When you are going to select green manure crops, just keep in mind some basic and important principles for getting best results. These principles are: fast growth and easily established; inexpensive; it should have the ability to generate good ground cover; should be multipurpose crop that grows on poor soils and not with the main crop; crop selected for green maunring should suit the soil and climatic condition of that region for example jantar and sunhemp is suitable for well irrigated crops, sesbania is suitable for rice growing areas, leaves of Gliricidia spium (cacao) for banana growing regions, guara for canal regions, arhar for barani areas, etc.
Green manuring has many advantages discussed as follows:
Improve physical condition of soil by the addition of organic matter and biological nitrogen fixation
Increase conservation and availability of nutrients
Control over disease, weed and erosion
Provide shade to soil to keep soil cool
As green manure crops grown as a cover or catch crop which results in utilization of excess fertilizer and minimize loss by leaching
Provides habitat for natural enemies and increase population of beneficial microbes by enhancing their growth
Incorporation of Pongamia and Neem leaves helps to control many insects/mite
Increase water holding capacity of soil, improve its aeration and increase porosity
Caution exchange capacity of soil is increased and toxic effects of heavy metals is decreased
How are green manures used? Farmers often see the benefits of green manure but many of them do not use because they do not know how to use and include in the farming system. Burying should be done at that time when crops attain maximum vegetative portion. Green manure crops are directly ploughed into soil about 3-4 weeks before the main crop. To get maximum benefit from green manure crops, it is important to plough them before they begin to flower or fruit. In the case of trees, the leaves and tender branches are cut and then ploughed into the soil. Crop is grown right there where it is utilized, that makes the costs negligible.
The writers are associated with the Department of Agronomy, University of Agriculture Faisalabad, Pakistan.