Green Manures Subsidiary Object and Advantages

Crops grown for the purpose of restoring or increasing the organic matter content in the soil are called green manure crops. Their use in cropping system is called ‘Green Manuring’ where the crop is grown in situ or brought from outside and incorporated.

Green leaf manuring consists of gathering green biomass from nearby location and adding it to the soil. In both, the organic material should be worked into the soil while they are fairly young for easy and rapid decomposition. Legumes are usually utilised as green manure crops as they fix atmospheric nitrogen in the root nodules through symbiotic association with a bacterium, rhizobium and leave part of it for utilization of the companion or succeeding crop.

B. Subsidiary Object of Green Manures

(a) Catch crops: Legumes are inter sown in the main standing crop a little before or after harvest. With a view to utilize the nitrates that might form during the off-season or the left over moisture in the soil profile. This may otherwise be lost. Such subsidiary crops are called ‘Catch Crops’. The catch crops ploughed in as green manures or grazed off. Utilizing the nitrates formed in the soil or residual moisture is a primary object of Green Manuring is only incidental.

(b) Shade crops: Green manure crops may be sown in young orchards with the object of shading the soil surface and preventing the rise of temperature. Otherwise the tender roots of fruit plants may be affected by the high soil temperature. In plantation crops like tea and coffee, Gliricidia is used as shade crop first and then incorporated as green manure.

(c) Cover crops: Green manure crops are sometimes grown with the object of clothing the surface with a vegetative cover especially in hill slopes during the rainy weather to avoid soil erosion and runoff. This may also done to check wind erosion. The crop chosen should be capable of covering the surface at the time of commencement of rainy or windy season. Later it is used as Green manure.

(d) Forage crops: Some legumes are also grown for taking a few cuttings of green fodder for cattle in every stages. For example, philippesara seeds are broadcasted in the standing rice crop (3–5 days before harvest) in coastal Andhra Pradesh. The early growth supplies fodder for cattle and the later growth is used for green manure purpose.

C. Advantages of Green Manuring

Green manuring has a positive influence on the physical and chemical properties of the soil. It helps to maintain the organic matter status of arable soils. Green manure serves as a source of food and energy for the soil microbial population, which multiplies rapidly in the presence of easily decomposable organic matter. The enhanced activities of soil organisms not only cause rapid decomposition of the green manure but also result in the release of plant nutrients in available forms of use by the crops.

Green manuring improves aeration in the rice soils by stimulating the activities of surface film of algae and bacteria. Many green manure crops have additional use as sources of food, feed and fuel.

(i) Soil structure and tilth improvement: Green manuring builds up soil structure and improves tilth. It promotes formation of crumps in heavy soils leading to better aeration and drainage. Depending on the amount humus formed, green manuring increases the water holding capacity of light soils. Green manure crops form a canopy cover over the soil and reduce the soil temperature and protect the soil from the erosive action of rain and water currents.

(ii) Fertility improvement of soils: Green manure crops absorb nutrients from the lower layer of soils and leave them in the soil surface layer when ploughed in, for use by the succeeding crops. Green manure crops prevent leaching of nutrients to lower layers. Leguminous green manure plants harbour nitrogen fixing bacteria rhizobia in the root nodules and fix atmospheric nitrogen. Green manure crops increase the solubility of lime phosphates, trace elements etc., through the activity of the soil microorganisms and by producing organic acids during decomposition. Single crop of green manure on an average is reported to fix 60–100 kg nitrogen/ha in single season under favourable conditions.

(iii) Amelioration of soil problems: Green manuring helps to ameliorate soil problems. Sesbania aculeata (dhaincha), when applied to sodic soils continuously for four or five seasons, improves the permeability and helps to leach out the harmful sodic salts. The soil becomes fit for growing crops. Green leaf manure from sources such as Argemone mexicana and Tamarindus indica has a buffering effect when applied to sodic soils.

(iv) Improvement in crop yield and quality: Green manuring increases the yield of crops to an extent of 15–20 per cent compared to no-green manuring. Vitamins and protein content of rice have been found to be increased by green manuring of rice crop.

(v) Pest control: Certain green manure like Pongamia and Neem leaves are reported to have insect control effects.