Sustaining agriculture through green manuring
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Applying organic matter and enhancing biological nitrogen fixation combined with mineral fertilizers help maintain soil fertility for sustainable farming.
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CONTINUOUS CROPPING and frequent cultivation of soil result in breaking down of crumbs of aggregates and destruction of organic matter leading to depletion of soil fertility. Intensive use of inorganic fertilizers degrades and emits toxicants of chemicals that enter the food chain endangering the whole life sustaining system through nitrate poisoning.
The use of chemical fertilizers is the quickest and the shortest way of boosting crop production. But fossil fuel resources are shrinking with escalating costs. Hence the need for alternate sources of plant nutrients.
Applying organic matters, recycling organic wastes and enhancing biological nitrogen fixation combined with the use of mineral fertilizers are a few measures to maintain an adequate level of soil fertility for sustainable farming.
Green manures can be defined as any crops or plants grown and ploughed into the soil to improve soil fertility by the addition of organic matter and nitrogen.
For green manuring, fast growing legumes with more vegetative growth should be used. Crotalaria juncea (Sunhemp) and Sesbania speciosa (Sesbania) are the common green manures in South India which produce relatively higher biomass and accumulate more nitrogen. S.acculeata (Daincha), a native of Africa, is more acceptable to Indian farmers and can accumulate 108 kg N/ha in 60 days of growth. It can be grown throughout the year and withstands a wide range of soil conditions like drought, salinity, alkalinity and water logging. S.rostrate (manila agathi), a recently introduced leguminous green manure crop to India from Senegal, is a special crop and it produces effective aerial rhizobial nodules on the surface of the stem too, besides the roots.
It is a quick growing green manure tolerant to flooding and water logging. It is capable of producing 25 tonnes of green biomass/ha.
In a field experiment conducted at Annamalai University, the productivity of certain green manure crops like Sesbania rostrata, S.acculeata and Crotalaria juncea was studied. It was found that S.rostrate produced high biomass with greater nitrogen accumulation which was closely followed by S.acculeata.
The sowing period February-May was more favourable to S.aculeate and yielded more biomass as compared to that S.rostrate.
The Crotalaria juncea was poor growing green manure with lesser growth attributes. The growth of green manures was good when the sowing time was during the month of June-August. But the October-December sown green manures yielded less biomass.
Green manuring should be encouraged where irrigation facilities are available. For effective green manuring, legume crops are grown for 40-50 days (before flowering) to attain full vegetative growth and then ploughed in the same field before sowing the next crop.
Growing of green manure crops in the off-season, reduces weed proliferation and weed growth. It helps in reclamation of alkaline soils.
Root knot nematodes can be controlled by green manuring. Hence, green manures are a useful tool for sustainability in productivity.
P. Panneerselvam
& R. Issac Manuel
Department of Agronomy
Annamalai University,
Annamalai Nagar 608002
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