Saturday, 13 August 2016

Biofertilizers

Dr Rashmi Tyagi
Associate Professor and HoD
The NorthCap University, Gurgaon

Biofertilizers are microorganisms (bacteria, cyanobacteria andfungi) which enhance the availability of nutrients, like nitrogen and phosphorus,to the crop plants. In addition to nitrogen/phosphorus they also provide organic carbon, various growth promoting substances and improves the physical properties of the soil. With the introduction of high-yielding crop varieties, demand for chemical fertilizers has been increased severalfolds. Further, crop plants are able to use only about 35% of the applied chemical fertilizer as considerable amount is continuously lost from the soil due to leaching, denitrification and volatilization. There are also many ecological limitations to the heavy use of chemical fertilizers - the concentration of toxic nitrate has been found to be increased in water bodies near heavily fertilized fields.Denitrification of nitrate produces nitrogen gas and small amount nitrousoxide (about 10 %) and the latter being a green house gas results in global warming and health hazards.

Unlike chemical fertilizers which have various environmental impacts,biofertilizers are economical, ecofriendly and have long - term sustainability. For optimum crop yields biofertilizers are generally used incombination with the chemical fertilizers. Some micro-organisms with practical or potential application as biofertilizer are Rhizobium, Nostoc, Azotobacter, etc.

Rhizobium sp. are symbiotic N -fixing 2bacteria forming pink coloured rootnodules in leguminous plants, like gram,mung, pea, pigeonpea (arhar dal) (Fig. 1).These bacteria inside the nodules fix atmospheric nitrogen into ammonia with the help of the enzyme nitrogenase. The estimated N fixed by Rhizobium ranges 2from 50-150 kg/ha/year and increases the yield by 10-15%. The culture of Rhizobiumis grown in incubators orfermenters and mixed with a carrier material like peat.Seeds coated with this biofertilizer are used for sowing in the fields.


Azotobacter
is a freelivingN -fixing 2bacterium used for the crops wheat, rice,and vegetables and is capable of fixing 15-30 kg N/ha/year. Azospirillumsp. are found in association with roots of many plants ofgrass family like jowar, wheat, bajra, etc.and provides about 15-30 kg N/ha/year.

Cyanobacteria (blue-green algae) arephotosynthetic, prokaryoticmicroorganisms and many of them fixatmospheric N , e.g. Nostoc, Anabaena, 2Aulosira, Tolypothrix, etc. (Fig.2).Cyanobacterial biofertilizer are preferably used in tropical rice fields where they contribute 20-30 kg N/ha/year. Moreover,after application for 3-4 successive years in the field, a sufficient population builds upand their further application in the field is not required. They are also useful inreclamation (curing) of alkaline and saline soils. Cyanobacterial biofertilizers are grown in small tanks or ponds in the sunlight and the sun-dried biomass is used to sprinkle the rice field.

Azolla is a water fern harbouring symbiotic cyanobacterium Anabaena azollaein thecavity of its leaves. It is widely used as biofertilizer in rice fields of temperate countries, like Vietnam (Fig. 3).


Phosphate Solubilizing Bacteria (PSB)are species of Pseudomonas, Bacillus,Thiobacillus, etc. which convert insolublenon-available inorganic phosphorus (rockphosphate) present in the soil intosoluble phosphorus utilized by the cropplants.

Vesicular Arbuscular Mycorrhizal(VAM) Fungi e.g. Glomus, are found inassociation with roots of plants; some atthe surface of roots (ectomycorrhizae)whereas others inside the roots(endomycorrhizae). They have multiple benefits converting non-available phosphorus into available forms,producing growth promoting substances and protecting crop plants against soil pathogens. These fungi are yet to be exploited on a commercial scaleand in USA they are applied in Citrus crop by inoculating the seedlings with VAM.


Thus, if a part of the chemical fertilizer demand of the crop plants could be met through biofertilizer, a major in put constraint can be overcome. Though,field inoculation experiments are limited,the biofertilizer is expected to make important contributions to agriculture. In the long term, the modern methods ofbiotechnology will greatly improve the biofertilizer technology. It would bepossible to impart herbicide and insecticide resistance to promote competition against weedy micro organisms in the fields. For wide rexploitation of the biofertilizer technology in agriculture, a coordinated research in the laboratory and field is necessary.





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