Biofertilizer is the biologically active products or microbial inoculants of bacteria, algae and fungi, which may help biological nitrogen fixation for the benefit of plants. Biofertilizers also include organic fertilizers (manures, etc.), which are rendered in an available form due to the interaction of microorganisms or due to the interaction of micro-organisms or due to their association with plants.
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Biofertilzers thus include (i) symbiotic nitrogen fixers e.g. Rhizobium spp.; (ii) asymbiotic free nitrogen fixer e.g. Azotobacter, Azospirillum, etc. (iii) algae biofertilisers e.g. blue green algae or BGA in association with Azolla; (iv) phosphate solubilising bacteria; (v) mycorrhizae; (vi) organic fertilizers.
The need for the use of biofertilzers has arisen, primarily of two reasons. First, because increase in the use of fertilizers leads to increased usage of chemical fertilizer leads to damage in soil texture and raises other environmental problems. Therefore, the use of biofertilzers is both economical and environments friendly. The pragmatic approach will be to develop the integrated nutrient supply system involving a combination of the use of chemical fertilizers and biofertilizers.
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Rhizobia spp
Rhizobia are able to enter into symbiotic relationship with legumes (pulses, etc.). They fix atmospheric nitrogen and thus not only increase the production of the inoculated crops, but also leave a fair amount of nitrogen in the soil, which benefits the subsequent crop. Following seven groups of Rhizobia have been recognised for inoculating legumes in India: R. legiuninosarum, R. japonicum and Rhizobium spp. The new technology for industrial preparation for rhizobial cultures included the following: (i) seed pelleting with gum arabic or carboxy methyl cellulose as percolated seed (ii) polyacrylamide entrapped rhizobia; (iii) freeze-dried or lyophilzed culture of rhizobia; (iv) Rhizobium paste; (v) inoculants in liquid/frozen concentrate form; (vi) granular soil inoculants to be applied by aerial application; (vii) natural peat granule as soil implant inoculum.
Asymbiotic Nitrogen-Fixers
Azotobacter and Azopirillum, when applied to rhizosphere, fix atmospheric Nitrogen and make it available to crop plants. They also synthesize growth promoting antibiotic substance, helpful to the plant. Most efficient strains of Azotobacter fix 30 kg of Nitrogen from 1000 kg of organic matter. When applied to fields, positive responses by field crops were observed leading to saving of 10-25 kg/ha of Nitrogen. Similarly Azospirllum with farmyard manure, led to saving of 15-25 kg equivalent to Nitrogen per hectare in crops like sorghum and other millets.
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Algal Fertilizers
Blue green algae (BGA) and Azolla constitute a system, which is the main source of algal biofertiliizer in south and Southeast Asia, particularly for lowland paddy. BGA inoculation with composite cultures of algal genera Anabaena, Nostoc, Plectonema, Aulosiro, Oscillatoria, Tolypothrix, etc. have been found to be more effective than single cultures. Besides being a source of Nitrogen, BGA use has the following advantages: (i) algal biomass accumulates as organic matter; (ii) growth promoting substances are produced, which stimulate growth of rice seedling; (iii) it provides partial tolerance to pesticides and fungicides; (iv) it also helps in reclamation of saline and alkaline soils.
Mycorrhizu
Mycorrhiza is a symbiotic association of fungi with roots of plants, so that the nutrients absorbed from soil by the fungus are released to the host eels and in turn, the fungus takes its food requirements from the host. Mycorrhizae are of two types’ viz., ectomycorrhiza and endomycorrhiza. Ectomycorrhiza are found on the roots of forest trees such as pine, oak, beech, eucalyptus, etc.
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They absorb nitrogen, phosphorus, potassium and calcium. They also convert complex organic molecules into simpler available forms, protect the roots from the pathogen, and produce growth promoting substances. Endomycorrhiza are found in the roots of most fruits and other horticultural crops such as coffee, pepper, cardamon betelvine, etc. They particularly help in phosphorus nutrition. They also produce growth promoting substances and offer resistance against pathogens.