Biofertilizers in agriculture

THE SIGNIFICANCE of the role of microorganisms like Rhizobium in fixing atmospheric nitrogen and hence enhancing soil fertility has been recognized since time immemorial, however, commercialization of their use is relatively a new innovation. Moreover, biofertilizers are apparently environment friendly and low cost or almost nonmonetary input which can play a significant role in plant nutrition. The demand for nutrients by higher yielding crops/varieties is high and therefore, biofertilizers cannot be a substitute to chemical fertilizers, rather with a ability to fix 15-40 kg nitrogen/ha, the biofertilizers can only be complimentary or supplementary to mineral nutrition demand of the crop. In general, increase in yield due to rhizobium may be to the order of 10-15 per cent.

The biofertilizers may be classified into following groups:

- Nitrogen fixing bacteria

- Symbiotic nitrogen fixation

- Non symbiotic or free living nitrogen bacteria

- Phosphorus or other nutrients solublising bacteria

Nitrogen fixing bacteria

Rhizobium - a symbiotic bacterium, colonizes on roots of the specific legumes to form root nodules. The nodule is a protective structure and bacteroids are the seat of nitrogen fixation. Nitroginase enzyme mediates the reduction of N 2 to NH3. Nitroginase is obtained by the disintegration of bacteroids by mechanical process. Nitrogen fixation is essentially an anaerobic process and hence nodules must have a mechanism to exclude oxygen from the bacteroid which is the site of nitrogen fixation. This is accomplished by the presence of leghaemoglobin around bacteroids enclosed by membranous envelope of host origin. The amount of legheamoglobin and the extent of bacteroid tissue in nodules has a direct bearing on the amount of nitrogen fixed by legumes. Rhizobium enter roots of legumes generally through root hairs. Effective nodules are well developed and posses pink colour due to the presence of a pigment leghaemoglobin. Rhizobium has the ability to fix nitrogen for the benefit of a leguminous host plant. In view of crop specificity of a Rhizobium, selection of an appropriate strain as an inoculant for a given host genotype is of utmost important for desirable results. The cross inoculation groups of Rhizobium are as following:

Nonsymbiotic or free living nitrogen bacteria

These bacteria are aerobic and free living. They fix nitrogen in rhizosphere and provide it to the plant. Positive response have been reported with the use of Azotobacters in cereals like wheat, rice, sorghum, pearl millet and cotton. Azotobacters get their energy from soil organic matter so they are relatively more effective in the soils rich in organic matter. Generally these are less effective in dry land areas because these soils are poor in organic matter and water content.

Azospirillum: It colonises the root mass and fixes nitrogen in loose association with plants in an environment of low oxygen tension. The bacteria induce the plant roots to secrete a mucilage which creates low oxygen environment and helps to fix atmospheric nitrogen. These organisms are found in association with the roots of cereals, millets, grasses etc. Azospirillum is specially effective in low land rice. These organisms require low energy, well effective even in saline alkali soils, perform well up to 30-402 C, and establish well in the roots of the crop. The response of these organisms in increasing grain yield have been found excellent in rice and good in other cereals like wheat, barley, oats and pearl millet.

Blue green algae (BGA): It is a free living nitrogen fixer often referred as `paddy organism' because of their abundance in paddy fields. Its potential in low land rice is well documented and contribute 25-30 kg N/ha. BGA use sunlight as a source of energy and water as a source of reductant for photosynthesis and nitrogen fixation. These are filamentous and consists of heterocyst which function as micro nodule for nitrogen fixation. The benefits of BGA depend mainly on the suitability of soil and environment which influence the growth, multiplication and activity of these organisms.

Azolla: It is an aquatic fern and is found in temperate and tropical ecosystems. It is prevalent in low land rice growing areas. It fixes atmospheric nitrogen in symbiotic association with heterocystous blue green algae (Anabena azollae). This association is a live, floating nitrogen factory using energy from photosynthesis to fix atmospheric nitrogen. Nitrogen fixed by azolla becomes available to rice through decaying of azolla crop in the form of biofertilizer. The growth of azolla will be proper if water is standing up to 5-10 cm, temperature is between 20-30 o C, pH is neutral and P2O5 availability is between 4-8 kg/ha.

Nutrient solubilising bacteria

Indian soils are generally low to medium in available phosphorus. The problem with phosphorus is that generally 10-15% of applied phosphorus is available to crop and the rest get converted in to relatively unavailable forms. Many heterotrohics soil bacteria and fungi bring sparingly soluble/unsoluble inorganic and/or organic phosphate in to soluble forms by secreting organic acids. These acids lower soil pH and dissolve immobile soil phosphate. Some hydroxy acids chelate Ca, Al, Fe and Mg resulting in effective availability of soil phosphorus. The response of phosphobacterin is high in soils rich in organic matter and low in available phosphorus. Microphos inoculant containing Pseudomonas striata and Bacillus polymyxa increased the yield of wheat, rice, chickpea, sugarcane and potato markedly.

S.K.Yadav

Department of Agronomy

CCS Haryana Agricultural

University, Hisar

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