Plants require nutrients for growth and other important cellular activities. They absorb the nutrients from the soil through the root.
Optimum nutrient absorption and assimilation promotes healthy growth and thereby increases productivity. The nutrients are absorbed and assimilated and then used for the biosynthesis of life supporting molecules like carbohydrates, proteins, lipids, vitamins, co-enzymes and nucleic acids.
These molecules are made from elements such as carbon, hydrogen, oxygen, nitrogen, phosphorus and occasionally sulfur. Metal ions are also required for important cellular activities. Nitrogen and phosphorus constitute important elements in proteins, nucleic acids, co-enzymes and some lipids. Nitrogen occurs in a gaseous state and constitutes approximately 80% of the atmosphere and phosphorus occurs as insoluble phosphates in the soil sediments.
Plants are not equipped with mechanisms for using the atmospheric molecular nitrogen and insoluble phosphates. They depend upon soil microorganisms for this basic necessity. These soil microorganisms trap.
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The atmospheric nitrogen and ‘convert it into nitrates by a process known as biological nitrogen fixation, however, a small fraction of the nitrogen, requirement is fulfilled by physical factors like lightning, which reduces atmospheric nitrogen into ammonia and then makes it available to plants as soluble nitrates through rain water. Other soil microorganisms solubilize the insoluble phosphates present in the soil sediments and then make it available ‘to plants.
Continuous crop cultivation reduces the nitrate and phosphate reservoir of the soil. Farmers have been using chemical fertilizers, over the years, for restoring the depleted soil fertility.
Chemical fertilizers are used as alternatives to the natural nitrates and phosphates fixed by soil microorganisms. Chemical fertilizers are manufactured in fertilizer industries by using natural resources and man power. For example, atmospheric nitrogen is industrially converted into ammonia by a process known as Haber-Bosch process.
This process requires a temperature of around 400-500° C and several hundred atmospheric pressure. Therefore, the cost of production of chemical fertilizers is very high. Despite this high cost, chemical fertilizers are made available to Indian farmers at a subsidized cost.
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The Government loses a major part of the revenue in granting the subsidy in fertilizer price. Secondly, continuous use, over a long period of time, has had detrimental effects on the soil.
The runoff water from the agricultural land contains an excess of soluble nitrates, which contaminates large bodies of water and consequently causes a serious water pollution crisis. Secondly, the nitrate content of the soil increases, thereby increasing the salinity of the soil.
There is a gradual depletion in the nutrient pool of the soil. After a period of time, the soil becomes completely sterile and becomes unsuitable for crop cultivation.
Farmers have been practicing two methods for circumventing these problems. Firstly, they adopt a crop rotation practice. Cereals and pulses are grown in alternate seasons. In doing so, the depleted nutrient reserve of the soil is replenished.
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Secondly, the discovery of biofertilizers has solved the foregoing problems to a great extent. Biofertilizers are biologically active products, including microorganisms (bacteria, algae and fungi), which provide nutrients to plants. Most biofertilizers belong to one of the two categories of microorganisms: (i) nitrogen fixing, (ii) phosphate solubilizing.
Chemical fertilizers account for half of the nitrogen supply to plants. The remainder is derived from nitrogen fixing (diazotrophic) bacteria, such as Rhizobium, Frankia. Azotobacter, Azospirillum, Klebsiella, Rliodospirillum and cyanobacteria (blue-green algae).
The process by which, the diazotrophic microorganisms make the atmospheric nitrogen available to plants for use,’ is known as biological nitrogen fixation. “A biofertilizer is an innoculum, containing one or a few of the above mentioned nitrogen fixing or phosphate solubilizing microorganisms, packaged in a carrier material, preferably sterilized soil”. This’method of producing a biofertilizer has been conventional over the years.
However, with the emergence of biotechnology, genetic engineers have tried to maneuver the nitrogen fixing property into non-nitrogen fixing crop plants. There could be three possible means for achieving this objective.
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1. Modify ehhet the microbe of the bev\ef\c\a\-\| plant so that each could benefit from an association with each other.
2. Modify non-nitrogen fixing bacteria, occurring in close association of the crop plants, which could fix nitrogen.
3. Genetically engineer crop plants by transferring nitrogen fixing (nif) genes from nitrogen fixing microorganisms, which could fix their own nitrogen from the atmosphere.
Let us consider the elements of nitrogen fixation, before we take up different methods employed in the manufacture of biofertilizers.