Bio-fertilizers: The Need of the Hour
for the Maintenance of Environmental Quality, Soil Health and Production of Nutritious Food Grains
-Dr. Arvind Singh
Bio-fertilizers are the organisms that bring about nutrient enrichment of the soil. There are a large number of bacteria and blue-green algae (cyanobacteria) which fix atmospheric nitrogen (conversion of elemental nitrogen to ammonia) either in association with some other organism or in a free state.
Why Bio-fertilizers are the need of the hour?
Chemical fertilizers used to raise the fertility status of the soil are costly and are manufactured from non-renewable petroleum feed stock which is gradually diminishing. The continuous use of chemical fertilizers is detrimental to soil health. For instance, excessive use of nitrogenous fertilizers destroys the soil structure thus making the soil prone to erosive forces like wind and water.
The fertilizers are also responsible for the surface and ground water pollution. Furthermore, the use of nitrogenous fertilizers makes the crop vulnerable to diseases and pests infestation. Fertilizer enriched soil does not support the microbial life and hence there are less humus and fewer nutrients in the soil. Indian soils are generally poor in organic matter and nitrogen because of the excessive use of chemical fertilizers to increase the crop yield. The application of excessive Superphosphate leads to copper and zinc deficiency in plants.
Fertilizers also alter the nutritional value of food crops. Excessive use of nitrogenous fertilizer urea causes a decrease in the potassium content of food grains. Similarly, excessive potash treatment decreases the vitamin C (ascorbic acid) and carotene contents in the plants. Nitrate fertilizers increase the total crop yield but at the cost of protein. Moreover, the balance of amino acids is disturbed within the protein molecule thus lowering the protein quality. Fertilizer use produces over-sized fruits and vegetables which are more prone to insects and other pests.
Benefits of Bio-fertilizers:
Bio-fertilizers provide the following benefits:
• Bio-fertilizers are cost-effective relative to chemical fertilizers. They have lower manufacturing costs.
• Bio-fertilizers add nutrients to soil through natural processes of nitrogen fixation, phosphorus solubilization and organic matter decomposition.
• Bio-fertilizers provide protection to plants against root pathogens.
• Bio-fertilizers restore the nutrient cycling of the soil and build organic matter.
• Bio-fertilizers stimulate plant growth through the synthesis of growth-promoting substances.
Types of Bio-fertilizers:
The bio-fertilizers can be categorized into three types as follows:
1. Nitrogenous bio-fertilizers: The bio-fertilizers which brings about the nitrogen enrichment of the soil are called as nitrogenous bio-fertilizers. Nitrogenous bio-fertilizers are important as they supply nitrogen to the soil. There are a large number of bacteria and blue-green algae in nature that fix atmospheric nitrogen. The important nitrogen-fixing bacteria are represented by Rhizobium, Azotobacter, Beijrinckia, Clostridium, Rhodospirillum, Herbaspirillum and Azospirillum. The Rhizobium forms symbiotic association (association in which both the organisms live together and raise mutual benefits from each other) with the roots of leguminous plants and fix atmospheric nitrogen.
The Herbaspirillum and Azospirillum are associative nitrogen fixers as they form a loose association with roots of plants and fix atmospheric nitrogen. These two bacteria inhabit the root zone (Rhizosphere) of tropical grasses and crops like maize and sorghum.
The Azotobacter, Beijrinckia, Clostridium and Rhodospirillum are a symbiotic nitrogen-fixing bacteria as they fix atmospheric nitrogen in free state in the soil.
The important blue-green algae which fix atmospheric nitrogen are represented by Nostoc, Anabaena, Aulosira, Cylindrospermum, Gloeotrichia Gloeocapsa, Calothrix, Tolypothrix and Scytonema. The amount of nitrogen fixed by blue-green algae range between 15 to 45 kg nitrogen per hectare. Standing water of 2 to 10 cm in the field is necessary for the growth of blue-green algae. They grow well in a pH range of 7 to 8.
The Anabaena also forms symbiotic association with a water fern Azolla and fix nitrogen. Hence Azolla is used as a potent bio-fertilizer for rice crops. A thick mat of Azolla supplies 30 to 40 kg nitrogen per hectare. Normal growth of Azolla occurs in temperature range between 20 to 30oC. It grows better during the rainy season.
What is Azolla-Anabaena symbiosis?
Azolla is a tiny aquatic fern with microphyllous leaves. The leaves are bilobed. The lower lobe is white in colour and is submerged in water while the upper lobe which floats on the surface of the water is green in colour (due to the presence of chloroplast) hence, it performs the function of photosynthesis. The upper green lobe has an algal cavity in which the nitrogen-fixing blue-green alga Anabaena azollae resides and fixes atmospheric nitrogen.
The Anabaena azollae is a Glutamine synthetase defective (GS defective) species of blue-green algae. Glutamine synthetase is a primary enzyme involved in assimilation of ammonia which is the end product of nitrogen fixation. Thus the nitrogen fixed in the form of ammonia by Anabaena azollae easily leaches out from heterocyst (specialized cells for nitrogen fixation) due to defective Glutamine synthetase enzyme which fail to assimilate the ammonia.
2. Phosphatic bio-fertilizers: These are those bio-fertilizers that solubilize bound phosphate in the soil thus ensuring its availability to the plants. Several phosphate solubilizing fungi like Acaulospora, Gigaspora, Endogone, Glomus, Sclerocystis, Amanita and Boletus forms symbiotic association with roots of plants and supply phosphate to the plants. This association of fungi with roots of plants is known as mycorrhizal association. Endomycorrhiza and ectomycorrhiza are two different categories of mycorrhizal association.
In endomycorrhizal association the fungal partner penetrates deep in the roots of plants. It is also known as vesicular arbuscular type of mycorrhiza (VAM) which is generally found in crop plants. VAM fungi enhance the frequency of nodulation in leguminous crop plants consequently their yield is increased. Acaulospora, Endogone, Glomus, Gigaspora, and Sclerocystis are examples of VAM fungi.
In ectomycorrhizal association, the fungal partner remain confined to the surface of plant roots. It is generally found in forest trees like pine, oak, beech etc. Amanita and Boletus are the examples of ectomycorrhizal fungi.
Besides fungi there are certain free-living bacteria like Bacillus subtilis, Pseudomonas fluoroscens and Pseudomonas putida which solubilize phosphate in the soil thus making it available to the plants.
3. Cellulolytic bio-fertilizers: The bio-fertilizers which enhance the rate of decomposition of organic matter thus facilitating the quick release of nutrients to the soil are known as cellulolytic bio-fertilizers. Cellulolytic bio-fertilizers are represented by fungi like Aspergillus, Trichoderma and Penicillium.
Conclusion:
It can be concluded that bio-fertilizers are not only economical but are environment-friendly, beneficial from a soil health point of view and capable of producing nutritious food grains. Therefore, the use of bio-fertilizers is the need of the hour for the maintenance of the environmental quality and soil health, and the production of nutritious food grains.
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Dr. Arvind Singh is M. Sc. and Ph. D. in Botany with an area of specialization in Ecology. He is a dedicated Researcher having more than four dozen published Research Papers in the Journals of National and International repute. His main area of Research is Restoration of Mined Lands. However, he has also conducted Research on the Vascular Flora of Banaras Hindu University-Main Campus, India.
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