Role of Microbes as Biofertilizers

Fertilizers are the nutrients required for plant growth, and by extension for the productivity of any cultivated crop; using them effectively is one of the central pillars of the Green Revolution's success. Fertilizers fall into two broad categories: inorganic (chemical) fertilizers, which are synthetic mixtures of mineral NPK (nitrogen, phosphorus, potassium) salts blended in defined proportions and applied to the field, and organic (biological) fertilizers — farmyard manure (FYM), compost and green manure — which are biological in origin. Non-judicious or excessive use of chemical fertilizers is a real hazard: it pollutes soil, air and groundwater, and it makes the soil progressively more acidic, whereas the organic alternatives generally build up soil fertility more gently and sustainably.

Biofertilizers are a further, specifically microbial category, increasingly used by farmers alongside organic farming practices for more sustainable agricultural production: they are commercial, ready-to-use preparations of living bacterial or fungal cultures — mostly, though not exclusively, nitrogen-fixing organisms drawn from bacteria, cyanobacteria and fungi — that, when applied to plants, soil or composting pits, enrich the soil's nutrient quality through their own biological activity. Compared with chemical fertilizers, biofertilizers are cost-effective and eco-friendly, and they play a genuinely important role in maintaining soil fertility and agricultural sustainability over the long term.

Classification by function. On the basis of what they actually do, biofertilizers are grouped into four types.

1. Nitrogen-fixing biofertilizers: microorganisms, called diazotrophs, that convert atmospheric nitrogen gas into usable nitrogenous compounds (nitrites and nitrates) via ammonia. These are of two kinds — symbiotic nitrogen-fixers, such as Rhizobium, Anabaena and Frankia, which are generally associated with the underground root systems of higher plants; and free-living or non-symbiotic nitrogen-fixers, such as Azotobacter, Azospirillum, Nostoc, Clostridium, Beijerinkia and Klebsiella, which fix nitrogen independently in the soil without needing to live inside a plant.
2. Phosphate-solubilising biofertilizers: bacterial species — Pseudomonas striata, Bacillus polymyxa, Agrobacterium, Micrococcus and Aspergillus species among them — that convert insoluble inorganic phosphate compounds, such as rock phosphate, into forms plants can actually absorb.
3. Compost-making biofertilizers: composting is the natural process that turns organic material into a dark, nutrient-rich substance called compost or humus, and it depends on a whole community of micro-organisms — bacteria, fungi, actinobacteria, protozoa and rotifers — to break down the raw organic matter into finished compost.
4. Cyanobacterial biofertilizers: many cyanobacteria (blue-green algae) — Anabaena, Nostoc, Plectonema and Oscillatoria among them — are aquatic or terrestrial, free-living or symbiotic, aerobic, photosynthetic, and capable of fixing nitrogen, whether they are heterocystous or non-heterocystous forms. Anabaena, Nostoc and Tolypothrix are associated with lichens, and Anabaena additionally forms a specific symbiosis with plants such as Azolla and Cycas.

Key biofertilizer organisms, in detail. Rhizobium species are rod-shaped, motile, aerobic, gram-negative, non-spore-forming, nitrogen-fixing bacteria that carry Nod genes and Nif genes; they form a symbiotic association specifically with the roots of leguminous plants, inducing the formation of root nodules, multiplying inside them, and fixing atmospheric nitrogen there into organic forms the plant can use as a nutrient — R. leguminosarum is specific to pea, and R. phaseoli is specific to beans, illustrating that different Rhizobium strains are matched to different legume hosts. Azotobacter is a well-known free-living, aerobic, non-photosynthetic, non-nodule-forming nitrogen-fixing bacterium closely associated with the roots of grasses and certain other plants; it is used as a biofertilizer for non-leguminous plants, especially rice, cotton and vegetables. Azospirillum is likewise a free-living, aerobic, nitrogen-fixing bacterium, associated instead with the roots of corn, wheat and jowar; it fixes a genuinely substantial quantity of nitrogen — roughly 20 to 40 kg per hectare — in non-leguminous plants such as cereals, millets, cotton and oilseed crops. Anabaena is a genus of multicellular, filamentous cyanobacteria that also exists as free-floating plankton; besides fixing nitrogen on its own, it forms symbiotic relationships with certain plants, notably the coralloid roots of Cycas and the thallus of Anthoceros. Nitrogen fixation in Anabaena takes place inside specialised, colourless cells called heterocysts. Azolla is a free-floating water fern, consisting of a floating rhizome (stem) bearing small, overlapping, bilobed leaves and trailing roots; each leaf shows a dorsal and a ventral lobe, and within the dorsal lobe's internal cavity, filaments of Anabaena live and fix nitrogen, in what is effectively a permanent plant–cyanobacterium partnership. Because of this, Azolla itself can be used directly as a biofertilizer in rice fields, where it grows on the flooded paddy water and its fixed nitrogen becomes available to the crop.

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