Q.How do biofertilisers enrich the fertility of the soil?

Biofertilisers are living microorganisms that enrich soil fertility by enhancing the availability of nutrients to plants through biological processes. They work in fundamentally different ways from chemical fertilisers, which simply add nutrients directly.

The main mechanisms are:

• Nitrogen fixation: Bacteria like Rhizobium (in legume root nodules), Azotobacter and Azospirillum (free-living in soil) convert atmospheric nitrogen into ammonia, which plants can absorb and use for protein synthesis.

• Phosphorus solubilisation: Fungi like mycorrhiza and bacteria dissolve bound phosphorus in the soil, making it available to plant roots. Mycorrhizae also increase the surface area for water and mineral absorption.

• Organic matter decomposition: Microbes break down complex organic matter in compost, releasing nutrients in forms plants can take up. …

Soil fertility depends not just on the minerals present but on whether plants can actually access them. Chemical fertilisers supply nutrients directly, but they come with costs — environmental damage, soil degradation over time, and expense. Biofertilisers offer a biological alternative: they are preparations containing live microorganisms that colonise the rhizosphere (the soil around roots) or the plant itself and enhance nutrient availability through natural processes.

The primary way biofertilisers work is through nitrogen fixation. Atmospheric nitrogen makes up about 78% of air, but plants cannot use it in gaseous form. Certain bacteria convert this inert nitrogen into ammonia, which plants absorb. Rhizobium bacteria form symbiotic nodules on the roots of leguminous crops like pulses, peas, and beans, fixing nitrogen directly where the plant needs it. Free-living bacteria like Azotobacter and Azospirillum also fix nitrogen in the soil without forming nodules, benefiting a wider range of crops. Cyanobacteria (blue-green algae) such as Anabaena are particularly important in paddy fields, where they fix nitrogen in waterlogged conditions.

Beyond nitrogen, biofertilisers also mobilise phosphorus. Phosphorus is often present in soil but locked in insoluble forms that roots cannot absorb. Mycorrhizal fungi form associations with plant roots, extending thread-like hyphae far into the soil. These fungi absorb phosphorus and other minerals from a much larger volume of soil than roots alone could reach, then transfer them to the plant. In return, the plant supplies the fungus with carbohydrates. This partnership dramatically improves phosphorus uptake, especially in phosphorus-deficient soils. …

Organise the answer around the three things an ordinary chemical fertiliser supplies (nitrogen, phosphorus, organic content) and assign the one microbial group that supplies each biologically -- Rhizobium/Azotobacter/cyanobacteria for nitrogen, mycorrhizal fungi for phosphorus, decomposer microbes for humus -- so the answer …