Microbes as Biofertilisers

NCERT grounding

This subtopic is Section 8.6 of NCERT Class XII Biology, Chapter 8, Microbes in Human Welfare — the final section of the chapter. NCERT introduces it against a backdrop of environmental concern: the use of chemical fertilisers to meet the ever-increasing demand of agricultural produce has contributed significantly to pollution, and there is large pressure to switch to organic farming through the use of biofertilisers. The section is short in the textbook, but every sentence in it has been examined by NEET, so it rewards a close, line-by-line reading rather than a skim.

NCERT defines the term precisely, names the three source groups, and gives the genus-level examples that the examiners draw upon. The definition below is the single most quoted line from the section and should be reproduced word-for-word in any descriptive answer.

What biofertilisers are and how they work

A biofertiliser is not a chemical input — it is a living organism, or a preparation containing living cells, that improves soil nutrient status when added to soil or applied to seeds and roots. The contrast with chemical fertilisers is the heart of the concept. A chemical fertiliser delivers a fixed dose of a soluble nutrient salt that the plant absorbs directly; a biofertiliser instead establishes a population of microbes that continuously convert nutrients into plant-available forms, season after season, while also adding organic matter to the soil. Because the microbe is alive, the benefit is renewable and self-sustaining in a way a single dose of urea can never be.

NCERT identifies three source groups — bacteria, fungi and cyanobacteria — and the two nutrients they target are nitrogen and phosphorus. Most biofertiliser microbes are nitrogen fixers: they convert inert atmospheric dinitrogen into organic, plant-usable nitrogen compounds. The fungal group is the exception — mycorrhizal fungi do not fix nitrogen at all; they mine phosphorus from the soil. Keeping the nitrogen group and the phosphorus group separate is the single most important organising idea for this subtopic, and it is exactly where NEET sets its traps.

The three groups can be set side by side as parallel items, since each is defined by a distinct organism type, a distinct mechanism, and a distinct set of textbook examples. The grid below is the compact reference for the whole subtopic; the sections that follow unpack each card in turn.

Why the shift away from chemical fertilisers

NCERT frames the entire section as a response to a problem. Chemical fertilisers raise yields, but their overuse leaches into ground water and runs off into water bodies, contributing significantly to environmental pollution. Biofertilisers offer a route out: they are non-polluting, they replenish soil nutrients naturally, and several of them add organic matter that improves soil structure. NCERT notes that in India a number of biofertilisers are now available commercially, and farmers use these regularly in their fields to replenish soil nutrients and reduce dependence on chemical fertilisers. The biofertiliser is therefore not merely an academic curiosity — it is the practical pillar of organic farming that the chapter has been building towards.

Bacterial biofertilisers

Bacteria are the largest and most heavily examined biofertiliser group, and all the bacterial biofertilisers named by NCERT are nitrogen fixers. They split cleanly into two categories defined by their lifestyle: symbiotic fixers that work only inside a plant partner, and free-living fixers that fix nitrogen on their own in the soil. NEET tests this symbiotic-versus-free-living split almost every time it touches this topic, so it deserves a careful, separate treatment.

Rhizobium — the symbiotic nitrogen fixer

Rhizobium is a bacterium that lives in a symbiotic association inside the root nodules of leguminous plants — plants of the family Fabaceae such as gram, pea, beans and groundnut. Within the nodules the bacterium fixes atmospheric nitrogen into organic forms, and this fixed nitrogen is used by the host plant as a nutrient. The relationship is mutually beneficial: the plant receives a private supply of nitrogen, and the bacterium receives carbohydrates and a protected, low-oxygen environment in which the nitrogen-fixing machinery can operate. When the crop is harvested or ploughed back, the nitrogen-enriched root residue raises the nitrogen status of the soil for the next crop, which is why legumes are central to crop-rotation practice.

The decisive point for NEET is the word symbiotic. Rhizobium cannot fix nitrogen meaningfully while drifting freely in the soil; it fixes nitrogen only after it has colonised the root nodule of a leguminous host. The process below traces that pathway in order.

Azospirillum and Azotobacter — the free-living nitrogen fixers

Not all nitrogen-fixing bacteria need a plant partner. NCERT names two free-living soil bacteria that fix atmospheric nitrogen without forming nodules — Azospirillum and Azotobacter. Living independently in the soil, they convert atmospheric nitrogen into combined forms and so enrich the nitrogen content of the soil directly. Because they do not depend on a leguminous host, they can benefit a much wider range of crops, including cereals, and they are applied to fields as soil or seed inoculants.

The functional outcome — added soil nitrogen — is the same as with Rhizobium, but the route is different, and that difference is exactly what examiners probe. The comparison below isolates the one variable that matters.

One memory note worth fixing early: Azospirillum and Azotobacter both begin with "Azo-", from the same root that gives us "azote", an old name for nitrogen. The "Azo-" prefix is a reliable cue that the organism is a nitrogen-related bacterium — a small hook that helps under exam pressure.

Fungal biofertilisers: mycorrhiza

The fungal group is the one that breaks the nitrogen pattern. Fungi are also known to form symbiotic associations with plants, and such an association is called a mycorrhiza — literally "fungus-root". NCERT states that many members of the genus Glomus form mycorrhiza. The crucial point is the nutrient: the fungal symbiont absorbs phosphorus from the soil and passes it to the plant. Mycorrhizal fungi are phosphorus biofertilisers, not nitrogen fixers — they do not fix atmospheric nitrogen at all.

The reason this association is so effective is structural. Phosphorus in soil is notoriously immobile; it does not diffuse readily to the root. The fine fungal threads of a mycorrhiza extend far beyond the root's own reach, vastly enlarging the volume of soil from which phosphorus can be drawn, and then channel that phosphorus into the plant. In return, the fungus receives carbohydrates manufactured by the plant — the answer to NCERT's own prompt about what advantage the fungus derives from the association.

The extra benefits of a mycorrhizal association

NCERT is explicit that mycorrhiza does more than feed the plant phosphorus. Plants having such associations show several other benefits as well, and these are frequently lifted verbatim into NEET options, so they should be memorised as a set:

• Resistance to root-borne pathogens — the mycorrhizal plant is better defended against soil-borne disease.
• Tolerance to salinity and drought — the association helps the plant cope with saline soils and water stress.
• An overall increase in plant growth and development.

Together, the phosphorus supply plus these three benefits make mycorrhiza one of the most valuable biofertiliser associations, especially on phosphorus-poor and stressed soils.

Cyanobacterial biofertilisers

The third group is the cyanobacteria — also called blue green algae. NCERT describes them as autotrophic microbes, widely distributed in aquatic and terrestrial environments. Being autotrophic, they photosynthesise and so do not depend on external organic matter; many of them additionally fix atmospheric nitrogen. The named examples are Anabaena, Nostoc and Oscillatoria — a trio worth committing to memory exactly as written.

Their most important agricultural role is in the paddy field. In the waterlogged conditions of rice cultivation, cyanobacteria serve as an important biofertiliser, fixing nitrogen for the rice crop. Beyond nitrogen, blue green algae also add organic matter to the soil and increase its fertility — a dual contribution that sets them apart from the bacterial fixers, which enrich nitrogen but do not, by NCERT's wording, add organic matter in the same way.

The diagram below places all three biofertiliser groups together, so the nitrogen pathways and the single phosphorus pathway can be seen at a glance.

With all three groups in view, the subtopic resolves into a clean grid: bacteria and cyanobacteria are nitrogen suppliers, fungi are phosphorus suppliers; Rhizobium is symbiotic while Azospirillum and Azotobacter are free-living; and cyanobacteria carry the extra distinction of adding organic matter and dominating the paddy field. Every NEET question on this section is a recombination of those few facts.

Worked examples

Common confusion & NEET traps

The biofertiliser section looks easy, but it generates predictable errors because several organisms sound similar and the nitrogen-versus-phosphorus split is easy to blur under time pressure. The traps below are the ones that cost marks.