Biofertilisers are living organisms that enrich the nutrient quality of the soil, mainly by fixing atmospheric nitrogen or adding phosphorus and organic matter. The three main sources are bacteria (Rhizobium, Azospirillum, Azotobacter), fungi (mycorrhiza) and cyanobacteria (Anabaena, Nostoc, Oscillatoria). Memory hook: "BFC feeds the field" — Bacteria, Fungi, Cyanobacteria are the three biofertiliser groups.
The three main sources of biofertilisers — bacteria, fungi (mycorrhiza) and cyanobacteria — and how each enriches the soil, as described in NCERT.
Your doubts, answered
Is a biofertiliser the same as a chemical fertiliser?
No. A biofertiliser is a living organism (a microbe) that enriches soil nutrients naturally. A chemical fertiliser is a man-made compound. NCERT says biofertilisers are part of organic farming and reduce the problems caused by overuse of chemical fertilisers.
What are the 3 main sources of biofertilisers?
Bacteria, fungi and cyanobacteria. This is a direct NCERT line and a very common NEET one-liner. Remember 'BFC': Bacteria (Rhizobium, Azospirillum, Azotobacter), Fungi (mycorrhiza), Cyanobacteria (Anabaena, Nostoc, Oscillatoria).
Do all biofertilisers work only by fixing nitrogen?
No. Most bacteria and cyanobacteria fix atmospheric nitrogen, but fungi (mycorrhiza) mainly supply phosphorus, resistance to pathogens, tolerance to salinity and drought. Cyanobacteria also add organic matter to the soil. So the benefit is nitrogen plus phosphorus plus organic matter.
Is Rhizobium a biofertiliser or a biocontrol agent?
Rhizobium is a biofertiliser, not a biocontrol agent. It forms nodules on legume roots and fixes atmospheric nitrogen into organic forms the plant can use. NEET often puts Rhizobium in a fake 'biocontrol' option to trap you — do not fall for it.
Why are cyanobacteria important in paddy (rice) fields?
Cyanobacteria such as Anabaena, Nostoc and Oscillatoria fix atmospheric nitrogen and also add organic matter, increasing soil fertility. In paddy fields they serve as an important biofertiliser. This exact context (paddy fields) is directly from NCERT and is heavily tested.
⚠️ The NEET trap ✗ Grouping Rhizobium, Nostoc or Azospirillum as biocontrol agents. ✓ Rhizobium, Nostoc, Anabaena and Azospirillum are BIOFERTILISERS (they fix nitrogen). Biocontrol agents are Trichoderma, Baculovirus and Bacillus thuringiensis. 🧠 Fix nitrogen = feed the plant = biofertiliser. Kill the pest = biocontrol. Do not mix the two lists.
Real NEET questions
NEET 2026
Among the following pairs of microbes, which pair has both the microbes that can be used as biofertilizers?
A · Aspergillus and Rhizopus
B · Rhizobium and Rhizopus
C · Cyanobacteria and Rhizobium ✓
D · Aspergillus and Cyanobacteria
Solution: Rhizobium fixes atmospheric nitrogen symbiotically in legume root nodules, and cyanobacteria (Anabaena, Nostoc, Oscillatoria) fix nitrogen and serve as biofertilisers in paddy fields. Aspergillus and Rhizopus are not biofertilisers, so only option C has both members as biofertilisers.
NEET 2025
Which of the following organisms cannot fix nitrogen? A. Azotobacter B. Oscillatoria C. Anabaena D. Volvox E. Nostoc
A · B only
B · E only
C · A only
D · D only ✓
Solution: Azotobacter is a free-living nitrogen fixer, while Oscillatoria, Anabaena and Nostoc are nitrogen-fixing cyanobacteria (biofertilisers). Volvox is a green alga that does NOT fix atmospheric nitrogen, so only D cannot fix nitrogen.
NEET 2019
Select the correct group of biocontrol agents.
A · Bacillus thuringiensis, Tobacco mosaic virus, Aphids
B · Trichoderma, Baculovirus, Bacillus thuringiensis ✓
C · Oscillatoria, Rhizobium, Trichoderma
D · Nostoc, Azospirillium, Nucleopolyhedrovirus
Solution: Trichoderma, Baculovirus and Bacillus thuringiensis are true biocontrol agents. The other options mix in biofertilisers (Rhizobium, Nostoc, Anabaena, Azospirillum) or pests (Aphids). This shows how NEET traps you by placing biofertilisers inside a biocontrol question.
Solved Microbes in Human Welfare NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A biofertiliser is an organism that enriches the nutrient quality of the soil. This is the exact NCERT definition and a common one-word NEET answer.
Name the three main sources of biofertilisers.
Bacteria, fungi and cyanobacteria are the three main sources of biofertilisers.
Which fungus type acts as a biofertiliser?
Mycorrhiza — a symbiotic association of fungi (many belong to the genus Glomus) with plant roots. It supplies phosphorus, resistance to pathogens, and tolerance to salinity and drought.
How do cyanobacteria act as biofertilisers?
Cyanobacteria such as Anabaena, Nostoc and Oscillatoria fix atmospheric nitrogen and add organic matter to the soil, increasing its fertility, especially in paddy fields.
Why are biofertilisers preferred over chemical fertilisers?
Overuse of chemical fertilisers causes soil and water pollution. Biofertilisers are part of organic farming, replenish soil nutrients naturally, and reduce dependence on chemicals.
