BiologyISC Class 12 · Microbes in Human Welfare

Microbes as Biofertilisers: ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

Biofertilisers are organisms that enrich the nutrient quality of soil. The main sources are bacteria, fungi and cyanobacteria: symbiotic Rhizobium in legume root nodules and free-living Azotobacter and Azospirillum fix nitrogen; mycorrhizal fungi such as Glomus absorb phosphorus for plants; cyanobacteria such as Anabaena and Nostoc fix nitrogen in paddy fields.

Understand Microbes as Biofertilisers

Why biofertilisers. Overuse of chemical fertilisers to meet the demand for farm produce has polluted soil and water. There is pressure to switch to organic farming, using biofertilisers: organisms that enrich the nutrient quality of the soil. Main sources: bacteria, fungi and cyanobacteria.

1. Nitrogen-fixing bacteria.

Symbiotic: Rhizobium forms nodules on the roots of leguminous plants (pea, gram, soyabean). It fixes atmospheric nitrogen into organic forms, which the plant uses as nutrient.
Free-living in soil: Azospirillum and Azotobacter fix atmospheric nitrogen and enrich the nitrogen content of the soil.

2. Mycorrhiza (fungi). A symbiotic association of a fungus with plant roots. Many members of the genus Glomus form mycorrhiza. The fungal partner absorbs phosphorus from the soil and passes it to the plant. It also helps the plant resist pathogens that attack roots, tolerate salty and dry soils, and grow and develop better overall. The fungus, in return, gets food (sugars) from the plant.

3. Cyanobacteria (blue-green algae). Autotrophic microbes in aquatic and terrestrial habitats; many fix atmospheric nitrogen: Anabaena, Nostoc, Oscillatoria. In paddy fields they are an important biofertiliser; they also add organic matter to the soil and increase its fertility.

Summary table.

Biofertiliser	Group	Mode	Benefit
Rhizobium	bacterium	symbiotic, legume root nodules	fixes N₂
Azotobacter, Azospirillum	bacteria	free-living in soil	fix N₂
Glomus (mycorrhiza)	fungus	symbiotic with roots	P uptake; pathogen, salt and drought tolerance; growth
Anabaena, Nostoc, Oscillatoria	cyanobacteria	free-living (paddy fields)	fix N₂; add organic matter

Advantages over chemical fertilisers. Cheap and renewable; no pollution of soil and ground water; improve soil structure and organic matter (and with it water-holding capacity); reduce dependence on chemicals; support soil life. Many biofertilisers are sold commercially and farmers use them to replenish soil nutrients.

Biofertilisers are not biopesticides (frequent ISC confusion).

Biofertilisers (nourish the crop)	Biocontrol agents (protect the crop)
Rhizobium, Azotobacter, Azospirillum, Glomus, cyanobacteria	ladybird (eats aphids), dragonfly (mosquitoes), Bacillus thuringiensis (caterpillars), Trichoderma (fungus against plant pathogens), Nucleopolyhedrovirus (baculovirus, species-specific insecticide)
add N or help P uptake	kill or control pests and pathogens

Harmful effects of chemical pesticides and IPM. Insecticides, pesticides and weedicides are toxic to humans and animals, pollute soil and ground water, contaminate fruits, vegetables and crops, kill useful as well as harmful organisms, and select resistant pests. Integrated pest management (IPM) combines biocontrol agents, knowledge of pests' life cycles and habitats, and need-based minimum chemical use, keeping pests at manageable levels rather than eradicating them, so that beneficial predators and parasites survive.

Flow chart (choosing a biofertiliser): legume crop → Rhizobium inoculation of seed; non-legume cereal → Azotobacter or Azospirillum; paddy → cyanobacteria (Anabaena, Nostoc); phosphorus-poor or dry, saline soil → mycorrhiza (Glomus).

Microbes as Biofertilisers: key facts

Nitrogen fixers: Symbiotic Rhizobium, in legume root nodules
Free-living Azotobacter, Azospirillum
Cyanobacteria Anabaena, Nostoc, Oscillatoria; All convert atmospheric N₂ into usable forms.
Mycorrhiza: Partners fungus (Glomus) and plant root
Plant gets phosphorus, resistance to root pathogens, salt and drought tolerance
Fungus gets sugars from the plant; Mutual benefit: fungus gets sugars.
Fertiliser versus protector: Biofertilisers (nourish the crop) Rhizobium, Azotobacter, Azospirillum, Glomus, cyanobacteria
Biocontrol agents (protect the crop) ladybird, dragonfly, Bacillus thuringiensis, Trichoderma, nucleopolyhedrovirus
Exam trap Bt, Trichoderma and NPV are never biofertilisers; Bt, Trichoderma, NPV are not biofertilisers.
Crop to biofertiliser: Legume (pea, gram) Rhizobium
Paddy cyanobacteria (Anabaena, Nostoc)
Non-legume cereal Azotobacter or Azospirillum
Phosphorus-poor or dry soil mycorrhiza (Glomus); Waterlogged paddy fields suit cyanobacteria.

Symbiotic	Rhizobium, in legume root nodules
Free-living	Azotobacter, Azospirillum
Cyanobacteria	Anabaena, Nostoc, Oscillatoria

Partners	fungus (Glomus) and plant root
Plant gets	phosphorus, resistance to root pathogens, salt and drought tolerance
Fungus gets	sugars from the plant

Biofertilisers (nourish the crop)	Rhizobium, Azotobacter, Azospirillum, Glomus, cyanobacteria
Biocontrol agents (protect the crop)	ladybird, dragonfly, Bacillus thuringiensis, Trichoderma, nucleopolyhedrovirus
Exam trap	Bt, Trichoderma and NPV are never biofertilisers

Legume (pea, gram)	Rhizobium
Paddy	cyanobacteria (Anabaena, Nostoc)
Non-legume cereal	Azotobacter or Azospirillum
Phosphorus-poor or dry soil	mycorrhiza (Glomus)

How to answer Microbes as Biofertilisers questions

ISC sets case-based items on biofertilisers (one benefit, one example), naming specific microbes, mycorrhiza's role, and the difference between biofertilisers and biopesticides.

1Name specific genera: Rhizobium, Azotobacter, Azospirillum, Glomus, Anabaena, Nostoc; not just 'bacteria' or 'fungi'.
2State the exact benefit: fixes atmospheric nitrogen, absorbs phosphorus, adds organic matter, improves water-holding capacity.
3Say whether the association is symbiotic or free-living.
4Do not give Bacillus thuringiensis, baculovirus or Trichoderma as biofertilisers; they are biocontrol agents.
5For paddy fields, give cyanobacteria; for legumes, Rhizobium.
6For advantages, contrast with chemical fertilisers: no pollution, renewable, cheap, improves soil.

Microbes as Biofertilisers in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Choosing the right biofertiliser

Suppose a farmer grows paddy in the rainy season and gram (a legume) in winter on the same field, and wants to cut his urea bill.

Suggest a biofertiliser for each crop and explain how it helps.

Try it first, then show the solutionSolution

Paddy fields stay waterlogged, which suits cyanobacteria such as Anabaena and Nostoc; they fix atmospheric nitrogen and add organic matter.
Gram is a legume, so its seeds can be coated with Rhizobium culture; the bacteria form root nodules and fix nitrogen for the plant.
Both supply nitrogen, so less urea is needed; soil organic matter and structure improve over seasons.
Mycorrhizal inoculum (Glomus) can be added if the soil is poor in phosphorus.

Answer: Cyanobacteria for paddy and Rhizobium for gram supply fixed nitrogen, reducing urea use.

Memory hook: Wet rice, blue-green; legume, Rhizobium.

Tricks to remember Microbes as Biofertilisers

Short memory aids. Say each one aloud once, then use it on a question the same day.

R-A-A-G-C
Rhizobium, Azotobacter, Azospirillum, Glomus, Cyanobacteria: the five ISC biofertilisers.
Use it when: Listing examples.
Glomus grabs phosphorus
G for Glomus, G for 'gets phosphorus'.
Use it when: Mycorrhiza function.
Feed or fight
Biofertilisers feed the crop; biocontrol agents fight the pests.
Use it when: Avoiding the biopesticide mix-up.

Microbes as Biofertilisers: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Define with examples · 2 marks · Short answerBoard level

What are biofertilisers? Name one symbiotic and one free-living nitrogen-fixing bacterium.

Try it first, then show the solutionSolution

Organisms that enrich the nutrient quality of the soil (bacteria, fungi, cyanobacteria) (1 mark).
Symbiotic: Rhizobium; free-living: Azotobacter or Azospirillum (1 mark).

Answer: Soil-enriching organisms; Rhizobium; Azotobacter.

Example 2: Mycorrhiza · 3 marks · Short answerBoard level

What is mycorrhiza? Name a genus that forms it and give three benefits to the plant.

Try it first, then show the solutionSolution

A symbiotic association between a fungus and the roots of a higher plant (1 mark).
Glomus (1 mark).
The fungus absorbs phosphorus from soil for the plant; gives resistance to root-borne pathogens; tolerance to salinity and drought; overall increased growth (any two) (1 mark).

Answer: Fungus-root symbiosis; Glomus; phosphorus uptake, pathogen resistance, salt and drought tolerance.

Example 3: Cyanobacteria in paddy · 2 marks · Short answerHarder board level

Why are cyanobacteria valuable biofertilisers in paddy fields? Name two nitrogen-fixing cyanobacteria.

Try it first, then show the solutionSolution

They are autotrophic and grow well in the waterlogged paddy field; many fix atmospheric nitrogen and also add organic matter, raising fertility (1 mark).
Anabaena and Nostoc (or Oscillatoria) (1 mark).

Answer: They fix N₂ and add organic matter in waterlogged fields; Anabaena, Nostoc.

Example 4: Biofertiliser or biopesticide · 3 marks · Short answerHarder board level

Classify the following as biofertilisers or biocontrol agents, and give the role of each: Bacillus thuringiensis, Azospirillum, Trichoderma, Glomus.

Try it first, then show the solutionSolution

Biofertilisers: Azospirillum (free-living, fixes nitrogen); Glomus (mycorrhiza, phosphorus uptake) (1½ marks).
Biocontrol agents: Bacillus thuringiensis (toxin kills caterpillars); Trichoderma (fungus that controls plant pathogens in the root zone) (1½ marks).

Answer: Biofertilisers: Azospirillum, Glomus. Biocontrol: Bt, Trichoderma.

Example 5: Chemical versus biological · 3 marks · Short answerHarder board level

Give three harmful effects of chemical fertilisers and pesticides, and three advantages of biofertilisers.

Try it first, then show the solutionSolution

Harmful: pollute soil and ground water; toxic residues in fruits, vegetables and crops; kill useful organisms along with pests (and select resistant pests) (1½ marks).
Biofertilisers: renewable and cheap; no pollution; improve soil organic matter, structure and water-holding capacity (1½ marks).

Answer: Chemicals pollute and poison; biofertilisers are cheap, clean and improve soil.

Example 6: A sandy, phosphorus-poor field · 4 marks · Short answerHOTS / competency

Suppose a maize farmer in a dry district has sandy soil that is poor in phosphorus and organic matter, and yields are falling. (a) Give two ways biofertilisers could help this field. (b) Name a suitable microbe for each problem and justify your choice. (c) A neighbour suggests Bacillus thuringiensis instead. Is the advice correct?

Try it first, then show the solutionSolution

(a) Mycorrhizal fungi absorb phosphorus for the roots and improve drought tolerance; nitrogen-fixing microbes add nitrogen, and regular biofertiliser use builds organic matter, improving soil structure and water-holding capacity (2 marks).
(b) Glomus (mycorrhiza) for phosphorus uptake and drought tolerance; Azotobacter or Azospirillum, free-living nitrogen fixers, because maize is a cereal and not a legume, so Rhizobium will not nodulate it (1 mark).
(c) No: Bt is a biocontrol agent whose toxin kills caterpillars; it does not nourish the soil or the crop (1 mark).

Answer: Phosphorus uptake and drought tolerance (Glomus) plus nitrogen fixation (Azotobacter or Azospirillum); Bt is a biopesticide, not a biofertiliser.

Where students lose marks in Microbes as Biofertilisers

The slips examiners see most often, and what to write instead.

Mistake: Confusing biofertilisers with biopesticides
In 2025 examiners reported methanogens, Agrobacterium, Bacillus thuringiensis or baculovirus named as biofertilisers.
Do this: Learn the five biofertiliser genera; Bt, Trichoderma and NPV are biocontrol agents.
Usually costs: 1 mark.
Mistake: Naming only 'bacteria' or 'fungi'
Examiners (2025) found general terms instead of specific microbes.
Do this: Write Rhizobium, Azotobacter, Glomus, Anabaena.
Usually costs: 1 mark.
Mistake: Wrong benefit of biofertilisers
Examiners (2025) saw claims that biofertilisers decrease water-holding capacity, kill microbes or protect from sunlight.
Do this: State: fix nitrogen, improve soil quality and water-holding capacity, improve root health.
Usually costs: 1 mark.
Mistake: Saying mycorrhiza fixes nitrogen
Mycorrhizal fungi help in phosphorus absorption, not nitrogen fixation.
Do this: Glomus = phosphorus.
Usually costs: 1 mark.

Exam tips

Match crop to biofertiliser: legume → Rhizobium; paddy → cyanobacteria.
Use 'atmospheric nitrogen' and 'fix' together.
Underline genus names; capitalise the genus.
Keep a two-column list: biofertilisers versus biocontrol agents.
Mention organic farming when asked why biofertilisers are promoted.

Practice questions on Microbes as Biofertilisers

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The fungal partner of mycorrhiza mainly helps the plant to absorb:
(a) nitrogen (b) phosphorus (c) potassium (d) iron
Hint
Glomus.
Answer
(b) phosphorus.
Q2 · 2 marks · Short answerBoard level
A farmer wants nitrogen added to soil without growing legumes. Suggest two bacteria and one blue-green alga he could use.
Hint
Azo-.
Answer
Azotobacter and Azospirillum; Anabaena (or Nostoc, Oscillatoria).
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Rhizobium is used as a biofertiliser for leguminous crops.
Reason (R): Rhizobium lives in root nodules of legumes and fixes atmospheric nitrogen into organic forms.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Symbiosis.
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
What does the fungus gain from a mycorrhizal association, and why is the association called symbiotic?
Hint
Both benefit.
Answer
The fungus obtains food (sugars made by photosynthesis) from the plant. Both partners benefit, the plant receiving phosphorus and protection, so it is a mutually beneficial symbiosis.
Q5 · 3 marks · Short answerHarder board level
What is integrated pest management (IPM)? Why does an organic farmer not aim to eradicate pests completely?
Hint
Checks and balances.
Answer
IPM combines biocontrol agents, knowledge of pest life cycles and habitats, and minimal need-based chemicals to keep pests at manageable levels. Complete eradication is undesirable because beneficial predatory and parasitic insects depend on the pests as food or hosts and would not survive; biodiversity keeps the system stable.
Q6 · 2 marks · Short answerHarder board level
Why are Nucleopolyhedroviruses considered good for IPM?
Hint
Narrow spectrum.
Answer
They are species-specific, narrow-spectrum insecticidal agents with no negative effects on plants, mammals, birds, fish or non-target insects, so beneficial insects are conserved.
Q7 · 3 marks · Short answerHOTS / competency
Suppose a farmer adds Rhizobium culture to a wheat field and sees no benefit, while his neighbour's pea crop with the same culture grows well. Explain, and suggest a biofertiliser for the wheat.
Hint
Host specificity.
Answer
Rhizobium fixes nitrogen in root nodules of leguminous plants; wheat is not a legume, so no nodules form and no nitrogen is fixed for it. Pea is a legume and benefits. For wheat, a free-living nitrogen fixer such as Azotobacter or Azospirillum (and mycorrhizal Glomus for phosphorus) is suitable.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose an agricultural officer finds a village using large amounts of urea and a broad-spectrum insecticide. Wells show nitrate contamination, and birds and frogs have declined.
(a) Link each problem to the chemical used. (1)
(b) Suggest one biofertiliser and one biocontrol agent to replace them. (1)
(c) Give one reason why the biocontrol agent you chose is safer. (1)
Hint
Leaching; non-target kills.
Answer
(a) Excess urea leaches nitrate into ground water; the broad-spectrum insecticide kills non-target animals such as insects that birds and frogs eat (and poisons them). (b) Biofertiliser: Azotobacter or cyanobacteria (Rhizobium for legumes); biocontrol: Bacillus thuringiensis or Nucleopolyhedrovirus (ladybirds for aphids). (c) Bt toxin acts in the gut of caterpillars and leaves other insects unharmed (NPV is species-specific), so non-target animals survive.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which biofertiliser forms root nodules on legumes?Show answer
Rhizobium.
2.Name the genus whose members form mycorrhiza.Show answer
Glomus.
3.Name two cyanobacteria used as biofertilisers.Show answer
Anabaena and Nostoc (also Oscillatoria).
4.Are Azotobacter and Azospirillum symbiotic?Show answer
No, they are free-living in the soil.
5.Which biocontrol agent controls aphids?Show answer
The ladybird beetle.
6.What are the main source groups of biofertilisers?Show answer
Bacteria, fungi and cyanobacteria.

Microbes as Biofertilisers: frequently asked questions

What are biofertilisers?

Organisms, mainly bacteria, fungi and cyanobacteria, that enrich the nutrient quality of the soil, for example by fixing nitrogen or helping in phosphorus uptake.

How does mycorrhiza help plants?

The fungal partner absorbs phosphorus from the soil for the plant and gives resistance to root pathogens and tolerance to salinity and drought.

Why are cyanobacteria used in paddy fields?

They grow well in waterlogged fields, fix atmospheric nitrogen and add organic matter, increasing fertility.

Is Bacillus thuringiensis a biofertiliser?

No. It is a biocontrol agent whose toxin kills caterpillars.

Why are biofertilisers better than chemical fertilisers?

They are renewable and cheap, do not pollute soil or water, and improve soil organic matter and structure.