7 min readConcept sheet1-mark MCQ or assertion-reason on Rhizobium, Glomus or cyanobacteria, a 2 or 3-mark question on types of biofertilisers; also a NEET favourite

Microbes as biofertilisers

Biofertilisers are living organisms that enrich soil nutrients: nitrogen-fixing bacteria and cyanobacteria, and mycorrhizal fungi that supply phosphorus.

BiofertilisersOrganisms that enrich the nutrient quality of the soil: bacteria, fungi and cyanobacteria

RhizobiumSymbiotic bacterium in root nodules of legumes; fixes atmospheric nitrogen

MycorrhizaFungus-root association, e.g. Glomus; fungus absorbs phosphorus from soil for the plant

CyanobacteriaAutotrophic nitrogen fixers such as Anabaena, Nostoc, Oscillatoria; important in paddy fields

Why biofertilisers?

Heavy use of chemical fertilisers pollutes soil and water and is costly. Biofertilisers are organisms that enrich the nutrient quality of the soil. Their main sources are bacteria, fungi and cyanobacteria. Many are now sold in the market, and farmers use them regularly to replenish soil nutrients and reduce dependence on chemical fertilisers, an important part of organic farming.

Types of biofertilisers

Group	Examples	What it does
Symbiotic bacteria	Rhizobium	Lives in root nodules of leguminous plants; fixes atmospheric nitrogen into organic forms the plant uses
Free-living bacteria	Azospirillum, Azotobacter	Fix atmospheric nitrogen while free-living in the soil, enriching its nitrogen content
Fungi (mycorrhiza)	Many members of genus Glomus	Fungus absorbs phosphorus from soil and passes it to the plant
Cyanobacteria (blue-green algae)	Anabaena, Nostoc, Oscillatoria	Autotrophic; fix nitrogen; add organic matter; key biofertiliser in paddy fields

Symbiotic vs free-living nitrogen fixers

SymbioticRhizobium lives inside root nodules of legumes such as pea, gram and groundnut. The plant gives it food and shelter; it gives the plant fixed nitrogen.

Free-livingAzotobacter and Azospirillum live on their own in the soil and fix nitrogen without forming nodules, so they help non-legume crops too.

Extra benefits of mycorrhiza

Better uptake of phosphorus from the soil.
Resistance to root-borne pathogens.
Tolerance to salinity and drought.
An overall increase in plant growth and development.

In real life

Indian rice farmers often grow blue-green algae in flooded paddy fields as a natural nitrogen source, and pulses like moong and chana enrich the soil through Rhizobium. Biofertilisers cut fertiliser bills and keep chemicals out of our food and water.

Exam angle

2 marks: “What are biofertilisers? Name one bacterial and one cyanobacterial biofertiliser.” Organisms that enrich the nutrient quality of the soil. Bacterial: Rhizobium (or Azotobacter, Azospirillum). Cyanobacterial: Anabaena (or Nostoc, Oscillatoria).

3 marks: “What is mycorrhiza? List three benefits a plant gets from it.” A symbiotic association between a fungus (e.g. Glomus) and plant roots. Benefits: phosphorus absorbed from soil; resistance to root-borne pathogens; tolerance to salinity and drought; better growth.

Remember it

N from Rhizobium, Azotobacter, Azospirillum and Anabaena; P from Glomus (mycorrhiza).