Biofertilizers
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Direct answer
Biofertilizers are living organisms — bacteria, cyanobacteria and fungi — that enrich the nutrient quality of soil, chiefly by fixing atmospheric nitrogen or improving phosphorus uptake, and thereby reduce dependence on chemical fertilisers. Rhizobium forms nitrogen-fixing nodules on the roots of legumes, free-living Azospirillum and Azotobacter and cyanobacteria such as Anabaena and Nostoc add nitrogen to the soil, and the mycorrhizal fungus Glomus partners with roots to absorb phosphorus. Their use underpins organic farming because they are pollution-free and inexpensive.
What you must remember
- Definition: organisms that enrich soil nutrients; the main ones in NCERT are Rhizobium, Azospirillum, Azotobacter, cyanobacteria and the mycorrhizal fungus Glomus.
- Rhizobium: symbiotic bacterium inhabiting the root nodules of leguminous plants such as pea, bean and gram, fixing atmospheric nitrogen for the host.
- Azospirillum and Azotobacter: free-living soil bacteria that fix nitrogen and can enrich the nitrogen status of cereal fields.
- Cyanobacteria: autotrophic blue-green algae such as Anabaena, Nostoc and Oscillatoria that fix nitrogen and add both nitrogen and organic matter, widely useful in paddy fields.
- Mycorrhiza: the symbiotic association of the fungus Glomus with the roots of higher plants; the fungal hyphae absorb phosphorus from soil and hand it to the plant.
- Mycorrhizal bonuses: resistance to root-borne pathogens and tolerance to drought and salinity, with overall better growth.
- Advantages: cuts chemical fertiliser use and the pollution and cost it brings — the reason biofertilizers are central to organic farming systems.
Common confusion
The nitrogen fixers and the phosphorus helper are often swapped: Rhizobium, Azospirillum, Azotobacter and cyanobacteria supply nitrogen, while Glomus mycorrhiza supplies phosphorus. Rhizobium is symbiotic inside root nodules, whereas Azospirillum and Azotobacter live free in the soil — another regular MCQ discrimination. Cyanobacteria are photosynthetic, so they add organic matter along with nitrogen, which pure bacteria do not.
Exam-focused takeaway
NEET-UG almost every year offers a match-the-following set linking microbe to nutrient — Rhizobium with nodules, Azotobacter with free-living nitrogen fixation, Anabaena with paddy fields, Glomus with phosphorus. Statement questions test the symbiotic versus free-living distinction and the mycorrhizal benefits of drought and salinity tolerance. Assertion-reason items connect biofertilizer use with reduced chemical pollution.
Frequently asked questions
What is a biofertilizer?
An organism, such as Rhizobium or Glomus, that enriches the nutrient quality of the soil and reduces the need for chemical fertilisers.
Which biofertilizer works with leguminous roots?
Rhizobium, which forms nodules on the roots of legumes and fixes atmospheric nitrogen symbiotically.
Name two free-living nitrogen-fixing bacteria.
Azospirillum and Azotobacter, both of which fix nitrogen in the soil without living inside plant roots.
How does Glomus help plants?
The mycorrhizal fungus absorbs phosphorus efficiently from soil and also protects the plant from root pathogens, drought and salinity.
Why are cyanobacteria useful in paddy fields?
Anabaena, Nostoc and Oscillatoria fix nitrogen and add organic matter in the waterlogged conditions where rice grows well.