UPSC Prelims 2013 Q33: Environment & Ecology – Sustainable Agriculture & Forestry
Consider the following organisms:
1. Agaricus
2. Nostoc
3. Spirogyra
Which of the above is/are used as biofertilizer/biofertilizers?
- (a) 1 and 2
- (b) 2 only
- (c) 2 and 3
- (d) 3 only
Answer: (b) 2 only
Explanation
Organisms used as biofertilizers
A biofertilizer is a living organism that adds nutrients, mainly nitrogen, to the soil.
| Organism | Used as a biofertilizer? | What it is |
|---|---|---|
| 1. Agaricus | No | The edible button mushroom, a fungus |
| 2. Nostoc | Yes | A blue-green alga (cyanobacterium) that fixes nitrogen, useful in paddy fields |
| 3. Spirogyra | No | A green pond alga; it does not fix nitrogen |
Common biofertilizers:
| Group | Examples |
|---|---|
| Symbiotic bacteria | Rhizobium in the root nodules of pulses |
| Free-living bacteria | Azotobacter, Azospirillum |
| Cyanobacteria | Nostoc, Anabaena, Oscillatoria |
| Fern | Azolla, which carries Anabaena |
| Fungi | Mycorrhiza, which helps roots take up phosphorus |
Answer: (b) 2 only.
Why each option is right or wrong
- (a) Incorrect. Agaricus is a mushroom, not a biofertilizer.
- (b) CORRECT. Only Nostoc is used as a biofertilizer.
- (c) Incorrect. Spirogyra does not fix nitrogen.
- (d) Incorrect. Spirogyra is not a biofertilizer; Nostoc is.
Key point
Biofertilizers: Rhizobium, Azotobacter, Azospirillum, cyanobacteria (Nostoc, Anabaena), Azolla, mycorrhiza. Agaricus is a mushroom; Spirogyra a pond alga.
Remember
Nostoc and Anabaena fix nitrogen.
Common mistakes
- Treating every alga as a nitrogen fixer.
- Not knowing that Agaricus is the button mushroom.
- Forgetting the Azolla-Anabaena pair used in rice fields.
Exam tip
Only blue-green algae fix nitrogen. Green algae such as Spirogyra do not.
Sources
- NCERT Class XII — Biology (Microbes in Human Welfare: microbes as biofertilisers)
- NCERT Class XI — Biology, Ch 2 (Biological Classification)