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UPSC Prelims 1994 · Environment · Species Watch
Which of the following is/are used as bio fertilizers?
- Azolla
- Blue green algae
- Alfalfa
- (a) II alone
- (b) I and II
- (c) I and III
- (d) I, II and III
Show the answer and explanation
Answer: (b) I and II
Azolla and blue-green algae are biofertilisers.
Statement by statement
1. ✓ YES Azolla, a water fern, hosts Anabaena and adds nitrogen to paddy fields.
2. ✓ YES Blue-green algae (cyanobacteria) fix nitrogen in rice fields.
3. ✗ NO Alfalfa is a fodder legume crop, not applied as a biofertiliser.
The idea
A biofertiliser is a living organism added to soil or seed that makes nutrients available to the crop. It is not a chemical and not a crop. Azolla and blue-green algae are grown in flooded rice fields, where they fix nitrogen from the air.
Nitrogen fixers for pulses: Rhizobium → Nitrogen fixers for cereals: Azotobacter, Azospirillum → For rice fields: blue-green algae, Azolla → Phosphorus helpers: mycorrhiza and phosphate-dissolving bacteria
The trap
Alfalfa fixes nitrogen as a legume, so it tempts. A biofertiliser is the microbe or fern you apply, not a crop you grow.
Also know
Azolla is a tiny floating fern that can double its mass in a few days; it is also fed to cattle and poultry. A legume crop ploughed back into the soil is green manure, a different thing.
🔒 Biofertilisers: Rhizobium, Azotobacter, Azospirillum, BGA, Azolla, mycorrhiza.