Biofertilisers: Rhizobium, PSB and Azotobacter, Used Right
Biofertilisers are living cultures of beneficial microbes that fix nitrogen and unlock phosphorus for a few rupees a packet. They work — but only if you match the culture to the crop and keep the packet alive. Here is how.
Technical Kisan Editorial3 min read
Biofertilisers are the cheapest input in organic farming and among the most misunderstood. A packet costs a few rupees and contains billions of live beneficial microbes — bacteria that either fix nitrogen from the air or unlock phosphorus already locked in the soil. Used properly they meaningfully cut a crop's nutrient needs. Used carelessly they do nothing, because the microbes are dead before they reach the root.
The whole trick is remembering they are alive.
Match the culture to the job
There are three you will actually use, and they are not interchangeable:
| Biofertiliser | Does what | Use on |
|---|---|---|
| Rhizobium | Fixes nitrogen in root nodules | Pulses and legumes only |
| Azotobacter / Azospirillum | Fixes nitrogen free in the soil | Cereals, vegetables, non-legumes |
| PSB (phosphate-solubilising bacteria) | Unlocks bound soil phosphorus | All crops |
The commonest waste is a mismatch. Rhizobium is crop-specific — the strain for gram is not the strain for groundnut, and none of them nodulate a cereal. On wheat or maize you want Azotobacter or Azospirillum for nitrogen, plus PSB for phosphorus. Pair the right nitrogen culture with PSB and you have covered the two nutrients that matter most.
The three ways to apply
Seed treatment — the default. Make a slurry of the culture in a little jaggery or rice-gruel water (the jaggery is a sticker), coat the seed evenly, dry it in the shade for 20–30 minutes, and sow the same day. This uses the least material and puts the microbes exactly where the roots will grow.
Seedling-root dip. For transplanted crops — rice, tomato, chilli — dip the roots in a culture slurry for a few minutes before planting.
Soil application. Mix the culture into well-rotted compost or FYM and broadcast. Uses more material but suits broadcast crops and standing orchards.
Keeping the packet alive
This is where biofertilisers succeed or fail. The cultures are live microbes and they die easily:
- Store in the shade, cool. Never leave a packet in the sun or a hot shed. Heat is the main killer.
- Use before expiry. An old packet is a packet of dead microbes. Check the date and buy fresh.
- Never mix with chemical fertiliser, pesticide or fungicide in the same slurry — they kill the culture. Keep at least a few days between a chemical seed treatment and a biofertiliser coating, or skip the chemical.
- Do not sow into hot, dry midday soil. Sow treated seed in the cool of the day into moist soil so the microbes survive the move.
What to expect
Realistic gains: a well-inoculated pulse crop can get much of its nitrogen from Rhizobium; a cereal gets a smaller but real share from Azotobacter; PSB frees up phosphorus you have already got. None of this replaces building soil organic matter — biofertilisers work with compost and green manure, feeding on the organic matter you have added.
For the cost — a few rupees an acre — there is no cheaper way to add nitrogen fixation and phosphorus solubilisation to an organic system. Just treat the packet like the living thing it is, and match the culture to the crop.
Frequently asked questions
Which biofertiliser is used for which crop?
Rhizobium for pulses and legumes (it forms root nodules that fix nitrogen). Azotobacter and Azospirillum for cereals, vegetables and most non-legumes. PSB (phosphate-solubilising bacteria) for every crop, to unlock soil phosphorus. Using the wrong nitrogen culture — Rhizobium on wheat, say — does nothing.
How are biofertilisers applied?
Three ways: seed treatment (coat seed with a slurry of the culture and jaggery water just before sowing), seedling-root dip for transplanted crops, and soil application mixed with compost. Seed treatment uses the least material and is the most reliable.
Do biofertilisers replace fertiliser?
They supplement it, not replace it. A good Rhizobium inoculation can meet much of a pulse crop’s nitrogen need; Azotobacter typically supplies a smaller share for a cereal. PSB unlocks phosphorus already in the soil rather than adding new phosphorus. They are cheap and worth using, but a crop still needs compost and organic matter behind them.
Why do biofertilisers sometimes not work?
Usually because the microbes were dead before they reached the soil. The cultures are living organisms: heat, direct sun and old stock kill them. Store in the shade, use before the expiry date, never mix with chemical fertiliser or pesticide in the same slurry, and do not sow treated seed into hot, dry midday soil.
Compiled by
Technical Kisan Editorial
Editorial Desk
Guides are compiled by the Technical Kisan editorial desk from ICAR and state agricultural university recommendations, and from central and state government scheme notifications. Every figure is labelled with the season it applies to. Always confirm against the official notification before acting on it.
- Compiled from ICAR and state agricultural university guidance
- Scheme details sourced from official notifications
- Figures labelled with the season they apply to
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