Biofertilizers — Types, Uses & India's 11 Notified Biofertilizers 🌱
Complete UPSC Notes — What are biofertilizers, how they differ from chemical fertilizers, 4 types of nitrogen fixers (Rhizobium, Azotobacter, Azospirillum, BGA/Azolla), phosphorus solubilizers, mycorrhiza. 11 notified biofertilizers under FCO 1985. INM strategy. With MCQs and memory aids.
🔥 What are Biofertilizers? — Made Simple
💡 The "Kitchen Helper" Analogy
Imagine your kitchen has a pantry full of ingredients (nutrients in soil), but many are locked in sealed containers that you can't open. A biofertilizer is like a kitchen helper who comes in and opens those sealed containers for you — making the ingredients available for cooking (plant growth). The helper doesn't bring new ingredients (unlike chemical fertilizers which add nutrients). Instead, the helper unlocks what's already there — converting atmospheric nitrogen into a form plants can absorb, dissolving locked-up phosphorus, and generally making the kitchen work better.
📊 Types of Biofertilizers — Complete Classification
| Biofertilizer | Type | Function | Crops | Key Facts for Exams |
|---|---|---|---|---|
| Rhizobium | Symbiotic N₂ fixer (bacteria) | Lives in root nodules of legumes. Fixes atmospheric N₂ → ammonia | Pulses, groundnut, soybean, all legumes | Most important biofertilizer. Symbiotic = needs host plant. Forms nodules on roots. |
| Azotobacter | Free-living N₂ fixer (bacteria) | Fixes N₂ independently — no host needed. Also produces growth hormones. | Wheat, maize, mustard, cotton, potato, vegetables | Free-living = does NOT need a plant host. Most abundant species: A. chroococcum. |
| Azospirillum | Associative N₂ fixer (bacteria) | Lives near roots (rhizosphere) but does NOT enter root. Fixes N₂ + produces growth hormones. | Sorghum, millets, maize, sugarcane, wheat | "Associative mutualism" — near roots but not inside. Used mainly for cereals and millets. |
| Blue-Green Algae (BGA) | Free-living / Symbiotic cyanobacteria | Fixes N₂ + adds organic matter to soil | Rice (paddy) | Nostoc, Anabaena, Aulosira. Specifically for rice/paddy fields (waterlogged conditions). |
| Azolla | Fern with symbiotic cyanobacteria | Azolla fern hosts Anabaena (N₂ fixer) in its leaf cavities. Also used as cattle feed. | Rice | Azolla is a FERN, not bacteria. Contains Anabaena azollae. Dual use: biofertilizer + cattle feed. |
| Mycorrhiza (VAM/AM) | Symbiotic fungi | Enhances phosphorus, water & mineral absorption. Protects roots from pathogens. | Most crops, forestry | Does NOT fix nitrogen. Fixes phosphorus. Obligate symbiont (needs host plant to survive). |
| Phosphate Solubilising Bacteria (PSB) | Bacteria/Fungi | Converts insoluble phosphorus → soluble form for plant uptake | All crops | Bacteria: Bacillus, Pseudomonas. Fungi: Penicillium, Aspergillus. Solubilise locked-up P. |
| Potassium Mobilizing Bacteria (KMB) | Bacteria | Makes potassium available from minerals in soil | All crops | Relatively newer. Included in FCO 1985 notification. |
| Zinc Solubilising Bacteria (ZSB) | Bacteria | Solubilises zinc from soil for plant uptake | All crops | Addresses zinc deficiency — a major micronutrient issue in Indian soils. |
| Acetobacter | N₂ fixer (bacteria) | Colonises crop tissues. Fixes N₂ + produces growth hormones. | Sugarcane, sugar crops | Specifically used in sugar-producing crops. Enters plant tissues (endophyte). |
🎯 Critical Biofertilizer–Crop Pairs (Most Asked)
Rhizobium → Legumes
Symbiotic. Root nodules. Pulses, groundnut, soybean. The most important and most tested biofertilizer in exams.
Azotobacter → Cereals
Free-living. No host needed. Wheat, maize, mustard, cotton, potato. Produces growth hormones.
Azospirillum → Millets
Associative. Near roots. Sorghum, pearl millet, finger millet, maize, sugarcane.
BGA/Azolla → Rice
Blue-green algae (Anabaena) in paddy. Azolla is a fern hosting Anabaena. Also cattle feed.
Mycorrhiza → P Uptake
Fungi. Enhances phosphorus (NOT nitrogen!). Also water and minerals. Protects roots.
Acetobacter → Sugarcane
N₂ fixer specific to sugar crops. Colonises plant tissues (endophyte). Produces growth hormones.
• Mycorrhiza does NOT fix nitrogen — it enhances phosphorus, water, and mineral uptake. If a question asks "which is NOT a nitrogen fixer?" → Mycorrhiza is the answer.
• Azolla is a FERN, not bacteria or algae. It acts as a biofertilizer because it hosts the cyanobacterium Anabaena in its leaf cavities.
• Agrobacterium is NOT a biofertilizer — it's used in genetic engineering (Ti plasmid for GM crops). Don't confuse with Azotobacter!
✅ Advantages & ⚠️ Limitations
✅ Advantages
• Improve yield by 10–25% (ICAR data)
• Can supplement 20–25% of chemical N & P fertilizers
• Eco-friendly — no soil/water pollution, no residues
• Improve soil structure, texture, and organic matter
• Low cost — affordable for small and marginal farmers
• Some protect roots from pathogens and nematodes
⚠️ Limitations
• Contain living organisms — sensitive to heat, sunlight, moisture
• Short shelf life — must be used before expiry
• Don't work well in extreme pH (too acidic/alkaline) or dry soils
• Cannot fully replace chemical fertilizers — only supplement them
• Contamination during manufacturing is a common issue
• Crop-specific — wrong strain = no benefit
📝 UPSC-Style MCQs
1. Rhizobium — symbiotic nitrogen fixer in legume root nodules
2. Azolla — a bacterium used as biofertilizer in paddy
3. Azotobacter — free-living nitrogen fixer that does not need a host
Which is/are correct?
🧠 Memory Aid
🔑 Lock These In for Prelims Day
❓ FAQs
What is the difference between biofertilizer, organic manure, and chemical fertilizer?
Why is Rhizobium specific to legumes?
Can biofertilizers completely replace chemical fertilizers?
🏁 Conclusion
🌱 Nature's Nutrient Unlockers
India's soil health is deteriorating — decades of chemical fertilizer overuse have depleted organic carbon, disrupted microbial communities, and led to nutrient imbalances. Biofertilizers offer a sustainable complement: Rhizobium in pulse fields fixing atmospheric nitrogen, Azotobacter in wheat fields working independently, blue-green algae in paddy fields enriching waterlogged soils, and mycorrhiza in orchards unlocking phosphorus from deeper soil layers. Together, the 11 notified biofertilizers under FCO 1985 represent India's biological toolkit for soil rejuvenation.
For UPSC: Memorise the 6 critical pairs: Rhizobium → legumes (symbiotic), Azotobacter → cereals (free-living), Azospirillum → millets (associative), BGA/Azolla → rice, Mycorrhiza → phosphorus (NOT nitrogen!), Acetobacter → sugarcane. Know the 3 traps: Agrobacterium ≠ biofertilizer, Azolla = fern (not bacterium), Mycorrhiza ≠ nitrogen fixer. And the ICAR stat: 10–25% yield improvement, 20–25% chemical fertilizer replacement.
