🦠Biofertilizers: Living Microbes for Sustainable Soil Fertility

Why Biofertilizers Matter in Agriculture

A soybean farmer in Madhya Pradesh inoculates seeds with Rhizobium culture before sowing. Within weeks, pink-coloured nodules appear on the roots — a sign of active nitrogen fixation. The crop fixes 80-100 kg N/ha from the atmosphere, reducing the need for urea by half. After harvest, 40-80 kg N remains in the soil for the next wheat crop. This is the power of biofertilizers — living microbes that convert free atmospheric nitrogen and locked-up soil phosphorus into plant-available forms at almost zero cost.

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What Are Biofertilizers?

Biofertilizers are preparations containing living cells or latent cells of efficient microbial strains that help crops take up nutrients through interactions in the rhizosphere (the soil zone directly around plant roots).

Key terms:

• Rhizosphere — the soil zone directly influenced by plant roots, root secretions, and associated microorganisms. This is the most biologically active zone in soil.
• Microbial inoculants — another name for biofertilizers

Application sequence mnemonic — FIR (like the tree):

Biofertilizers are a key component of integrated nutrient management (INM) — cost-effective, renewable, and environment-friendly.

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Classification of Biofertilizers

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Nitrogen-Fixing Biofertilizers

These microorganisms convert atmospheric N₂ (which plants cannot use) into ammonia (NH₃) (which plants can use). They are classified by their relationship with plants:

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(a) Rhizobium — The Most Important Biofertilizer

• Type: Symbiotic N₂-fixer (forms root nodules with legumes)
• Organism: Rhizobium spp. — aerobic, heterotrophic bacteria
• N-fixation: 50-200 kg N/ha/year
• Crops: All leguminous crops — pulses (moong, arhar, lentil, chickpea), groundnut, soybean, berseem, lucerne
• Residual benefit: Leaves 40-80 kg N in soil for subsequent crop
• Yield increase: 25-30%

How it works: Rhizobium forms nodules on legume roots. Inside the nodules, Leghemoglobin (a pinkish-red pigment) acts as an oxygen carrier, delivering oxygen slowly to the bacteria while protecting the Nitrogenase enzyme from oxygen inactivation. Nitrogenase is the enzyme that actually converts N₂ to NH₃.

Important: Rhizobium is host-specific — different legumes need different Rhizobium strains. Optimum pH for nodulation is >5.0; legumes fail to develop nodules when pH is less than 5.0.

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(b) Azotobacter — For Non-Legumes

• Type: Free-living, aerobic N₂-fixer
• N-fixation: 5-20 kg N/ha/year
• Crops: Rice, Cotton, Sugarcane and other non-leguminous crops
• Species: Azotobacter chroococcum and Azotobacter vinelandii
• Optimum pH: 6.5-8.0 (does not perform well in acidic soils)
• Also produces growth regulators (IAA, IBA, NAA, GA) and vitamins
• Secretes antibiotics acting as biopesticides
• Application: Mix 3-5 kg inoculum with 5 tonne FYM per hectare

Think of Azotobacter as the biofertilizer for crops that Rhizobium cannot help — all the non-legumes like rice, cotton, and sugarcane.

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(c) Azospirillum — For Grasses and Cereals

• Type: Associative symbiotic N₂-fixer
• Crops: Sorghum and other Poaceae (grasses) — rice, wheat, corn, oats, barley
• Fixes nitrogen within the rhizosphere but has no direct physical contact with plants — called associative symbiosis ^{RRB SO-20}
• Does not form nodules
• Also produces growth hormones and improves root development

The key difference from Rhizobium: Azospirillum lives near roots but never enters them or forms nodules.

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(d) Blue Green Algae (BGA / Cyanobacteria) — For Paddy

• Type: Free-living, photosynthetic (autotrophic) N₂-fixer
• N-fixation: 20-25 kg N/ha in rice fields
• Crops: Primarily paddy (rice) — thrive in waterlogged, flooded conditions
• Key genera: Anabaena, Nostoc, Aulosira, Tolypothrix, Calothrix
• Fix nitrogen using heterocysts (specialized thick-walled cells)
• Also supply growth regulators (IAA, IBA, NAA, GA₁ to GA₃₃) and vitamins

BGA are unique because they are both photosynthetic (make their own food) and nitrogen-fixing — completely self-sustaining in flooded rice fields.

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(e) Azolla-Anabaena Association — The Floating Nitrogen Factory

• Type: Symbiotic — water fern Azolla pinnata with cyanobacterium Anabaena azollae
• N-fixation: 25-35 kg N/ha/year (some sources: 30-40 kg N/ha)
• Optimum temperature: 20-30 degree C
• pH: 5.5-7.0
• Unlike BGA, Azolla thrives well at low temperature

How it is used in rice cultivation:

1. As green manure: Azolla grows on flooded fields for 2-3 weeks before transplanting, then water is drained and Azolla is incorporated by ploughing
2. As dual crop: 1000-5000 kg/ha of Azolla is applied one week after transplanting with 25-50 kg/ha SSP and 5-10 cm standing water. When a thick mat forms, it is incorporated by trampling.

Anabaena colonizes cavities formed at the base of Azolla fronds.

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Nitrogen Fixation: The Big Picture

Nitrogen fixation converts atmospheric N₂ into ammonia (NH₃) that plants can use. Two pathways exist:

Biological fixation is nearly 4 times greater than industrial fixation — underscoring the importance of biofertilizers.

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Symbiotic N-Fixation

Symbiosis = living together for mutual benefit. The plant provides sugars from photosynthesis and the microbe provides fixed nitrogen in return.

Nodule-Forming Symbiosis

(i) With legumes (Rhizobium):

• Boussingault first proved experimentally that legumes use atmospheric N₂
• Leghemoglobin carries O₂ to bacteroids slowly, protecting nitrogenase from oxygen inactivation
• Rhizobium is aerobic and heterotrophic
• Range: Green gram fixes 20 kg N/ha; Lucerne fixes more than 250 kg N/ha

(ii) With non-legumes (Frankia):

• About 160 species from 13 genera of non-legumes
• The actinomycete Frankia forms root nodules on trees like Casuarina and Alder

Actinomycetes are intermediate between true fungi and bacteria. They grow best in moist, aerated soil. Growth ceases below pH 5.5 (unlike fungi, which tolerate acidity).

Factors Affecting Nodulation and N-Fixation

Non-Nodule Symbiosis (Azolla-Anabaena)

• Anabaena azollae lives in leaf cavities of water fern Azolla pinnata
• Fixes 30-40 kg N/ha
• Optimum: 20-30 degree C, pH 5.5-7.0
• Thrives at lower temperatures than BGA

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Non-Symbiotic (Free-Living) N-Fixation

These organisms fix nitrogen independently, without any plant partnership.

Heterotrophic Fixers (need organic matter for energy)

All fix 5-20 kg N/ha/year. Applied by mixing 3-5 kg inoculum with 5 tonne FYM per hectare.

Autotrophic Fixers (make their own food using light)

• Rhodospirillum — anaerobic photosynthetic bacterium
• Blue green algae — photosynthetic, fix 20-25 kg N/ha in rice fields

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N-Fixation Rates: Complete Comparison

Summary: N-fixation rates by different biofertilizers

Biofertilizer	N-fixation (kg N/ha/year)	Type
Lucerne (with Rhizobium)	>250	Symbiotic
Rhizobium (general range)	50-200	Symbiotic
Azolla-Anabaena	25-40	Symbiotic (non-nodule)
BGA (Blue Green Algae)	20-25	Free-living (autotrophic)
Green gram (with Rhizobium)	20	Symbiotic
Free-living bacteria (Azotobacter etc.)	5-20	Free-living (heterotrophic)

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Phosphorus-Related Biofertilizers

Phosphate Solubilizing Bacteria (PSB)

Most soil phosphorus exists in insoluble forms that plants cannot use. PSB organisms produce organic acids (citric, oxalic, gluconic acid) that dissolve these insoluble phosphate compounds.

Key PSB organisms:

• Bacillus megaterium var. phosphaticum, B. subtilis, B. circulans
• Pseudomonas striata

PSB fungi: Penicillium sp., Aspergillus awamori

PSB can also solubilize P from rock phosphate, slag, and bone meal — an economical and environment-friendly alternative to chemical P-fertilizers.

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Mycorrhizae — Fungi That Extend Root Reach

Mycorrhiza (literally “fungus-root”) is a mutualistic association between plant roots and fungal hyphae. The plant feeds the fungus sugars; the fungus returns phosphorus and other nutrients from soil volumes far beyond root reach.

Consider a nursery raising citrus seedlings: mycorrhiza-inoculated seedlings establish faster, absorb more phosphorus, and survive transplantation shock better than uninoculated ones.

VAM (Vesicular Arbuscular Mycorrhiza) is the most common type, associating with approximately 80% of all land plants.

Types of Mycorrhizae

Ectomycorrhiza details:

• Sheath/Mantle increases absorbing surface area and protects roots
• Hartig net acts as storage and transport organ for P

Benefits of VAM inoculation:

1. Improved mineral nutrition — especially P, Zn, Cu, K, S, NH₄⁺
2. Greater soil exploration by fungal hyphae
3. Protection against root pathogens
4. Improved water relations
5. Better tolerance to salinity and heavy metal stress
6. Protection against transplantation shock

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Silicate and Zinc Solubilizers

• Bacillus sp. solubilizes silicate and zinc in soil
• Azozink — zinc-solubilizing bacterial biofertilizer developed for zinc-deficient soils
• Important because 50% of Indian soils are zinc-deficient

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Plant Growth Promoting Rhizobacteria (PGPR)

• Key organism: Pseudomonas fluorescens
• Promote growth through nutrient solubilization, plant growth hormones, and biocontrol of pathogens
• Act as both growth promoter and disease suppressor

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Organic Matter Decomposers (OMD)

• Cellulolytic fungi: Aspergillus, Penicillium, Trichoderma (also a biocontrol agent)
• Lignolytic organisms: Arthrobacter, Agaricus
• Break down complex organic matter for quick nutrient release — useful as compost accelerators

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Benefits of Biofertilizers

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Application Methods and Rates

(a) Seed Inoculant (Seed Treatment)

• 20 g of Rhizobium culture treats 1 kg seed
• One packet = 200 g of culture

(b) Soil Inoculant

• 10 packets (2 kg/ha) of Azotobacter/Azospirillum mixed with 25 kg FYM + 25 kg soil and broadcast before transplanting

Biofertilizer Application Rates - Quick Reference for Exams

Crop Type	Biofertilizer	Rate per ha	Packets/ha
Small-seeded pulses (moong, arhar, lentil)	Rhizobium	500 g	2.5
Soybean, Bengal gram	Rhizobium	1 kg	5
Groundnut	Rhizobium	1.5 kg	7.5
Non-legumes (rice, cotton, sugarcane)	Azotobacter	2 kg + 25 kg FYM + 25 kg soil	10
Grasses/cereals (sorghum, wheat)	Azospirillum	2 kg + 25 kg FYM + 25 kg soil	10
Rice (Azolla as dual crop)	Azolla	1000-5000 kg/ha + 25-50 kg SSP	-
Free-living bacteria	Azotobacter/others	3-5 kg + 5 tonne FYM	-

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Exam Tips and Mnemonics

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Summary Table

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Summary Cheat Sheet