🌱 Biofertilizer Formulations — Types and Production

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

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Biofertilizer Formulations — Types and Production

Types of Biofertilizer Formulations

Biofertilizers are produced in three primary physical formats, each with distinct advantages, production processes, and suitability for different cropping conditions.

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1. Carrier-Based (Powder) Biofertilizers

Carrier-based biofertilizers are the most widely produced and used format in India. A carrier material is mixed with a broth culture of the microorganism to produce a powder or semi-moist product.

Carrier Materials

Carrier	Properties	Suitability
Peat soil	Excellent moisture retention, high organic matter	Rhizobium — gold standard
Lignite	Cheap, widely available; slightly lower CFU retention	Azotobacter, PSB
Vermiculite	Sterile, neutral pH, good aeration	VAM, fungi-based
Talc	Fine powder, good flow; inert	Trichoderma, Pseudomonas
Charcoal	High porosity, good for anaerobic organisms	Specialized use
Press mud	Sugarcane by-product; nutrient-rich	Azospirillum

Production Steps

1. Carrier sterilization — Autoclaving at 121°C for 30 min (2–3 times on alternate days) to eliminate competing microorganisms
2. Culture preparation — Target organism grown in appropriate broth to 10^9–10^10 cells/ml
3. Mixing — Culture broth mixed with carrier at a ratio ensuring final CFU ≥ 10^8/g
4. Moisture adjustment — Final moisture content maintained at 35–40% (critical for cell survival)
5. Packaging — 200g or 500g polyethylene pouches, heat-sealed
6. Curing — Stored at 25–28°C for 3–7 days to allow cell multiplication in carrier
7. Quality check — CFU count, contamination test, pH check before dispatch

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2. Liquid Biofertilizers

Liquid biofertilizers are broth-based formulations containing high populations of target microorganisms suspended in a liquid medium with additives to extend shelf life.

Advantages of Liquid Biofertilizers

• Longer shelf life — 12–24 months (vs 6 months for carrier-based)
• Higher population density — 10^9 cells/ml consistently maintainable
• Better colonization — Liquid form allows uniform coating on seeds; better root penetration
• No carrier quality variation — Carrier quality varies batch to batch; liquid avoids this problem
• Fertigation compatible — Can be applied through drip irrigation systems
• No contamination risk from poorly sterilized carriers

Composition

• Broth culture (YEM, Okon's, Ashby's depending on organism)
• Osmoprotectants: glycerol, polyethylene glycol, sucrose (protect cells during desiccation)
• Surfactants: Tween 80 for uniform dispersion
• Gum arabic or carboxymethyl cellulose (CMC) as viscosity modifier
• pH buffer: 6.8–7.2 optimal for most nitrogen fixers

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3. Granular Biofertilizers

Granular biofertilizers are designed for soil application, particularly where seed treatment is not practical (transplanted crops, perennials, forest plantations).

Types

• Calcium alginate beads — Microorganism encapsulated in alginate gel; excellent protection; controlled release
• Peat-based granules — Compressed peat mixed with culture; 1–3mm granules
• Clay granules — Bentonite or kaolin-based; good for broadcasting

Advantages

• Uniform soil distribution; no need for mixing
• Protected from UV radiation and soil temperature fluctuations
• Can be applied through fertilizer applicators
• Suitable for high soil temperature conditions

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Production of Key Biofertilizer Formulations

Rhizobium Inoculant

Rhizobium is the most important biofertilizer globally; it forms nitrogen-fixing nodules on legume roots.

Strain selection criteria:

• High nitrogen fixation ability (acetylene reduction assay)
• Competitive nodulation ability
• Host-strain specificity (e.g., R. leguminosarum for pea, Bradyrhizobium japonicum for soybean)
• Tolerance to heat, acidity, and pesticides

Culture medium: Yeast Extract Mannitol (YEM) broth

• Mannitol: 10g/L (carbon source)
• Yeast extract: 0.4g/L
• K₂HPO₄: 0.5g/L, MgSO₄: 0.2g/L, NaCl: 0.1g/L

Quality standard: Minimum 10^8 CFU/g (BIS IS:10453); no contamination; pH 6.5–7.0

Host-strain specificity table:

Rhizobium species	Host legume
R. leguminosarum bv. viciae	Pea, lentil, vetch
R. leguminosarum bv. phaseoli	French bean
Bradyrhizobium japonicum	Soybean
B. elkanii	Soybean, cowpea
Mesorhizobium ciceri	Chickpea
Sinorhizobium meliloti	Alfalfa, berseem

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Azotobacter Formulation

Azotobacter chroococcum is the primary free-living nitrogen fixer used for non-legume crops.

• Growth requirements: Aerobic; optimal temperature 25–30°C; optimal pH 7.0–7.5
• Crops: Wheat, sugarcane, cotton, vegetables, flower crops
• Culture medium: Ashby's Mannitol broth (mannitol as sole carbon source; nitrogen-free)
• Nitrogen fixation capacity: 20–30 kg N/ha/year under optimal conditions
• Additional benefits: Phytohormone production (IAA, GA, cytokinin), antifungal metabolites

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Azospirillum Formulation

Azospirillum brasilense and A. lipoferum are associative nitrogen fixers colonizing the rhizosphere of grass family crops.

• Mode of association: Root surface colonization (not intracellular); associative symbiosis
• Crops: Rice, sorghum, maize, sugarcane, wheat
• Benefits: N₂ fixation (10–20 kg N/ha) + IAA production → improved root architecture
• Culture medium: Okon's semi-solid nitrogen-free malate medium (microaerophilic conditions essential for nitrogenase activity)
• Optimal temperature: 28–32°C; optimal pH 6.8–7.2

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Phosphate Solubilizing Bacteria (PSB) Formulation

PSB make fixed phosphorus available to plants by producing organic acids that dissolve insoluble phosphates.

Key organisms:

• Bacillus megaterium var. phosphaticum — most widely used
• Pseudomonas fluorescens — dual role (PGPR + PSB)
• Bacillus subtilis

Phosphate solubilization mechanism:

• Production of organic acids: gluconic acid (most important), citric acid, oxalic acid, tartaric acid
• Acid reduces soil pH → dissolution of Ca₃(PO₄)₂, AlPO₄, FePO₄
• Also: production of phosphatases and phytases to release organic phosphorus

Culture medium: Pikovskaya's broth (tricalcium phosphate as P source; glucose as C source)

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VAM/AMF (Vesicular Arbuscular Mycorrhizae) Formulation

AMF are obligate biotrophs — they cannot be cultured on artificial media and must be produced on living host roots.

Production methods:

1. Pot culture method: Host plant (sorghum, maize, Sudan grass) grown in sterilized soil + infested soil; roots and soil harvested as inoculum after 3 months
2. Aeroponics system: Roots grown in mist chambers; cleaner inoculum with higher spore counts
3. In vitro culture: On transformed root cultures (Ri T-DNA-transformed roots)

Inoculum types:

• Root pieces containing colonized cortex
• Infected rhizosphere soil (spores + infected roots)
• Carrier-based pellets (alginate encapsulated spores)

Quality assessment: Spore count (≥300 spores/g), colonization percentage on test roots

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Zinc Solubilizing Bacteria (ZSB) Formulation

ZSB solubilize insoluble zinc compounds (ZnO, ZnS, ZnCO₃) through acid production and chelation.

• Thiobacillus thiooxidans — produces sulfuric acid, dissolving ZnS
• Bacillus subtilis — produces chelating compounds (gluconate)
• Dose: 5 kg/ha (carrier-based) as soil application
• Particularly beneficial in calcareous and alkaline soils of Indo-Gangetic Plains

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Quality Standards for Biofertilizers

BIS Standards

Biofertilizer	BIS Standard	Minimum CFU	Contamination
Rhizobium	IS:10453	10^8 CFU/g	Absent
Azotobacter	IS:10892	10^7 CFU/g	Absent
Azospirillum	IS:11232	10^7 CFU/g	Absent
PSB	IS:13584	10^7 CFU/g	Absent
Biofertilizer mixtures	IS:14579	As per component	Absent

Shelf Life Requirements

• Carrier-based: Minimum 6 months from date of manufacture at specified CFU
• Liquid: Minimum 12–24 months
• All products must maintain CFU at time of sale, not just at manufacture

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Inoculation Methods

Method	Description	Crops	Dose
Seed coating	Mix biofertilizer + adhesive (jaggery solution) + seed	Most field crops	200–250g/10 kg seed
Seedling dip	Root dip in slurry for 30 min before transplanting	Rice, tomato, onion	2.5 kg/ha in 10L water
Soil application	Mix with compost/FYM; broadcast or furrow	Perennial crops, vegetables	5 kg/ha
Fertigation	Dilute liquid biofertilizer; apply through drip	High-value crops	2–4 L/ha
Biopriming	Pre-soak seeds in biofertilizer solution	Vegetable crops	10^8 cells/ml

Critical note: Biofertilizers should never be mixed with chemical fertilizers or fungicides immediately before application. Allow a gap of at least 7 days between fungicide seed treatment and biofertilizer application.

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Summary Cheat Sheet

Topic	Key takeaway
Main focus	Carrier-based, liquid, and granular biofertilizer formulations — production methods, quality standards, and inoculation techniques.
Section context	Revise this lesson with the rest of Biofertilizer Technology for stronger conceptual continuity.