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Microbial Bio-fertilizers (Rhizobium, PSB, KSB, VAM)

Microbial bio-fertilisers are live or dormant inoculants of beneficial bacteria and fungi formulated to supplement soil nutrient availability through biological processes. The category is regulated in India under the Fertiliser Control Order, with technical specifications maintained by the National Centre of Organic and Natural Farming (NCOF) and ICAR-NBAIM.

Composition

The principal regulated categories are:

  • Rhizobium: symbiotic nitrogen fixer for legume crops (pulses, soybean, groundnut, lucerne); strains are species-specific.
  • Azotobacter and Azospirillum: free-living and associative nitrogen fixers for non-legumes; together contributing 15-20 kg N per hectare. See Azotobacter Biofertilizer.
  • Phosphate-solubilising bacteria (PSB): Bacillus megaterium, Pseudomonas striata and related strains that solubilise insoluble soil phosphates.
  • Potassium-solubilising bacteria (KSB): Frateuria aurantia and similar strains that mobilise non-exchangeable potassium.
  • Vesicular-arbuscular mycorrhiza (VAM): Glomus and related symbiotic fungi that extend root reach and improve P uptake; see Vam Mycorrhiza.

FCO specifications require minimum N-fixation of 10 mg per gram of carbon source, defined viable count thresholds, and absence of contaminants at standard dilutions.

Mode of action

Each category acts through a distinct biological mechanism: nitrogen fixers convert atmospheric N2 to ammonium; P-solubilisers release organic acids and chelators that mobilise tricalcium phosphate and other locked-up P; K-solubilisers release K from mica and feldspar; mycorrhizal fungi extend the absorbing surface of the root system through extramatrical hyphae. Most strains additionally produce IAA, gibberellins or siderophores that promote root growth.

Target use and dose

Application formats include seed treatment (10-25 g powder per kg seed, or 5-10 ml liquid per kg), seedling root dip (paddy nurseries, vegetables), soil application with FYM (2-4 kg powder per acre), and drip fertigation (1 litre liquid per 200 litres water). Carrier-based powder products have a shelf life of approximately one year; liquid formulations match this and tolerate field heat better.

Safety and regulatory status

Notified under FCO Schedule III; non-pathogenic when strain identity is confirmed and quality is tested. Should not be tank-mixed with chemical fungicides, copper sprays or hot fertiliser slurries that kill the inoculum.

See also: Azotobacter Biofertilizer, Liquid Organic Biofertilizer Consortia, Vam Mycorrhiza, Waste Decomposer Ncof, Pseudomonas Fluorescens.

References

  1. Biofertilizers. ICAR Crop Science Division.
  2. BioBacter Liquid Azotobacter Biofertilizer. Office of the Principal Scientific Adviser, Government of India.