Biofertilizers and Soil Amendments
Questions here test which organism does what, inoculant quality standards, and the correct choice of lime, gypsum or pyrite for acid, sodic and alkali soils.
Core rules
- Rhizobium forms symbiotic nodules on legumes and can fix 50–200 kg N/ha; the strain must match the host species.
- Azotobacter is free-living, Azospirillum is associative with cereals, and blue-green algae and Azolla are used in flooded rice.
- Phosphate-solubilising bacteria release P from insoluble calcium phosphates; mycorrhizal fungi (Glomus) extend the P and Zn exploration zone.
- Carrier-based inoculants use lignite or peat and should carry at least 10^7 viable cells per gram at field use, with a shelf life of about six months.
- Seed treatment is typically 10–20 g/kg seed; inoculant should not be mixed directly with fungicide or concentrated fertilizer.
- Lime (CaCO3) raises pH on acid soils and removes Al toxicity. Gypsum (CaSO4·2H2O) supplies Ca to displace Na on sodic soils. Elemental S and pyrite acidify alkali soils.
Worked example
Gypsum requirement = (initial ESP − target ESP) × CEC × 0.00172 t/ha for a 15 cm furrow slice. For ESP falling from 25 to 10 at CEC 20 cmol/kg: 15 × 20 × 0.00172 = 0.52 t/ha. For reclamation to 45 cm depth, scale by three, giving about 1.5 t/ha before allowing for field efficiency and subsoil sodium.
Traps that cost marks
- Prescribing lime for a sodic soil or gypsum for an acid soil - the two are not interchangeable.
- Treating biofertilizer as a replacement for major nutrient doses.
- Using an unmatched Rhizobium strain, which nodulates poorly or not at all.