Soil Fertility, Soil Health and Nutrient Management
Soil Fertility, Soil Health and Nutrient Management tests the essential elements, how soil supplies them, and how you convert a soil-test recommendation into bags of fertiliser. Expect both recall (17 essential elements, biofertiliser names) and calculation.
Nutrient basics
- 17 essential elements: C, H, O, N, P, K, Ca, Mg, S (macro) and Fe, Mn, Zn, Cu, B, Mo, Cl, Ni (micro). C, H and O come from air and water.
- Liebig's law of the minimum: growth is limited by the scarcest nutrient, not by the total supply.
- Availability is pH-dependent; P is locked up in both very acid and very alkaline soils, and Zn deficiency is common in light, alkaline soils.
- Soil health also needs organic matter, good structure, CEC and biological activity, not just NPK.
Inputs and INM
- FYM (~0.5% N, 0.2% P₂O₅, 0.5% K₂O), compost, green manure, crop residues.
- Biofertilisers: Rhizobium (legumes), Azotobacter and Azospirillum (non-legumes), phosphate-solubilising bacteria, mycorrhiza.
- Integrated nutrient management blends organic, inorganic and biological sources.
Worked example
A crop needs 120 kg N/ha and you have urea (46% N). Urea = 120/0.46 = 260.9 kg/ha. Reasoning: divide the nutrient dose by the nutrient fraction of the fertiliser. If DAP (18-46-0) supplies the P, remember its 18% N counts towards the nitrogen total.
Traps
Dividing fertiliser by nutrient instead of the reverse; quoting P as P₂O₅ when the recommendation is elemental P; forgetting DAP's nitrogen; assuming more fertiliser always means more yield.