Manures and Fertilizers
Domain 3 covers materials that supply plant nutrients: bulky organic manures, concentrated organic sources, chemical fertilisers and biofertilisers, plus the arithmetic that converts a recommendation into bags of product.
Organic sources
- Farmyard manure: about 0.5% N, 0.2% P2O5, 0.5% K2O, and it adds humus.
- Compost, vermicompost, green manure (sunhemp, dhaincha), oilcakes, bone meal, poultry manure.
- Biofertilisers: Rhizobium for legumes, Azotobacter and Azospirillum for non-legumes, phosphate-solubilising bacteria, mycorrhiza.
Chemical fertilisers
- Straight: urea (46% N), single superphosphate (16% P2O5), muriate of potash (60% K2O).
- Complex: DAP (18-46-0, i.e. 18% N and 46% P2O5), NPK grades such as 19:19:19.
- Methods: broadcast, band placement, ring placement, fertigation, foliar spray.
Worked example
Recommendation: 60 kg N, 40 kg P2O5, 40 kg K2O per hectare. DAP at 40/0.46 = 87 kg supplies 40 kg P2O5 and 87 x 0.18 = 15.6 kg N. Balance N = 60 - 15.6 = 44.4 kg, so urea = 44.4/0.46 = 96.5 kg. MOP = 40/0.60 = 67 kg. Reasoning: credit the nitrogen hidden in DAP before calculating urea.
Marks-losing traps
- Forgetting DAP's nitrogen.
- Confusing P with P2O5 (multiply P by 2.29) and K with K2O (multiply K by 1.21).
- Treating biofertilisers as full nutrient substitutes.
- Mixing incompatible fertilisers such as calcium ammonium nitrate with single superphosphate.