Fertilizers and Manures
Fertilizer classification
Straight fertilizers supply one nutrient (urea 46-0-0; SSP 0-16-0 plus 11% S and 19% Ca; MOP 0-0-60). Complex fertilizers contain two or more nutrients combined chemically with a defined grade (DAP 18-46-0; 10-26-26). Mixed or blended fertilizers are physically combined, and a "complete" fertilizer contains N, P and K. Grade figures always mean %N, %P2O5, %K2O in that order.
Manures and biofertilizers
Manures are bulky organic materials with low nutrient concentration but long residual value: FYM (about 0.5% N, 0.2% P2O5, 0.5% K2O), compost, vermicompost, green manures such as dhaincha and sunhemp, and oil cakes. Biofertilizers are living inoculants: Rhizobium, Azotobacter, Azospirillum, blue-green algae and Azolla for nitrogen; phosphate-solubilising bacteria and mycorrhiza for phosphorus.
Worked example
Recommendation: 60 kg N, 30 kg P2O5 and 30 kg K2O per hectare. Using DAP for phosphorus: 30 ÷ 0.46 = 65.2 kg DAP/ha. This also supplies 65.2 × 0.18 = 11.7 kg N. Balance N = 60 − 11.7 = 48.3 kg; urea needed = 48.3 ÷ 0.46 = 105 kg/ha. MOP = 30 ÷ 0.60 = 50 kg/ha. Reasoning: always subtract the N carried by DAP before calculating urea, otherwise you over-apply nitrogen.
Traps
Ignoring the nitrogen in DAP; confusing P2O5 with elemental P (P = P2O5 × 0.44) or K2O with K (K = K2O × 0.83); calling urea a complex fertilizer; assuming FYM alone can meet a high-yielding crop's demand; applying nitrate fertilizers to flooded rice, where denitrification losses are high.