Fertilizers: Types, Composition and Reactions in Soil
The exam checks fertilizer grade notation, the composition of common straight and complex fertilizers, and the chemical fate of N and P in soil.
Core rules
- Grades are N–P2O5–K2O. Conversions: P2O5 × 0.44 = P; K2O × 0.83 = K. To go the other way multiply P by 2.29 and K by 1.205.
- Common N fertilizers: urea 46-0-0, ammonium sulphate 21-0-0, ammonium chloride 25-0-0, CAN about 26-0-0. Phosphatics: SSP 16% P2O5 with 11% S and gypsum, TSP 46% P2O5, DAP 18-46-0, MAP 11-52-0. Potassic: MOP 60% K2O, SOP 50% K2O with 18% S.
- Urea hydrolyses by urease to ammonium carbonate, then to NH3 and CO2; volatile loss is high on warm, high-pH surfaces, which is why urea is incorporated.
- Ammonium fertilizers acidify soil through nitrification; ammonium sulphate is the most acidifying conventional source.
- DAP is alkaline in reaction, MAP is acidic. Water-soluble P is fixed as Fe/Al phosphates in acid soils and as increasingly basic calcium phosphates up to hydroxyapatite in calcareous soils, so band placement beats broadcasting.
Worked example
Target 60 kg N, 60 kg P2O5, 40 kg K2O per ha. DAP needed = 60/0.46 = 130.4 kg, which carries 130.4 × 0.18 = 23.5 kg N. Balance N = 60 − 23.5 = 36.5 kg, supplied by 36.5/0.46 = 79.3 kg urea. MOP = 40/0.60 = 66.7 kg.
Traps that cost marks
- Reporting elemental P or K when the question asks for P2O5 or K2O, or the reverse.
- Forgetting the N credit carried by DAP.
- Using ammonium chloride or MOP on chloride-sensitive crops such as tobacco and grapes.