Soil Science and Fertility
Soil science and fertility questions test soil texture, pH, CEC, nutrient forms, and fertilizer calculations. The exam expects you to convert recommended nutrients into fertilizer grades and to link soil properties to nutrient availability.
Core rules
- Texture affects water holding, aeration, and CEC; clay and organic matter have higher CEC than sand. Base saturation is the proportion of CEC occupied by bases.
- Soil pH controls availability: most nutrients are available near pH 6.5–7.5; phosphorus is fixed in acidic and calcareous soils; micronutrients such as iron and zinc become less available at high pH.
- Essential nutrients: N, P, K, Ca, Mg, S are macronutrients; Fe, Mn, Zn, Cu, B, Mo, Cl, Ni are micronutrients. P is expressed as P2O5 and K as K2O in fertilizer recommendations.
- Fertilizer grades: urea 46% N; DAP 18% N and 46% P2O5; SSP 16% P2O5 plus sulphur and calcium; MOP 60% K2O. INM combines organic manure, biofertilizers, and mineral fertilizer.
- Soil testing guides dose; split nitrogen application reduces leaching and volatilization.
Worked example
A recommendation is 120 kg N/ha. How much urea is needed? Urea is 46% N, so urea = 120 ÷ 0.46 = 260.9 kg/ha. If DAP is also used at 130 kg/ha for 60 kg P2O5, it supplies 130 × 0.18 = 23.4 kg N/ha, so urea should be reduced to (120−23.4) ÷ 0.46 = 209.9 kg/ha. Reasoning: account for nutrients supplied by all fertilizers, not just the straight fertilizer.
Traps
Treating P2O5 as elemental P; using 46% for DAP N; forgetting DAP supplies N; adding instead of dividing when converting nutrient to fertilizer; assuming lime lowers pH; ignoring calcareous soil effects on zinc.