Soil Science and Soil Fertility Management
Soil science links soil physical, chemical, and biological properties to fertility. The exam tests soil profile, texture, pH, fertilizer grades, and nutrient management calculations.
Soil properties and fertility
- Profile horizons: O (organic), A (topsoil), B (subsoil), C (parent material), R (rock).
- Texture refers to sand, silt, and clay proportions; loam is favourable for most crops.
- pH below 6.5 is acidic, 6.5–7.5 near neutral, above 7.5 alkaline. EC indicates soluble salts.
- Tamil Nadu soils include red loam, black vertisols, alluvial delta soils, laterite, and coastal saline soils.
- Organic matter improves structure, water holding, and nutrient exchange. FYM, compost, and green manure are organic sources.
- Biofertilizers: Rhizobium for legumes, Azospirillum for cereals, phosphobacteria for phosphorus, mycorrhiza for phosphorus and water uptake.
Fertilizer grades and calculations
Urea = 46% N; SSP = 16% P2O5; MOP = 60% K2O; DAP = 18% N and 46% P2O5. Fertilizer requirement = recommended nutrient ÷ nutrient fraction in the fertilizer.
Worked example
For 100 kg N, 50 kg P2O5, 50 kg K2O per hectare: urea = 100 ÷ 0.46 = 217 kg; SSP = 50 ÷ 0.16 = 312.5 kg; MOP = 50 ÷ 0.60 = 83.3 kg. Apply phosphorus and potassium basally; split nitrogen into basal and top dressings.
Traps that cost marks
Do not treat P2O5 as elemental P. Do not use DAP and SSP interchangeably without recalculating nitrogen. Lime raises pH in acid soils; gypsum is for sodic soils. Zinc deficiency in paddy is not cured by adding more urea.