Soil Science and Soil Fertility Management
What the exam tests
Soil Science and Soil Fertility Management tests soil physical properties, pH and liming, cation exchange, organic matter, diagnostic soil testing, and fertilizer calculations for Nepal's Terai alluvial soils, hill residual soils and mountain soils.
Core rules
- Soil texture is the proportion of sand, silt and clay; structure is aggregation. Clay has the highest CEC and water-holding capacity but low infiltration.
- CEC (cmol(+)/kg) measures exchangeable cations. Base saturation = exchangeable bases ÷ CEC × 100; high base saturation generally means less lime requirement.
- Soil pH controls nutrient availability. Acid hill soils may need lime; lime requirement should be based on buffer pH or exchangeable acidity, not guesswork.
- Fertilizer recommendations are usually given as N, P₂O₅ and K₂O. Convert to fertilizer products using their nutrient percentage.
Worked example
Recommended dose = 100 kg N/ha. Urea contains 46% N. Urea required = 100 ÷ 0.46 = 217 kg urea/ha. If the recommendation were 60 kg P₂O₅/ha and DAP is 18% N and 46% P₂O₅, DAP = 60 ÷ 0.46 = 130 kg/ha. This also supplies 130 × 0.18 = 23.4 kg N/ha, which must be deducted from the urea dose. Reasoning: work in elemental or oxide form consistently, then convert at the end.
Traps
Mixing N with P₂O₅; forgetting the N credit from DAP; treating CEC as a percentage; and applying lime without considering pH buffering.