Soil Science and Soil Management
Soil Science and Soil Management examines the soil as a physical, chemical and biological system and asks you to convert laboratory values into management decisions - lime, fertilizer, drainage and erosion control.
Core rules
- Texture: sand 2.0–0.05 mm, silt 0.05–0.002 mm, clay <0.002 mm; the textural class comes from the three percentages on the triangle.
- Bulk density (BD) = oven-dry mass ÷ total volume; particle density is about 2.65 g/cm³; total porosity = 1 − (BD ÷ PD).
- Cation exchange capacity (CEC) in cmol(+) kg⁻¹; base saturation = (sum of exchangeable bases ÷ CEC) × 100. Below pH 5.5, exchangeable Al³⁺ and Mn²⁺ become the main constraints and lime is justified.
- Fertilizer grades: urea 46 % N, TSP 46 % P₂O₅, MOP 60 % K₂O. Convert oxide to element: P = P₂O₅ × 0.44, K = K₂O × 0.83.
Worked example
A hectare of soil, 0–15 cm, BD 1.3 g/cm³. Soil mass = 10 000 m² × 0.15 m × 1300 kg/m³ = 1.95 × 10⁶ kg. The recommendation is 90 kg N/ha; using urea, mass = 90 ÷ 0.46 = 195.7 kg/ha. Porosity = 1 − (1.3 ÷ 2.65) = 0.51, i.e. 51 % - a compacted layer at BD 1.6 gives only 40 %, restricting root aeration.
Traps that cost marks
Using fresh (field-moist) mass for BD; mixing cmol with meq (numerically equal for monovalent ions but not for Al³⁺); liming on pH alone without checking exchangeable Al; treating P₂O₅ as elemental P; applying the whole N dose at planting when leaching and volatilisation losses matter.