Soil Science and Land Management
Domain 3: Soil Science and Land Management
This domain tests soil formation, physical and chemical properties, fertility and conservation. The exam expects you to link soil diagnosis to practical land-management decisions on smallholder farms.
Core rules and concepts
- Texture: sand, silt and clay proportions determine water holding, drainage and workability. Loams are generally best for crop production.
- pH: affects nutrient availability. Most crops grow best at pH 5.5–6.5; acid soils may need lime, and the pH scale is logarithmic.
- Cation exchange capacity (CEC): clay and organic matter hold nutrients such as calcium, magnesium and potassium.
- Fertilizer grades: N:P₂O₅:K₂O percentages tell the nutrient content of a fertilizer bag. Urea is about 46% N; compound maize fertilizers vary.
- Erosion control: contour ridges, box ridges, vetiver strips, mulching and agroforestry reduce runoff and soil loss.
Worked example
A recommendation is 92 kg N/ha. How much urea (46% N) is needed? Urea required = 92 ÷ 0.46 = 200 kg/ha. The reasoning: the grade is a fraction of the product, not the product mass. If a 50 kg bag is used, 200 ÷ 50 = 4 bags/ha. Apply in split doses so the crop uses nitrogen efficiently and losses are reduced.
Traps that cost marks
- Treating 46% N as 46 kg N per bag regardless of bag size.
- Assuming lime raises pH by equal steps; pH is logarithmic.
- Calling sheet erosion the same as gully erosion.
- Recommending fertilizer without considering soil test or organic matter.