Soil Science: Formation, Properties, Fertility, and Conservation
Soil Science covers how soils form, why they behave differently, and how fertility is managed without degradation. The exam links soil properties to practical recommendations for tillage, irrigation, lime or gypsum, and fertilizer use.
Formation and properties
- Soil forms through the interaction of parent material, climate, organisms, relief and time. Climate and organisms drive weathering and organic matter accumulation.
- Texture is the proportion of sand, silt and clay. Structure is the arrangement of particles into aggregates. Texture is not easily changed; structure can be improved with organic matter and reduced tillage.
- Cation exchange capacity (CEC) is the soil's capacity to hold exchangeable cations. Clay and humus increase CEC. A high CEC generally means a greater nutrient-holding buffer.
- Soil pH affects nutrient availability. Acid soils may need lime; sodic soils may need gypsum and drainage. Apply amendments on the basis of soil test, not guesswork.
Worked example
A recommendation is 60 kg N/ha. Urea contains 46% N, so urea required = 60 ÷ 0.46 = 130.4 kg/ha. In 50-kg bags this is 2.61 bags, so about 3 bags in practice, but the field dose remains 130 kg urea. Reasoning: fertilizer grades give nutrient percentage, not product percentage. DAP is 18-46-0; its phosphorus is expressed as P2O5, so convert to P when comparing with elemental recommendations.
Traps that cost marks
Do not confuse texture with structure. Do not treat pH as a linear scale. Remember that CEC is expressed per 100 g or per kg of soil. In conservation questions, match the practice to the erosion type: contour farming for slopes, gully control for channels, cover crops for surface protection.