Soil Science and Soil Management
This domain tests your understanding of how soil forms, what its physical and chemical properties mean for plant growth, and how soils are conserved and improved.
Formation and profile
Soil forms from parent rock through weathering, acted on by climate, relief, organisms and time. A profile shows the A horizon (topsoil, humus-rich), B horizon (subsoil, illuviation of clay and iron) and C horizon (weathered rock). Organic matter improves structure, water retention and cation exchange capacity in both sandy and clayey soils.
Texture, structure and pH
Texture is the proportion of sand, silt and clay and is permanent. Structure is how particles aggregate and can be changed by cultivation, organic matter and compaction. Clay holds water and nutrients but drains poorly and waterlogs; sand drains freely but leaches nutrients. pH controls nutrient availability: most crops prefer pH 5.5–7.0; below that, lime raises pH and above it, gypsum or organic matter may help. Erosion by water (sheet, rill, gully) and wind is controlled by contour ploughing, terracing, mulching, cover crops, strip cropping and windbreaks.
Worked example
Particle-size analysis gives 40% sand, 40% silt and 20% clay. Plotting on a texture triangle places it in the loam class. Reasoning: no single fraction dominates, so the soil balances drainage and aeration (from sand) with water and nutrient retention (from silt and clay). Management: suitable for most arable crops, but maintain organic matter because loams are easily compacted by machinery on wet soil.
Traps that cost marks
Clay is not automatically fertile - it may be acid or nutrient-poor. Adding sand to clay can create a concrete-like mixture, not a loam. Organic matter helps sandy and clayey soils in opposite ways. Do not confuse texture (fixed) with structure (manageable).