Soil systems
Domain 5 focuses on soil as a dynamic system: formation, texture, structure, fertility, erosion and management. The exam tests soil profiles, particle-size effects, cation exchange, nutrient cycling and conservation.
Core rules
- Soil forms from parent material through weathering, with additions (organic matter), removals (erosion, leaching), translocations and transformations.
- Texture is the sand-silt-clay proportion. Sand drains fast and holds little water or nutrients; clay holds water and nutrients but drains slowly and compacts; loam balances both.
- Cation exchange capacity (CEC) is the soil's ability to hold positively charged nutrients. Clay and organic matter raise CEC.
- Soil pH affects nutrient availability; most crops favour pH 6-7.
- Erosion by water and wind is accelerated by bare ground, steep slopes, overgrazing and poor tillage. Conservation includes cover crops, contour ploughing, terracing and windbreaks.
Worked example
A soil sample is 60% sand, 30% silt and 10% clay. Using a texture triangle, it falls in the sandy loam class. Reasoning: sand dominates, so infiltration is rapid and water-holding capacity is moderate to low; nutrients leach more readily than in clay. Management should add organic matter to raise CEC and water retention, and use cover crops to reduce erosion. If the same field were 20% sand, 30% silt and 50% clay, it would be clay loam, with slower drainage and higher CEC but greater compaction risk.
Traps
- Saying clay is always more fertile; it holds nutrients but can waterlog and lack aeration.
- Confusing soil texture (particle size) with structure (aggregation).
- Treating soil erosion as only a wind problem; water erosion is often greater.
- Ignoring salinization from irrigation, which raises salinity and reduces yields.
- Assuming organic matter only adds nutrients; it also improves structure and water retention.