Key Idea: Topic 5.1 explains what soil is, how it forms, what controls its properties, how soil affects ecosystem productivity (NPP), and why soil degradation is a major sustainability issue.
🧱 Soil formation + composition
- Soil forms via weathering + organic matter (humus) over long timescales
- CLORPT controls soil formation: Climate, Organisms, Relief, Parent material, Time
- Typical soil: ~45% minerals, ~25% air, ~25% water, ~5% organic matter
Soil is effectively non-renewable on human timescales (forms extremely slowly).
🧪 Soil properties + horizons
- Texture (sand/silt/clay) controls drainage + nutrient retention
- pH affects nutrient availability (many crops prefer ~6–7)
- Porosity controls air + water movement; humus improves structure
- Soil profile horizons: O, A, B, C, R; processes include leaching, eluviation, illuviation
🌱 Soil + productivity (NPP)
- Higher nutrients + water-holding capacity → higher plant growth → higher NPP
- Soil organisms (decomposers, nitrogen-fixers, mycorrhizae, earthworms) recycle nutrients and support productivity
For 5–7 marks, write cause → effect chains (e.g., humus ↑ → CEC ↑ → nutrients held ↑ → plant uptake ↑ → NPP ↑).
⚠️ Soil degradation
- Types: erosion, salinization, compaction, nutrient depletion, contamination, desertification
- Causes: deforestation, overgrazing, intensive farming, irrigation mismanagement, urbanization
✅ Exam Checklist
- Define weathering + humus + leaching
- Name CLORPT factors
- Link soil property → productivity
- Name 3 degradation types + a cause each