Soils
Core concepts
Soil is a mixture of mineral particles, organic matter, air, water and living organisms. Know the three mineral fractions by size: sand (largest, drains fast, low water and nutrient holding), silt (intermediate) and clay (smallest, high water and nutrient holding, poor drainage, sticky when wet). Learn soil texture (proportions of these particles) versus soil structure (how particles clump into aggregates). Good structure gives friable crumbs with pores for air and water.
Understand soil profile horizons (topsoil, subsoil, parent material), soil pH and its effect on nutrient availability, and cation exchange capacity as the soil's ability to hold nutrients. Know the difference between soil texture and structure, and how organic matter improves both.
Traps in questions
A common trap is confusing texture with structure: adding sand changes texture slowly and can make clay soils harder, while adding organic matter improves structure. Another trap is assuming clay soils are fertile in practice; they hold nutrients but poor aeration can limit roots. Candidates also forget that pH affects availability, not total nutrient content.
Worked example
A gardener has sticky, waterlogged clay. The best practical answer is to improve structure by incorporating well-rotted organic matter and avoiding working the soil when wet, rather than adding large amounts of sand. Rule of thumb: feed the soil structure with organic matter; texture is very hard to change.