Soils: Formation, Texture, Structure and Water
What you must understand
Know the soil profile: O (organic litter), A (topsoil, humus-rich), B (subsoil, accumulation), C (weathered parent material), R (bedrock). Describe the three soil particles - sand (large, drains fast, low water holding), silt (medium), clay (tiny, high water holding and nutrient exchange) - and read a textural triangle.
Understand structure (aggregates, crumb, blocky, platy), porosity (macropores for air and drainage, micropores for water retention), bulk density, field capacity, permanent wilting point, gravitational water and capillary water. Know cation exchange capacity (CEC): clay and organic matter hold cations (Ca²⁺, Mg²⁺, K⁺, NH₄⁺) and release them to roots. Understand soil pH effects: acid soils lock up phosphorus and molybdenum and free aluminium; alkaline soils lock up iron and zinc. Know soil organisms (earthworms, bacteria, fungi, mycorrhizae) and the causes of erosion (wind, water, poor cover).
Traps in questions
- Clay holds the most water but drains slowest; sand dries fastest.
- Water held between field capacity and wilting point is plant-available; gravitational water drains away.
- Liming raises pH; gypsum adds calcium without raising pH much.
- Aluminium toxicity is an acid-soil problem, not an alkaline one.
- Compaction lowers porosity and increases bulk density.
Rule of thumb
Loam - roughly 40% sand, 40% silt, 20% clay - is the target texture for most field crops because it balances drainage, water holding and workability.
Worked example
A soil holds 32% water at field capacity and 14% at wilting point. Available water = 32 − 14 = 18% by mass. For a 0.3 m root zone with bulk density 1.3 g/cm³, that is about 70 mm of available water, enough for roughly a week of moderate crop demand.