Soil Science and Soil Management
The soil system
Soil is a three-phase system - mineral particles, organic matter, and pore space holding air and water. You must know the soil profile horizons (O organic, A topsoil, B subsoil/zone of accumulation, C weathered parent material, R bedrock) and how they differ in colour, texture and organic matter.
Texture is the proportion of sand, silt and clay; structure is how those particles clump into aggregates. These two are routinely confused in questions. Sandy soils have large pores, drain quickly, hold little water and have a low cation exchange capacity (CEC), so nutrients leach - the Kalahari sands are the classic local example. Clay soils hold water and nutrients but are poorly aerated and difficult to work when wet. Adding sand to clay does not “lighten” it; it produces a hard, concrete-like mixture. The reliable rule is to add organic matter - manure, compost, mulch - to both extremes.
Understand soil pH as a logarithmic scale (pH 5 is ten times more acidic than pH 6), its effect on nutrient availability, and the correctives: lime raises pH, while sulphur and acidifying fertilizers lower it. Learn the erosion types (sheet, rill, gully, wind) and their controls: contour ploughing, terracing, mulching, cover crops, strip cropping, windbreaks and conservation tillage.
Worked example - available water: a soil holds 30% water at field capacity and 15% at permanent wilting point, so available water is 15% by volume. In a 0.2 m root zone, 0.15 × 200 mm = 30 mm of water is available to the crop.