Soil Science and Soil Management
Soil formation and profile
Soil forms from parent rock through weathering, acted on by climate, organisms, relief and time. A soil profile has horizons: O (litter), A (topsoil, humus-rich), B (subsoil, accumulation), C (weathered parent material) and R (bedrock). Candidates must name horizons and state one property of each; a frequent trap is to call the B horizon the most fertile layer - fertility and organic matter peak in the A horizon.
Texture, structure and water
Texture is the proportion of sand, silt and clay; structure is how particles clump into aggregates. Sandy soils drain fast, hold little water and leach nutrients; clay soils hold water but may waterlog and bake hard. Know field capacity, permanent wilting point and available water, and the meaning of porosity and bulk density. Traps: confusing texture with structure, and assuming all water in soil is available to plants.
Fertility and pH
Macronutrients are N, P, K, Ca, Mg and S; micronutrients include Fe, Mn, Zn, Cu, B and Mo. pH controls nutrient availability: most crops do best near pH 6–7. Nitrogen supports leafy growth, phosphorus roots and flowering, potassium fruit quality and disease resistance. Organic matter improves structure, water holding and nutrient supply.
Conservation and management
Cover the causes of erosion (wind, water, overgrazing, clean cultivation) and controls (contour ridges, mulching, cover crops, terracing, windbreaks, crop rotation, minimum tillage, manure and compost).
Worked example
A field of deep sandy soil gives poor maize yields. Reasoning: low water-holding capacity and rapid leaching of nitrogen. Practical response: add manure or compost to raise organic matter, split nitrogen applications, mulch to reduce evaporation, and grow a drought-tolerant crop such as sorghum.