Learning objectives
- Draw and label a soil profile
- Compare sandy, loamy and clayey soils for farming
- Explain soil pH and describe five conservation practices
1. The soil profile
From the surface downwards: the O horizon (litter and humus), the A horizon or topsoil (dark, rich in humus and soil organisms, where most roots feed), the B horizon or subsoil (accumulated clay and leached minerals, few roots), the C horizon (weathered parent rock fragments) and R, the unweathered bedrock.
- O — organic litter and decomposing matter
- A — topsoil, humus-rich, main root zone
- B — subsoil, leached clay and iron oxides
- C — weathered parent material; R — bedrock
2. Soil types and texture
Sandy soil has large particles: it drains fast, warms early and is easy to till, but leaches nutrients and dries out. Clayey soil has tiny particles: it holds water and nutrients well but is heavy, poorly aerated and cracks when dry. Loam is the balanced mixture of sand, silt and clay and is the best agricultural soil.
An ideal soil is roughly 45% mineral matter, 5% organic matter, 25% water and 25% air. Structure — how particles clump into crumbs — matters as much as texture, because crumb structure gives both drainage and water retention.
3. Soil pH, erosion and conservation
Most crops grow best at pH 6.0-7.0, where phosphorus and micronutrients are most available. Acid soils are corrected by liming with calcium carbonate; alkaline soils by adding organic matter, sulphur or gypsum. Very acid soils lock up phosphorus and free toxic aluminium, which shows as stunted, purple-tinged seedlings.
Erosion may be sheet, rill, gully or wind. Control it with contour ploughing, terracing, cover cropping, mulching, strip cropping, planting windbreaks, and avoiding bush burning and over-grazing.
- Sheet erosion — uniform removal of topsoil, easy to miss until yields drop
- Rill and gully erosion — visible channels on slopes
- Conservation — mulching, cover crops, contour bunds, terraces, agroforestry
Key definitions
- Soil texture
- The relative proportion of sand, silt and clay particles in a soil.
- Leaching
- Washing down of soluble plant nutrients beyond the root zone by percolating water.
- Liming
- Addition of calcium compounds to raise the pH of an acid soil.
Exam tips
- When comparing soils, always answer in terms of drainage, aeration, nutrient retention and workability — those are the mark points.
- Label a soil profile from the surface downwards, and never omit the parent rock.
Practice exam questions & model answers
Q1.Why is loamy soil considered the best agricultural soil?
Loam contains a balanced mixture of sand, silt and clay, so it drains freely yet retains enough water, is well aerated, holds nutrients and humus, and is easy to cultivate — supporting good root growth and high yields.
Q2.State four practices used to conserve soil on a sloping farm.
Contour ploughing across the slope, construction of terraces or contour bunds, mulching and cover cropping to protect the surface, strip cropping, and planting trees or grass strips as windbreaks and barriers.
Q3.How does soil pH affect crop production?
pH controls nutrient availability and microbial activity. Below about pH 5.5 phosphorus is fixed and aluminium and manganese become toxic, stunting crops; above pH 7.5 iron, zinc and manganese become unavailable, causing chlorosis. Most crops therefore yield best between pH 6.0 and 7.0, and acid soils are corrected by liming.
Continue revising
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Put the theory to work — diagnose a real leaf photo or browse the disease library.