(a) Outline four components of soil. (b) Describe, using a diagram, the soil profile. (c) Highlight any three farming practices that can lead to soil erosio...

(a) Four components of soil

1. Mineral (inorganic) matter: weathered rock particles (sand, silt and clay) that form the bulk of the soil and supply nutrients such as potassium, calcium and phosphorus.
2. Organic matter (humus): decomposed remains of plants and animals which give the soil its dark colour, improve fertility and help retain moisture.
3. Soil water (moisture): water held in the pore spaces which dissolves nutrients and makes them available to plant roots.
4. Soil air: gases occupying the pore spaces, supplying oxygen for respiration of roots and soil organisms. (Living organisms such as bacteria, earthworms and fungi are also present.)

(b) The soil profile

A soil profile is a vertical section through the soil from the surface down to the parent rock, showing a succession of layers called horizons.

• O horizon (litter layer): topmost layer of fresh and decaying leaves and organic debris.
• A horizon (topsoil): dark, rich in humus and living organisms; the zone from which minerals are leached downward.
• B horizon (subsoil): lighter coloured zone of accumulation where clay, iron and minerals leached from above are deposited.
• C horizon: partly weathered, broken parent rock (regolith).
• D horizon (bedrock): the solid unweathered parent rock beneath.

See the labelled soil-profile diagram showing these horizons in order from the surface downward.

(c) Three farming practices that lead to soil erosion

1. Bush burning: burning of vegetation before cultivation removes the protective plant cover, exposing bare soil to rain-splash and wind.
2. Overgrazing: keeping too many animals strips the grass cover, loosens the topsoil and allows it to be washed or blown away.
3. Cultivation on steep slopes / ploughing up and down the slope: tilling along the direction of the slope creates channels that guide run-off, accelerating gully and sheet erosion. (Continuous cropping without fallow and shifting cultivation also expose and exhaust the soil.)