ZIMSEC O Level · Agriculture · Form 1, Form 2
Soil is the upper layer of the Earth's crust in which plants grow.
Formation and composition
Soil forms from weathering of parent rock by physical, chemical and biological agents over a long time. It contains mineral particles (about 45%), air and water (about 25% each) and organic matter (about 5%), plus living organisms.
Soil texture and structure
- Texture is the proportion of sand, silt and clay. Sandy soils drain fast but hold few nutrients; clay soils hold water and nutrients but can waterlog and are hard to work; loam is best.
- Structure is how particles group into crumbs. Good crumb structure allows air and water movement and root growth.
Soil pH
Most crops grow best at pH 5.5–7. Acidic soils are corrected by liming (agricultural lime).
Soil fertility and fertilisers
- Major nutrients: nitrogen (leaf growth), phosphorus (roots, flowering), potassium (disease resistance, fruit quality).
- Organic manures: kraal manure, compost, green manure – improve structure and supply nutrients slowly.
- Inorganic fertilisers: Compound D (basal NPK), ammonium nitrate (top dressing, N).
Soil erosion and conservation
Causes: overgrazing, deforestation, poor ploughing, veld fires. Conservation: contour ridges, grass waterways, mulching, cover crops, crop rotation, minimum tillage, Pfumvudza/Intwasa planting basins.
Exam tip: Link each soil property to its effect on crops – e.g. “clay holds water, so crops survive dry spells longer”.
Check yourself
- Name the three mineral particles that make up soil texture.
- How can an acidic soil be corrected?
- State the main function of phosphorus in plants.
Show answers
- Sand, silt and clay.
- By applying agricultural lime.
- Root development (and flowering/seed formation).
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