Composition of Soil
Soil is a mix of mineral particles, organic matter, water, air and living organisms.
The five components of soil:
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Mineral particles β Weathered rock fragments of varying sizes. Classified as:
- Clay (< 0.002 mm) β Very small particles; high surface area; holds water and nutrients well; can become waterlogged and compacted; slow drainage.
- Silt (0.002β0.05 mm) β Medium-sized; moderate water retention; reasonable drainage.
- Sand (0.05β2 mm) β Large particles; drains quickly; holds little water or nutrients; does not compact easily.
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Humus β Partially decomposed organic matter from dead plants and animals. Humus:
- Binds mineral particles into aggregates (crumbs), improving soil structure.
- Increases water-holding capacity.
- Provides a reservoir of slowly available nutrients as it decomposes further.
- Darkens the soil (improves heat absorption).
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Water β Found in pore spaces between soil particles. Essential for plant roots to absorb dissolved nutrients.
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Air β Also found in pore spaces. Oxygen is needed by plant roots and soil organisms for aerobic respiration.
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Living organisms β Bacteria, fungi, earthworms, mites, nematodes, beetles. They decompose organic matter, mix soil layers, aerate and improve soil structure.
Soil profile: Soil is organised into horizontal layers called horizons:
- A horizon (topsoil): Rich in humus; darkest layer; where most roots grow; most biological activity.
- B horizon (subsoil): Lighter in colour; less organic matter; some roots penetrate here.
- C horizon: Partially weathered parent rock.
- R horizon (bedrock): Unweathered rock.
Importance of topsoil: The fertile topsoil (A horizon) is only a few centimetres to a few tens of centimetres thick. It takes hundreds of years to form 1 cm of topsoil. Losing it through erosion or degradation is a serious long-term threat to agriculture.
- 5 components: mineral particles, humus, water, air, living organisms.
- Clay: holds water/nutrients but waterlogged. Sand: drains fast, low fertility. Silt: intermediate.
- Humus: improves structure, water retention, slow-release nutrients.
- 1 cm of topsoil takes hundreds of years to form.
- Soil profile: A (topsoil) β B (subsoil) β C (weathered rock) β R (bedrock).