(a) With the aid of a well-labelled diagram, describe a typical soil profile (b) Highlight the importance of soil Use the climatic data in the table below t...

Answer Details

(a) A typical soil profile

A soil profile is a vertical section through the soil, from the surface downwards to the underlying parent rock. It reveals a series of distinct layers, called horizons, which differ in colour, texture, humus content and mineral composition. A well-labelled diagram of a typical soil profile is shown below.

Working downwards, the horizons are:

• O horizon (litter layer): the thin surface layer of fresh and partly decayed plant and animal remains (leaf litter and humus).
• A horizon (topsoil / zone of eluviation): a dark, humus-rich mineral layer where biological activity is greatest. Most plant roots and soil organisms occur here, and soluble minerals are washed downward (leached) out of this zone.
• B horizon (subsoil / zone of illuviation): lighter in colour because it contains less humus. Clay and minerals leached from the A horizon (chiefly iron, aluminium and calcium compounds) accumulate here, making it denser and more compact.
• C horizon (regolith): partly weathered, broken fragments of the parent rock from which the true soil above has developed; it acts as a reservoir of fresh minerals.
• R horizon (bedrock): the solid, unweathered parent rock at the base of the profile.

(b) Importance of soil

1. It is the main medium of plant growth, supplying nutrients, water and anchorage for roots.
2. It forms the basis of agriculture and grazing, and therefore of food production; soil type largely determines the kind of farming practised.
3. It is a habitat for micro-organisms, earthworms, reptiles and other soil organisms that recycle nutrients.
4. It provides raw materials for industry, such as sand, clay and limestone.
5. It stores, filters and releases water, and holds the essential minerals needed by plants.
6. It serves as a firm foundation for buildings, roads and other structures.