Soils

What is soil?

Soil is a thin layer of loose sediments (usually very fine-grained) and organic matter that lies above a layer of partly-weathered broken rock (called subsoil). Subsoil lies above the bedrock. The term regolith describes all of the loose, incoherent sequence of rock fragments, sand, alluvium, and soil that rests upon solid rock (also known as bedrock).

Soil layers

The soil layers are called horizons. These are known as:

• A-horizon: The top layer of soil. This is usually the darkest as it contains the greatest concentration of organic material. This layer mainly consists of clay minerals and quartz with soluble minerals being absent.
• B-horizon: The second soil layer, which contains little organic matter but abundant iron oxide and soluble minerals.
• C-horizon: The bottom soil layer. This is really just slightly altered bedrock (i.e. subsoil) and contains broken fragments of the bedrock, along with clay minerals.

It is very important to conserve the soil horizons as it takes many thousands of years to develop a mature soil.

Formation of soils

The A-horizon can take up to 10 000 years to fully develop and the B-horizon may take as long as 100 000 years. A mature soil is one that is in chemical equilibrium. As the soil is slowly eroded from the top by natural processes, it is gradually deepened by chemical reaction. However, if the erosion is rapid, the rate of chemical decay is not fast enough and the soil is drastically thinned. Soil formation involves the response of mineral decay and mechanical erosion to variabilities in time, temperature, rainfall and biological activity. Palaeosols are ancient soils that are recognised in the sedimentary rock record.

Soil types

Three major soil types can be classified based on the minerals present in their A and B horizons:

• Pedalfers: (Ped is Greek for soil, al is for aluminium and fer for iron). In areas of high rainfall, the A and B horizons of the soil are leached, resulting in the soil being rich in quartz, clay minerals and iron oxide minerals. These soils are rich in aluminium and iron.
• Pedocals: In warm and dry climates, the soils are enriched in soluble minerals, particularly calcium carbonate.
• Laterites: In tropical regions, the soils are so highly leached that all silicate minerals are completely altered leaving the soil enriched in aluminium and iron oxides. This type of soil is usually deep red in colour.