Soils: The Critical Resource Higher Geography Biosphere

Introduction ‘We build on it, we farm on it, we play on it, we could not survive without it, and yet, we abuse it. Soil arguably, is our most important non- renewable resource. Home to countless organisms, it forms a desperately thin layer lying on bedrock’

Soils and environmental inputs

Soil Content In reality the amounts do vary, not least the ever changing amounts of water and air (gases).

Mineral matter This consists of minerals derived from parent material by physical and chemical weathering. This consists of minerals derived from parent material by physical and chemical weathering. By volume this is the most important component in soils. The parent materials are fragments of rock called regolith which can come from underlying bedrock or glacial deposits. By volume this is the most important component in soils. The parent materials are fragments of rock called regolith which can come from underlying bedrock or glacial deposits.

Organic material This is mainly derived from decaying roots, leaves, needles and remains of dead organisms. This is mainly derived from decaying roots, leaves, needles and remains of dead organisms. All such material is broken down by the action of countless micro-organisms and larger life forms such as worms and moles. All such material is broken down by the action of countless micro-organisms and larger life forms such as worms and moles.

Air and water These co-exist in an ever changing volume and occupy the voids (the spaces in the soil). These co-exist in an ever changing volume and occupy the voids (the spaces in the soil). In a well drained soil, water forms a film around the mineral particles and allows space for the entry of air (gases). In a well drained soil, water forms a film around the mineral particles and allows space for the entry of air (gases). Soil air lacks light and has a higher percentage of carbon dioxide compared to ‘atmospheric air’. Soil air lacks light and has a higher percentage of carbon dioxide compared to ‘atmospheric air’.

DEVELOPMENT OF A SOIL

Soil Formation and Soil Profiles Pedology (soil science) is a very demanding and complicated subject. Pedology (soil science) is a very demanding and complicated subject. Therefore most models in soil formation are very simplified. Therefore most models in soil formation are very simplified.

A Soil Profile A soil profile is a vertical section through the soil from surface vegetation to the bedrock. A soil profile is a vertical section through the soil from surface vegetation to the bedrock. By examining the (sometimes) distinctive layers or horizons, it is possible to work out the type of soil. By examining the (sometimes) distinctive layers or horizons, it is possible to work out the type of soil. Four horizons are picked out in the model profile. These differ in soil texture (the ‘feel’ of moist soil), colour and chemical composition. Four horizons are picked out in the model profile. These differ in soil texture (the ‘feel’ of moist soil), colour and chemical composition.

A Soil Profile

The Ao Horizon The Ao horizon, when present, is the surface organic layer. It can be subdivided into three layers: The Ao horizon, when present, is the surface organic layer. It can be subdivided into three layers: L (litter) which may consist of leaves and pine needles. L (litter) which may consist of leaves and pine needles. F (fermentation layer) where the organic material starts to decompose. F (fermentation layer) where the organic material starts to decompose. H (humus) the decomposed remnants of vegetation, animals and bacteria along with all their waste products. H (humus) the decomposed remnants of vegetation, animals and bacteria along with all their waste products.

The A Horizon The A horizon is the main top layer and consists of a mixture of organic and inorganic material. The A horizon is the main top layer and consists of a mixture of organic and inorganic material. It is here that the organic material is introduced from the Ao layer. It is here that the organic material is introduced from the Ao layer. It is usually nutrient rich and fine textured, it is referred to as topsoil. It is usually nutrient rich and fine textured, it is referred to as topsoil.

The B Horizon The B horizon is the subsoil, which contains less organic matter and is coarser in texture, reflecting the importance of weathering. The B horizon is the subsoil, which contains less organic matter and is coarser in texture, reflecting the importance of weathering. Soluble soil material containing nutrients may be leached out of the A horizon into he B horizon. Leaching is the removal of soluble materials and humus. Soluble soil material containing nutrients may be leached out of the A horizon into he B horizon. Leaching is the removal of soluble materials and humus.

The C Horizon The C Horizon is the zone of regolith whose large particles sit upon the underlying bedrock. The C Horizon is the zone of regolith whose large particles sit upon the underlying bedrock. Physical and chemical weathering of parent material is a further source of nutrients. Physical and chemical weathering of parent material is a further source of nutrients.

Depth of soil profiles Soil profiles vary in depth. In the humid tropics they can be up to 50m deep, in Britain they average around 1.5m. Soil profiles vary in depth. In the humid tropics they can be up to 50m deep, in Britain they average around 1.5m.