Soils:Soils:The Critical The Critical ResourceResource

Higher GeographyHigher Geography

BiosphereBiosphere

IntroductionIntroduction

‘‘We build on it, we farm on it, we play We build on it, we farm on it, we play on it, we could not survive without it, on it, we could not survive without it,

and yet, we abuse it. and yet, we abuse it.

Soil arguably, is our most important Soil arguably, is our most important non-renewable resource.non-renewable resource.

Home to countless organisms, it forms a Home to countless organisms, it forms a desperately thin layer lying on desperately thin layer lying on

bedrock’bedrock’

Soils and environmental Soils and environmental inputsinputs

Soil ContentSoil Content

In reality the amounts do vary, not least the ever changing amounts of

water and air (gases).

Mineral matterMineral matter

This consists of minerals derived This consists of minerals derived from parent material by physical and from parent material by physical and chemical weathering.chemical weathering.

By volume this is the most important By volume this is the most important component in soils. The parent component in soils. The parent materials are fragments of rock materials are fragments of rock called called regolithregolith which can come from which can come from underlying bedrock or glacial underlying bedrock or glacial deposits.deposits.

Organic materialOrganic material

This is mainly derived from decaying This is mainly derived from decaying roots, leaves, needles and remains of roots, leaves, needles and remains of dead organisms.dead organisms.

All such material is broken down by All such material is broken down by the action of countless micro-the action of countless micro-organisms and larger life forms such organisms and larger life forms such as worms and moles.as worms and moles.

Air and waterAir and water These co-exist in an ever changing volume These co-exist in an ever changing volume

and occupy the voids (the spaces in the and occupy the voids (the spaces in the soil).soil).

In a well drained soil, water forms a film In a well drained soil, water forms a film around the mineral particles and allows around the mineral particles and allows space for the entry of air (gases).space for the entry of air (gases).

Soil air lacks light and has a higher Soil air lacks light and has a higher percentage of carbon dioxide compared to percentage of carbon dioxide compared to ‘atmospheric air’.‘atmospheric air’.

DEVELOPMENT OF A SOILDEVELOPMENT OF A SOIL

Soil Formation and Soil Soil Formation and Soil ProfilesProfiles

PedologyPedology (soil science) is a very (soil science) is a very demanding and complicated subject.demanding and complicated subject.

Therefore most models in soil Therefore most models in soil formation are very simplified.formation are very simplified.

A Soil ProfileA Soil Profile A soil profile is a vertical section through A soil profile is a vertical section through

the soil from surface vegetation to the the soil from surface vegetation to the bedrock.bedrock.

By examining the (sometimes) distinctive By examining the (sometimes) distinctive layers or layers or horizonshorizons, it is possible to work , it is possible to work out the type of soil.out the type of soil.

Four horizons are picked out in the model Four horizons are picked out in the model profile. These differ in soil texture (the profile. These differ in soil texture (the ‘feel’ of moist soil), colour and chemical ‘feel’ of moist soil), colour and chemical composition.composition.

A SoilA SoilProfileProfile

The Ao HorizonThe Ao Horizon The The Ao horizonAo horizon, when present, is the , when present, is the

surface organic layer. It can be subdivided surface organic layer. It can be subdivided into three layers:into three layers:

L (litter)L (litter) which may consist of leaves and which may consist of leaves and pine needles.pine needles.

F (fermentation layer)F (fermentation layer) where the where the organic material starts to decompose.organic material starts to decompose.

H (humus)H (humus) the decomposed remnants of the decomposed remnants of vegetation, animals and bacteria along vegetation, animals and bacteria along with all their waste products.with all their waste products.

The A HorizonThe A Horizon

The The A horizonA horizon is the main top layer is the main top layer and consists of a mixture of organic and consists of a mixture of organic and inorganic material.and inorganic material.

It is here that the organic material is It is here that the organic material is introduced from the Ao layer. introduced from the Ao layer.

It is usually nutrient rich and fine It is usually nutrient rich and fine textured, it is referred to as textured, it is referred to as topsoiltopsoil..

The B HorizonThe B Horizon The The B horizonB horizon is the subsoil, which is the subsoil, which

contains less organic matter and is contains less organic matter and is coarser in texture, reflecting the coarser in texture, reflecting the importance of weathering.importance of weathering.

Soluble soil material containing Soluble soil material containing nutrients may be leached out of the A nutrients may be leached out of the A horizon into he B horizon. horizon into he B horizon. LeachingLeaching is is the removal of soluble materials and the removal of soluble materials and humus.humus.

The C HorizonThe C Horizon

The The C HorizonC Horizon is the zone of is the zone of regolith regolith whose large particles sit whose large particles sit upon the underlying bedrock.upon the underlying bedrock.

Physical and chemical weathering of Physical and chemical weathering of parent material is a further source of parent material is a further source of nutrients.nutrients.

Depth of soil profilesDepth of soil profiles

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