Soils: The Critical Resource

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SoilsThe Critical Resource

• Higher Geography
• Biosphere

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Introduction

• Soil is a mixture of particles of weathered
  rock, decayed organic matter, water and gases in
  which living organisms are present.

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

• 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.

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Organic material

• 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.

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Air and water

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

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Soil Formation and Soil Profiles

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

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Simplified stages in soil formation (1)
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Simplified stages in soil formation (2)
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Simplified stages in soil formation (3)
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A Soil Profile

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

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A SoilProfile
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The Ao Horizon

• 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.
• 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.

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A SoilProfile
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The A Horizon

• 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 usually nutrient rich and fine textured, it
  is referred to as topsoil.

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A SoilProfile
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The B Horizon

• 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 the B horizon.
  Leaching is the removal of soluble materials and
  humus.

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A SoilProfile
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The C Horizon

• 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.

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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.