Transcript Soils: The Critical Resource

Higher Geography
Biosphere
Soils:
The Critical Resource
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 nonrenewable 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
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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.
Organic material
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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.
Air and water
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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’.
DEVELOPMENT OF A SOIL
Soil Formation and Soil Profiles
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Pedology (soil science) is a very
demanding and complicated subject.
Therefore most models in soil formation
are very simplified.
A Soil Profile
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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.
A Soil
Profile
The Ao Horizon
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The Ao horizon, when present, is the surface
organic layer. It can be subdivided into three
layers:
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L (litter) which may consist of leaves and pine
needles.
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F (fermentation layer) where the organic
material starts to decompose.
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H (humus) the decomposed remnants of
vegetation, animals and bacteria along with all
their waste products.
The A Horizon
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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.
The B Horizon
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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.
The C Horizon
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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.
Depth of soil profiles
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Soil profiles vary in depth. In the humid
tropics they can be up to 50m deep, in
Britain they average around 1.5m.