Biosphere
Soil
What is soil?
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Soil is a mixture of particles of
weathered rock, decayed organic
matter, water and gases in which living
organisms are present.
Decaying roots, leaves, animals
broken down by organisms and
micro-organisms
Water
Organisms
Occupy the
spaces in
the soils
Lacks light,
Varies in
proportions
Minerals
Minerals from the physical / chemical
weathering of the parent rock
Air
Mineral matter
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Minerals derived from parent material
by physical and chemical weathering
Largest component in terms of volume
45% in a typical topsoil
Organic Material
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Mainly derived from decaying roots,
leaves, needles and remains of dead
organisms
Broken down by micro-organisms,
worms and moles
5% volume in a typical topsoil
Air and water
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Ever changing volumes which fill the
voids - the spaces in the soil
Soil Profile
Soil Profiles
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A soil profile is a vertical section through
the soil from surface vegetation to the
bedrock
Soils can be identified by looking at
distinct layers or horizons
Usually 4 distinct horizons can be
identified in a soil
Soil Profile - Ao Horizon
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Surface organic layer - decaying vegetation.
Subdivided into 3 layers
L(litter) - leaves , pine needles, cones or dead
heather shoots
F(fermentation layer) - organic material starts
to decompose
H(humus) - decomposed remnants of
vegetation, animals and bacteria. Important
source of nutrients for soil below
Soil profile - A Horizon
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Main top layer
Consists of a mixture of organic and
inorganic material.
Organic material is introduced from Ao
layer
Usually nutrient rich and fine textured
Eluviation - washing out of minerals
The topsoil
Soil profile - B Horizon
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The subsoil
less organic material and courser in
texture due to importance of weathering
Soluble material can be leached out of A
horizon into B horizon
Leaching is the removal of soluble
minerals
Illuviation - washing in of minerals
Soil profile - C Horizon
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Zone of the regolith
large particles sit upon the bedrock
Physical and chemical weathering of
parent material is source of nutrients
Soil Formation - Factors
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5/6 factors are seen to be important in
influencing the formation of a soil
Soil Formation – 1)Parent material
(13 bullet points)
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Significant in early stage of soil
development
can vary from solid bedrock to deposits
such as alluvium and glacial till
Soil Formation - Parent material
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Rate of weathering
Hard rocks such schist (metamorphic
rocks) weather slowly leading to thin
soils
Softer sedimentary rocks such as shale
weather more quickly
Soil Formation - Parent material
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Chemical composition and soil colour
soils from granite have high silica
content and are acidic
Soils on chalk and limestone are
alkaline
Silica rich soils are light in colour
Rocks such as basalt have high iron
content and form dark soils
Soil Formation - Parent material
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Soil texture
the feel factor is influenced by the size
of the soil particles
determines permeability of soil
Soil formation – 2) Biotic factors
(1 bullet point)
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Involve the action of vegetation and
organisms. They interact, influenced by
climate to produce humus. This may lie
below the L and F layers of the Ao
horizon or mixed through the whole A
horizon
Soil formation - 3) Climate
(5 bullet points)
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At low temperatures soil formation is
slower particularly organic
decomposition
Warm temperatures encourage
decomposition and production of
organic material in the soil
Soil formation - Climate
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Water percolation also affects soil
Where precipitation exceeds
evaporation, leaching is an important
process
Where evaporation exceeds
precipitation water and minerals are
drawn to the surface by capillary
movement
Soil formation – 4) Relief
(3 bullet points)
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Upper slopes - runoff and through flow
of water
Lower slopes gain water organic and
mineral material
Shady north facing slopes are colder
and wetter than south facing slopes
Soil formation – 5) Time
(3 bullet points)
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Critical to development of soil
When soils are young they retain
features of parent material
Scottish soils are young mostly as a
result of last glaciation
Soil formation – 6) Human Activity
(3 bullet points)
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Prehistoric people began to fell
woodland around 3000 BC
Accelerated naturally occurring soil
erosion
More recently planting blanket
coniferous forest and the application of
fertilisers have altered soil
characteristics