Introduction
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Soils are a fertile, natural
resource.
Soils develop / form from the
weathering of rocks in one
place and from re-deposited
weathered materials.
Composition of Soil
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Soil is composed (made-up) of:
Mineral Particles (pieces) weathered
or eroded from the soil’s parent rock
(original) over a long period of time.
Mineral particles make up about 45%
of a soil.
Examples of mineral particles:
calcium from limestone (original
rock).
Air
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Air is found in spaces (pores)
between soil particles.
Air supplies oxygen and
nitrogen that help plants to
grow.
Air makes up 25% of a soil
Water
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Water is found also in spaces (pores)
between soil particles.
Water dissolves soluble minerals and
moves fertile minerals to roots in
plants.
Water generally makes up 25% of a
soil. – in deserts there is so little water
that no vegetation can grow.
Humus (Organic Matter
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The remains of dead plants and animals.
Dead plants are broken down into humus
by micro-organisms (earthworms, fungi).
Humus darkens the soil and increases
fertility and is found near the surface.
Organic matter makes up about 5% of a
soil usually.
Soil Characteristics
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There are 6 major factors taken into account when
describing the characteristics (qualities) of soil.
1 - Colour
2 - Structure
3 - Texture
4 - Organic content (Humus)
5 - PH Value (acidity)
6 - Water (content and retention)
3 major characteristics:
1 - Texture
 2 - Structure
 3 - Water content and
retention.
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The texture refers to the smoothness or coarseness
(roughness) of soil.
The texture depends on the size of the soil particles,
which affects the pore spaces (gaps) between the
soil particles.
The different pore spaces mean that soils will have
different:
Aeration (amount of air)
 Drainage (water passing through)
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Sandy
soils
Clay soils
Loam soils
Rough / coarse and loose textures.
 Large pores between particles so air
and water can pass through them.
 Little water logging so the particles
don’t stick together.
 Leaching of nutrients can occur and
during a dry spell there may be a
shortage of water.
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Smooth with small soil particles and are
packed closely together (tight-fitting
particles).
Very tiny pores so they prevent water and air
passing through them.
Waterlogged in winter due to heavy rain
(heavy / sticky) and hard, dry cracked
surfaces in summer.
High in nutrients as they don’t allow leaching.
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Form from roughly equal amounts of
sand and clay particles.
Moderate pore sizes allow air and
drainage.
They usually don’t become waterlogged
in winter or too dry in summer.
Light soils and high in nutrients so ideal
for agriculture.
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Soil structure describes how soil grains are lumped
(cemented) together by humus and water.
The grains of soil are in small lumps (cling together)
called Peds.
The structure of the soil depends on the shape of the
peds.
The spaces (pores) between the peds allow the soil
to:
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Hold air and water or
Let air or water through.
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main soil structures are:
 Crumb (granular) – allow air
and water to pass through.
 Platy – where the peds are
flat and overlap and prevent
water from passing through
(water logging occurs).
The amount of water in a soil or
the amount of water a soil can
retain (hold in) depends mainly
on:
 Texture
 Structure
 Humus.
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 Coarse
sandy soils have large
pores so they allow water to
pass through (little retention).
 Small grained clay soils don’t
allow water to pass through
so they are poorly drained.
 Crumb
(granular) structure allow
water to pass through, so they
can dry up quickly.
 Platy structure restrict water
movement so they have a large
amount of water.
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Soils rich in humus can hold more
water than those with a low humus
content.
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Soils vary – no two soils are identical in texture or
appearance.
Processes occurring within soils produce differences
in texture, structure, colour etc.
The major processes are:
1 - Leaching and podzolisation
2 - Laterisation
3 - Humification
4 - Weathering.
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Leaching occurs when heavy rainfall percolates
(soaks) down through soils.
The rainwater:
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Washes soluble substances down through the soils
Can dissolve minerals and humus from the upper layers.
Though leaching is needed to bring humus to lower
layers, it is often so great that the nutrients are
brought so far down in soils that plant roots can’t
reach them – making the soils infertile.
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This can happen in sandy soils
(permeable, loosely packed grain
structure) as rainwater can pass through
very quickly.
Podzolisation is leaching that occurs in
regions where there is:
 Heavy rainfall e.g. Irish mountains.
 A large amount of humic acids e.g.
Under bogs or coniferous forests.
The humic acids cause the rainwater to
become so acidic that it dissolves most of
the soil nutrients.
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Laterisation is an extreme form of leaching that
occurs in equatorial and tropical regions due to a
combination of:
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The heavy rainfall is the major factor as it washes the
nutrients down through the soil.
The high temperatures:
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Heavy rainfall
High temperature.
Speed up the chemical reactions of water on the minerals
Break down iron into iron oxide (rust).
The iron oxide gives the soil a red, rusty colour and
the soils are sometimes called tropical red soils.
Sugar cane thrives in
some red soils
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There is dead organic matter (remains of
dead plants and animals) in most soils.
Micro-organisms (bacteria, fungi) cause
the dead organic matter to decay slowly
into a black gel.
Oxygen (from air in the soil) also helps in
the breakdown of organic matter in
humus.
Rain washes the humus into the soil and
it is used by plant roots as a nutrient.
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Weathering helps to break down the
parent (original) rock into the mineral part
of the soil:
Physical / mechanical weathering breaks
the rock into minerals e.g. Freeze-thaw in
Irish mountains.
Chemical weathering causes rocks to
decay / decompose e.g. Carbonation
causes calcium carbonate in limestone to
dissolve.