Chapter 5.2: Soils
Soil: A Product of Weathering
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Weathering is normally thought of as destructive.
However, if it were not for weathering, we would
not have soils, which are an important natural
byproduct of weathering.
Imagine a world without soils. Without soils, there
would be virtually no life on land.
No terrestrial plants and animals.
Soils
Soil Composition
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Soil is made up of four parts. These four parts are
mineral matter, organic matter, water, and air.
The mineral matter of soil is the part that comes
from broken rock.
The organic matter of soil is composed of dead leaf
litter, dead vegetation, dead animals etc.
Another name for organic matter is humus.
4 Parts of Soil
Soil Texture and Composition
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One way to
determine a soil's
composition and
texture is to use a
soil-texture
diagram like the
one shown here.
Soil Formation
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The five most important factors that contribute to
the way soils are formed are...
Parent material – the source of the soil material.
Where did the soil come from? Did it come from
granite, sandstone, glacial till?
Time – The longer a soil has been forming, the
thicker it becomes; the more mechanical and
chemical weathering it has been exposed to.
Parent Material and Time
Soil Formation
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Climate – The affect of climate on weathering is
similar to the affect of climate on soils.
Hot, humid climates have the most intense chemical
weathering and as a result the soils are generally
unproductive (poor in nutrients).
Colder climates sometimes experience mechanical
weathering (frost wedging, contraction/expansion
etc.), but there is very little chemical weathering, so
the soils are often more productive (nutrient rich).
Climate and Soils
Soil Formation
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Organisms – Living organisms also contribute to
soil formation.
Bacteria and fungi can help to break down the dead
plants and animals and put nutrients into the soil.
Earthworms and other burrowing animals help to
aerate and hydrate the soil with air and water.
Slope – The steepness of a slope can also play a
factor in soil formation. Steeper slopes are affected
more by erosion and have less soil than flat ground.
Soil and Organisms
Soil and Slope
Soil Profiles
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Some soils change gradually from the top to the
bottom. Others have distinct regions or layers that
vary in texture and composition.
One way to analyze soil layers is to look at a vertical
section or column of soil called a soil profile.
Most soils have a top layer of mineral matter mixed
with organic matter. This top layer of soil is called
the A horizon or the topsoil layer.
Soil Profiles
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The next layer down, located below the A horizon
is called the B horizon.
The B horizon is often called the subsoil.
The B horizon contains fine clay sediment that
washed out of the A horizon.
Sometimes the B horizon forms a hard
impenetrable layer called hardpan.
Soil Profiles
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The final layer is called the C horizon.
The C horizon barely resembles what you would
typically think a soil would look like.
The C horizon is broken down solid bedrock.
Soil Types
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Because there are so many different types of
climates around the world, there are also many
different types of soils.
The three main types of soils are...
Pedalfers – These soils form in temperate climates
that receive moderate rainfall (63 cm). Generally
nutrient-rich soils with lots of organic matter. This
type of soil is most commonly found in the eastern
half of the United States.
Pedalfers
Soil Types
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Pedocals – This soil type is found in the western
United States that have grass and brush vegetation.
Pedocals are typically found in dryer climates so
chemical weathering is minimal.
Because chemical weathering is small, Pedocal
soils contain less clay and more calcite.
Laterites – This soil type is found in hot, humid,
tropical areas. Chemical weathering is severe.
Very nutrient-poor soils.
Pedocals
Laterites