Soil
Section 5.2
What is Soil?
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Soil is the part of the regolith that
supports the growth of plants.
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Regolith is the layer of rock and mineral
fragments that covers most of Earth’s land
surface.
Soil is the result of weathering.
What is Soil?

Soil has four major components:
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Mineral matter (broken down rock);
Organic matter (humus);
Water; and
Air.
These four components must be present
to be considered a soil!
Composition of Good Quality Soil! Not all
soil has the same percentage of air,
water mineral matter and organic matter!
Humus

Humus consists of decayed remains of
animal and plant life and pore spaces
where air and water circulate.
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Source of plant nutrients;
Increases soil’s ability to retain water.
Soil Water and Air

Water
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Provides moisture needed for chemical
reactions that sustain life.
Provides nutrients to plants.
Air

Source of carbon dioxide plants use to
produce sugar during photosynthesis.
Soil Texture

Texture refers to the proportions of different
particle sizes.
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Sand (large)
Silt
Clay (small)
Loam (a mixture of all three sizes) is best suited
for plant life.
Texture influence a soil’s ability to support life.
Soil Texture Triangle
What are the percentages of
sand, silt and clay at point A?
20% sand, 20% silt, and
60% clay
What type of soil has 30%
clay, 50% sand, and 20%
silt?
Sandy clay loam
Soil Formation

The most important factors in soil
formation are:
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Parent Material
Time
Climate
Organisms
Slope
Soil Formation

Parent material

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Residual soil—parent material is the bedrock
Transported soil—parent material has been carried
from elsewhere and deposited
Soil Formation

Time
The longer a soil has been forming, the
thicker it becomes.
Soil Formation
Climate
• Has the biggest effect on soil formation!
• Different climates form different soil types.
Pedalfer
Humid
Pedocol
Arid
Laterite
Tropical
Soil Formation

Organisms
• Plants are the main source of organic matter
in soil.
•
Burrowing animals mix mineral and organic
matter in soil.
Holes made by burrowing animals also help water
and air to penetrate soil.

Slope

Soil Formation
Angle
Steep slopes often have poorly
developed soils.
 Flat land can have too little erosion and
poor drainage.
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Orientation, or direction the slope is
facing, influences soil formation.
Soil temperature
 Soil moisture
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The Soil Profile
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The A horizon is
commonly know as topsoil.
The B horizon is subsoil
and contains clay particles
washed out from the A
horizon.
The C horizon is between
B horizon and unaltered
parent material.
The Soil Profile
Soil Types
There are 12 soil types recognized by the United
States Geological Survey.
They fall into three broad categories based on
climate:
1.
2.
3.
Pedalfers (temperate regions with moderate
rainfall)
Pedocals (dry regions)
Laterites (hot, wet climates)
Soil Types
Pedalfers
•
In temperate areas receiving >63 cm (>30 inches) rain/year
•
Present in much of the eastern USA, mostly in forested areas.
•
Contain aluminum rich clay.
Pedocals
•
Found in the drier western USA <63 cm (< 30 inches) rain/year;
•
Contain less clay (chemical weathering slower in dry climates);
•
Contain abundant calcite.
Soil Types
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Laterites
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Form in hot, wet tropical areas, where chemical
weathering is intense.
When dried, become very hard and almost
waterproof!
Used as building material (laterite bricks)
Contain almost no organic matter because
plants decompose too quickly, and nutrients
released during decomposition absorbed by
living plants.
Angkor Wat, Cambodia
Built between 1113 and 1150)
Soil Erosion: Water, Wind, Ice

Water is the biggest agent of erosion!
A raindrop can splash soil particles more than a meter away
from where it strikes the soil!
How Water Erodes Soil:
Sheet Erosion
How Water Erodes Soil:
Sheet Erosion
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Water flowing across land surface carries away dislodged
soil particles in thin sheets of water, hence the name
“sheet erosion.”
Can eventually form rills, which can form deeper gullies.
Rates of Erosion
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Human activities that remove natural
vegetation, such as farming, logging, and
construction, have greatly accelerated erosion.
Case in Point:
The American Dust Bowl
Sediment Pollution
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Eroded soil is deposited in rivers, lakes,
reservoirs, and navigation waterways.
Sediments are contaminated by pesticides and
fertilizers.
Soil Degradation
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Organic and inorganic
nutrients are returned
naturally to the soil when
plants shed their leaves,
or when they die.
Most farmed crops are
totally removed from the
ground, leaving no source
of nutrients to replenish
the soil.
Topsoil is a Valuable Resource
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Productive soils ~ 10% of Earth’s land surface
but produce 97% of the world’s food
Soil Conservation

Controlling Erosion
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Planting rows of trees
called windbreaks
Terracing hillsides
Plowing along the
contours of hills
Contour Plowing
Windbreaks
Terracing
Soil Conservation
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Reducing Soil Degradation
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Rotating crops
Planting cover crops
Year 1
Year 2
Year 3