Transcript Formation of Soil

Pg. 73
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Soil is:
◦ Loose rock particles
◦ Decaying organic matter (called humus)
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Result of chemical and mechanical
weathering
Begins with mechanical weathering and then
animals and bacteria live in weathered
material and create the humus
Forest soils contain more organic matter than
desert soils
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Soil forms in layers as it develops
The bedrock that the first weathered rocks
come from is the parent rock
Soil above the parent rock is called residual
soil
Topsoil lies above residual soil (subsoil)
Chemical weathering can alter the
components of soil
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The vertical sequence of soil layers is called a
soil profile
Poorly developed soils show little distinction
between layers
A distinct layer is called a soil horizon
Three major horizons
◦ A –high organic matter; dark colored
◦ B- subsoils enriched with clay minerals
◦ C- subsoils that contain weathered parent material
Subsoils are usually reddish brown
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Affects the thickness of soil
Sloped areas  coarser particles remain
Soils in lower areas (like valleys) are thick
and fertile
South facing slopes receive more sunlight so
they have more vegetation and thicker soils
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Soils vary greatly depending on climate
Soils are classified based on the climates
from which they form
Four major types of soil:
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Polar
Temperate
Desert
tropical
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Form at high latitudes and high elevations
Greenland, Canada, Antartica
Good drainage but no horizons because they
are shallow
Permanently frozen ground (called
permafrost) is present under polar soils
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Vary greatly
Annual rainfall greater than 50-60 cm
Grasslands- abundance of humus; rich fertile
soils
Forests- less deep, less fertile soil; aluminum
rich clays
Prairies- support growth of grasses and
bushes
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Low levels of precipitation
High level of salts
Can support a limited amount of vegetation
Little to no organic matter but abundant
nutrients
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High temps and heavy rainfall
Infertile soil
High bacterial activity that leaves the soil with
few nutrients
Characteristic red color because of the
oxidation of iron
Iron ore is mined from these soils (Brazil,
Australia, and Jamaica)
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Classified according to size (clay, silt, or
sand)
Clay is smallest; sand is largest
The proportions determine a soil’s texture
Soil textural triangle is used to determine the
texture of a soil sample
Affects moisture and ability to support plant
growth
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Measure of how well soil can support growth
Factors that affect fertility:
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Availability of minerals
Number of microorganisms
Amount of precipitation
Topography
Level of acidity
Use fertilizers to replace minerals
Planting of legumes allows bacteria to grow
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Top soil is usually dark
Red and yellow soils are the result of
oxidation of iron minerals
Yellow soils are an indication of
environmental problems
Grayish or bluish soils are usually found in
regions where it is constantly wet and there is
a lack of oxygen