Soil
Characteristics of Soil
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Inorganic Materials
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About half of most soils are inorganic
materials, such as the products of weathered
rock, pebbles, sand, silt, and clay particles.
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Study Tip
Soil is a complex mixture of
different materials. When soil
scientists want to precisely
determine soil type, they
measure the percentage of
sand, silt, and clay.
Organic Materials
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About half of all soils are organic materials, formed from the partial breakdown and
decomposition of plants and animals. Organic materials are necessary for a soil to be
fertile. The organic portion provides the nutrients, such as nitrogen, needed for strong
plant growth.
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Air and Water
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In between the solid pieces, there are tiny spaces filled with air and water.
Texture
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Earth Science Study Guide
Surface Processes and Landforms
A permeable soil allows water to flow through it easily because the spaces between
the inorganic particles are large and well connected.
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Soils that have lots of very small spaces are water-holding soils.
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When a soil contains a mixture of grain sizes, the soil is called a loam
Classification
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Soil types are determined by their percentages of sand, silt, and clay.
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The percentages of sand, silt, and clay correspond to the type of soil in the diagram below
Soil Ecosystem
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There are thousands to millions of living organisms in soil such as earthworms, ants, bacteria,
or fungi
Factors in Forming Soil
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Climate
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More rain yields greater chemical
reactions to weather minerals and rocks
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Increased rainfall increases the amount of
rock that is dissolved because as materials
are carried away new surfaces are
exposed, which increases the rate of
weathering
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Increased temperature increases the rate
of chemical reactions
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In warmer climate areas plants and
There are many different types of soil
that are created by different
environmental factors.
bacteria grow faster and help to weather materials and produce soils
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Rock Types
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Residual soils form in place. The underlying rock breaks down to form the layers of soil that
reside above it
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Transported soils have been transported in from somewhere else. Sediments can be
transported by glaciers, wind, water, or gravity.
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Slope
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The steeper the slope, the less likely material will be able to stay in place to form soil.
Material on a steep slope is likely to go downhill. Materials will accumulate and soil will form
where land areas are flat or gently undulating.
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Time
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Soils thicken as the amount of time available for weathering increases. The longer the amount
of time that soil remains in a particular area, the greater the degree of alteration.
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Biological Activity
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The decayed remains of plant and animal life are called humus, which is an extremely
important part of the soil.
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Humus coats the mineral grains. It binds them together into clumps that then hold the soil
together, creating its structure.
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Humus increases soils water holding capacity and buffers extreme acid changes. It also helps
soil to hold its nutrients, thus making it fertile.
Soil Horizons and Profiles
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Soil horizons develop as each layer of soil becomes progressively altered. The greatest degree of
weathering is in the top layer. Each successive, lower layer is altered just a little bit less. This is
because the first place where water and air come in contact with the soil is at the top.
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The simplest soils have three horizons:
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Called the A-horizon, the topsoil is usually the
darkest layer of the soil because it has the highest
proportion of organic material.
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The B-horizon or subsoil is where soluble
Study Tip
minerals and clays accumulate. This layer is lighter
brown and holds more water than the topsoil
because of the presence of iron and clay minerals.
There is less organic material.
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The C-horizon is a layer of partially altered bedrock.
There is some evidence of weathering in this layer,
but pieces of the original rock are seen and can be
identified.
A soil profile shows each of the
different horizons, or layers of
the soil in an area.
Types of Soils
Types of Soil
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There are thousands of types of soil, each with their
own characteristics and names.
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Soil that isn’t weathered much is usually thinner and
rich in nutrients; heavily weathered soil is very thick
yet nutrient-poor.
The Al in pedalfer is the
chemical symbol of the
element aluminum. The Fe in
pedalfer is the chemical
symbol for iron. Both of these
metals are commonly found in
this type of soil.
Three Major Types
Three types of soil
Description
Pedalfer
Found in moist forest areas especially in
many areas of the temperate, eastern part
of the United States.
Dark brown and black color comes from
aluminum clay and iron oxide. Very fertile.
A great deal of rainfall is common in is
climate.
Image
Earth Science Study Guide
Surface Processes and Landforms
Pedocal
Found in dry, temperate areas where
grasslands and brush are the main forms of
vegetation.
Unique due to its calcite layer formed from
calcium carbonate.
There is less chemical weathering compared
to pedalfer soils, resulting in its lower
fertility.
Laterite
Found in tropical regions, nutrient poor due
to chemical weathering, which strips the
soil of its nutrients.
Usually red from the iron oxide left behind.
There is practically no humus since much of
the soluble minerals are stripped away.
Climate and Soil Type
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The types of soil present in a certain area are heavily dependent on the area’s climate conditions.
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Where there is less weathering, soils are thinner but soluble minerals are more present
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Soil development may take hundreds to thousands of years for good topsoil to form.