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HOW SOIL FORMS
6.E.2.3 Explain how the formation of soil is related
to the parent rock type and the
environment in which it develops.
Soil
Thin soil overlaying bedrock
Soil is the
loose,
weathered
material on
Earth’s surface
in which plants
can grow.
Bedrock is the
solid layer of
rock beneath
the soil.
Soil Composition
Soil is the mixture of
rock particles,
minerals, decayed
organic material, air,
and water.
Together, sand, silt,
and clay make up the
portion of soil that
comes from
weathered rock.
Soil Composition
Loam, a type of soil, is made up
of air, water, and organic matter
as well as materials from
weathered rock.
Humus
The decayed organic material in soil is
humus, a dark colored substance that
forms as plant & animal remains decay.
Humus helps create spaces in soil for air
and water that plants must have.
The fertility of soil is a measure of how
well the soil supports plant growth.
Soil Texture
Gravel
Silt
Soil texture depends on the size of
individual particles, followed by silt
particles.
The largest soil particles are gravel.
Next in size are sand particles,
followed by silt particles.
Clay particles are the smallest.
Clay
Soil Texture
(Cont.)
• Texture is important for
plant growth.
• Plants can drown for lack
of air in clay soil, and they
may die from lack of water
in sandy soil.
• The best soil for growing
most plants is loam, which
is soil that is made up of
equal parts of clay, sand &
silt.
Loam
Soil
Soil particles range in size from gravel to clay particles too
small to be seen by the unaided eye. The sand, silt, and
clay shown here have been enlarged.
Soil Formation
• Soil forms as rock is broken down by
weathering and mixes with other
materials on the surface. It is
constantly formed wherever bedrock
is exposed.
• Soil formation continues over a long
period of time.
• A soil horizon is a layer of soil that
differs in color and texture from layers
above or below it.
Soil Formation
(Cont.)
A horizon- made of
topsoil, crumbly, dark
brown soil. (humus, clay,
& other minerals)
B horizon- subsoil,
consists of clay & other
particles washed down
from A horizon, but little
humus.
C horizon- contains only
partly weathered rock.
The Process of Soil Formation
Soil forms as rock is broken down by weathering and mixes with
other materials on the surface. Soil is constantly being formed
wherever bedrock is exposed.
Soil Formation
(Cont.)
• The rate at which soil forms depends on
the climate and type of rock. Remember
that weathering occurs most rapidly in
areas with a warm, rainy climate. As a
result, soil develops more quickly in these
areas. In contrast, weathering and soil
formation take place slowly in areas where
the climate is cold and dry.
Soil Formation
(Cont.)
• Some types of rock weather and form soil
faster than others. For example,
limestone, a type of rock formed from the
shells and skeletons of once-living things,
weathers faster than granite. Thus, soil
forms more quickly from limestone than
from granite.
Soils are very different, depending on how they form
Soil Types
• Scientist classify different types of soil into
major groups based on climate, plants,
and soil composition.
• The most common plants found in a region
are also used to help classify the soil.
• Major soil types in North America include
forest, prairie, desert, mountain, tundra, &
tropical soils.
Soil Types
(Cont.)
Scientists classify the different types of soil into major groups
based on climate, plants, and soil composition.
An area’s climate and plant life help to determine what type of
soil forms from bedrock.
Living Organisms in Soil
Some soil organisms make humus, the material that makes soil fertile.
Other soil organisms mix the soil and make spaces in it for air and
water.
Humus
• Plants contribute most of the organic
remains that form humus. The leaves that
plants shed form a loose layer on the
ground called litter.
Oak tree litter makes the soil soft ...
Decomposers
• Humus forms in a process called
decomposition, in which organisms that
live in the soil turn dead organic material
into humus.
• The organism that break the remains of
dead organisms into smaller pieces and
digest them with chemicals are called
decomposers.
Decomposers
• Fungi, bacteria, worms,
and other organisms are
the main soil
decomposers.
• Earthworms do most of the
work of mixing humus with
other materials in the soil.
• Earthworms & burrowing
animals also help aerate or
mix air into, the soil.
Questions
What is the decayed organic material in
soil?
A. Rotten dirt
B. Dark matter
C. Humorous
D. Humus
What are the smallest particles of soil?
A. Gravel
B. Sand
C. Silt
D. Clay
What is a layer of soil that differs in color
and texture from layers above or below it?
A. Soil Layers
B. Soil Horizon
C. Soil Horizontal
D. Soil Verical
The End