What Is Soil Made Of?
Soil begins to form when
bedrock is broken apart into
small pieces of rock and
minerals. Rain, ice, wind,
freezing, and thawing can do
this. Chemical changes can
do this, too.
Plants and animals that live in
small rock pieces help break apart
rocks. As plant roots grow down,
they pry apart rocks. Burrowing
animals, such as earthworms and
ants, create tunnels between rock
pieces. Some of these tunnels fill
with air and water. Water expands
as it freezes, further breaking apart
the rocks.
How Soil Forms
Bacteria and fungi also help create soil. They
decompose dead plants and animals for energy.
The leftover plant and animal matter is called
humus.
Humus becomes mixed with
the rock pieces. Finally, a
material that can be called soil is
produced. Soil is a mixture of tiny
rock particles, minerals, humus,
water, and air. Soil takes a long
time to form. It may take
hundreds to thousands of years
for one inch of soil to form.
As soil forms,
different layers
result. A layer of
soil differing from
the layers above
and below it is called
a horizon. Soils
typically have three
horizons. From the
top down, they are
A, B, and C. Each
horizon has certain
characteristics.
The A horizon is made up
of topsoil. Topsoil is the top
layer of soil. It is rich in humus
and minerals. Topsoil is
usually dark in color. Most
plants grow here. Many
organisms live here, too.
The B horizon is known as
subsoil. Subsoil is normally a
fairly hard layer. It is made of
clay particles and minerals that
have filtered down from the A
horizon. It is usually light in color.
Sturdy plant roots may grow down
into the B horizon.
The C horizon is
made up of coarse
material broken down
from the underlying
bedrock. It is typically
beyond the reach of
plant roots.
The soil horizons
make up a soil profile.
A soil profile is a
vertical section of soil
from the surface
down to bedrock.
Main Idea and Details
• What is the main idea of this passage?
• What is one important detail that tells more
about the main idea?
• What is another detail that tells more about the
main idea?
• Write a summary of the passage in one or two
sentences.
What are three soil horizons?
• Answer the question…
– With a partner
– In your Learning Notebook
humus
• Leftover
decomposed
plant and
animal
matter.
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horizon
• A layer of soil
differing from
the layers
above and
below it.
Back to text
topsoil
• The dark, top
layer of soil, rich
in humus and
minerals, in which
many tiny
organisms live
and most plants
grow.
Back to text
subsoil
• A hard layer
of clay and
minerals that
lies beneath
topsoil.
Back to text
soil profile
• A vertical section
of soil from the
surface down to
bedrock. The
more horizons in a
soil profile, the
greater the relative
age of the soil.
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