Download What Is Soil Made Of?

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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