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 FormsHow 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 DetailsMain 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?What are three soil horizons?

• Answer the question…– With a partner– In your Learning Notebook

humushumus

• Leftover decomposed plant and animal matter.

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horizonhorizon

• A layer of soil differing from the layers above and below it.

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topsoiltopsoil

• The dark, top layer of soil, rich in humus and minerals, in which many tiny organisms live and most plants grow.

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subsoilsubsoil

• A hard layer of clay and minerals that lies beneath topsoil.

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soil profilesoil 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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