How Soil Forms

What is soil?

• Rocks and sediments might not seem to have

much in common with soil, but soil is what

they become when they are weathered.

• Soil is the loose, weathered material on

Earth’s surface in which plants can grow!

• One of the main ingredients in soil comes

from bedrock.

• Bedrock is the solid layer of rock beneath the

soil.

• Once bedrock is exposed to the surface, it

weathers away into small particles that form

soil.

Bedrock!

Soil Composition

• Soil is made up of more than just weathered rock.

• Soil is a mixture of particles, minerals, decayed organic material, water, and air.

• The decayed organic material is called humus.

Humus

• Humus is a dark-colored substance that forms as plant and animal remains decay.

• It helps create spaces in soil for the air and water.

• It also contains substances called nutrients (nitrogen, sulfur, phosphorus, & potassium)

• These nutrients are needed for plants to grow.

Fertility

• When a soil has a lot of nutrients, it is fertile.• Fertility of soil is a measure of how well the

soil supports plant growth.• Soil that is rich in humus has high fertility.• Sandy soil containing little humus has a low

fertility.

Soil Texture• Sand feels coarse and gritty, but clay feels smooth and silky.

• These differences are known as the soil’s texture.

• Soil texture depends on the size of the individual particles.

Pg. 187

Soil Texture & Plant Growth

• Soil texture is very important to plant growth.• If it has too much clay, it will hold too much

water and drown the plant.• If it has too much sand, it will not hold very

much water, and the plant will die.• The best soil for planting is called loam.

Loam

• Loam is made up of nearly equal parts of clay, silt, and sand.

• The clay allows the soil to retain water.• The sand allows air to get into the soil.• It is for these reasons that loam is the best soil

for growing most plant types.

The Process of Soil Formation

• Soil forms as rock is broken down by weathering and mixes with other materials on the surface.

• Soil formation continues over a long period of time.

Soil Horizons• Gradually, soil develops layers called soil horizons.• There are three soil horizons.• The A-Horizon, the B-Horizon, and the C-Horizon.

A-Horizon

• The A-Horizon is made of topsoil.• Topsoil is a crumbly, dark brown soil that

contains a mixture of humus, clay, and other minerals.

B-Horizon

• The B-Horizon is often called subsoil.

• Subsoil usually consists of clay and other particles that have washed down from the A-Horizon.

• Subsoil contains very little humus (just what washes down from the A-Horizon).

C-Horizon

• The C-Horizon only contains partly weathered rock.

Video of How Horizons and Soil Form

• http://www.phschool.com/atschool/phsciexp/active_art/soil_layers/index.html

Soil Types

• If you looked at the soil in north-central Georgia, the clay would be made of red clay.

• In other parts of the country, soils can be black, brown, yellow, or gray.

• In the U.S. alone, there are thousands of different types of soil.

• Scientists classify the different types of soil into major groups based on climate, plants, and soil composition.

Soils of North America

Pg. 189

Living Organisms in Soil• Soil is teeming with living things!• Some soil organisms make humus, the material that

makes soil fertile.• Other soil organisms mix the soil and spaces in it for air

and water.

Pg. 190-191

Forming Humus

• Plants contribute most of the organic remains that form humus.

• As plants shed their leaves, they form a loose layer called litter.

• The plants themselves and their roots become litter after the plant dies.

• To form humus, the organic remains go through a process called decomposition.

Decomposition

• Organisms called decomposers are the ones who break down the remains of dead organisms into smaller pieces and digest them with chemicals.

• Soil decomposers include fungi, bacteria, worms, and other organisms.

Fungi• Fungi are organisms such as mold and

mushrooms.• Fungi grown on, and digest, plant remains.

Bacteria

• Bacteria are microscopic decomposers that cause decay.

• Bacteria attack dead organisms and their wastes in soil.

Soil bacteria

Other Living Organisms in Soil• Very small animals, such as mites and worms,

also decompose dead organic material and mix it with the soil.

Mixing the Soil

• Earthworms do most of the work of mixing the soil.

• As earthworms eat their way through the soil, they carry humus down to the subsoil and subsoil up to the surface.

• Earthworms also pass out the soil they eat as waste.

• The waste soil is enriched with substances that plants need to grow, such as nitrogen.

Mixing the Soil (continued)

• Many burrowing mammals such as mice, moles,

prairie dogs, and gophers break up hard,

compacted soil and mix humus through it.

• These animals also add nitrogen to the soil when

they produce waste, and they add organic

material to the soil when they die and decay.

• Soil is aerated for roots thanks to these living

organisms.