AIM: What is SOIL and how does it form?

AIM: What is SOIL and how does it form?

• Soil: a combination of weathered bedrock and organic material.

–Bedrock: solid layer of rock beneath the soil

–Humus: decayed plant and animal material found in soil. •Soil that contains 20-30% humus is considered rich soil for plant growth.

• Soil contains:–Decayed organic

material (humus)

–Rock particles

–Minerals

–Water

–Air

How is soil created?• Soil forms as rock is weathered and mixes with

other materials at the surface. –Constantly being formed wherever bedrock is

exposed.

1. The surface of the rock gets attacked by weather and broken into smaller pieces.

2. Over many years, you wind up with heavily weathered sediments at the surface.

3. As you dig deeper, you find less and less weathering and the materials become coarser.

TIME

Soil Layers1. Topsoil or A Horizon = the top

layer of soil that contains more humus than the layers below.

2. Sub soil or B Horizon = consists of clays and dissolved minerals that have been washed down from above. Contains less humus.

3. C Horizon = consists of weathered rock fragments, usually from the parent rock below.

4. Bedrock = the layer of rock beneath the soil. Frequently the parent rock of the soil above.

Soil Types• Scientists classify the different types of soil into

major groups based on climate, plants, and soil composition.

How do plants and animals affect soil?• Forming Humus:– As plants shed leaves or die, they

form a loose layer called litter.– Humus forms when litter decomposes.– Decomposers: the organisms that break the remains of

dead organisms into smaller pieces and digest them with chemicals.• Fungi (mold, mushrooms)• Bacteria• Worms

How do plants and animals affect soil?• Mixing the Soil:– Earthworms mix humus with other materials in soil– Burrowing mammals such as mice, moles, gophers

break up hard soil and mix humus through it.• Also add nitrogen to the soil when they produce

waste, and add organic material when they die and decay.

How do plants and animals affect soil?• Earthworms and burrowing animals also help to

aerate (mix air into) the soil.

Why is soil valuable?• Natural resource: anything in the environment

that humans use.• Soil is one of Earth’s most valuable natural

resources.– Everything that

lives on land, including humans, depends directly or indirectly on soil.

Why is soil valuable?• There is a limited supply– Less than 1/8th of land on Earth has soils well-suited for

farming.– Can take 100’s of years for just centimeters of soil to

form.

The world as an apple

Soil Damage and Loss• The value of soil is reduced when it loses its

fertility and when topsoil is lost to erosion.

George Washington Carver (1864 – 1943) taught new methods of soil conservation to help restore soil fertility in the South. Peanut plants have small lumps on their roots that contain nitrogen-fixing bacteria adding nitrogen, an important nutrient, to the soil.

Soil Damage and Loss• Loss of topsoil: exposed soil (without plants) can

be removed by wind or water.– Ex: Dust Bowl in the 1930’s

A giant dust storm blacks out the sky of Goodwell, Oklahoma during the Dust Bowl.

How can we conserve soil?• Soil conservation: the management of soil to

prevent its destruction.1. Contour plowing2. Conservation Plowing3. Crop Rotation