GRADE: Seventh

CONTENT STANDARD: Earth and Space Science

CONTENT TOPIC: Structure of the Earth

CONCEPT: Soil is made through the actions of nature's forces on rocks.

CONTENT OBJECTIVE: 7I1 To learn how soil is formed

INSTRUCTIONAL OBJECTIVE: The learner will:

1.01 explain how soil is formed as a product of weathering and organic processes.
1.02 describe differences in soils in terms of texture, mineral content, and where the soil was formed.
1.03 identify and describe the parts of a soil profile.

I. Formation of soil
A. Bedrock
B. Bacteria and decay
C. Animals
II. Characteristics of soil
A. Texture
1. Sandy
2. Sand
3. Clay
III. Mineral content
IV. Location of soil formation
A. Transported soil
B. Residual soil
V. Soil profile
A. A horizon
B. B horizon
C. C horizon

TN COMPONENT OF SCIENCE: Process of Science

TN GOAL:

To enable students to demonstrate the processes of science by posing questions and investigating phenomena through language, methods and instruments of science.

1.5 EXPLAINING - Phenomena and related information are made understandable through discussion that culminates in a higher level of learning.

1.5c Information should be related to prior knowledge.

BENCHMARK Prior knowledge provides a foundation for new learning experiences.

TIME REQUIRED:

3 days

Plastic cups, sand, clay, gravel, dirt, small pots, permanent marker, magnifying glass or low-powered microscope, and white paper.

We are going to discover of what soil is made, and how each of these components are found in layers with their own composition and texture.

If you were to go out and dig a hole right now, what would you discover about the soil? Is the soil on the top the same as the soil on the bottom of the hole? Is it easier to dig at the top of the hole or the bottom? Where do you think the soil is looser? (the top) Where do you think the soil is more tightly packed? (the bottom)

If you dig down far enough, you will hit a layer of very solid rock called bedrock. Bedrock is the layer from which all of the soil is formed. A process called weathering breaks down the bedrock into smaller and smaller particles.

The type of bedrock also determines the type of minerals that will be found in the soil. Minerals are very important to soil because it helps determine the kind and amount of plant life that will survive.

Soil comes not only from bedrock, but it also comes from animals! What do you think happens when dead animals and plants are lying in the soil? Small organisms called bacteria help to break down, or decay, that plant or animal. When these do decay, they release an acid that helps in the breakdown of the soil. Humus is the decayed remains of plants and animals, and it is the organic material in the soil. There are also animals that live in the soil, such as earthworms, ants, and moles that burrow through the ground. They help to break apart the large pieces of soil, and they also allow for more water to enter the soil. All of these things help in the break down of the rock.

Therefore, soil is a mixture of pieces of weathered rock, minerals, and humus. All of these parts have a different texture. The texture of a piece of soil is described by its size. Sandy soils have a coarse texture. Sand particles are anywhere from 0.05 mm to 2 mm in diameter. Silt particles are the next size down. They are usually from 0.002 mm to 0.05 mm in diameter. The finest particles are clay. They are too small to be measured, but when they become wet, they feel sticky and muddy.

Soil is usually characteristic of the area from which it is found. For example, if you live in a farming area, there will probably be a large amount of humus available because of the decaying remains of crops and from the farm animals. If you were to travel to a desert area, you would not find a large amount of humus because there is not a large number of plants and animals that can survive in that environment. Sometimes soil is carried away from the area in which it is formed. It can be moved by running water, glaciers, wind and waves. Soil that is moved from the place it was formed is called transported soil. The transported soil will be different in texture from the soil that was originally in that area.

If the soil is not transported and stays on top of the bedrock of which it was formed, then it is called residual soil. Residual soils have a chemical makeup like that of their parent material.

(Do the activity Observing Soil)

Remember that bedrock is a mixture of different minerals. Knowing which minerals make up a rock can help you identify and classify it. (Have students look at different rocks such as quartz, mica, hornblende, feldspar, and granite. Ask students to point out their differences.)

Minerals are often removed from soils by plants and by a process called leaching. Leaching is the washing away of minerals by water. The minerals are not usually "washed away," but they are washed deeper into the soil. On farmland, the minerals will need to be replaced; therefore, farmers add fertilizers to replenish the soil.

(Do activity Plants and Soil)

Soil forms in layers called horizons. There are three horizons, and they are named by capital letters. The A horizon is the top layer, beneath it the B horizon, and the C horizon is on the bottom. If you were able to cut into the soil and look at it from the side, you would be able to see all the horizons in what is called a soil profile. A perfect place to see a large picture of the soil profile of the western United States is the Grand Canyon. As the Colorado River ran through the area, it cut into the rock, and the result was the Grand Canyon. (Show slides or pictures of the Grand Canyon.)

Soil does not automatically have three horizons to it. That can take soil up to hundreds of years to reach that point. If a soil does have three horizons, it is called mature soil, and if it has two layers, it is called immature soil. As the immature undergoes more decaying and weathering, it will eventually become mature soil.

The A horizon is just below a thin layer of organic material. This thin layer of organic material is called the O horizon. The A horizon is what we commonly call topsoil. Topsoil is primarily made up of humus and tiny particles of rock. The humus is what gives the topsoil its dark color. Most of the plants and animals live in the topsoil.

The B horizon is the horizon just beneath the A horizon. It is called the subsoil and is primarily made of clay, small pieces of weathered rock, and minerals. During leaching, many of the minerals move into this area of the horizon. This layer is much harder and has a lot more clay than the A horizon. Therefore, only the roots of very large plants are able to reach into the B horizon.

The C horizon is the bottom layer of the horizon. It is made up of large pieces of rock. The weathering of these rocks is the first step in the formation of soil. At the bottom of this layer is the bedrock.

(Do the activity Modeling a Soil Profile)

Observing Soil - On a piece of paper draw the chart below. Spread a small amount of soil on the paper. Let this be sample 1 and describe its color on the chart. Using a magnifying glass or low-powered microscope, look at the particles of the sample. Draw a diagram of the particles you see. Pinch and squeeze the sample and see if the sample sticks together or if it changes in any way. Return the sample to its container. Do the same procedures for the other three samples of soil.

Sample Number

1

2

3

Color

Diagram

Other Observations

Questions:

1. Are all of the samples the same color?
2. While looking at the sample through the lens, did you see any organic material?
3. What happened to the samples when you squeezed them together?

Plants and Soil - Divide your class into several groups and provide each group with samples of clay soil, sandy soil, and soil rich with humus. Repeat these samples, but add fertilizer to each pot. Have each group plant one plant in each pot. Label the pots and place them where they will receive adequate sunlight. Check on them everyday for about 10 days. Record all observations.

Modeling a Soil Profile - In a clear plastic cup, place some clay, sand, gravel and dirt in the cup to represent the layers of the soil. Be sure to let the clay represent the bedrock. Label each layer as to whether it is the A, B, C horizon and the bedrock.

Questions:

1. Which layer represents the A, B, and C horizons?
2. Does your soil show mature or immature soil?
3. Which layer is the topsoil and which is the subsoil?

1. Sketch a soil profile. Label and describe the properties of each layer of the horizon.

2. Contact your library or the county office of the USDA Soil Conservation Service to discover the types of soil found in your area.

We have been talking about soil over the past few days. You have learned how soil forms from bedrock, its importance, and how soil forms in layers called horizons.

bedrock - solid rock that lies beneath the soil

horizon - soil layer

humus - decaying remains of plants and animals

leaching - removing or washing away of minerals in the soil

residual soil - soil that remains on top of the bedrock from which the soil was formed

soil profile - all the layers that make up the soil in an area

texture - size of soil particles

transported soil - soil that has been moved from above the bedrock from which the soil was formed

This is the time this file has been accessed since 04/02/98.

The University of Tennessee at Martin is not responsible for the information or views expressed here.

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