2001 Volunteer Leader Training Guide
Soil and the Underground Horizons

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Target Audience

4th-6th grade students

Targeting Life Skills Model

• Critical thinking

• Decision making

• Problem solving

Goal

To introduce the concepts of soil profiles to elementary students using hands-on learning techniques.

Objectives

• To understand the definition of the soil profile.

• To identify the three horizons of the soil profile.

Introduction

Some have said the soil is to the earth as the peel is to the orange. While this analogy is acceptable to some, the soil scientist views the soil as a complex system that dwells beneath our feet. The soil is made up of subdivisions of soil. Therefore, the soil defines the overall soil covering of the earth while a soil is a subdivision having distinct characteristics. For example, a soil type prevalent in Arkansas is Crowley (DeWitt) silt loam.

Soil changes from site to site, but it also changes from top to bottom. What a soil is like depends on what rock it is made from (parent material), the climate, the topography, how long it has taken to form and how it has been changed by human activity. Some places may have shallow soils, like on top of a mountain, and some places may have deep soils, like those found in a river flood plain. Soils can also differ in composition, color, texture, structure, pH, temperature, the amount of organic material they contain, the amount of organisms in the soil, how they have been used and the amount of air space in the soil.

The best way to start a study of soil is to find an exposed soil bank or to dig a soil pit and expose the different layers of soil. A cross section of soil is called a soil profile, and each layer in the profile is called a horizon. To determine a soil horizon, mark where the soil changes color and general appearance. Many soils have three major layers or horizons: the top soil, the subsoil and the parent material.

The Soil Profile

A vertical section of a soil in the field reveals several somewhat distinct horizontal layers. The section of the soil is often referred to as a soil profile while the individuals layers are known as horizons. Figure 1 details three horizons – Horizon A, B and C.

Horizon A contains the topsoil which is at the surface. The soil tends to be darker in color because it usually is high in organic matter. With abundant roots from plants at the surface, the soil is high in biotic activity.

Horizon B is also called the subsoil. It is just below plow depth and has fewer roots than Horizon A. The lower subsoil is often more yellowish and has less clay, fewer roots and less aeration than above.

Horizon C is just above the bedrock and contains weathered bedrock from which soil develops. There is usually no biotic activity and few roots. Below this horizon lays the bedrock or consolidated rock.

Understanding the Soil Profile

This activity will reinforce the concept of different layers of soil.

Materials Needed:

• 16-ounce clear plastic cups (one for each student)

• Spoons (one for each student)

• At least 4 various breakfast cereals
  (Each cereal should be a different size, shape and color from the next.)

Activity:

1. Discuss the topics in the introduction.

2. Discuss the characteristics of the soil profile. (You can also use the graphic provided)

3. Hand each student a plastic cup.

4. Allow the students to make their own soil profiles using the cereals provided.

5. Discuss the differences in each cereal profile. (Suggested questions are listed.)

6. Pour milk over the cereal and discuss the similarities between the milk in cereal and water in the soil.

7. Pass out spoons and enjoy the treat!

Suggested Questions:

• What would grow well in this soil profile?

• Where would you find a profile like this?

• Does this soil have any organic matter? How much?

• How well does the soil drain?

• Did the milk change colors? How is the milk like our groundwater?

References

Books

• Agricultural Water Management in the Mississippi Delta Region of Arkansas by H. Don Scott, James A. Ferguson, Linda Hanson, Todd Fugitt, and Earl Smith. Arkansas Agricultural Experiment Station Research Bulletin 959, University of Arkansas, 1998. ISSN: 0099-3491 CODEN: AKABA7.
   

• The Nature and Properties of Soil by Nyle C. Brady and Ray R. Weil, Prentice Hall, Inc., 1996, Upper Saddle River, NJ 07458; ISBN: 0-02-313371-6, 11th Edition.
   

• Soil Science Simplified by Milo I. Harpstead, Francis D. Hole and William F. Bennett. Iowa State University Press, 1988, Ames, Iowa 50010. ISBN: 0-8138-1514-2, 2nd Edition, (Advanced).

Internet

• Soil Experiments by Cranfield University
   
• Soil Science Education Homepage by NASA

Fact Sheets

• Soil Erosion and Clear Stream Water, FSA1028
   

• Test Your Soil for Plant Food and Lime Needs, FSA2121
   

• Understanding the Numbers on Your Soil Test Report, FSA2118

Willa L. Williams, Youth Agriculture – Assistant Specialistst

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