Lesson 3: Topsoil Dessert

SUBJECT: Science

OBJECTIVE: Students will create two models, one they can EAT that demonstrates a complete soil profile, and the other that demonstrates the structure of particles and pore spaces in the topsoil. They will also see the effect of water in the soil.

EVALUATION: Students can visualize and describe a soil profile and the different sized particles of which soil is composed. They will also be able to describe the problems of too much, or too little, water.

BACKGROUND FOR TEACHERS:

The soil profile has three main layers.

The soil profile has three main layers. The BEDROCK LAYER is the bottom layer, and is also called the "parent material." It is the rock from which the soil was made hundreds, thousands or millions of years ago. It might be dozens of feet below the surface of the soil, or very near the surface, depending on the geology of the region. (The chocolate chips in the dessert are the "rocks" in this layer.)

The SUBSOIL LAYER is the layer above the bedrock. If the topsoil is eroded, the subsoil may be at, or very near, the surface of the soil. If the topsoil is rich and deep, the subsoil may be several feet below the surface.

The TOPSOIL is where roots of corn and other plants live. This area is filled with millions of plants and animals (see Unit 2, Lesson 1), and is the area where nutrients feed the plants (see Unit 2, Lesson 2). Wind or water erosion can wash away this valuable layer; that's why farmers work hard to protect it (see Unit 2, Lesson 4).

It takes nature over 500 years to replace one inch of topsoil!

The topsoil is composed of three kinds of particles: sand, silt and clay. The worksheet picture shows the relative sizes.

Clay particles are smaller than 0.002 millimeters in diameter. It would take more than 12,500 clay particles side by side to form a line one inch long. This line would be so thin that it would take more than 250 lines side by side to form a line as wide as a pencil mark.

Silt particles are from 0.002 to 0.05 millimeters in diameter. Sand ranges from 0.05 to 2.0 millimeters. Particles larger than 2.0 millimeters are called gravel or stones.

Most soils contain a mixture of sand, silt and clay in different proportions.

The amount of open space, called "pores" between the particles affects how fast water moves through a soil, and how much water it will hold. Too much clay, in proportion to silt and sand, causes a soil to absorb water very slowly. These soils stay wet longer, and are sticky when wet. They often need to be drained by tile or ditches.

Sandy soils cannot hold as much water, so plants cannot live long in them without frequent rains or irrigation.

The pore spaces between soil particles are also very important because they hold the oxygen that plants need to live (See Unit 2, Lesson 2). If the soil has no pore space because of the kind of soil, or because the soil was compacted by machinery, or if that space is filled with water, plants will not do well. They might even die.

STUDENT ACTIVITIES:

1.Ask students to read Ann Learns to Plow, paying close attention as she describes the characteristics of the topsoil in her field- its color, and how it looks shiny and moist when it's turned. Note also the paragraph in which "she remembered the science fair project she'd done in the sixth grade." where she describes the many different kinds of soil in her state. Discuss the color and other characteristics of the topsoil in your part of the country.

2. Help students each make a Topsoil Dessert. (recipe courtesy of Illinois Ag in the Classroom; Illinois Farm Bureau):

• Gather the following ingredients:
  See-through plastic cups (1 per person)
  1 large package chocolate sandwich-type cookies (OreoTM is an example)
  3/4 pound gummy worms (See Unit 9 for another story about gummy worms)
  12 oz. miniature chocolate chips
  green-colored coconut
  4 Tablespoons butter or margarine
  8 oz. cream cheese
  1 cup powdered sugar
  3 1/2 cups milk
  2-3 oz. pkg. Vanilla instant pudding
  12 oz. whipped topping (Cool WhipTM is an example)

• Crush and set aside cookies.

• Cream together butter, cream cheese and powdered sugar.

• Mix together milk, pudding and whipped topping.

• Mix together the creamed mixture and the pudding mixture.

• Set the plastic cups out. Fill with layers as described in steps 6-8.

• Bottom layer: Begin with a layer of crushed cookies. Mix chocolate chips with half of the blended pudding mixture and smooth it over the cookies.

• Middle layer: Add more crushed cookies, then a blended pudding layer and the gummy worms. Save one worm for the top.

• Top layer: Finish with a layer of crushed cookies. Sprinkle with green coconut "grass" and poke a gummy worm through the top to peek out of the soil.

3. Explain that the dessert is a model of a soil profile (see Example 1), and ask students to explain what they've learned about each layer. (See the teacher's background for more information.)

4. Show students the diagram of soil particles, highlighting the size differences, then ask them to complete the worksheet.(Worksheet 1)

5. To illustrate these particle sizes, fill a fruit jar about two-thirds full of marbles or small pebbles (representing sand particles), then add sand (representing silt or clay particles). Tap the jar on the table a few times as you add the sand.

6. To demonstrate water movement in the soil, reread the story, emphasizing the paragraph where Ann explains drainage of the land, "Those tiles helped drain water down through the soil; otherwise rushing water could carry soil off the fields. They also helped drain excess water after heavy rains; otherwise the corn could drown. Roots needed oxygen to live, just like people." (See Unit 2, Lesson 2 for more information on roots and their oxygen requirements.) When the sand completely covers the marbles, and ask students if there is any more space. Proceed to add water, pouring from a measuring cup so you know the amount added.

• Ask students what would happen to roots in the dry soil (before you add the water), and what would happen to the plant and animal life if the soil were totally dry. This is important to discuss, especially if you live in an area where irrigation is used in crop production.

• Ask them to predict how much water can be added.

• Ask them what would happen to roots when the soil is completely saturated with water. Ask them what would happen to the plant and animal life (See Unit 2, Lesson 1 to learn about the plant and animal life) in the soil with no oxygen.

Last reviewed May 7, 2004