Students observe soil changes in relationship to depth and identify factors associated with soil formation. Grades 3-5

bedrock: a solid, anchored rock; may be beneath the soil or exposed at the surface

horizon: soil layer parallel to the soil surface that is different (visibly, chemically, and/or physically) from the layers above and below

infiltration: to pass through a substance by filtering or permeating

parent material: the soil horizon just above bedrock that contains broken up pieces of rock and will eventually break down into soil

percolation: the movement of water within and through the soil

pore space: the spaces between soil particles and between soil aggregates; pores can be filled with air or water

soil profile: vertical cross section of the soil from the ground surface to the underlying bedrock; composed of horizons

subsoil: the soil horizon between topsoil and parent material that is low in organic matter and poorly suited to growing crops

topsoil: the upper layer of soil that is rich in organic matter and best suited to growing healthy crops

If you were to dig deeply enough, you would hit solid rock. This is called bedrock. But first, you would have to dig through three or four different layers of soil. These layers are called horizons, and their arrangement in the soil is called the soil profile. Examining the horizons in a soil profile can reveal a lot about how water will move through the soil, what nutrients are in the soil, and what kinds of plants will grow well there. The top layer in the profile is mostly organic matter in varying states of decay. Organic matter comes from plants and animals and their wastes. In general, the arid western United States has soils low in organic matter, and in some dry, rocky places this layer may be nonexistent. You will find deeper organic layers in regions with more moisture that support more plant life.

The first soil horizon under the organic layer is called topsoil. The topsoil is where plants spread their roots. In this layer, roots can most readily absorb water, nutrients (minerals), and air (carbon dioxide and oxygen). If enough of the topsoil blows or washes away we are left with subsoil, the next horizon below the topsoil. Subsoil is usually lighter in color and less productive than topsoil. Minerals here are not in a form that is easy for plants to use. The subsoil is mostly made up of sand, silt, or clay and has very little organic material. Plants grow poorly in subsoil. That’s why farmers must work hard to conserve their topsoil. Between the subsoil and the bedrock is the last horizon in the profile. This layer is composed of small rocks that have started to break off the bedrock, and it is called the parent material of the soil. That is because most of what makes up the soil was once part of the rock.

It can be difficult to imagine how solid rock turns into soil, but rocks are the parent material of every soil. Both physical forces and chemical weathering play a role in soil formation. Water from rain flows into the cracks of rocks. Water expands when it freezes, forcing the cracks in the rocks to get bigger and little bits of the rock break off. Sometimes the roots of plants will grow into cracks in rock causing them to break. Rocks can be broken apart by lichens—tiny crusty coral-like organisms (green, orange, gray, etc.) that grow on rocks and secrete an acid that dissolves some minerals. Plant roots and many microorganisms also secrete organic acids that break down rocks in soil, like the vinegar in Activity 3. These processes take many years and are further affected by relief in the landscape, which directs where water flows and affects exposure to sun and wind. Overall, soil formation is driven by five factors: type of parent material, passage of time, climate, activity of plants and animals, and lay of the land.

In many parts of the United States these factors combined favorably thousands of years ago, and left us with fertile soils well-suited to growing crops. Topsoil is the foundation of agriculture, and it is just a thin layer that sustains life. Topsoils range from several feet deep in grasslands to 12 inches or less in many Western states. Because our topsoils are so important, farmers, ranchers, and others who are charged with caring for the land must use practices that conserve topsoils and hold them in their place. Soil is considered a nonrenewable resource because it takes so long to form.

1. Ask your students, "What would life be like without soil?" Provide further guiding questions and prompts to help them realize that much of the food we eat grows in soil, the clothes we wear (cotton) are produced by plants that grow in soil, and the animals that provide milk, meat, and eggs to our diet rely on food that was grown in soil as well. Without soil, we would have virtually no food supply.
2. To illustrate this concept, show the beginning of the video clip Dirt: Secrets in the Soil (stop at 3:10).
3. After showing the video clip, explain to the students that they will be learning more about how soil is made and how it is formed in layers.

Activity 1: Soil Profiles

Note: Prepare by asking students to bring in a sample of cereal from home (in a sealable plastic bag), or purchase three to five different kinds of bagged bulk cereal. Other food or nonfood items may be substituted for the cereals.

1. Begin with some opening questions:
   • When you dig into the ground under the grass in your yard, you’ll find soil. But what happens if you keep on digging?
   • If you dug far enough, would you run out of soil?
   • How far would you have to dig before you ran out?
   • And what would you find there?
2. Using the Soil Profile handout and the background information, explain the differences in each layer of the soil profile. Use minutes 3:22–5:43 of the video Dirt, Secrets in the Soil to show an example profile. Utah teachers should continue playing the video for specific rock and soil formations in Utah.
3. Explain to students that they will be creating an edible soil profile out of cereal. Have them begin the activity by washing their hands because they will be eating their creation.
4. Place the cereals that were brought in (or that you have purchased) on a table. Set out the plastic cups and Ziploc bags.
5. Instruct students to each take a plastic cup and layer different cereals in it to represent the three main soil horizons: parent material at the bottom (rocky), subsoil in the middle, and topsoil on top (usually darker and finer than other layers).
6. Use the Ziploc bags to crush the cereal that represents topsoil. Students can also mix cereals to get their desired colors and textures. Allow each student to share their “profile.”
7. Next, milk can be poured onto the cereal to represent water and illustrate how pore space is taken up by the liquid (milk or water in soil) and how percolation occurs. This illustration will work best if students have crushed their top layer of cereal finely and the milk is poured slowly.
8. Pass out the spoons and bon appetit! As students eat, discuss the following:
   • Some layers in some soil profiles are difficult to see because the colors are very similar. What other ways could you determine where one layer begins and another ends? (By the amount of organic matter and rocks. Chemical tests could also be used.)
   • Where do you think most soil life exists and why? Can a soil profile tell you how well plants might grow in that soil?

Activity 2: Making Soil

Note: For this activity, you will need some sandstone. Sandstone is relatively easy to find; however, if you have difficulty finding sandstone, pieces of brick or concrete can be substituted.

1. Share the Factors that Build Our Soil handout and discuss how soil forms as parent material and is broken down over many years by chemical and physical processes.
2. Demonstrate how parent material can be broken into smaller pieces by rubbing two pieces of sandstone together over a white piece of paper. Particles of sand will fall off (if you have enough sandstone for groups of students, they can conduct the activity on their own with a little guidance).
3. Explain to students that you are using sandstone because it is easiest to experiment with; other rocks need more force to break down. Nature provides this force over time in the environment, but your school day will end in a few hours, not a few centuries.
4. Place the sandstone into some water, rub the stone with your finger. Particles come off. You can also demonstrate how water running over the stone will wash away mineral particles. Water erosion on rocks takes gallons of water and many years.
5. Show students the river rock and ask them how it’s different. Point out the smoothness.
6. Heat a small piece of sandstone or limestone on a hot plate. You may want some safety glasses for this next part.
7. Ask everyone else to stand back. Pick up the rock with metal tongs and quickly drop it into ice water. The rock should break or crack as it contracts after its expansion by heating.
8. Put some small pieces of limestone or sandstone in a little vinegar in a small pot. Heat the vinegar on a hot plate and notice how bubbles form on the pieces of stone. These bubbles are filled with carbon dioxide gas, which is being released by a chemical reaction between the stone and the acid in the vinegar. If you continued this process for a long time, the entire stone would gradually break down.
9. Optional: To further demonstrate how rocks break down, you can completely fill a small glass jar with water, cap it tightly, and place it in a freezer. When the water freezes, it will expand and break the glass jar (put the jar in a bag so pieces of glass won’t get all over the freezer.)
10. Discuss the following:
    • How long does it take to make soil?
    • Is soil a renewable resource?
    • What factors help to make soil?
    • How do these factors contribute to the variability in soils found in different locations?
    • How do plants and other organisms contribute to soil formation?
    • Why would it be useful for a farmer to understand his or her soil profile?

After conducting these activities, review and summarize the following key concepts:

• Soil is a natural resource used in farming.
• Farmers could not grow the food we eat without soil.
• There are different layers of soil called horizons that make up a soil profile.
• The composition of a soil's profile affects how water moves through the soil.

1. http://www.blm.gov/nstc/soil/Kids/facts.html
2. http://extension.psu.edu/plants/crops/soil-management/soil-quality/some-facts-about-soil-basics

• A Handful of Dirt
• Dirt: Secrets in the Soil (DVD)
• Dirt: The Scoop on Soil
• Mountains of Jokes About Rocks, Minerals, and Soil
• Rocks and Soil
• SOIL Reader
• Sand and Soil: Earth's Building Blocks
• Seed, Soil, Sun: Earth's Recipe for Food
• Soil Health Education Resources
• Soil Science Society of America
• Soil! Get the Inside Scoop
• Soil, Not Dirt
• Under Your Feet: Soil, Sand and Everything Underground
• You Wouldn't Want to Live Without Dirt!