What's in Soil?
Students identify the components of soil and demonstrate that soil contains air and water. Grades 3-5
- Soil Ag Mag
- Paring knife
Activity 1: Soil Inventory
- 2-cup soil samples, 1 per pair of students
- Hand lenses
Activity 2: Soil Moisture
- Tissue paper or thin paper towel (the thick, brown paper towels common in schools do not work)
- Overhead projector
Activity 3: Soil Air
- 1-cup measuring cups, 1 per pair of students
- 2-cup containers, 1 per pair of students (preferably clear, e.g. plastic cups, mason jars)
- Soil Pie: Components of the Soil handout, for display
- Piece of yarn
infiltration: to pass through a substance by filtering or permeating
mineral matter: small pieces of weathered rock (parent material) that has been broken down into small particles over thousands of years
organic matter: a soil component derived from the decay of once-living organisms like plants and animals
percolation: the movement of water within and through the soil
pores: the spaces between soil particles and between soil aggregates; pores can be filled with air or water
saturation: occurs in a soil when all of the pores are filled with water, leaving no air
soil structure: the arrangement of soil particles into aggregates, which contain solids and pore space
Did You Know?
- Soil contains all three states of matter; solids (minerals and organic matter), liquids (water), and gases (carbon dioxide, oxygen).1
- Soil is one of the most important natural resources known to man.3
- Soil scientists study the upper few meters of the Earth’s crust.1
- A single shovelful of soil can contain more living things (organisms) than live in the Amazon rain forest.2
- Just a teaspoonful of forest soil can hold more than 10 miles of fungi (mushrooms, mold, yeasts, and mildews).2
Background Agricultural Connections
Soil is a naturally occurring, loose material at the surface of the earth that is capable of supporting plant and animal life. In simple terms, soil is comprised of three phases: solid, liquid, and gas. The solid phase, which accounts for approximately 50% of the volume in a typical soil, gives soil its mass and is a mixture of mineral and organic matter. Soil particles fit together loosely (depending on particle size and aggregation), leaving “empty” pore spaces. The pore spaces are then filled with water (liquid) and air (gas). The water and air in an average soil make up the other half of the soil’s volume.
All soils are made up of mineral matter, organic matter, air, and water, although the proportions of each will vary. An average soil contains approximately 45% mineral matter by volume and 1 to 5% organic matter. The mineral component is derived from the weathering of rock at the earth’s surface. Mineral particles, which are classified as sand, silt, or clay based on their size, determine the texture of a soil. Weathering of minerals and decomposition of organic matter are key sources of nutrients for life in the soil.
The organic matter component of soil consists of decomposing plant and animal remains. Organic matter acts like glue, binding mineral particles together into aggregates that give a soil structure. Soils high in organic matter do not compress as tightly, meaning they have more pore space. Organic matter also absorbs water, improving the water holding capacity of a soil. Sand like you find in a sandbox does not hold water well because there is no organic matter and the pores between particles are all very large—the water just slips right through.
Pore space occupies approximately 50% of the volume in a typical soil. Water and air, which are both important to soil life, move through the pores in the soil. Larger pores allow air and water to move through the soil quickly, while small pores can hold reserves of air or water. In an “average” soil, half of the pore space is occupied by air and half by water. Of course, these proportions can change drastically during a drought or a flood. During a drought, plants will pull all the water that they can out of the soil, leaving pore spaces filled mostly with air. Water carries nutrients to plants and prevents them from wilting. Extended drought can wreak havoc on farmers.
Somewhat unintuitively, prolonged rain that saturates the soil can also wreak havoc on farmers. When a soil is saturated, all the pore spaces are filled with water, leaving no air. Most crop plants will drown if the soil is saturated for too long—their roots need oxygen from the air just like our lungs do. Some plants have adapted to growing in water. For example, rice has developed a special way to transport oxygen from its leaves to its roots. Rice is often grown in flooded fields, where soil saturation prevents weeds from growing.
- Begin a discussion with students and ask, "Why is soil important?" Accept all answers and record them on chart paper or somewhere in the classroom where the responses are visible. Next, display the Illinois Ag in the Classroom Ag Mag, titled Soil on the whiteboard. Paper copies can be printed if a whiteboard is not available. Have a student read the section on the first page that addresses the importance of soil. Compare the students' responses to those found in the reading. Ask another student to read aloud the section on page 2 that addresses soil parts and add more information to the list as to why soil is important.
- Review all responses and ask, "How much soil exists on earth that farmers can access to grow crops or raise livestock?" Accept all answers and add these responses to the chart paper. To respond to this question, refer the students to the section of the Ag Mag, titled A Slice of Soil found on page 3. You will need an apple and a paring knife to demonstrate the activity and answer the question as to how much farmable soil is available to farmers.
- Challenge students to think of ways farmers can conserve soil to produce enough food to feed the world. If time permits, play one of the videos on the last page of the Ag Mag that interviews two individuals who work in the field of soil science to help students understand the importance of soil and ways farmers practice conservation. The videos can be shown from an interactive whiteboard. Look for the blue video cameras to play each video.
Explore and Explain
Activity 1: Soil Inventory
- Divide students into pairs. Give each pair a soil sample. Ask students to pour their sample out onto a newspaper.
- Note: Students may bring samples from home, but instruct them to collect true soil, not a soilless media. Many potting mixtures are made mostly of organic matter and contain little or no mineral matter. Ensure that samples are collected from a variety of locations.
- Using a hand lens, ask each pair of students to observe their sample and write a description, noting everything that they see or feel. Mineral matter will look like small, bead-like, particles, or little rock-like pieces. Leaves, sticks, roots, and living creatures like worms and beetles comprise the organic matter component. Water content can be felt as moisture. Students probably will not identify air as a component in their soil because they cannot see it.
- Show students the Soil Pie: Components of the Soil handout. Tell them that the four components of soil are mineral matter, organic matter, air, and water. Everything they observed in their soil sample should fit into one of these categories.
Activity 2: Soil Moisture
- Pick one of the soil samples that feels moist or appears to have the most organic matter.
- Place the tissue paper or paper towel on an overhead projector. Notice that it blocks out the light. Place a drop of water on the paper towel to illustrate that some of the light now shines through.
- Place the moist soil sample on the paper towel, and after a few minutes, depending on how wet your sample is, pour the soil off. Replace the paper towel on the projector to see if the soil left any more moisture in the towel. This will illustrate that soils do hold moisture.
- Repeat with different soil samples to show that the moisture content of soils varies.
Activity 3: Soil Air
- Ask each student pair to measure out one cup of dry soil from their samples and place it into a two-cup container. A clear container will allow students to watch how the water infiltrates and then percolates through the soil.
- Next, students should fill their measuring cups with one cup of water and slowly pour it into the soil container until the soil is saturated. Stop pouring as soon as the sample is saturated, no more air bubbles emerge, and water has just begun to pool on the surface of the soil without infiltrating.
- Students should measure the amount of water left and subtract it from one cup.
- Lead students to infer that the amount of water added to the soil sample represents approximately the amount of air that was displaced.
- Have students compare the amounts of water that they were able to pour into their soil samples. There will be differences depending on the soil texture and organic matter content. Calculate the percentage of air in the soil samples. For example, if 1/4 cup of water was needed to saturate the soil, the sample contained 25% air.
- Tape one end of the yarn to the center of the Soil Pie: Components of the Soil handout. Take the loose end and move it up and down between the water and air components to show how they displace each other. Discuss climatic conditions that would lead soils to contain more air or water.
- Discuss what you have learned:
- What are the components of soil? (mineral matter, organic matter, water, air)
- Which two components are the most variable? (water and air) Why?
- What does it mean when a soil is saturated?
- What would happen to a farm field if it was saturated for a very long time? (pore spaces completely filled with water; crops may die or be very stunted)
For an activity illustrating soil particle sizes, see the lesson Soil Texture and Water Percolation.
Read the Illinois Soil Ag Mag to learn more about soil.
After conducting these activities, review and summarize the following key concepts:
- Soil is a natural resource that farmers use to provide our food.
- Soil is necessary to grow the plants which provide food for humans and animals. Soil is also used to grow plants which provide clothing (cotton) and fuel.
- Soil contains mineral matter and organic matter. It also contains water and air.
Recommended Companion Resources
- Dirt: Secrets in the Soil (DVD)
- Dirt: The Scoop on Soil
- Jump Into Science: Dirt
- Life in a Bucket of 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 Center
- Soil Health Education Resources
- Soil Science Society of America
- Soil! Get the Inside Scoop
- The Dirt Book: Poems About Animals That Live Beneath Our Feet
- Topsoil Tour
- Under Your Feet: Soil, Sand and Everything Underground
- You Wouldn't Want to Live Without Dirt!