BioEd Online

Teacher Background

Carbon, oxygen, and hydrogen are the building blocks of the molecules that make up our bodies, our foods, and even the fuels we burn. These elements are combined during photosynthesis to make energy-rich materials, such as sugars and other carbohydrates (starches). Plants obtain hydrogen from liquid water (H₂O). They obtain carbon from carbon dioxide gas (CO₂) in air. Oxygen is part of both water and carbon dioxide and is present as oxygen gas (O₂) in air. However, all living things, including plants, require additional materials to carry out the chemical processes necessary for life.

Where do these other materials come from? Most of them are released into water from soil. Plants and plant-like organisms, such as algae, absorb nutrients dissolved in water. Examples of these nutrients include nitrogen, phosphorous, and potassium. Non-photosynthetic organisms obtain the minerals and more complex molecules that they need by consuming plants and other living things. Thus, the nutrients in soil are important not only for supporting plant growth but also for ensuring that other organisms are able to grow and survive.

Soil has both living and nonliving components. It constantly changes through the action of weather, water, and organisms. Soil formation takes a very long time—up to 20,000 years to make 2.5 cm of topsoil! (This is only as deep as a quarter standing on its side!)

The nonliving parts of soil originated as rocks in the Earth’s crust. Over time, wind, water, intense heat or cold, and chemicals gradually break rocks into smaller pieces, a process known as weathering. The size and mineral composition of the tiny rock particles determine many of the properties of soil.

Most soils are enriched by decomposed plant and animal material. Soil is home to many kinds of organisms: bacteria, fungi, algae (plant-like organisms that live in water or moist  enviornments), earthworms, insect larvae, and plant roots, to name a few. Soil also contains many tiny air spaces. Typical garden soil is 25% water, 45% minerals, 5% material from living organisms, and 25% air.

Objectives and Standards

Concepts

• Soil is a combination of many different living and nonliving things.

• Soil provides raw materials needed by all living things.

Science, Health, and Math Skills

• Observing

• Measuring

• Recording observations

• Drawing conclusions

Materials and Setup

Teacher Materials

• 2 cups of natural soil

• 2 paper plates

• 1/2 tsp of alum

• clear soft drink bottle with screw-on cap, 2-liter size

• measuring cup

• metric ruler

• newspapers to cover work area

• copy of "Soil Observations" sheet

Materials per Student

• craft stick, toothpick, or coffee stirrer

• hand lens (magnifier)

Setup

1. This activity works best with a recently dug sample of natural soil (from a field, yard, garden, or the playground). About one large shovelful will be enough for the entire class. Collect the soil 24 hours or less before conducting the activity, and store it in a large plastic bag (do not seal completely).

2. Have students work in groups of 2–4 to conduct the activity.

Safety

You may want to sterilize soil by baking it at 375°C for 30–40 minutes, or microwave loosely covered damp soil at full power for seven minutes. You also can create your own soil mix for students by combining packaged top soil and sand with a small amount of mulch.

Procedure and Extensions

Time

Two 30-minute sessions

Session 1: Looking at soil

1. Direct students to cover their work areas with newspapers. Have the Materials Manager from each group measure about 2 cups of soil onto a paper plate and bring the soil back to their group.

2. Have students place about 1/2 of their group’s soil in the center of their work area. Have them take turns describing the soil, using all of their senses except taste. Ask, What does the soil look like? How does it smell? How does it feel?

3. Ask each student to write three words that describe some aspect of the soil sample on his or her "Soil Observations" sheet.

4. Next, direct students to spread out the sample (using toothpicks, popsicle sticks, etc.) and to observe the different components of the soil sample. Ask, What are some of the things that you can see in the soil? Possibilities include twigs, pieces of leaves, plant roots, insects, worms, small rocks, and particles of sand. Ask, What are some things in soil that we can’t see? Answers may include air, water, and microorganisms.

5. Have students list or draw the different things they find in their soil samples. Suggest that they think about and classify the different components of soil as coming from living or nonliving sources.

Session 2: Soil texture

1. Each group will need a soft drink bottle (with cap) and the other half of its soil sample.

2. Ask students to describe the different components of the soil they investigated during the previous session. Tell them they are now going to observe the make-up of soil in a different way.

3. Have each group add about 1/2 cup of soil and 1/2 teaspoon of alum to the soft drink bottle, then add water until the bottle is 3/4 full. If students have difficulty pouring soil into the bottle, have them make a paper funnel by rolling a sheet of paper into a cone shape.

4. Direct students to cap the bottles tightly and shake the bottles for about one minute.

5. Next, have students place the bottles in the centers of the groups’ work areas and observe how quickly or slowly the different types of particles settle.

6. When layers are visible at the bottom of the bottle, have students measure and mark the layers and draw their observed results on their student sheets. To facilitate accurate measuring, you may want to instruct students to fold a sheet of paper lengthwise, hold it against the side of the bottle, and mark the boundaries of each layer on the paper.

7. After students have completed their observations, invite the groups to share their observations. Ask, How many different layers did you find? What was on the bottom? What was on the top? The heaviest particles, such as sand and rocks, usually will make up the bottom layer, followed by fine sand and silt. Some clay particles are so tiny that they will remain suspended in the water. Plant and animal material also may remain floating at the top of the water. You might also ask, What do you think soil is mostly made of?

Extensions

• Create unique soil samples for each group by mixing varying amounts of soil and sand from different sites. Have students compare their results and discuss which samples might be the best to use in a vegetable garden. Have them test their predictions by putting the different kinds of soils in pots or cups and planting flower or vegetable seeds in each one.

• Provide samples of pure sand and pure dry clay for students to examine with their magnifiers. Have them write about the difference between the samples.

• Try making your own pH paper to test soil acidity. Place about 1 cup of sliced purple cabbage into a sealable bag filled with warm distilled water. When the water is dark blue or purple, pour it into a container. Cut white coffee filters into 1-inch wide by 6-inch long strips. Dip the strips into the cabbage water and allow them to dry on a hard surface. Test the pH strips in vinegar (weak acid) and water with baking soda (weak base) to see how they change color. Measure 1/2 cup of soil into 2 cups of distilled water. Test the water using the pH strips. Compare several soils from different locations.

Handouts and Downloads