Introduction to Soil

 

STUDENTS WILL BE ABLE TO:
- Name the four components of soil: inorganic material, organic material, air, and water.
- Examine soil organisms.
- Explain how inorganic and organic material is broken down into soil.

GUIDING QUESTIONS:
What is soil?
What is in soil?

MATERIALS:
- Tubs of water (one per group)
- Yogurt or other plastic containers with lids
- Glass jars
- Salt
- Soil samples from different areas (one per group): Ideally, soil samples should include a representation of some soil macroinvertebrates. The size of the necessary sample will vary with soil type, but a gallon is a good general estimate.
- Sieve
- Microscopes
- Magnifying Glasses
- Plates
- Scales (optional)
- Oven to dry soil (optional)

CLASS ACTIVITIES:
1. Soil contains four main components: inorganic material, organic material, air, and water. Open class by leading students through a few short demos to identify these components. If you wish, draw an unlabeled pie chart on the board at the opening of class that shows the relative proportions of these ingredients in your local soil. As you complete the demos have students determine what the labels are and in which area they belong. If you do not want to focus on the proportions, write "Four Ingredients of Soil" on the board, and let students know that you will be filling in the "recipe" during class.
2. Have groups of roughly four students group around each tub of water. Students should fill a glass jar 2/3 full of water, add a tablespoon of salt and mix thoroughly, and then add roughly a cup of soil. Most of the soil will fall to the bottom, but some things will float. Ask your students to explain the difference between what has fallen and what floated. (The inorganic material will fall to the bottom; the organic material will float.)
3. Write inorganic material and organic material on the board. Develop a working definition of the two. Does your soil have more inorganic material or organic material?
4. Have students fill the plastic containers with soil and put the lid on. When doing so, they should take care not to shake the containers - the soil should not settle. Then they should immerse the container in water, and remove the lid. What happens? (If the soil has not settled, air bubbles will rise as water flows into the soil.) Where did the air come from?
5. Add air to the list of soil ingredients.
6. If your soil is relatively moist, your students may be able to determine that the last component is water. Ask students to feel the soil - is it cool? If so, why? Does it stick to their hands? Do their hands feel damp after touching it? What do plants get from soil? An alternate experiment, which must be completed overnight, is to have students weigh a sample of soil. Take these home overnight and dry in an oven. (Muffin tins can be a useful way to sort small soil samples for drying. Samples should be spread out for fastest drying.) In the next class, weigh the soil again. Why does it weigh less? What is missing?
7. Add water to the list of soil ingredients.
8. Now each group should explore the soil to see what is living in it. Organic material is broken down by members of the F.B.I.: Fungus, Bacteria, and Invertebrates. Pairs of students should put a soil sample (1-3 cups) on a plate. Can students find all members of the F.B.I. in their soil sample? Why might they not be able to find all three? (Also remind students that non-living forces of nature, such as weather and fire, also play key roles in decomposition.)
9. Students can spend the rest of the class period examining these organisms. They should make a rough sketch of each organism they find, noting the size of the organism.