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 Mixing It Up

This activity aligns with the following National Science Education Standards: Content Standard C: Life Science–Populations and Ecosystems and Content Standard D: Earth and Space Science–Structure of the Earth System.

Background

A variety of human activities pose serious threats to life in the Mojave Desert. Use of desert lands for livestock grazing can strip land of most of its vegetation. Farming and urbanization both result in increased use of groundwater that can leave desert plants and animals without the water they need to survive. The growing popularity of off-highway vehicles (OHVs) causes further damage to plants and fragile desert soils. When desert plants are damaged, the thin soil begins to erode, making the area unsuitable for living things. Once the damage is done, the process of recovery is extremely slow. There are many reasons for this, including temperature extremes, sunlight intensity, high winds, and lack of rainfall. Another factor is the sparse vegetation that is characteristic of the desert. Fewer plants mean less humus, which is formed from the decomposed remains of plants and animals. Humus provides many of the nutrients plants need to survive.

By examining the composition of soil and making soil mixtures of our own, we can begin to understand ways in which soil contributes to plant growth and the long process that is involved in its formation. We’ll also come to appreciate why it’s important to protect the thin, fragile layer of soil in the desert.

Materials Needed

  • natural soil samples, at least 0.25 L per student, collected from several outdoor locations (make sure you have permission to dig samples if you’re not digging on school property)
  • an assortment of “ingredients” for students to make their own soil mixtures, such as leaves, small rocks, broken shells, sand, clay, grass clippings, small sticks, etc., (assemble enough so that each student will be able to make about 0.5 L of soil) water
  • plastic trays, cardboard box tops, or sections of newspaper, at least 25 cm by 40 cm containers of varying sizes, such as large coffee cans and plastic cups or planting pots, with a capacity of at least 0.25 L
  • hand lenses, spoons, toothpicks, and other “excavating” devices
  • fast-germinating seeds, such as zucchini, beans, zinnia, and cosmos

Procedure and Questions

1. Students spread samples of the natural soil out on plastic trays and use hand lenses to examine them. Can they see pieces of rock or sand? Any living things? Encourage students to smell and feel their soil samples as well, and to describe their samples as precisely as possible.

2. Remind students that soil contains both organic and inorganic materials and that its formation involves a long, complex process.

3. Next, students develop their own “recipes” for soil mixtures that they think will be capable of supporting plant life. The goal should be to produce about 0.5 L of soil mix. One approach could be to divide the class in half, with one group of students attempting to replicate desert soil and the second group working on mixtures that are more likely to be found in a forest environment. Before they begin mixing their ingredients, students need to consider the following questions:

  • How would the balance of ingredients differ between forest soils and desert soils? (Desert soils would contain more sand, while forest soils are likely to contain more organic matter and, depending on where they come from, possibly more clay.)
  • What ingredients would provide organic matter to plants? (From the above list of materials, leaves, grass clippings, and sticks would provide organic matter.) What ingredients would provide minerals? (rocks and shells) 
  • What materials would improve drainage? (sand and other larger particles)
  • What materials would help retain water? (clay and other smaller particles)
  • What materials would help anchor plant roots? (clay)

Following a discussion of these questions, students adjust their recipes if they wish.

4. Students then combine their ingredients to make the soil, adding a few tablespoons of water to each mixture. Stir until the ingredients are thoroughly moistened.

5. Discuss with the class the ways in which their mixtures are similar to and differ from the natural soil samples they examined in Step 1. What might account for differences? (Answers will vary depending on where the natural samples came from and what ingredients were used in students’ recipes, but the major factor accounting for differences is time: The natural processes that create soil occur slowly.)

6. How do students think plants will grow in the soils they made? To test their ideas, have students put their soil mixtures into plastic pots and plant seeds in them. Seeds should also be planted in the natural soils for comparison. Place all the pots in a well-lighted area, and water every couple of days with equal amounts of water.


Keeping Cool

This activity aligns with the following National Science Education Standard: Content Standard C: Life Science – Diversity and Adaptations of Organisms

Background

Desert animals have developed many ways to cope with the heat and sun. Many come out only at night or during the early morning and late evening. They spend their days in the shade or they burrow into the sand, where temperatures are cooler. Students can easily test the value of such strategies by measuring temperature differences in the schoolyard or in their own backyards.

Materials Needed

  • two indoor-outdoor or classroom thermometers, 15–20 cm long
  • 5 kg of sand
  • a deep box or small wastebasket, about 15 L in capacity
  • plenty of sunshine

 Procedure and Questions

1. Students place one thermometer out in the bright sun for at least an hour, and a second one in the shade of a tree or large boulder for the same length of time. (Make sure that this second thermometer stays in the shade for the entire testing period.) At the prescribed time, read the temperatures on the two thermometers. Students should discover that the temperature in the shady area is considerably lower than that in the sunny location.

2. Next, fill the box or small wastebasket with sand. The sand should be about 25–30 cm deep. Put the container in an area where the sun will shine on it all day. In the afternoon, push the end of one thermometer just slightly into the sand. After a minute or so, record the temperature. Place the second thermometer deep in the sand. Again, wait about a minute and record the temperature. How much lower is the temperature beneath the sand?


Extension

Assign each student or student group a specific desert animal to research and report on. Students should focus on ways in which their animal is adapted to its environment. Students should be encouraged to use the Internet as a resource in their research; some good websites to use as starting points are:

Mojave National Preserve–Desert Plants and Animals at http://www.nps.gov/moja/naturescience/index.htm

Desert USA-Desert Animals and Wildlife at http://www.desertusa.com/animal.html