Erosion: Does soil type affect the amount of water that runs off a hillside?

Purpose/Hypothesis

In this experiment, you will find out how the type of soil affects how much erosion can occur. Soil is a mixture of inorganic materials (rocks, sand, silt, or clay) and organic materials (decomposing leaves and organisms). The ratio of these components to each other determines the kind of soil and its texture. In turn, the texture of soil determines how well the soil can support plants and withstand erosion. Before you begin, make an educated guess about the outcome of this experiment based on your knowledge of soils and erosion. This educated guess, or prediction, is your hypothesis. A hypothesis should explain these things:

• the topic of the experiment
• the variable you will change
• the variable you will measure
• what you expect to happen

A hypothesis should be brief, specific, and measurable. It must be something you can test through observation. Your experiment will prove or disprove your hypothesis. Here is one possible hypothesis for this experiment: "The looser and coarser the texture of the soil, the less runoff and erosion will occur."

In this case, the variable you will change will be the texture of the soil, and the variables you will measure are the amount of water that runs off and the amount of soil it carries with it, judged by the color of the runoff water. You expect the looser and coarser soils to have less water runoff and less soil erosion.

Setting up a control experiment will help you isolate one variable. Only one variable will change between the control and the experimental soil pans, and that variable is the kind of soil used. For the control, you will use potting soil. For your experimental soil pans, you will use sand, clay, and neighborhood soil.

You will measure the amount of water that runs off your soil pans and how much erosion occurs. If the looser- and coarser-textured soils have less runoff, your hypothesis is correct.

Level of Difficulty

Moderate, because of materials needed.

Materials Needed

• 2 to 3 pounds (1 to 1.5 kilograms) of purchased potting soil
• 2 to 3 pounds (1 to 1.5 kilograms) sand
• 2 to 3 pounds (1 to 1.5 kilograms) clay
• 2 to 3 pounds (1 to 1.5 kilograms) neighborhood soil
• 4 shallow pans. Cookie sheets with 0.5 to 1.0 inch (1.25 to 2.5 centimeters) high edges work well.
• 4 glass jars, approximately 24 fluid ounces (680 milliliters)
• scrap lumber
• a sprinkler can or hose nozzle with mist setting
• water
• measuring cup
• labels
• outdoor area to conduct experiment, since it may be messy
• a baking dish, approximately 9 x 13 x 2 inches (23 x 33 x 5 centimeters)
• magnifying glass (optional)

Approximate Budget

$10 if soils must be purchased.

Timetable

2 to 3 hours.

How to Experiment Safely

Wash your hands carefully after you handle soil, especially if you are using soil from outdoors. Be careful when digging to avoid broken glass or other trash in the soil.

Step-by-Step Instructions

1. First, examine your soils. You may want to look at their particles with a magnifying glass. On your chart (see illustration) record your soils in the order of their textures, from coarse to fine. If you cannot see separate particles, then the texture is very fine.
2. Place your shallow pans in a row and place a different kind of soil in each one. Fill each pan evenly up to its edges all around.
3. Prop one end of your potting soil pan on a board to simulate a hill. The exact slope is not important, but you must use the same slope for each pan.
4. Place the bottom end of the pan so it is resting in the baking dish.
5. Measure 3 cups of water into your sprinkler can.
6. Sprinkle the water over your "hillside," mostly from the top edge, and watch what happens.
7. After the can is empty, wait 5 minutes.
8. Pour the water from the baking dish pan into a glass jar. Look at its color and measure how much you have collected. The darker the water, the more soil has run off.
9. Label the jar with the type of soil.
10. Repeat the procedure for sand, clay, and neighborhood soil.

Summary of Results

Record your results on a chart like the one illustrated.

Compare the amounts and colors of water in each jar. The darker the water, the more soil has run off in it. What have you discovered? Did coarser soils have less runoff? Was your hypothesis correct? Fill in your chart carefully and summarize what you found.

Source: Experiment Central. U·X·L, 2000.