Environmental Science Resources
Plants and Erosion: How do plants affect the rate of soil erosion?
Purpose/Hypothesis
Soil is an important part of an ecosystem. An ecosystem is a community of plants, animals, and microorganisms considered together with their environment. Because soil is the foundation for life on Earth, erosion can be a serious problem for the living beings that depend upon it—including humans.
In this experiment, you will explore how the rate of soil erosion is affected by plants growing on the soil. Plant cover — either growing plants or fallen leaves and branches — protects soil from erosion by slowing down flowing water or absorbing the impact of rain drops. Roots of trees and other plants help to prevent erosion by holding the soil in place. Roots absorb water and provide stability to the soil. Before you begin, make an educated guess about the outcome of this experiment based on your knowledge of soils, plants, 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 whether your hypothesis is correct. Here is one possible hypothesis for this experiment: "Less soil will erode from a hillside with plant cover (a layer of leaves or growing grass) than from a hillside with no plant cover."
In this case, the variable you will change is the amount of plant cover, and the variables you will measure are the amount of water that runs off and the color of the soil that runs off. You expect the looser and coarser soils to have less water runoff and soil erosion.
Setting up a control experiment will help you isolate one variable. Only one variable will change between the control and the experimental trays, and that variable is the presence or absence of growing plants or plant cover. For the control, you will use potting soil without any vegetation. For your experimental trays, you will use grass and leaf litter (leaves and/or grass clippings).
You will measure how much erosion occurs in each of the trays by measuring water that runs off and comparing the color of the water. If the experimental trays show less erosion than the control tray, then your hypothesis was correct.
Level of Difficulty
Moderate, because of materials and time required.
Materials Needed
- 2 to 3 pounds (1 to 1.5 kilograms) purchased potting soil
- 1 to 2 pounds (0.5 to 1.0 kilograms) small gravel
- leaf litter (fallen leaves, twigs, and grass clippings)
- grass seed
- 3 shallow pans or trays (plant trays from a garden shop are designed to allow drainage; you may wish to use glass casserole dishes that allow you to observe the roots; otherwise, cookie sheets with edges will work.)
- 4 glass jars, approximately 24 fluid ounces (680 milliliters)
- a sprinkler can or hose nozzle with mist setting
- water
- labels
- measuring cup
- board or scrap lumber
- an area with adequate light for growing grass
- an outside area or other place for conducting the experiment, which may be messy
- a baking dish, approximately 9 x 13 x 2 inches (23 x 33 x 5 centimeters) or a dish pan to collect water that runs off
Approximate Budget
$10 if soil and plant trays are purchased.
Timetable
Approximately 2 weeks.
How to Experiment Safely
Be careful when collecting fallen leaves or grass clippings, as broken glass or other trash might be in the leaves or grass. Wash your hands thoroughly afterward. If you collect soil from your neighborhood rather than using potting soil, use caution when collecting and handling the soil. Do not dig soil where you do not have permission to do so.
Step-by-Step Instructions
- Prepare three trays by putting an equal amount of potting soil in each tray. If you are using pans or cookie sheets, spread a layer of gravel on the bottom of the pan before adding the soil. This will allow for drainage since you will be watering all three pans while the grass is growing.
- Set Tray 1 aside. In Tray 2, cover the soil with a layer of leaves and grass clippings. In Tray 3, sprinkle grass seed on the top of the soil.
- Place the three trays in a place where they are level and have similar light and temperature conditions. (The temperature must be above 50°F (10°C) for the grass to grow.)
- Use the sprinkling can to give each tray the same amount of water. Continue watering all three trays approximately every 3 days until the grass in Tray 3 is about .5 inches (1.25 centimeters) tall. This may take one week or longer. You may have to adjust your watering schedule depending on how fast the soil dries. Check the soil daily to see if it looks and feels moist.
- When the grass has grown, you are ready to do the erosion test. Prop the end of Tray 1 (soil only) on a board to simulate a hill. The exact slope is not important, but you must use the same slope for each tray.
- Place the bottom end of the tray so it is resting in the baking dish or dish pan.
- Measure 3 cups of water into the sprinkler can.
- Sprinkle the water over your "hillside," mostly from the top edge, and watch what happens.
- When the can is empty, wait 5 minutes.
- Pour the water from the baking pan into a glass jar. Look at its color and measure how much you have collected.
- Label your jar (Tray 1: soil only).
- Repeat procedure for Tray 2 (soil with leaf litter) and Tray 3 (soil with grass). Be sure to label each jar so you can compare the quantity and color of the water.
Summary of Results
Record your results on a chart like the one illustrated.
When you have finished, compare the amounts and colors of water in each jar. The darker the water, the more soil has run off. What have you discovered? Did the trays with leaf litter and grass have less runoff than the control tray? Did the tray with grass have less runoff than the tray with leaf litter? Was your hypothesis correct? Fill in your chart carefully and summarize what you found.
Source: Experiment Central. U·X·L, 2000.