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:
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.
Moderate, because of materials and time required.
$10 if soil and plant trays are purchased.
Approximately 2 weeks.
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.
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.