To explore and verify that soil erosion is affected by the makeup of the soil using plant roots, rocks, and the slope of the land as experimental factors.
Middle-school students should be able to explain how soil is formed by the breakdown of a variety of organic and inorganic materials, sediments of sand, and smaller particles (sometimes containing the remains of organisms).
In grades 3-5, students learned that waves, wind, water, and ice shape and reshape the earth's land surface by eroding rock and soil in some areas and depositing them in other areas, sometimes in seasonal layers.
This lesson briefly focuses specifically on soil erosion caused by water run off. Students are now ready to learn how topsoil is formed over hundreds of years and this lesson will focus on how, if not properly anchored, it can be washed away quickly. (Benchmarks for Science Literacy, p.73.)
Soil erosion is a naturally occurring process on all land. The agents of soil erosion are water and wind, each contributing a significant amount of soil loss each year. Soil erosion and degradation are so severe worldwide that it threatens our agricultural base.
Soil erosion may be a slow process that continues relatively unnoticed, or it may occur at an alarming rate causing serious loss of topsoil. The loss of soil from farmland may be reflected in reduced crop production potential, lower surface water quality, and damaged drainage networks.
The rate and magnitude of soil erosion by water is controlled by these factors: rainfall intensity and runoff, soil erodibility, slope gradient and length, and vegetation.
Factors affecting erosion can be summarized as follows:
- heavy rains on weak soil: rain drops loosen soil particles and water transports them down hill
- vegetation depleted by drought: rain drops are free to hit the soil, causing erosion during rainfall; winds blow away the fine particles during droughts
- steep slopes: gravity “pulls harder;” water flows faster; soil creeps, slips, or slumps downhill
- sudden climate change
- rainfall: erosion increases unexpectedly rapidly as rainstorms become more severe
- drought: water dries up and the soil becomes a play ball of winds; soil biota die; a sudden rain causes enormous damage
- changing winds: areas previously sheltered become exposed
- change of land (deforestation): the land loses its cover, then its soil biota, porosity, and moisture
- intensive farming: the plough, excessive fertilizer, and irrigation damage the land, often permanently
- housing development: soil is bared; massive earthworks to landscape the subdivision; soil is on the loose
- road construction: roads are cut; massive earthworks, leaving scars behind; not enough attention paid to rainwater flow and maintenance of roadsides
To begin this lesson, have students use their Soil Erosion student esheet to go to California Industry, Homes Damaged on the CBS News site, to read this article about the effects of a mudslide on a community.
Once students have had a chance to read the article, discuss real-life stories about mudslides they may have seen on the news.
Then have students perform a Google search for mudslide images. Once students have had a chance to look at these images, conduct a class discussion about what they saw and read. Ask these types of questions:
- What might go through your mind if you were in a house that started creaking and breaking apart?
- Would you know what to do?
- What might be an option for you and your family and what would the consequences be?
Next, have students use their esheet again to go to Landslide Events on the USGS site. Students should use this site to locate recent landslide activity. They should use their Landslides student sheet to answer these questions:
- Where did the landslide happen?
- When did it happen?
- Why did it happen?
- Could something have been done to prevent this from happening?
Follow up this activity with a discussion using these questions:
- Predict how many landslides you think you will find that happened in the last two weeks.
- Where do you predict these landslides will be?
- Have you heard of floods in the last few days? In the U.S.? Internationally?
Tell students that in this part of the lesson they will explore erosion further by taking part in a hands-on activity. Before they begin this activity, write the central question for this lesson in a visible place: “What can be used on the hillside to slow or stop the soil from eroding? Can it be stopped? To what extent can erosion be slowed or stopped?”
Before students start the hands-on activity, they should use their esheet to visit In the Time it Took to form One Inch of Soil. Ask them: “How many years did it take to create one inch of topsoil according to this website?”
Now, have students use their esheet to go to Soil Erosion and WEPP Technology to read about the different types of erosion caused by water runoff. Students should use their Erosion by Water student sheet to record the descriptions of each type of erosion. They also should be prepared to discuss the descriptions in class.
Part I: Watching Water Erosion
Now, as a class, engage in a hands-on demonstration of soil erosion. Show students a pile of dirt (about a gallon) on a large piece of plastic (or do this outside with a mound of dirt and a hose with a spray nozzle). Place little plastic figures like plastic people, tiny houses, model trees, etc., in different places around the mound of dirt. Use a spray bottle or hose to spray the mound until the dirt starts to erode. Watch the people, houses, and trees slide down the mound. Measure the amount of runoff. Drain the water and only measure the amount of soil runoff.
Begin a discussion about what the water is doing to the mound. Ask, “Can you see examples of the different types of erosion? If so, point them out and discuss the characteristics.” Talk about ways to slow this erosion down so the people can live on the mountainside.
Part II: Using Plants to Slow Erosion
In this part of the lesson, divide students into groups. Each group will create a hillside garden that they feel will work to slow erosion on the hillside using the material listed on the Planning a Hillside Garden student sheet. Students should follow these steps:
- 1. Use the activity sheet to plan their hillside garden. Save the sheets for final discussion.
- 2. Using measured proportions, plant their garden in the foil tin following their garden plan.
- 3. Grow their gardens with equal amounts of water for each garden. Decide as a group what that measurement should be for all gardens.
- 4. Once the garden has grown to the point where the roots have been established, two weeks should be plenty, test their garden for erosion.
To perform the erosion test, students should place their gardens on a block so that one end of the tray is elevated 3-4 inches. Then, they should place the opposite end on a catch bin (use aluminum foil or some other type of material that will catch the dirt and water runoff.)
Students should sprinkle the gardens with equal amounts of water and collect the runoff in the tray at the bottom. The amount of water will depend on how much and how long they choose to pour. Several cups of water should produce enough runoff to test but they can add more if needed.
Now students should measure the amount of soil/plant runoff from each garden. Have them record this amount on their Planning a Hillside Garden student sheet. Then they should drain off the water and only measure the soil/plant runoff. Again, they should record this amount on their student sheet.
Refresh everyone’s memory as to the central question for this lesson by writing it in a visible place: “What can be used on the hillside to slow or stop the soil from eroding? Can it be stopped? To what extent can erosion be slowed or stopped?”
Discuss each group’s results and have students write these on the garden plan sheets. Use these questions to stimulate discussion:
- Which group had the least amount of runoff? Which group had the most?
- How did your garden plans differ?
- What might be the cause of this difference?
- Can you see examples of the specific types of erosion you studied? Describe.
- How would you plant your garden differently next time to more effectively limit the amount of lost soil?
- How does this experiment relate to the real world and the use of vegetation and rocks to reduce runoff?
- Can you see any examples of this on or around your school grounds?
Soil erosion can be caused by many factors. This experiment dealt only with water, vegetation, and the slope of the land. By effectively placing plants, shrubs, trees, and rocks on a slope, soil erosion can be managed.
To evaluate the students’ new knowledge, give each student a new garden plan sheet and have them design their garden again. Have students label at least four sections of the plan with numbers and then write a summary as to how each area will be affected by rainfall.
In the Science NetLinks lesson, How Sedimentary Rocks are Formed, students learn more about the rock cycle.
The Field Museum's Underground Adventure site takes students underground to see what life is like under the soil.
The Soil Education page from the USDA Natural Resources Conservation Service provides resources for educators and various grade levels.