Summary
Introduction
You pick up litter, you reduce, reuse and recycle but did you ever consider conserving the soil? It might seem farfetched, but it turns out that soil erosion—or the wearing away of soil by water, wind and other natural forces—is a problem. Productive farmlands can disappear as nutrient-rich topsoil in fields washes away when heavy rains hit them. Waterways can get polluted as pesticides and fertilizers wash into them. Even human life and property are jeopardized, as soil erosion causes mudslides and landslides. The economic cost of soil erosion is estimated to be several billion dollars every year for the United States of America alone. This is a serious problem; can you help solve it? Do this activity and learn how to save the soil with nothing more than a few plants!
Background
Soil erosion is partially caused by runoff washing away the soil after a heavy downpour. Runoff refers to the water that flows over the surface of the soil. It occurs when the soil is saturated or unable to absorb any more water. One way to help combat soil erosion is through the use of plants. Plants have extensive root systems that can help to “grab on” to soil and keep the soil clumped together. You might have seen this when you pulled a plant out of the soil and a clump of soil was stuck around its roots. Plants also help absorb some of the water in the soil. These effects make it harder for water to wash away the soil. Note that plants also help reduce soil erosion in other ways, like breaking the wind.
Now that you know what soil erosion is, let’s observe how rain contributes to water erosion.
Materials
- Scissors or sharp knife
- Clean, empty one gallon container with lid
- Water
- Two aluminum bread pans. Alternatively, you can use two sturdy shoeboxes that are similar in size. The boxes need to be at least three inches or seven centimeters deep.
- Dirt, potting soil works well
- Two aluminum nine by thirteen inch cake pans. Alternatively, you can use shallow plastic containers that are about one inch or three centimeters deep, and wide enough that the short side of the bread pans or shoeboxes can rest in the containers.
- Twelve to fourteen plastic forks (note that metal forks do not work as well).
- Two blocks, shallow plastic containers or other objects that can be used to prop up one edge of a bread pan to a height of roughly three to five centimeters. Both items need to be identical in height. Do not use an object that you do not want to get wet, like a book.
- Outdoor test area with a flat, level surface where it is easy to clean up spilled water and soil. If you must do the project inside, consider a bathtub.
Preparation
Safety Tip: Have an adult assist while using the scissors or sharp knife.
- In the next three steps, you will transform a one gallon container into a watering can that simulates rain. If you have a watering can with a rain sprout that simulates rain well (not one that pours the water in a single stream), you can skip these steps and use the watering can to simulate rain.
- Ask an adult to use the scissors or knife to poke one hole just above the handle of the one gallon container. This hole will allow air to enter the watering can as water pours out. It can be quite big, a half to one centimeter works well.
- Ask an adult to poke holes in the top of the jar on the opposite side of the handle for the water (rain) to come out. Three rows of five holes each works well. These holes need to be a little smaller, about two millimeter diameter works well. Do not worry if the holes are not exactly the same in size, or are not neatly organized in rows. You can test how it simulates rain and make adjustments if needed. Do you think the number of holes and /or the size of the holes will change the type of rain you will create?
- To test your watering can, fill it halfway with water, close the container and hold it tilted over the sink so water can pour out of the smaller holes. Does it mimic rain well? Play around with the angle at which you tilt the container. If water runs down the side of the container, tilt the container more. If it comes out of the big hole as well as the little holes, you tilted the container too far. What type of rain do you get: heavy downpour, a steady rain, a drizzle? Make adjustments to the holes if needed. The goal is to get a steady, heavy rain.
Instructions
- You will likely spill water and soil in this activity. Be sure to protect your surface or use a surface that can get wet and dirty.
- Fill the two bread pans with soil. While you do so, pulverize any clumps of soil that are present. Leave a little space at the top so the soil so runoff does not flow over the edges. The layer of soil needs to be at least six centimeters deep.
- Ideally, you would grow plants in one pan. This takes time. To complete this project in an hour, you will use plastic forks to simulate plants. The tines and curved area of the forks will simulate the root system. Do you think the tines and curved area of the forks can hold as much soil as roots from a well-developed plant can?
- Plant forks in one box. Spread the forks around so the area is evenly covered. Plant the forks deep into the soil so the curved areas of the forks with the tines are in the soil and only the handles stick out.
- In this step, you will remove the top half of one short side of each bread pan. To do so, make two vertical cuts on one short side of a bread pan where it meets the longer sides, then fold down or remove a little over half of that short side to expose the soil. This will allow soil to flow out of the bread pan when you do your erosion test. Repeat this step with the other bread pan.
- Before you set up your test area, make sure your surface is protected or can get dirty and wet.
- Start by placing a block with a height of three to five centimeters next to an empty cake pan on a flat work surface. The cake pan is there to collect erroded soil.
- Place the cut end of a bread pan into the cake pan and push the block back until the other end of the bread pan rests on the block. The surface of the soil will be at an angle. This allows you to mimic a small slope like the slope of a hill. Do you expect runoff to be a bigger problem on hillsides compared to flat surfaces?
- Repeat the previous step with the other cake pan, block and bread pan so you have two identical setups, one where the soil contains forks (plants) and another with bare soil. Leave some space between the setups so spilled water and soil from one setup does not interfere with the other.
- Fill your watering container three-fourthsfull with water.
- Use your watering container to simulate rain over one bread pan or shoebox. Hold the watering container up high enough so rain falls on the entire width of the bread pan at once. You might not be able to cover the entire length of the bread pan at once. If so, concentrate on the upper part of the bread pan. Be prepared, as water will go to the sides of the pan and soil might splatter out of the pan, which is why you needed to protect your work surface. Make observations as rains comes down on the soil. What happens initially? Do you see changes in the soil surface? Do you see splats of soil? Does runoff (water flowing over the soil surface) occur instantly? Do you see a sudden change after a little while? Why would this change happen?
- Fill your watering jar again three-fourths full and use it to simulate rain in a smiliar way over the other bread pan or shoebox. Make sure you fill the watering container to a similar level, tilt the container in a similar way and hold it at a similar height. Why do you think this is important? Would changing these varialbles considerably simulate rain with a different intentsity? Do you think the intentsity of the rain influences the type and amount of erosion damage caused by the rain?
- Water the second bread pan or shoebox until the container is empty. What do you observe this time? Do you see any differences between the two expriments?
- Depending on the type of your soil and the size of your bread pans or shoeboxes, you might not see sizable erosion occur after the first downpour. If so, fill you your watering jar halfway and simulate another downpour on your bread pans or shoeboxes, one at a time. If needed, repeat until you observe sizable erosion happening in at least one bread pan or shoebox. Does erosion happen after the same amount of rain for both experiments?
- To see the amount of eroded soil in your cake pan, slightly tilt the cake pans (one at a time) to drain the water. Some soil suspended in the water will drain out with the water, which is fine. The majority of the eroded soil will settle to the bottom and stay in the pan.
- Compare the amount of eroded soil collected for both experiments. Do you find more, less or equal amounts of eroded soil? Would you conclude that the forks helped reduce the amount of eroded soil due to rain?
- How do you think forks are similar and how are they different from small plants? Do you think having small plants in the bread pan or shoebox would have helped reduce erosion?
- How do you think real-world situations might be similar to, or different from, your experiment?
- The next time you go for a nature walk, look around. Can you find signs of water or wind erosion?
Extra: Try the experiment with different types of soil. Would some types of soil be more susceptible to erosion than others?
Extra: Grow radish sprouts or other fast growing plants in a bread pan. Redo the test, now comparing bare soil with soil covered with sprouts. Be sure to use a bread pan for this, as a cardboard shoebox will soften when filled with moist soil for a longer time. Would plant sprouts used in this test be a good substitute for full-size plants or trees in real situations? Which one do you expect to hold soil best, forks, sprouts or full grown plants?
Extra: Look up more details about the different types of water erosion. Sheet, rill, gully and splash are some examples. What type of erosion do you observe during your experiment?
Observations and Results
Did you find more soil was washed away from the bread pan or shoebox filled with bare soil?
Initially, you might have seen that the impact of heavy raindrops falling on the soil caused pieces to break away from the soil, leaving little dips in the soil. Maybe you observed soil splatting away as a result.
At the start, the soil could probably absorb most of the rain. It acted like a sponge, holding the water. With a very hard downpour, the soil is not always able to absorb the water fast enough, and you instantly observed excess water flowing on top of the soil. Sooner or later, you should have observed excess water flowing down, as eventually, the soil gets saturated and cannot absorb any more water.
The water flowing on top of the soil is called runoff. As it flows, it carries loose soil with it. The amount of soil transported depends on the speed and volume of the flowing water, as well as the type of soil.
Although forks cannot hold on to soil as well as plants do, they do help hold the soil together and ward off layers of soil sliding down. As a result, you probably collected more eroded soil from rain falling on the bare soil.
Ask an Expert
Additional Resources
- Can Plants Stop Soil Erosion?, from Science Buddies
- Erosion, from Plant and Soil Sciences eLibrary
- 'Slow, insidious' soil erosion threatens human health and welfare as well as the environment, Cornell study asserts, from Cornell Chronicle
