Grass might not be the most exotic looking plant in the world, but it is one of the most important. Its pretty green color comes from the chlorophyll inside, which is a pigment that plants use in photosynthesis. Photosynthesis is the process in which plants convert light energy from the sun to chemical food energy, and chlorophyll is essential to the first steps of that process. Grass plants store the food energy, and when grazing animals like buffalo, deer, or cows eat the grass. If you eat beef or drink milk, you are getting energy from the sun that was originally stored as food energy by grass!
Grass is also important because it reduces the erosion of soil. Erosion involves the carrying away of earth materials by water, ice, and wind. Soil can take millions of years to form, so we certainly don’t want it carried away, and humans have spent years learning how to prevent erosion. In this experiment, you are going to see how the presence of grass affects the rate of erosion, but first you need to learn a bit more about the structure of grass.
Grass belongs to the herbaceous plant group, which consists of plants that have soft stems—not tough ones like woody plants have. Grass plants also have fibrous roots, meaning they don’t have a big central root like a carrot plant does. Instead, their roots spread out in all directions.
Below is a diagram of the grass plant. Note the soft central stem and the attached leaves. The place where the grass leaves attach to the stem are called the nodes. Besides having regular vertical stems, grass plants have horizontal stems. If the horizontal stem grows above the ground, it is called a stolon or runner, if the horizontal stem grows below the ground, it is called a rhizome. Sometimes, instead of starting from a seed, new grass plants grow from stolons or rhizomes.
How does grass prevent soil erosion?
- 3 Styrofoam egg carton tops
- Grass seed
- 2 paper or plastic cups, twelve ounces
- Duct tape
- Skewer or other hole poking tool
- Shallow baking pan or cookie sheet
- White paper
- Fill one of the egg carton lids to the brim with soil.
- Sprinkle with grass seeds.
- Water and place in a sunny location.
- Water your grass regularly and wait for it to grow to a height of ¼”. This can take more than a week. Make sure your grass is not too wet for the day of your experiment!
- Place duct tape over the holes on the sides of all the egg carton lids.
- Cut off one small side of each egg carton lid. This will make it easier for water to flow out.
- Fill the other two egg carton lids to the brim with soil.
- Using the skewer or other hole-making tool, poke about six holes in the bottom of only one of your plastic cups. This will be your rainmaker.
- Find some grass (not from your grass tray!) and tear it up into smaller pieces.
- Sprinkle the grass over the top of one of the soil-covered lids.
- Position the egg carton lid filled with soil only so that the end that was not cut off rests on the rim of the pan. The whole lid should be slightly slanted because the side resting on the rim of the pan is elevated.
- Place a sheet of white paper on the other side of the baking dish. This will make it easier to see the color of the emerging water.
- Fill the non-holed cup with water.
- Position the cup with holes about 10 cm above the slanted pan.
- Transfer the water into the cup with holes.
- Move the cup with holes over the soil surface to mimic a rain storm.
- Carefully observe the color and amount of water emerging from the open end of the egg carton lid.
- Remove the soil only lid and replace it with the lid that has soil and grass clippings.
- Wipe up all water in the baking dish and replace your sheet of paper with a fresh one.
- Repeat steps 11-19 using the egg carton lid filled with dirt and grass clippings.
- Repeat steps 11-19 using the egg carton lid you grew your grass in.
The water emerging from the soil only is likely to have the most soil particles in it. It might look muddy, or have pieces of soil floating in it. The water emerging from the soil covered by grass clippings is likely to look clearer, with less soil in it. The water emerging from the grass planted lid is likely to have the smallest amount of visible soil. It is also likely that the water will take the longest to emerge from the grass-filled lid.
Since the grass clippings covered the soil, some of falling water flowed over the grass instead, reducing the amount of soil erosion. The lid planted with grass prevented erosion of soil even more because the fibrous roots, rhizomes, and stolons helped hold the soil in place while the water traveled over and through it. The grass plants themselves also provide an alternative surface for the water to travel over.
If desired, you could continue the experiment using another cup or two of rain. If this experiment reminded you of landslides, there’s a good reason for that—and from observing the results of your trials, you may have already figured out how landslides can be prevented!