Have you ever taken a step onto what appeared to be dry ground, only to find yourself ankle deep in mud? Yikes! When you walk through damp soil, it can be a very messy experience. How can you tell if soil is wet or dry before you step on it? In this science activity you will investigate whether the color of soil can help you determine how dry or wet it is.
The amount of water present in a soil sample is called the moisture content. Moisture is very important and its amount needs to be in careful balance—not too dry and not too wet—for organisms to thrive. Specifically, the soil’s moisture content needs to match the needs of the plants, animals and other organisms living in the habitat. Some organisms, such as ferns and salamanders, need a lot of moisture. Others, such as cacti and some snakes, are adapted to desert habitats and need very little water. Most often soil with evenly balanced moisture is a haven for plants and small, soil-dwelling animals.
Moisture conditions affect soil structure in many ways, too. Soil that is too wet or does not drain properly can suffer from erosion. That which is too dry can become hard and compacted. Additionally, different types of soil respond to moisture differently. Sandy soil will drain water quickly but clay soil will prevent water drainage and become soggy.
- One cup of dry soil (Gather this with permission either from your backyard or another outdoor location or from a plant nursery or garden center. If the soil is moist, dry it using an oven, a small baking dish and a cookie sheet following the steps given in the Preparation section.)
- Seven identical cups or glasses
- Desk, counter or table
- Measuring spoons
- Sheet of paper
- If your soil is not completely dry, you will have to remove the moisture. With an adult's help, pour the soil into a small baking dish, place it on a cookie sheet then put the cookie sheet (with dish) in the oven and bake it at low heat (200 degrees Fahrenheit) for two to three hours. This will evaporate the water from your soil. When the soil is dry, carefully remove the cookie sheet and dish from the oven and allow the soil to cool completely.
- Place two tablespoons of soil into each of the seven cups or glasses.
- Line up the cups in a row on a desk, counter or table.
- To the far left cup, do not add any water. Into the next cup to its right, add half a teaspoon of water to the soil. To the next cup on the right of that, add one teaspoon of water. And to the next one on the right, add one and a half teaspoons of water. Continue increasing by half a teaspoon the amount of water you add to each cup as you move to the right; you should end up adding three teaspoons of water to the far right cup. How does the soil in each cup look as increasing amounts of water are added?
- Now mix the water in with the soil in each cup that includes water. How do the different cups look as you mix in the water?
- Place all of the mixed cups on a white sheet of paper. (Placing them on a solid white background will help you compare the colors of the soil in each cup.) Look straight down at the soil in each cup. Which soil is lightest? Which is darkest? Do you see a correlation between how much water you added and how light or dark the soil is?
- Now take a tablespoon of each soil sample—starting with the dry soil and working toward the one with the most water—and place the sample on a clean area of the paper. Feel the soil with your fingers. How does each feel? What textures would you expect from each?
- Do you think you could use your results to determine the moisture level of other soil samples?
- Extra: You could try to quantify the results from this activity by taking a picture of the damp soil in all of your cups, printing the picture out in gray scale and comparing it with a gray-scale color bar (one that has the different amounts of black or white labeled with percentages). When you quantify your results, just how different are the soil samples? Are some much darker or lighter than others?
- Extra: In this science activity you only tested one type of soil. Try sampling different areas of your yard or another location (with permission) and, without drying them, compare these new samples both with one another and the soil in the original series of cups. Can you use your original soil series to estimate the moisture content of soil from different places in your yard?
- Extra: Try estimating the moisture of soil that is in the same location but on different days. Does soil moisture change with the weather, such as on hot days compared with cool ones and cloudy days compared with those that are sunny? How much does the moisture vary from day to day?
Observations and results
Did you see that the soil became darker as you added water to it?
Soil color can vary depending on the type that is used but, in general, dry soil becomes darker in color when water is added to it. The color of the dry soil depends on what little particles comprise it—specifically what minerals and proteins (organic matter) it contains. When water is added to soil, it can replace the oxygen that is naturally in the soil, and make the soil darker. This means wetter soil will have less oxygen compared with drier soil. When enough water is added, soil can become saturated and the water will start to form a layer on top of it. You may have seen this happen in the cups with the most water added to them. As mentioned earlier, different organisms need soils with different moisture contents. If the soil is too dry, plants, animals and microorganisms that need more water will not survive in it, and the soil may also become hard and compacted. If soil is too wet, however, it will not contain enough oxygen for some organisms to survive in it either.
More to explore
Comparing Earth and Mars; Classifying Soils, from Kids Do Science, The University of Georgia
Soil Color and Moisture, from Science Buddies
Soil Moisture and Evaporation Modeling, from the U.S. Geological Society
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies