Soil Science: How Moist is that Mud?
Have you ever taken a step onto what appeared to be dry ground, only to find yourself ankle-deep in mud? Yuck! 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 the soil can help you determine how dry or wet it is.
The amount of water present in a sample of soil is called the moisture content. Moisture is very important and the amount of moisture needs to be in careful balance, not too dry and not too wet, for organisms to thrive in it. Specifically, the moisture content of the soil needs to match the needs of the plants, animals and other organisms living in the habitat. Some organisms need a lot of moisture, like ferns and salamanders. Others, like cacti and snakes, are adapted to desert habitats and need very little water. Most often, evenly moist soil is a haven for plants and small, soil-dwelling animals.
Moisture conditions affect the soil structure in many ways too. Soil that is too wet or does not drain properly can suffer from erosion. Soil that is too dry can become hard and compacted. Also, different types of soil respond to moisture differently. A sandy soil will drain water quickly, but a clay soil will prevent water drainage and become soggy.
- One cup of dry soil; from your backyard, another outdoor location, or a plant nursery. If the soil is not dry, dry it using an oven, small baking dish and 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, dry it with an adult’s help by pouring the soil into a small baking dish, placing the dish on a cookie sheet, and placing the cookie sheet (with dish) in the oven and baking it at low heat (200 degrees Fahrenheit) for two to three hours. This will evaporate all of the water from your soil. When it is done, remove the soil from the oven and allow it to cool completely.
- Place two tablespoons of soil into each of the seven cups or glasses.
- Line the cups up in a row on a desk, counter or table.
- To the far left cup, do not add any water. To the next cup on the right, add half a teaspoon of water to the soil. To the next cup on the right, add one teaspoon of water. To the next cup on the right, add one and a half teaspoons of water. Continue adding half a teaspoon of extra water to the cups 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 with the increasing amounts of water being added?
- Now mix the water in with the soil in each cup. (Skip the far left cup, which has no water added.) 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 soil is darkest? Do you see a correlation between how much water you added and how light or dark the soil is?
- 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 grayscale, and comparing it to a grayscale 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 and compare them to each other 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 to cool days, and cloudy days compared to sunny ones? 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?
The color of soil can vary depending on the type of soil 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 make up the soil, specifically what minerals and proteins (i.e., organic matter) it contains. When water is added to soil, the water can replace the oxygen that is naturally in the soil, and make the soil darker. This means that wetter soil will have less oxygen compared to drier soil. When enough water is added, the soil can become saturated and the water will start to form a layer on top of the soil. 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. However, if soil is too wet, it will not have enough oxygen in it for some organisms to survive in it either.
Ask an Expert
- Soil Color and Moisture, from Science Buddies
- Science Activities for All Ages!, from Science Buddies