Soil Sleuths

By Eve Pranis

The following time-honored activities can provide springboards for engaging students in exploring soils and how they "act."

If It Feels Good

Invite students to examine the texture of moist soils as scientists do. Set up stations with soil samples with very different textures: a sandy soil, a clay soil, and one with a lot of organic matter, for instance. Have students visit each station in small groups. Allow them to use a mister or squeeze bottle to moisten each sample until it is putty-like, then challenge them to try to form a ball with the soil. Next, ask them to press the samples between their index fingers and thumbs and try to form ribbons. Ask: How did the soils differ? Which soil would you want to grow a plant in and why? After discussing and sharing information on different types of soil particles, ask: Which seems to contain more clay? more sand? (The firmer and stickier the ball or longer the ribbon you can form, the more clay a soil contains. If a soil feels rough or gritty, and easily breaks apart or won't hold together in a ball, it contains sand. A loam will hold together at first when squeezed in your fist, but crumble apart when lightly touched.)

Cultivating Compost

No exploration of soils would be complete without examining how once-living materials decay and become part of the soil, releasing nutrients that other organisms need to survive. Consider creating small compost bags that students can observe in the classroom over time: Punch air holes into resealable plastic bags, then add moist soil and organic materials such as vegetable scraps (or other materials students want to test) before resealing each bag. Be sure to leave the soil out of one bag for comparison!

Projects that inspire kids to explore decomposition can range in size from small indoor "decomposiums" to a classroom worm bin or school-wide compost pile. Outdoor gardens present abundant opportunities to experiment with different ways of increasing organic matter and testing its effects on plant growth.

Once students have explored decomposition up close, they may want to compare qualities of soils from an area with lots of available organic matter, such as the woods, with soils in an area with little organic matter, such as a building site. They might also set up indoor or outdoor experiments to test the effects of worm- or human-made compost on soils and, ultimately, on plants.

Shake It Up

Invite students to further explore different soil components by creating "mudshakes" and watching components settle out. For each soil sample, have students fill a clear container about two-thirds full of water, then add enough soil to nearly fill the jar. Also add a pinch of alum (found at pharmacies) to help the soil components separate more markedly. Shake the jar vigorously, then observe the next couple of days as the particles begin to settle into layers. The larger particles (gravel and sand) will settle at the bottom, with progressively smaller particles above, as illustrated. The clay may stay suspended and cloud the water for a long time. Organic matter will float on or just below the water surface.

Have students describe the different layers. Ask: Are they consistent with your touch tests? Mark each jar or make a soil chart representing the width of each layer. Students can use their math skills to measure, then calculate the percentage of each particle size by measuring the height of each layer and dividing it into the height of the whole sample. Share that ideal garden soil contains 40 percent sand, 40 percent silt, and 20 percent clay and ask students which sample is closest to that ideal. Ask: Where did it come from? Which sample has the most organic matter? Does there seem to be a connection between the proportions of different-sized particles and where soils originated? How can you test your inferences?

Consider having students compare "mystery" soil samples. After revealing where samples came from, ask students to match the samples in the jars with locations on the list and then defend their choices (e.g., "this soil must be from the woods because it has a bigger layer of organic matter").

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