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Grainy: What Kinds of Particles Make up Soil?

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Author: Janice VanCleave

Problem

What kinds of particles make up soil?

Materials

  • Sheet of paper
  • 1 teaspoon
  • (5 ml) of dry soil (a sample can be collected from outside or from an indoor flower pot)
  • Magnifying lens

Procedure

Grainy

Grainy

  1. Place the sheet of typing paper on a table near a window with direct sunlight.
  2. Spread about ½ teaspoon (2.5 ml) of soil onto the paper.
  3. Observe the soil through the magnifying lens. Note the differences in the shapes, sizes, and colors of the soil particles. Determine which type of particle is most abundant in the sample.
  4. Rub the soil between your thumb and fingers. Observe how the soil feels as you rub it between your fingers.
  5. Allow the particles to fall onto the paper.
  6. Study the soil again through the magnifying lens.

Results

The description will vary depending on the type of soil. Generally the sample will contain small twigs; tiny, irregularly shaped stones (usually brown or grey); small, irregularly shaped, clear glassy pieces; tiny balls; and specks of brown dirt. The texture or feel of the soil changes as you rub it between your thumb and finger. At first, it feels smooth, but it changes to a grainier texture. The twigs, stones, and glassy pieces are surrounded by tiny specks of brown dirt.

Why?

The texture of the soil changes after being rubbed because the smooth dirt balls break up and fall away, leaving the hard stones and sand grains behind. Soil contains particles of various sizes, which determine the texture of the soil. The largest particles are stones and gravel; these are irregularly shaped pieces of rock and are larger than .08 inch (.2 cm) in diameter. The next smallest-sized particles are sand grains; these particles look like pieces of clear, rounded glass and are smaller than .08 inch (.2 cm).

Some of the particles have diameters less than .00016 inch (.0004 cm) and would require a powerful microscope in order to be seen. The amount of each kind of particle in a soil sample not only determines the name given to the soil, but how it behaves. An example is soil that is called sandy. Sandy soil contains at least 70 percent sand and does not hold water well because there are large spaces between the grains. Sandy soil is loose, allowing water to drain through rapidly.

Try It With A Microscope

Microscope Procedure

  1. Rub a soil sample between your thumb and finger, allowing the particles to fall onto a microscope slide.
  2. Place a coverslip over the particles and press gently to flatten them.
  3. Position the desk lamp so that the slide is brightly lit from above, and use the mirror or light attachment on the microscope to project light onto the slide from below.
  4. Move the slide around to study all areas.

Microscope Results

Some of the particles look transparent and others are opaque (materials that light does not pass through). The sizes and shapes of the particles vary.

Let's Explore

Repeat the original experiment using samples of a soil collected from different locations. Samples taken from house plants may contain small, round pieces of white plastic foam. Science Fair Hint: Collect soil samples from different locations while on a vacation or ask friends and relatives to mail you some. Use information learned about the different samples, their location, and their appearance under a magnifying lens and/or a microscope as part of a project display.

Show Time!

Use an earth science text to discover more about soil horizons, and the differences in soil color, texture, and composition in each layer. Collect samples of soil at different depths and study their appearances with magnifying instruments. Look for a place where a road has been cut into a hill or mountain, because this provides a good cutaway section of different soil horizons. Photographs of this roadside area, along with drawings of magnified views of the soil samples, can be displayed.

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