Where does soil come from?

Time: 20 minutes
Best location: indoors
  • 2 pieces of limestone or sandstone
  • paper
  • heat source (hot plate)
  • ice water
  • small glass jar
  • vinegar
The teacher/leader can begin by discussing how soil is formed:
  • Rocks rubbing together very slowly over thousands of years.
  • Changes in temperature. The expansion and contraction of rocks due to freezing and heating chips rocks. Freezing water expands with tremendous force. Water that finds its way into small cracks freezes and breaks the rocks into even smaller pieces.
  • Plant acids. A plant's roots take in oxygen and give off carbon dioxide. This gas is one of the important parts of decaying organic materials. It dissolves in soil moisture, forming weak acids. These acids will decompose rock. When you put limestone into vinegar, you are duplicating this process.

The teacher/leader then proceeds with the following experiments and discuss the results with the children.

  1. Rub two pieces of limestone or fine sandstone together, and collect the small particles of debris on a sheet of paper. If natural stone is not available, pieces of building brick or concrete will do. Note how long it took to rub off the particles.
  2. Heat a piece of limestone over a flame or on a hot plate. Drop it quickly into a pan of ice water. What happened to the rock? Did it break or crack? Why?
  3. Fill a small glass jar with water and put a tight lid on it. Let it freeze in a freezer. Talk about what happened to the jar.
  4. Put some vinegar in a small pan. Add a piece of limestone, and heat the vinegar on a hot plate or stove. Did bubbles form on the rock? Why?