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Soil Horizon

based on 3 ratings
Author: Tricia Edgar
Topics: Fifth Grade, Ecology

Soil is made up of different layers. How far down do you need to dig to find the soil horizon, or the place where bits of leaves and branches become soil?

Problem:

How deep do you need to go to find the place where organic material starts to turn into soil?

Materials

  • Trowel
  • Shovel
  • Flashlight
  • Ruler
  • Notebook and pencil

Procedure

  1. Choose your environment. You might have a garden or a park nearby, or perhaps you live near a wild place such as a forest.
  2. Create a hypothesis, your best guess as to what is going to happen. How deep will the leaves and sticks go before you reach more consistent soil?
  3. Now, dig! If the soil is easy to dig in, you can take a big shovelful out. Try to do this cleanly, without digging a lot and mixing the layers of soil. Push the shovel down, pull it out with the soil, and then look into the hole. If it’s hard to get a shovel in, you can use a trowel.
  4. Take a look into the hole you’ve made. You may want to use a flashlight, because it can make it easier to look closely at the side of the hole.
  5. Find the place where you stop seeing chunks of organic material, where the soil looks more consistent. Use your ruler to measure how deep this is. Record this in your notebook.
  6. At what depth does the organic material start to become soil?
  7. If you wish, you can go to a different location and try the same experiment.  Is the other place different? The same?

Results 

Generally, the horizon that contains organic material is about two inches deep, but this depends on where you’re doing the experiment.

Why?

Even though it looks like a big bunch of brown stuff on the ground, soil is actually quite organized. It naturally forms horizons, or layers of soil.

If you’re in a forest, a garden, or other natural system that’s not a dry desert, you’ll generally see organic material like leaves on the ground. This is future soil. Over time, bugs munch the leaves and other materials, fungi decompose them, and microbes do the rest, composting this organic matter into a layer of rich soil.

Generally, this layer of organic material—the O horizon—is about two inches deep. However, this changes from place to place and from season to season. Imagine a garden with big trees in the fall. When all of those leaves fall down, that’s a lot of organic material!

Under the organic matter is the A horizon, which is the surface soil. This is a little different from the organic material, because it’s full of minerals as well. The A horizon looks less chunky and more like what you’d find if you took a shovel and dug into the ground.

How deep was the O horizon in your study area? How do you think that this would change from one season to the next? Do the same experiment in a different environment. How are your results different?

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