If you dig deep enough, you can see that soil is organized in layers. The top layer (topsoil) is dark and rich with decomposed plant material -- this is where most roots grow and where earthworms live. Below that is subsoil, which is lighter in color and contains more clay and minerals washed down from above. Beneath the subsoil is a layer of broken rock fragments. At the very bottom is solid bedrock -- the unbroken rock that everything sits on.
Show a cross-section diagram or photo of a soil profile with labeled layers. If possible, visit a spot where a hillside or road cut exposes the soil layers naturally. Create a soil layer model in a clear jar by layering different colored materials (dark compost on top, lighter clay, pebbles, a flat rock for bedrock). Discuss why each layer is different.
If you could slice the ground like cutting a cake and look at it from the side, you would see that soil is not the same all the way down. It is organized in layers, each one different from the one above and below it. Scientists call these layers horizons, and together they make up what is called a soil profile.
The top layer is called topsoil. It is usually the darkest layer because it is full of humus -- decomposed leaves, roots, dead insects, and other organic matter. Topsoil is where the action happens. Most plant roots grow here because it is packed with the nutrients plants need. Earthworms, beetles, fungi, and billions of bacteria live in this layer, constantly breaking down dead material and recycling nutrients. Topsoil is typically only a few centimeters to about 30 centimeters deep, depending on the location.
Below the topsoil is subsoil. This layer is lighter in color -- often tan, orange, or reddish -- because it contains less organic matter and more minerals. Water percolating down from the topsoil carries dissolved minerals into the subsoil, where they accumulate. This layer tends to have more clay. Plant roots can reach into the subsoil, but fewer organisms live here compared to the topsoil.
Below the subsoil is a layer of broken rock fragments -- pieces of rock that are in the process of breaking down but have not yet become fine soil particles. These fragments range from small pebbles to larger chunks. And at the very bottom is bedrock -- the solid, unbroken rock that forms Earth's crust. Bedrock is the original material from which the soil above it has slowly formed over hundreds or thousands of years. Every soil profile tells the same basic story: rock breaks down from the bottom up, organic matter builds from the top down, and the layers in between represent different stages of that process.
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