How Is Dirt Made?

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Dirt is mostly made of bits and pieces of this rock, which is broken down into smaller and smaller pieces.
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“Ask a Science Teacher” by Larry Scheckel answers over 200 common questions about science.

Ask a Science Teacher(The Experiment, 2011) by Larry Scheckel is sure to resolve the everyday mysteries you’ve always wondered about. You’ll learn how planes really fly, why the Earth is round, how microwaves heat food, and much more — before you know it, all your friends will be asking you! This section answers how dirt is created.

Dirt is the thin layer of soil that covers our planet. In most places, it is just a few feet thick, because nearly all of the Earth is a big, hard, solid rock, with an inner liquid core.

Dirt is mostly made of bits and pieces of this rock, which is broken down into smaller and smaller pieces because of weathering and microorganisms breaking down plant matter. Moisture, temperature, wind, rain, freezing, and thawing are all part of the weathering process. Over hundreds of years, rocks break down into tiny grains, and these small grains, mixed with plant and animal matter — decayed roots, leaves, dead bugs and worms, and other organic matter thrown in, along with water and air — is what we call dirt or soil.

The type of rock determines the alkalinity and texture of the soil. Limestone produces soils that are fertile, neutral (not base or acid), and finely textured. We have a lot of limestone-based soils where I’m from in Wisconsin. Soft shale rock yields a heavy clay soil. Sandstone becomes a coarse, sandy soil. Granite gives a sandy loam and acidic soil.

Dirt that is dark and black has a lot of old plants in it. The dark soils of southern Minnesota are some of the richest on Earth. Dirt that is light-colored contains a lot of silicate, or sand. Sandy soils drain quickly and tend to need a lot of water to grow productive crops, which is why you often see irrigation systems on land with sandy soil.

Clay soils are composed of extremely fine minerals and flat particles that pack together tightly. Clay soils tend to be reddish, harden when dry, and drain poorly. They tend to feel sticky when wet. The southern states of Georgia and Alabama are examples of areas with clay-based soils.

On a lighter note, dirt is what you track in on your mother’s floor. Soil is what plants grow in. Dirt is what you get on your uniform sliding into second base. Soil is vital to the crops that feed people. Every state has selected a state soil, twenty of which have been established by their states’ legislatures. In my home state of Wisconsin, the 1983 legislature named Antigo silt loam as the official state soil. It is a well-drained soil suited for forests, dairy, and potatoes.

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Excerpted from Ask a Science Teacher: 250 Answers to Questions You’ve Always Had About How Everyday Stuff Really Works © Larry Scheckel, 2011, 2013. Reprinted by permission of the publisher, The Experiment. Available wherever books are sold.