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Unless you've spent a lot of time underground like we do, you might not think very often about dirt. But dirt is awesome! It's everywhere. And just about every living thing on Earth depends on it. From the plants that grow in it to the animals that eat the plants, to the animals that eat those animals, that's why we're really glad that one of our viewers, 4 year old Caleb from Mississippi, wrote to ask us: What is dirt? Great question!

First of all, the stuff that fills our flower pots and baseball fields may just be dirt to us, but scientists have a different name for it. They call it top layer that covers most of the land, soil. So what is soil? Well it's a mixture of things. How soil looks and feels depends on where you are. So some very dry places have a lot of gritty sand in the soil. While other parts of the world might have hard rocks, soft clay, or even pieces of seashell in it. Usually though, soil is made of 4 main things.

Mostly soil's made up of minerals. Minerals are hard stony substances that form things like rocks, sand, and clay. The next big part of soil is water. That's right! Even if a clump of dirt doesn't seem like mud to you, it actually has water in it. All living things need water to survive and soil does a great job of holding on to water for plants, animals, and other things that live in it. And you know what else is in the soil that you probably can't tell is there? Air! Just like water, air in the soil is important because living things need air too. It fills the spaces between all those little bits of minerals, water, and other parts of the soil. And the beauty of it is, the animals that live in the soil actually help more water and air get inside it. 

Creatures like worms and insects and even small mammals like moles and mice dig tunnels down in the dirt. Those tunnels let air and water trickle in and flow around so that even deep down the soil has what living things need to make a nice home there. 

And this my friends brings us to probably the most important part of soil. In fact, it's some of the most important stuff on Earth. Scientists call it organic matter. Organic matter includes anything that's alive. Like those worms and insects, but also parts of plants and living things that we can't even see, like bacteria. Plus organic matter also includes things that used to be alive, like dead plants and animals. After those things die, other living things like fungi, bacteria, insects, and worms, eat what's left of those plants and animals, and break them down into very small pieces. This broken down material puts nutrients back into the soil. And plants then use those nutrients to grow. So soil is where plants get their food Scientists call this special, nutrient-rich part of the soil by its own name, humus. 

Humus is what gives soil its dark brown color. And it even sometimes contains special bacteria that makes soil smell, well, dirty. Now all of these things are mixed together to make soil. But the mixture isn't always the same. Scientists have found  that soil has many layers to it. 

The top layer is all made up of organic matter, with lots of that good useful humus. Beneath that is the layer that we often call topsoil. Topsoil is made mostly of humus with a little bit of minerals sprinkled in. This layer is where seeds start to grow. As you go further down away from the surface, you find that the deeper layers of soil contain more minerals than humus. And once you get through the bottom layer, you run into a layer of solid rock. So now you have the dirt on soil. And you know it's made up of minerals, water, air, and organic matter. 

Thank you for asking, Caleb. Does anybody else have a question? Let us know by either leaving a comment or emailing us at kids@thescishow.com. Thanks guys and see you next time on Sci Show Kids. 

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