Dirt On Dirt

What’s in dirt anyway?

Admit it.

After sinking the shovel or trowel into the ground to plant something, you end up looking at the dirt that is pulled up and asking yourself, “Just what is in dirt that helps plants to grow?”

We’ve all asked the question at some point. So, to keep things really simple, the answer is that there is a lot of 'stuff' in dirt — including rocks, sand, clay and organic matter. The United States Environmental Protection Agency says the average soil sample is 45% minerals, 25% water, 25% air, and 5% organic matter.

Dirt, or soil, makes up the outermost layer of the planet and topsoil is the most productive soil layer. Topsoil is also a very thin layer. Five tons of topsoil spread over an acre of land is only as thick as a dime.

Soil changes depending on where you live with the weather playing a big role in determining the combinations of rocks, clays and sand in the area and what kinds of plants, insects, and animals live there.

The weather in an area breaks down solid rock. Freezing and thawing, rainy and dry spells, and even wind variables all impact how quickly rocks are broken down into smaller and smaller particles. Different sized mineral particles — such as sand, silt and clay — give soil its texture.

Organic matter is the source of most nutrients in the soil; and those nutrients are released during the decaying process of organic matter. Living creatures also play a part in making up the soil because animals such as moles and earthworms help aerate the soil. Plant roots also add to soil development, breaking up rocks that become part of the soil. In addition, those plant roots help aerate the soil as well as hold it together to prevent erosion.

Over the span of thousands of years, all of those factors work together to create the dirt your new plant is going to call home.

— Frank Graff

Frank Graff is a producer/reporter with UNC-TV, focusing on North Carolina Science Now, a weekly science series that airs Wednesdays, beginning in August 2013, as part of North Carolina Now on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog!

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