Topsoil Color Explained

On a recent road trip from Kansas to the Southwest, I couldn’t help but notice the changes in the color of the soil. When I went to sleep (someone else was driving) the ground was the comfortable, sandy brown of Western Kansas; when I woke up the landscape was similar, but the dirt was definitely red (leading me to comment, “We must be in Oklahoma”). As we continued southwest through Texas, New Mexico and Arizona, the red stayed with us, though joined by yellow and ochre (a color that I can identify only because it’s a Crayola crayon). Other places famed for red dirt are diverse: Hawaii, North Carolina, Prince Edward Island, India and Australia.

But what is it that causes the soil (my editor and FFA soil-judge father aren’t fans of the “dirt” word) to be red? The simple answer is the presence of iron, but the hows and whys are pretty interesting.

We tend to think of soil as being something that is added to from above. Leaves fall and add organic matter, we add fertilizer, we plant things. But the answer to the mystery of red dirt comes from below.

Sometime in grade school, no doubt, you had a unit on the layers of the soil, starting with the organic layer (or humus), progressing downward through creative names like topsoil and subsoil to the bedrock beneath. Soil forms when the rocks below the surface are broken down, and the color of the soil is affected by the type of rock that forms it.

The breakdown of the rock is accomplished through weathering. As the bedrock interacts with its environment, it gets broken down further and further. In physical weathering, heat, water, ice and pressure act on the rock to break it down. Water enters the cracks and freezes, causing it to break apart. Heat causes the outside of the rock to expand faster than the cooler interior, causing fractures.

But what we’re really interested in is chemical weathering, when water and various chemicals in the atmosphere, or from organic matter, interact with the rock to break it down. One kind of interaction is oxidation, when oxygen from the air or from water interacts with a compound. One of the most obvious examples of oxidation is rust, when the iron in an old car, chain or nail interacts with water and then turns red and crumbly. The same process happens to iron-rich rocks in the soil. Sometimes soil can be so filled with iron oxide that it coats other minerals in the soil and causes it all to look red.

If you look at soil samples from lots of places (http://soils.usda.gov/gallery/state_soils/), you’ll notice that red is a common color for soil layers, just as iron is a fairly common element. The Oklahoma state soil, called Port, is reddish all the way to the top. The bedrock below the surface is an iron-rich clay, making their dirt very red.

Soil color is affected by other things, too – organic matter makes a contribution – but that’s a subject for another day. In the meantime, I’ll be working on washing the burnt sienna mud off the white minivan we drove.

Published on Apr 7, 2009

Grit Magazine

Live The Good Life with GRIT!