How Dirt Got That Way

'Every morning," says dirt expert Francis Hole, "you should get up and say, 'Thank you, soil, thank you, earth beneath my feet.' " Professor Hole is a retired pedologist (one who studies soil) from the University of Wisconsin in Madison. Why should we be grateful? Soil supports plant life, and plant life provides both food and oxygen for humans' use.

"Turn a rock into powdered material and you've got soil," Dr. Hole says. Rocks disintegrate into ever-smaller particles. Sand turns into silt, and silt becomes clay. (Yes, clay is actually tiny pieces of rock.)

You can see how a rocky surface "grows" soil in places like northern Canada. The ground is covered mostly with rocks. Still, small spruce and pine trees take root in places where the rock has crumbled enough to nurture a seed. The trees attract insects and migratory birds. The soil is enriched by decaying plants and animals.

Worms go to work. So do other, little-known subterranean creatures. Hole has spent nearly half a century studying soils, "and I don't know much about these animals," he says. "There are about a thousand species that live in soils."

A good place to study soil is where road crews have cut through the earth. That gives you a window on what's underground. Another good place is where a beach meets the land, leaving earthen cliffs. Or look in your own backyard. A shovelful may reveal color differences caused by various excavators. (When workers dug the hole for your basement, what happened to all that dirt?)

Making soil is the work of centuries. The color of soil may indicate how old it is.

Near the equator, the soil is often bright red, Hole notes. You get duller soil colors as you move away from the equator. The red color is the result of oxygen combining with iron particles in the soil. The soil is rusty, in other words, and rust is red. Rust means the soil has been exposed to the air longer than duller soils have. (Can you figure out why soil along the equator has been exposed to air longer than soil in, say, Michigan or Maine? Think about the glaciers of the ice age.) Iron-rich soil will redden, eventually.

On the other hand, when not much of the iron in soil has oxidized (rusted), it gives the soil a blue color. Soil that's been cut off from the air - maybe by being in a river or lake bed - may have a blue hue.