Look up “dirt” in your dictionary, and you’ll find a list of unpleasant-sounding definitions. When we think of dirt, we usually think of something that’s filthy — “dirty.” Dirt is something your parents ask you to wash off your hands before you eat dinner. You are supposed to clean the dirt off your shoes before you come into the house. We use vast amounts of soap, water, and energy to get rid of dirt.
“Soil” is in a totally different league. It’s the top layer of Earth — the one in which plants can grow. This rich layer, called topsoil, can be less than a few centimeters thick, or it can extend down several meters. (See the Measurements page to convert metrics to U.S. equivalents.) This is where farmers plow to plant their crops, where larger animals dig and search for food. And it’s the place where millions of creatures live.
Soil is full of tiny tunnels, some far too small for us to see and some quite obvious. There are also little pockets of empty space, and patches of gluey substances that never appear on the surface. And in those places lives a vast population of creatures, most of them small. They represent all sorts of species, from field mouse to microscopic nematode to viruses.
All those organisms, alive and once-alive and about to be born, together with the ground-up rock and decomposing plant vegetation in which they live and the waste they excrete as they move through the soil, form a layer that is usually brown in color and that easily sticks to your shoes, hands, and clothes. Then we talk about being “dirty.” On this Web site we're going to show a little more respect for this stuff and call it soil.
