Most people use the terms dirt and soil interchangeably. But soil scientists are not most people. To them, soil is special — because it knows its place. Or at least they know its place.
Dirt is different from soil, says J. Patrick Megonigal. As he explains it: “Dirt is displaced soil.” Megonigal is a research biogeochemist. He’s also deputy director of the Smithsonian Environmental Research Center in Edgewater, Md. And he knows a lot about soil. Indeed, in 2008 he was the curator of a major exhibit on soil at the Smithsonian National Museum of Natural History, in Washington, D.C.
In essence, dirt is what ends up on your shoe. Or it’s that muddy residue that the dog tracks into the house on a rainy day. Soil is the same stuff — except it’s still in the ground in your backyard, along a roadside or perhaps in a farm field.
As Megonigal describes it, soil is the mix of minerals, air, water, animals and other living matter (and their wastes or decaying bodies) that accumulate in layers. Season by season, layers of crumbly, carbon-rich material form and become compacted. Scientists refer to layers of soil that have roughly the same ingredients as “horizons.” And the composition of each horizon can vary dramatically over time and space.
When particles of that soil erode or are dug up, they lose the “history” of their place, Megonigal says. By this, he means they lose their association with particles that might have been above, below and to their sides.
So soil is the diverse but integrated community of living and nonliving things that make up the ground beneath our feet. It develops as rocks erode into tiny particles and living things rot into the minerals and other building blocks of their cells.
And dirt? It’s a group of runaways or kidnapped soil particles that can no longer be easily associated with the site in which they developed. In a sense, they’re soil-like particles that have been rendered anonymous.