By Janet Raloff

Web edition: July 17, 2008

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What Is This Stuff?

This must be dirt, since "soil" would appear to be something you couldn't hold in your hands.

Credit: L. Clarke/Corbis

A number of speakers at this morning’s media preview of the Smithsonian’s soils exhibit acknowledged how they initially didn’t know that soil and dirt were not synonymous. Of course, none went on to explain the difference either. So when the Q&A period opened, my hand shot up with the first question: “What IS the difference between dirt and soil?”

Elizabeth Duggal, associate director of the museum, demurred — and then turned to Pat Megonigal for the answer. A good choice since he’s the exhibit’s curator and one of some 6,000 dues-paying members of the Soil Science Society of America. In a nutshell, he explained that “Dirt is displaced soil.”

Uh, what? Like when it’s on my shoe it’s dirt, and when it’s on the ground it isn’t? This simplistic answer didn’t quite satisfy.

So when I was able to corner Megonigal, a half hour later, I asked for some clarification. As he described it, soil is the compilation of minerals, air, water, animals and other living matter (and their wastes or decaying bodies) that accumulate in layers and become compacted over time. Indeed, soils are laid down in discrete horizons (his name for those layers) and whose compositions vary over time and space.

When particles of that soil erode or are dug up, they lose the “history” of their place, he says — essentially their associations with particles that might have been above, below, and to their sides.

It sounds like he’s saying soil is the diverse but integrated community of living and inanimate things that make up the ground beneath our feet. And dirt? It’s a group of runaways or kidnapped individuals that can’t easily be associated with where they were born and grew up. In a sense, they’re particles that have been rendered anonymous.

As my toxins rant indicated, a few days ago, I think people should use words carefully and appropriately. If, however, the distinction is all but moot, let’s not get too silly about this. When I got back to my office, this afternoon, I did look up both terms in my trusty desk dictionary (a reporter’s best friend) and found one definition of dirt as “loose or packed soil or sand: EARTH.” And a definition for soil was “firm land: EARTH.”

If one of you agronomists or soil scientists out there cares to weigh in, please be my guest.