Explainer: What makes dirt different from soil | Science News for Students

Explainer: What makes dirt different from soil

Dirt is kidnapped soil that has lost its history
Nov 16, 2017 — 6:30 am EST
hands holding dirt

In ordinary soil, scientists can find clues to past civilizations, evidence to catch criminals and bacteria that might help fight global warming.

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If you reached into a bog’s soil and extracted a handful, this is what it would look like. It’s a rich source of nutrients. But once in someone’s hands, this “soil” gets a new name.

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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.

Power Words

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carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

curator     Someone who manages a collection of items, for instance in a museum, library or art gallery. This person’s primary job is to design exhibits, organize and acquire collections and do research on the artifacts included in the collection.

erode     Gradual removal of soil or stone, caused by the flow of water or the movement of winds.

matter     Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."

mineral     Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.

particle     A minute amount of something.

waste     Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.