2. Past: What not to do with dirt

3. Present: On thin ground?

4. Future: Dig in

More than dirt: This handful includes nutrients, air, water, and a billion bacteria. Not to mention worms.. Photo: NOAA

This soil profile from the hills of Turkey shows a distinct layers (horizons), including the bottom rock bed, a red mineral layer, and a dark layer rich with organic material near the top. Photo: USGS.

POSTED 8 APR 2004

Why bother with soil?
Everyone wants clean air. We recycle to save energy and worry about pollution in the water supply. We fear the ever-rising global thermostat. But one fragile -- and fundamental -- piece of the environment is prone to invisibility.

Here's a hint: You're stepping on it.

Soil is as precious a resource as any, a panel of scientists reported earlier this year at the annual meeting of the American Association for the Advancement of Science. And, they said, it should be protected as such.

If you doubt the value of dirt, consider history, says Jared Diamond. The University of California geographer and author of Guns, Germs and Steel is known for his sweeping explanations of why, and how, civilizations fall. Societies are forged on soil, and often die when it does, Diamond says. He cites Easter Island, the Fertile Crescent of the Middle East and the Mayans of pre-Columbian Mexico and Guatemala as examples of once-thriving societies that collapsed when their soils were spent.

"Soil problems are rarely front page news because they are not spectacular. But they result in a steady decline in agricultural productivity, and serious environmental effects beyond the farm," Diamond says.

Damaged soil is an obvious loss for agriculture but it can also spoil the surrounding air and water. Heavily eroded soil (we'll get back to that later) leaches pollutants into groundwater, and the heavy machinery that carves the earth on many large farms releases great quantities of carbon dioxide into the air, aggravating the greenhouse effect.

The Green Revolution of the 1960s and 70s gave a big boost to crop yields and stamped out hunger in many parts of the world, but it relied on soil-squandering methods like tillage, heavy irrigation and chemical fertilizers to do it. Since then, farmers and scientists have been devising ways to maintain food production (or enhance it) without screwing up the soil.

It's no easy task. The earth's skin is thin -- notoriously slow to replenish, sensitive to chemicals and a changing climate, and easily damaged beyond repair. Despite the tools of modern agriculture and international conservation standards, the demand for food and living space still depletes soil faster than it can recover in many parts of the world.

So why, despite massive public information campaigns on air and water, is everyone mum on dirt? Turns out, not everyone is. (Here's an example: An astonishing web gadget that shows soils of the United Kingdom.) For this Why File, we talked to the people who are shouting loudest (and returned our phone calls).

But first things first
In case you aren't acquainted with the stuff you walk on, let's get introduced. Soil is more than dirt. Pinch an inch of it and you're cradling some one billion individual living organisms. Soil is a living ecosystem, made up of weathered rocks and minerals, decaying plants and animals, nutrients, roots, microbes and bugs, earthworms and -- critically -- pockets of air. A cross-section usually reveals distinct layers called horizons, each of which is composed of some mixture of rocks, organic matter and the four kinds of soil particles:

Clay: Particles are smaller than 0.002mm in diameter; is sticky when wet. Lots of clay makes a heavy, dense soil. The spaces between particles are tiny, and most plant roots can't push through it. Little or no air can get in. But clay is important because it can change the soil chemistry. Clays give off minerals and absorb acids.

Silt: Particle size between 0.05 and 0.002mm; soft like flour. Silt, the soil component that makes mud, is commonly found in floodplains. Silty soills make excellent farm land but erode easily. Silt is easily blown away in dust storms and carried down stream in floods.

Sand: 2 - 0.05mm; feels gritty. Sandy soils have big spaces between the particles, and typically don't hold water or nutrients. Plant roots can't hold onto this soil easily. But there are some plants that are able to grow in sandy topsoil by putting their roots deep, through sandy layers to the soil below.

Gravel: larger than 2mm; feels coarse. Useful in parking lots.

In a farmer's perfect world, all soil would have an equal mix of sand and silt, with a touch of clay. Large and small spaces in this type of soil -- known as loam -- allow air and water to seep in, and the clay sticks everything together with organic matter. The space between clumps provides passage for plant roots. Fields of loam are agricultural heaven.

The U.N. has categorized soil into 26 types and mapped it for all to see that soil is anything but simple Map: U.N. Food and Agriculture Organization.

Here on Earth, there are dozens of soil types, each with a different blend of talents and needs. Take the U.N. Soil Map of the World, for which scientists painstakingly mapped where the 26 types of soils show up. Multiply these differences by climate, the other big variable that determines what can grow where, and you have a lot of diversity. Accordingly, there are umpteen ways to work with dirt. We can't list them all here, but we can share the big ones, which enhance the services that good soils provide naturally:

SOILS:

offer nutrients, air and water to plant roots;

recycle carbon and nutrients from dead plants and animals, and animal scat;

buffer temperature change and changes in acidity;

and regulate the flow of rainfall into ground water and rivers.

What happens when agriculture gets it wrong?

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Terry Devitt, editor; Sarah Goforth, feature writer for this story, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

©2004, University of Wisconsin, Board of Regents.