North America is blessed with a disproportionate share of the world's best agricultural soil. It is no coincidence that the U.S. is one of the few countries that’s a net exporter of food – North America has 17% of the world's arable land, but less than 7% of the world's population. As the human population approaches 10 billion people later this century, productive farmland will become an increasingly strategic resource as the amount per capita declines by as much as half.

Researching my book, Dirt: The Erosion of Civilizations, opened my eyes to how much humanity takes soil for granted – and how much trouble that has gotten society after society into in the past. A scant 3% of the world's total surface area has soil suitable for intensive cultivation.

And yet, how much do most of us know about where soil comes from? Our most essential and undervalued resource, soil is where the living world of biology meets the dead world of geology.

The world's best agricultural soils are those that developed along with the native forests (ultisols and alfisols) and grasslands (mollisols) that long covered continents in the temperate regions. In these locations the addition of organic matter from leaf fall in forests or root growth in grasslands built up fertile soils with high agricultural capacity. And the best of these soils for agriculture are those developed on deposits of loess, windblown silt such as that covering much of the American Midwest.

Central North America, eastern Europe, northern China and the Argentine Pampas are the backbone of the world's agricultural production. In all of these areas, loess from a few feet to hundreds of feet thick blankets continental bedrock. With a high proportion of finely ground, fresh mineral grains, soils developed on loess are renowned for their fertility. Loess is so fertile that it can be farmed productively even after the topsoil is eroded off. The same can't be said for soils in most of the world, where rock lies just one to several feet below ground. North America’s tremendous agricultural productivity reflects a disproportionate share of the world's loess.

But American agriculture’s foundation did not originate in place. Loess blew in on the wind. If you fly over northern Canada on a clear day you can see vast areas of naked rock, the geological scar of where it all came from. Over the past several million years, glaciers repeatedly overran most of modern Canada. As they grew and pushed south, the great ice sheets scraped off weak layers of soil and fractured, weathered rock, stripping the land down to fresh, hard rock. Bulldozed up by the ice, Canadian soils were carted to the melting front of the ice sheet and deposited as broad outwash plains built by rivers of meltwater. Strong winds then spread this fertile blanket of fine, glacier-ground silt across the American Midwest.

The windblown origin of loess helps explain why the 1930s’ Dust Bowl was so severe. Soil-forming material formerly anchored by native prairie roots was vulnerable to high winds blowing across plow-bared fields during the next severe drought.

The Dust Bowl produced dramatic soil loss and societal dislocation, but even ancient societies that mined their soil far more slowly were eventually reduced to destitution. Consider how images of modern Iraq don't exactly evoke the Garden of Eden.

As I looked back through history, I learned how the Greeks, Romans, Easter Islanders and many other societies declined as they lost their agricultural soils after cultivation practices exposed soil to accelerated erosion. Societies that plowed through their soil cashed in their future. In Syria, the foundations of Roman towns and farms standing on bare rock offer mute testimony to the former prosperity implied by Roman tax records that tell of bountiful harvests in areas now lacking soil.