most of recorded history, soil occupied a central place in human cultures. Some of the earliest books were agricultural manuals that passed on knowledge of soils and farming methods. The first of Aristotle's fundamental elements of earth, air, fire, and water, soil is the root of our existence, essential to life on earth. But we treat it as a cheap industrial commodity. Oil is what most of us think of as a strategic material. Yet soil is every bit as important in a longer time frame. Still, who ever thinks about dirt as a strategic resource? In our accelerated modern lives it is easy to forget that fertile soil still provides the foundation for supporting large concentrations of people on our planet.
Geography controls many of the causes of and the problems created by soil erosion. In some regions farming without regard for soil conservation rapidly leads to crippling soil loss. Other regions have quite a supply of fresh dirt to plow through. Few places produce soil fast enough to sustain industrial agriculture over human time scales, let alone over geologic time. Considered globally, we are slowly running out of dirt.
Should we be shocked that we are skinning our planet? Perhaps, but the evidence is everywhere. We see it in brown streams bleeding off construction sites and in sediment-choked rivers downstream from clear-cut forests. We see it where farmers' tractors detour around gullies, where mountain bikes jump deep ruts carved into dirt roads, and where new suburbs and strip malls pave fertile valleys. This problem is no secret. Soil is our most underappreciated, least valued, and yet essential natural resource.
Personally, I'm more interested in asking what it would take to sustain a civilization than in cataloging how various misfortunes can bring down societies. But as a geologist, I know we can read the record previous societies left inscribed in their soils to help determine whether a sustainable society is even possible.
Historians blame many culprits for the demise of once flourishing cultures: disease, deforestation, and climate change to name a few. While each of these factors played varying—and sometimes dominant—roles in different cases, historians and archaeologists rightly tend to dismiss single-bullet theories for the collapse of civilizations. Today's explanations invoke the interplay among economic, environmental, and cultural forces specific to particular regions and points in history. But any society's relationship to its land—how people treat the dirt beneath their feet—is fundamental, literally. Time and again, social and political conflicts undermined societies once there were more people to feed than the land could support. The history of dirt suggests that how people treat their soil can impose a life span on civilizations.
Given that the state of the soil determines what can be grown for how long, preserving the basis for the wealth of future generations requires intergenerational land stewardship. So far, however, few human societies have produced cultures founded on sustaining the soil, even though most discovered ways to enhance soil fertility. Many exhausted their land at a rate commensurate with their level of technological sophistication. We now have the capacity to outpace them. But we also know how not to repeat their example.
Despite substantial progress in soil conservation, the United States Department of Agriculture estimates that millions of tons of topsoil are eroded annually from farmers' fields in the Mississippi River basin. Every second, North America's largest river carries another dump truck's load of topsoil to the Caribbean. Each year, America's farms shed enough soil to fill a pickup truck for every family in the country. This is a phenomenal amount of dirt. But the United States is not the biggest waster of this critical resource. An estimated twenty-four billion tons of soil are lost annually around the world—several tons for each person on the planet. Despite such global losses, soil erodes slowly enough to go largely unnoticed in anyone's lifetime.
Even so, the human cost of soil exhaustion is readily apparent in the history of regions that long ago committed ecological suicide. Legacies of ancient soil degradation continue to consign whole regions to the crushing poverty that comes from wasted land. Consider how the televised images of the sandblasted terrain of modern Iraq just don't square with our notion of the region as the cradle of civilization. Environmental refugees, driven from their homes by the need to find food or productive land on which to grow it, have made headlines for decades. Even when faced with the mute testimony of ruined land, people typically remain unconvinced of the urgent need to conserve dirt. Yet the thin veneer of behavior that defines culture, and even civilization itself, is at risk when people run low on food.
For those of us in developed countries, a quick trip to the grocery store will allay fears of any immediate crisis. Two technological innovations—manipulation of crop genetics and maintenance of soil fertility by chemical fertilizers—made wheat, rice, maize, and barley the dominant plants on earth. These four once-rare plants now grow in giant single-species stands that cover more than half a billion hectares—twice the entire forested area of the United States, including Alaska. But how secure is the foundation of modern industrial agriculture?
Farmers, politicians, and environmental historians have used the term soil exhaustion to describe a wide range of circumstances. Technically, the concept refers to the end state following progressive reduction of crop yields when cultivated land no longer supports an adequate harvest. What defines an adequate harvest could span a wide range of conditions, from the extreme where land can no longer support subsistence farming to where it is simply more profitable to clear new fields instead of working old ones. Consequently, soil exhaustion must be interpreted in the context of social factors, economics, and the availability of new land.
Various social, cultural, and economic forces affect how members of a society treat the land, and how people live on the land, in turn, affects societies. Cultivating a field year after year without effective soil conservation is like running a factory at full tilt without investing in either maintenance or repairs. Good management can improve agricultural soils just as surely as bad management can destroy them. Soil is an intergenerational resource, natural capital that can be used conservatively or squandered. With just a couple feet of soil standing between prosperity and desolation, civilizations that plow through their soil vanish.
As a geomorphologist, I study how topography evolves and how landscapes change through geologic time. My training and experience have taught me to see how the interplay among climate, vegetation, geology, and topography influences soil composition and thickness, thereby establishing the productivity of the land. Understanding how human actions affect the soil is fundamental to sustaining agricultural systems, as well as understanding how we influence our environment and the biological productivity of all terrestrial life. As I've traveled the world studying landscapes and how they evolve, I've come to appreciate the role that a healthy respect for dirt might play in shaping humanity's future.
Viewed broadly, civilizations come and go—they rise, thrive for a while, and fall. Some then eventually rise again. Of course, war, politics, deforestation, and climate change contributed to the societal collapses that punctuate human history. Yet why would so many unrelated civilizations like the Greeks, Romans, and Mayans all last about a thousand years?
Clearly, the reasons behind the development and decline of any particular civilization are complex. While environmental degradation alone did not trigger the outright collapse of these civilizations, the history of their dirt set the stage upon which economics, climate extremes, and war influenced their fate. Rome didn't so much collapse as it crumbled, wearing away as erosion sapped the productivity of its homeland.
In a broad sense, the history of many civilizations follows a common story line. Initially, agriculture in fertile valley bottoms allowed populations to grow to the point where they came to rely on farming sloping land. Geologically rapid erosion of hillslope soils followed when vegetation clearing and sustained tilling exposed bare soil to rainfall and runoff. During subsequent centuries, nutrient depletion or soil loss from increasingly intensive farming stressed local populations as crop yields declined and new land was unavailable. Eventually, soil degradation translated into inadequate agricultural capacity to support a burgeoning population, predisposing whole civilizations to failure. That a similar script appears
