Dirt: The Erosion of Civilizations
David R. Montgomery
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could be kept in continuous production without compromising soil fertility. But the population was still subject to the whims of the climate. A few bad years, or even a single disastrous one, could be catastrophic. Extended drought severely reduced crop yields; a peasant revolt during one from about 2250 to 1950 BC toppled the Old Kingdom. Still, the generally reliable Nile sustained a remarkably successful agricultural endeavor. Unlike in Mesopotamia, regulating the distribution of the river's
blazing tropical sun. Suddenly I sank in to my ankles, then my knees, before settling waist deep in hot sand. While my waders began steaming, my graduate students went for their cameras. After properly documenting my predicament, and then negotiating a bit, they pulled me from the mire. Few things can make you feel as helpless as when the earth gives way beneath your feet. The more you struggle, the deeper you sink. You're going down and there's nothing you can do about it. Even the loose
Peter…to be obtain'd in Plenty, we should need but little other Composts to meliorate our Ground.”4 Well ahead of his time, Evelyn anticipated the value of chemical fertilizers for propping up—and pumping up—agricultural production. By the start of the eighteenth century, improving farmland was seen as possible only through enclosing under private ownership enough pasture to keep livestock capable of fertilizing the plowed fields. Simply letting the family cow poop on the commons would not do.
nutrients and impurities to seeping water, leaving behind a deeply weathered iron crust. Aluminum and iron ore can form naturally through this slow weathering process. Over geologic time, the ample rainfall and hot temperatures of the tropics can concentrate aluminum and iron as chemical weathering leaches away almost everything else from the original rock. Although it may take a hundred million years, it is far more cost-effective to let geologic processes do the work than to industrially
frost and freezing. Soil thickens until it reaches a balance between soil erosion and the rate at which soil-forming processes transform fresh rock into new dirt. This time Darwin got it right. Soil is a dynamic system that responds to changes in the environment. If more soil is produced than erodes, the soil thickens. As Darwin envisioned, accumulating soil eventually reduces the rate at which new soil forms by burying fresh rock beyond the reach of soil-forming processes. Conversely, stripping