The first Christian to write in Latin, Quintus Septimius Florens Tertul- lianus (known today simply as Tertullian), lived in Carthage around AD 200. In describing the end of the Roman frontier in North Africa he warned of overtaxing the environment. "All places are now accessible, well known, open to commerce. Delightful farms have now blotted out every trace of the dreadful wastes; cultivated fields have overcome woods.... We overcrowd the world. The elements can hardly support us. Our wants increase and our demands are keener, while Nature cannot bear US."
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Remnants of hillslope soils and archaeological evidence show that since the Bronze Age there have been centuries-long periods with high settlement density, intensive farming, and accelerated soil erosion separated by millennia-long periods of low population density and soil formation... Regional climate changes cannot explain the boom-and-bust pattern of human occupation in ancient Greece because the timing of land settlement and soil erosion differed around the region. Instead, modern geoarchaeological surveys show that soil erosion episodically disrupted local cultures, forced settlements to relocate, led to changes in agricultural practices, and caused periodic abandonment of entire areas.
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Lowdermilk estimated than a foot of topsoil had been lost from hundreds of millions of acres of northern China. He found exceptions where Buddhist temples protected forests from clearing and cultivation; there the exceptionally fertile forest soil was deep black, rich in humus. Lowdermilk described how farmers were clearing the remaining unprotected forest to farm this rich dirt, breaking up sloping ground with mattocks to disrupt tree roots and allow plowing. At first, plowing smoothed over new rills and gullies, but every few years erosion pushed farmers farther into the forest in search of fresh soil. Seeing how colonizing herbs and shrubs shielded the ground as soon as fields were abandoned, Lowdermilk blamed the loss of the soil on intensive plowing followed by overgrazing. He concluded that the region's inhabitants were responsible for impoverishing themselvesjust too slowly for them to notice.
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Comparison of ancient agricultural soils and uncultivated soils in New Mexico and Peru shows that agricultural practices need not undermine societies. Soils at a site in the Gila National Forest, typical of prehistoric agricultural sites in the American Southwest, were cultivated between AD 1100 and 1150, at the peak of Pueblo culture, and subsequently abandoned. Soils of sites cultivated by the Pueblo culture are lighter colored, with a third to a half of the carbon, nitrogen, and phosphorus content of neighboring uncultivated soils. In addition, cultivated plots had gullies-some more than three feet deep-that began during cultivation. Even today, little grass grows on the ancient farm plots. Native vegetation cannot recolonize the degraded soil even eight centuries after cultivation ceased. In contrast, modern farmers in Peru's Colca Valley still use ancient terraces cultivated for more than fifteen centuries. Like their ancestors, they maintain soil fertility through intercropping, crop rotations that include legumes, fallowing, and the use of both manure and ash to maintain soil fertility. They have an extensive homegrown system of soil classification and do not till the soil before planting; instead they insert seeds into the ground using a chisel-like device that minimally disturbs the soil. These long-cultivated soils have A horizons that are typically one to four feet thicker than those of neighboring uncultivated soils. The cultivated Peruvian soils are full of earthworms and have higher concentrations of carbon, nitrogen, and phosphorus than native soils. In contrast to the New Mexican example, under traditional soil management these Peruvian soils have fed people for more than fifteen hundred years. The contrast between how the Pueblos and the Incas treated their dirt is but another chapter in the broader story of how the rise of agriculture set off a perpetual race to figure out how to feed growing populations by continuously increasing crop yields. Sometimes cultures figured out a way to muddle through without depleting soil productivity, often they did not.
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The valley of the Nile provides a notable exception to the generality that civilizations prosper for only a few dozen generations... The floodplain of the Nile proved ideal for sustained agriculture. In contrast to Sumerian agriculture's vulnerability to salinization, Egyptian agriculture fed a succession of civilizations for seven thousand years, from the ancient pharaohs through the Roman Empire and into the Arab era. The difference was that the Nile's life-giving flood reliably brought little salt and a lot of fresh silt to fields along the river each year... The longevity of Egyptian agriculture reflects a system that took advantage of the natural flood regime with minimal modification... Egyptian agriculture remained remarkably productive for thousands of years until people adopted new approaches out of tune with the river's natural rhythm. Desire to grow cotton for export to Europe brought aggressive year-round irrigation to the Nile in the early nineteenth century. Just as in the scenario that unfolded thousands of years earlier in Mesopotamia, salt began to build up in the soil as the water table rose below overly irrigated fields.In the past half century, civilization finally acquired the engineering skill to cripple an almost indestructible land. After four years of work, Egyptian president Gamal Abdel Nasser and Soviet premier Nikita Khrushchev watched Soviet engineers divert the Nile in May 1964 to build the Aswan High Dam... After advancing for thousands of years since sea level stabilized, the Nile delta is now eroding, cut off from a supply of silt. Although the dam allows farmers to grow two or three crops a year using artificial irrigation, the water now delivers salt instead of silt. A decade ago salinization had already reduced crop yields from a tenth of the fields on the Nile delta. Taming the Nile disrupted the most stable agricultural environment on Earth... As the renowned fertility of the Nile valley began to fall, agricultural output was sustained with chemical fertilizers that peasant farmers could not afford. Modern farmers along the Nile are some of the world's foremost users of chemical fertilizers-conveniently produced in new factories that are among the largest users of power generated by Nasser's dam. Now, for the first time in seven thousand years, Egypt-home of humanity's most durable garden-imports most of its food.
Dirt: The Erosion of Civilizations (David R. Montgomery)
