Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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Soil erosion rates from West African fields range from about threequarters
of an inch per century on savanna cropland to an extreme of more
than ten inches a year on bare-plowed fields in steep formerly forested areas.
Some estimates put average erosion rates for Sahel cropland at about an inch
per year. In many parts of West Africa the topsoil is only six to eight inches
thick. Cultivation after forest clearing quickly strips it off. Maize and cowpea
yields in southwestern Nigeria dropped by 30 to 90 percent with the
loss of less than five inches of topsoil. As the Nigerian population increased,
subsistence farmers moved to steeper land that could not support sustained
cultivation. Cassava plantations on land with slopes steeper than eight
degrees lose soil more than seventy times faster than fields sloping less than
a degree. Soil erosion rates on Nigerian hillslopes planted in cassava reach
over an inch a year, far beyond any conceivable replacement rate.
Social conventions hindered soil conservation. Subsistence farmers were
reluctant to invest in erosion control because they moved their fields every
few years. Erosion problems were most severe in areas where communal
land ownership discouraged individual efforts to conserve soil. In many
West African countries, tractor-hiring schemes are heavily subsidized, so
farmers get their fields plowed regardless of the steepness, soil type, or
cropping system. Soil erosion rates in sub-Saharan Africa increased twentyfold
in the past thirty years. The rapid soil erosion typical of West
African agriculture means that only a few years of cultivation are needed
to ruin the soil. This, in turn, fuels the drive to clear more land.
In the late 1970s University of Washington professor Tom Dunne and
two of his graduate students—one of whom was my graduate advisor—
compared recent and long-term erosion rates on the gentle slopes of semiarid
grazing lands in Kenya by using the height of dirt pedestals where vegetation
of known (or reasonably estimated) age still held the soil on
denuded slopes as well as the amount of incision into land surfaces of
known geologic ages. Remnant mounds of soil standing eight inches above
the general ground surface around the base of fifteen- to thirty-year-old
dwarf shrubs indicated modern erosion rates on the order of a quarter to
half an inch per year.
Dunne’s team determined that the average rate of erosion since the time
of the dinosaurs averaged about an inch every three thousand years. The
average erosion rate over the past several million years was about an inch
every nine hundred years, a little higher than their estimated rate of soil
formation of no more than about an inch every two thousand five hundred
dust blow