Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

president Gamal Abdel Nasser and Soviet premier Nikita Khrushchev
watched Soviet engineers divert the Nile in May 1964 to build the Aswan
High Dam. Two and a half miles across, and more than seventeen times as
massive as the Great Pyramid, the dam impounds a lake 300 miles long
and 35 miles wide that can hold twice the river’s annual flow.
The British hydrologists who controlled Egypt’s river until the 1952 coup
that brought Nasser to power opposed building the dam because evaporation
would send too much of the huge new lake back into the sky. Their
fears were well founded. Under the desert sun six feet of water evaporates
off the top of the lake each year—more than fourteen cubic kilometers of
water that used to head down the river. But a greater problem was that the
130 million tons of dirt that the Nile carried off from Ethiopia settled out
at the bottom of Lake Nasser.
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. Still, the remarkable
longevity of Egyptian civilization is a primary exception to the general riseand-fall
of ancient civilizations.
The history of Chinese agriculture provides another example where, as
in Mesopotamia, dryland farmers from the uplands moved down onto
floodplains as the population exploded. Unlike the Sumerians who appear
to have treated all soils the same, the Yao dynasty (2357–2261 bc) based taxation
on a soil survey that recognized nine distinct types of dirt. A later soil
classification, dating from 500 bc, codified older ideas based on soil color,
texture, moisture, and fertility.
Today, the Chinese people overwhelmingly live on the alluvial plains
where great rivers descending from the Tibetan Plateau deposit much of
their load of silt. Flooding has been a problem for thousands of years on the
Huanghe, better known in the West as the Yellow River, a name imparted
rivers of life 43