Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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of a high load of dissolved salt in irrigation water, high temperatures during
the irrigation season, and increasingly intensive cultivation pumped
ever more salt into the soil.
Temple records from the Sumerian city-states inadvertently recorded
agricultural deterioration as salt gradually poisoned the ground. Wheat,
one of the major Sumerian crops, is quite sensitive to the concentration of
salt in the soil. The earliest harvest records, dating from about 3000 bc,
report equal amounts of wheat and barley in the region. Over time the proportion
of wheat recorded in Sumerian harvests fell and the proportion of
barley rose. Around 2500 bc wheat accounted for less than a fifth of the
harvest. After another five hundred years wheat no longer grew in southern
Mesopotamia.
Wheat production ended not long after all the region’s arable land came
under production. Previously, Sumerians irrigated new land to offset
shrinking harvests from salty fields. Once there was no new land to cultivate,
Sumerian crop yields fell precipitously because increasing salinization
meant that each year fewer crops could be grown on the shrinking amount
of land that remained in production. By 2000 bc crop yields were down
by half. Clay tablets tell of the earth turning white in places as the rising
layer of salt reached the surface.
The decline of Sumerian civilization tracked the steady erosion of its
agriculture. Falling crop yields made it difficult to feed the army and maintain
the bureaucracy that allocated surplus food. As their armies deteriorated,
the independent city-states were assimilated by the younger Akkadian
empire from northern Mesopotamia at the time of the first serious
decline in crop yields around 2300 bc. During the next five hundred years
the region fell to a succession of conquerors. By 1800 bc crop yields were
down to a third of the initial yields and southern Mesopotamia declined
into an impoverished backwater of the Babylonian Empire. Salinization
that destroyed the Sumerian city-states spread northward, triggering an
agricultural collapse in central Mesopotamia between 1300 and 900 bc.
Mesopotamian agricultural practices also spread west into North Africa
along the Mediterranean coast and into Egypt. The valley of the Nile provides
a notable exception to the generality that civilizations prosper for
only a few dozen generations. The first farming settlements in the Nile
delta date from about 5000 bc. Farming and livestock herding gradually
replaced hunting and gathering as silt carried by the river began building
a broad, seasonally flooded, and exceptionally fertile delta once the postglacial
sea level’s rise slowed enough to let the silt pile up in one place. At
rivers of life