Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

Figure 5. Ancient Egyptian plow (Whitney 1925).
first Egyptian farmers simply cast seeds into the mud as the annual flood
receded, harvesting twice the amount of grain used for seed. Thousands of
people died when the water drained too quickly and crops failed. So farmers
started impounding water behind dikes, forcing it to sink into the rich
earth. As the population grew, innovations like canals and water wheels
irrigated land higher and farther from the river, allowing more people to
be fed.
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 geography of the river’s two great tributaries mixed up the ideal formula
to nourish crops. Each year the Blue Nile brought a twentieth of an
inch (about a millimeter) of silt eroded from the Abyssinian highlands.
The White Nile brought humus from central Africa’s swampy jungles.
Fresh silt replaced mineral nutrients used by the previous crop and the
influx of humus refreshed soil organic matter that decayed rapidly under
the desert sun. In addition, the heavy rains that fell during June in the
uplands to the south produced a flood that reliably reached the lower Nile
in September and subsided in November, just the right time for planting
crops. The combination produced abundant harvests year after year.
Egyptian irrigation exploited a natural process through which overflow
channels spread floodwaters across the valley. Irrigating fields did not
require elaborate canals; instead the river’s natural levees were breached to
rivers of life 41