few million acres of land between the Tigris and Euphrates rivers fed a succession of civilizations as the rich valley turned one conquering horde after another into farmers. Empires changed hands time and again, but unlike soils on the mountain slopes where agriculture began, the rich floodplain soil did not wash away when cleared and planted. Coalescence of Sumerian cities into the Babylonian Empire about 1800 BC represented the pinnacle of Mesopotamian organizational development and power. This merger solidified a hierarchical civilization with formalized distinctions recognizing legal classes of nobility, priests, peasants, and slaves.
But the irrigation that nourished Mesopotamian fields carried a hidden risk. Groundwater in semiarid regions usually contains a lot of dissolved salt. Where the water table is near the ground surface, as it is in river valleys and deltas, capillary action moves groundwater up into the soil to evaporate, leaving the salt behind in the ground. When evaporation rates are high, sustained irrigation can generate enough salt to eventually poison crops. While irrigation dramatically increases agricultural output, turning sun-baked floodplains into lush fields can sacrifice long-term crop yields for short-term harvests.
Preventing the buildup of salt in semiarid soils requires either irrigating in moderation, or periodically leaving fields fallow. In Mesopotamia, centuries of high productivity from irrigated land led to increased population density that fueled demand for more intensive irrigation. Eventually, enough salt crystallized in the soil that further increases in agricultural production were not enough to feed the growing population.
The key problem for Sumerian agriculture was that the timing of river runoff did not coincide with the growing season for crops. Flow in the Tigris and Euphrates peaked in the spring when the rivers filled with snow melt from the mountains to the north. Discharge was lowest in the late summer and early fall when new crops needed water the most. Intensive agriculture required storing water through soaring summer temperatures. A lot of the water applied to the fields simply evaporated, pushing that much more salt into the soil.
Salinization was not the only hazard facing early agricultural societies. Keeping the irrigation ditches from silting up became a chief concern as extensive erosion from upland farming in the Armenian hills poured dirt into the Tigris and Euphrates. Conquered peoples like the Israelites were put to work pulling mud from the all-important ditches. Sacked and rebuilt repeatedly, Babylon was finally abandoned only when its fields became too difficult to water. Thousands of years later piles of silt more than thirty feet high still line ancient irrigation ditches. On average, silt pouring out of the rivers into the Persian Gulf has created over a hundred feet of new land a year since Sumerian time. Once a thriving seaport, the ruins of Abraham's hometown of Ur now stand a hundred and fifty miles inland.
As Sumer prospered, fields lay fallow for shorter periods due to the growing demand for food. By one estimate almost two-thirds of the thirty-five thousand square miles of arable land in Mesopotamia were irrigated when the population peaked at around twenty million. The combination 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 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.
Figure 5. Ancient Egyptian plow (Whitney 1925).
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 direct water to particular places on the floodplain. After the annual flood, the water table dropped more than ten feet below the valley bottom, eliminating the threat of salinization. In contrast to the experience of Mesopotamian farmers, Egyptian wheat harvests increased over time. The longevity of Egyptian agriculture reflects a system that took advantage of the natural flood regime with minimal modification.
Fresh dirt delivered by predictable annual floods meant that fields could be kept in continuous production without compromising soil fertility. But the population was still subject to the whims of the climate. A few bad years, or even a single disastrous one, could be catastrophic. Extended drought severely reduced crop yields; a peasant revolt during one from about 2250 to 1950 BC toppled the Old Kingdom. Still, the generally reliable Nile sustained a remarkably successful agricultural endeavor.
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