Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

46
out the region, he concluded that the whole region had been cultivated at
some time in the past. The contrast of a sparse population and extensive
abandoned irrigation systems told of better days gone by.
Lowdermilk had first recognized the impact of people on the lands of
northern China at a virtually abandoned walled city in the upper Fen River
valley. Surveying the surrounding land, he discerned how the first inhabitants
occupied a forested landscape blanketed by fertile soil. As the population
prospered and the town grew into a city, the forest was cleared and
fields spread from the fertile valley bottoms up the steep valley walls. Topsoil
ran off the newly cleared farms pushing up the mountain sides. Eventually,
goats and sheep grazing on the abandoned fields stripped the
remaining soil from the slopes. Soil erosion so undercut agricultural productivity
that the people either starved or moved, abandoning the city.
Lowdermilk estimated than a foot of topsoil had been lost from hundreds
of millions of acres of northern China. He found exceptions where
Buddhist temples protected forests from clearing and cultivation; there the
exceptionally fertile forest soil was deep black, rich in humus. Lowdermilk
described how farmers were clearing the remaining unprotected forest to
farm this rich dirt, breaking up sloping ground with mattocks to disrupt
tree roots and allow plowing. At first, plowing smoothed over new rills and
gullies, but every few years erosion pushed farmers farther into the forest
in search of fresh soil. Seeing how colonizing herbs and shrubs shielded the
ground as soon as fields were abandoned, Lowdermilk blamed the loss of
the soil on intensive plowing followed by overgrazing. He concluded that
the region’s inhabitants were responsible for impoverishing themselves—
just too slowly for them to notice.
Over the next three years, Lowdermilk measured erosion rates from protected
groves of trees, on farm fields, and from fields abandoned because
of erosion. He found that runoff and soil erosion on cultivated fields were
many times greater than under the native forest. Farmers in the headwaters
of the Yellow River were increasing the river’s naturally high sediment
load, exacerbating flooding problems for people living downstream.
Today the cradle of Chinese civilization is an impoverished backwater
lacking fertile topsoil, just like Mesopotamia and the Zagros Mountains.
Both of these ancient civilizations started off farming slopes that lost soil,
and then blossomed when agriculture spread downstream to floodplains
that could produce abundant food if cultivated.
Another commonality among agricultural societies is that the majority
of the population lives harvest-to-harvest with little to no hedge against
rivers of life