Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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Enough American farms disappeared beneath concrete to cover
Nebraska in the three decades from 1945 to 1975. Each year between 1967
and 1977, urbanization converted almost a million acres of U.S. farmland
to nonagricultural uses. In the 1970s and 1980s over a hundred acres of
U.S. cropland was converted to nonagricultural uses every hour. Urban
expansion gobbled up several percent of the best European farmland in the
1960s. Already, urbanization has paved over 15 percent of Britain’s agricultural
land. The growth of urban areas continues to consume farmland
needed to feed cities.
During the cold war, the Department of Agriculture developed tolerance
values for soil loss to evaluate the potential for different soils to sustain
long-term agricultural production. These values were based on both
technical and social inputs—what was considered economically and technically
feasible in the 1950s. Soil conservation planning based on this
approach typically defines acceptable rates of soil erosion as 5 to 13 tons per
hectare per year (2–10 tons per acre per year), equivalent to a loss of an inch
of soil in 25 to 125 years (0.2 to 1 mm per year). However, agronomists generally
argue that maintaining soil productivity requires keeping erosion
under 1 ton per hectare per year—loss of less than one inch in 250 years—
2 to 10 times lower than USDA soil loss tolerance values.
Until recent decades, little hard data were available on rates of soil production.
So it was hard to know how big a deal to make out of this problem.
Farms were losing soil faster than desired, but it was easy to lose sight
of the big picture when farmers were struggling to deal with overproduction
and food was cheap. Recent studies using a variety of methods, however,
all point to soil production rates much lower than USDA soil loss tolerance
values. A review of soil production rates from watersheds around
the world found rates of from less than 0.1 to 1.9 tons per hectare per year,
indicating that the time required to make an inch of soil varies from about
160 years in heather-covered Scotland to more than 4,000 years under
deciduous forest in Maryland. Likewise, a global geochemical mass balance
based on budgets of the seven major elements in the earth’s crust,
soils, and waters pegs the average global rate of soil production at between
an inch in 240 years to an inch in 820 years (equivalent to an erosion rate
of 0.37 to 1.29 tons per hectare per year). For the loess soils of the Great
Plains a soil replacement rate of an inch every 500 years is more realistic
than the USDA’s acceptable soil loss rates. Hence, the currently “acceptable”
rates of soil loss are unsustainable over the long run, as they allow soil
erosion to proceed four to twenty-five times faster than soil production.
dust blow