From the Malthusian Files: World's topsoil gone in 60 years?
This is an intensely sobering article. Seems like a classic Malthusian limit may be coming into view...
http://www.eclectications.com/
How much erosion is acceptable? For the better soils of the Midwestern corn belt, about five tons per acre per year -- in theory. The Soil Conservation Service, who are now the Natural Resources Conservation Service (NRCS), speak of the acceptable erosion rate as the Soil Loss Tolerance Rate, or sometimes the "T factor."
NRCS reported in 2007 that only 28% of US cropland was eroding faster than the acceptable rate "T" -- not an ideal situation, but apparently much better than "gone by 2075." Yet the more detailed NRCS data don’t fully support this statement. And the T factor itself is a puzzling legacy..
NRCS publications don’t say how T was calculated, though it’s tabulated in the soil survey for every soil in every county of the US. The calculation took into account the natural rate of soil formation, but only as one factor among many. Judging from some discussions back in 1956, T was mainly designed to keep the topsoil deep enough for crop production, free of serious gullying, and capable of holding most of the nutrients applied to it. The soil formation rates considered may have been some early, highly optimistic estimates.
Current estimates suggest that soil forms at about half a ton per acre per year in some representative Iowa and Minnesota soils. Even the lowest T values are many times higher. Soil scientist Leonard Johnson reviewed the research behind the T factor in 1986. He concluded that erosion control based on T values "..should be considered as provisional or short-range."
Conservation planning is difficult because the accepted method of estimating soil erosion in a given field, refined and extended since the 1930s, is still inadequate. New studies, in which actual runoff from test fields is collected and measured, show that actual soil erosion is 100-200% worse than the most sophisticated estimate. (One of the test fields is being farmed by the best no-till methods.)
With actual soil erosion this severe, we may be losing topsoil at 10 to 30 times the rate it is forming. That range echoes the 10x-40x estimate of an Australian soil scientist, Prof. John Crawford, for the entire world’s cropland. He told Time magazine in 2012 that
"A rough calculation of current rates of soil degradation suggests we have about 60 years of topsoil left. Even the well-maintained farming land in Europe, which may look idyllic, is being lost at unsustainable rates."
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