"The long-term erosion rates are also one to four orders of magnitude lower than the assumed 1 mm yr soil loss tolerance value assigned to these locations by the U.S. Department of Agriculture."
So this paper shows the erosion is actually slower (better!) than we expected.
From the conclusion:
> We find that pre-agricultural erosion rates in the midwestern United States are on the order of 0.0001–0.1 mm yr–1. Soil loss tolerance values of 1 mm yr–1 are one to four orders of magnitude higher than pre-agricultural erosion rates. Similarly, the agricultural erosion rates are 10–1000 times greater than the preagricultural erosion rates. Our results indicate that tolerable soil erosion, as currently defined, will lead to the depletion of midwestern soils.