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This information is irrelevant without knowing the current size of top soil.

Also important is whether the erosion is accelerating, constant or decelerating.

Those numbers are easy to find on google! Iowa 14-18" average topsoil depth in 1900, 6-8" in 2000. I'm not sure anyone could tell acceleration/deceleration over this timescale.

And topsoil is not very significant anyway. Corn doesn't need it, much.

My field was eroded clay when I bought it 20 years ago. Left fallow (well, planted in cover crops) it now has 4" topsoil. It can also rebuild you see.

There was almost no erosion before modern farming, say 120 years ago. The high rates of erosion are a purely man-made phenomenon

Without modern farming most people wouldn't be able to eat. Soil erosion doesn't worry me nearly as much as the constant transformation of farmland into crappy suburban developments, with pretty lawns that contribute nothing to the eco system or food supply.

And no one talks about the overpopulation enabled by the petrochemicals and the Green Revolution.

On geological timescales the rate of erosion is almost instantaneous. We're talking about running out of topsoil in a single lifetime.

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