[ljw1004]

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.

[JoeAltmaier]

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.