The last time a lot of that land was productive was before the Civil War when it still had that layer of humus, with a light clay underlayer. When the forests were cleared, it went away pretty fast, at least in geological terms. Probably in less than 2 generation the land was pretty much down to what's there now.

There's a great but hard-to-find book by John Hebron Moore called, "The Emergence of the Cotton Kingdom in the Old Southwest: Mississippi, 1770 to 1860" .

With plowing and intensive cropping - often not on contour - that top layer vanished pretty quickly. "When exposed to water, this clay dissolved almost as readily as sugar," he writes pretty early in the book.