Actually I think that particular proposition isn't particularly difficult to prove or disprove. All you have to do is collect soil samples from various uncompromised contexts and devise a fertility test. I would say from my experience as an archaeologist that that statement is true for riverine contexts where most paleolithic people tended to live. But then, alluvial soils are constantly renewed and their fertility tends to remain high over the long term.
Incidentally, The Secrets of the Soil book does have 9 pages of references, I have not checked the bios of the authors, but the reviews at Amazon are consistently favorable.
Just because somebody reviews a practice -- such as meditating over the soil, doesn't mean the advocate that practice.
