We reached out to Anna Cates,Extension Specialist in Soil Health, Assistant Professor, Department of Soil, Water, and Climate University of Minnesota for data on the organic matter lost since the advent of industrial farming. This is what she had to share with us:
The basic story is that prairie soils have lost about half of the organic matter or carbon they had since farming started. So you could just double your current number as a ballpark estimate. Oak savannahs probably went in and out of prairie over the years, so they may not have started quite so high. Doubling your current number might be a little bit of an overestimation, but for something pretty much unknowable, it's not a bad guess.