I have read the whole paper of Peltzer et al. and two additional paper on the topic. Now I understand the concept of retrogression better. Of course, I had read that soils of the Amazon Basin and Australia, for example, are old and poor, but I hadn't understood the inevitability of soil retrogression. The concept is actually very logical and even simple: during millennia nutrients are "washed" away or below the rooting zone. I would have two questions to you.
First, I find the boreal retrogression, you described for Minnesota and Petzer et al. for Sweden, is fundamentally different from most of the others Petzer et al. describe. In these boreal forests, the reason for retrogression is the development of a thick humus layer which prevents roots from reaching mineral soil, but the nutrients are still there. Add million years without glaciations/volcanic events, and the mineral soil would retrogess even if there were fires with a sufficient return period which remove the humus layer. Is this correct? Perhaps this "boreal retrogression" could be called "secondary retrogression" and that in Australia, for example, "primary retrogression".
Second, how about the prairies of the US? Large areas were unglaciated but they are still very fertile. Petzer et al. say dry areas do not necessarily retrogess, but I have read there were boreal forests in those areas in the glacial time, and thus they were not so dry during most of the last 2 million years. So, why are the prairies so fertile? Or is some of the facts above incorrect?