So, let's say that there is a soil with half the right amount of magnesium in relation to the amount of calcium present. There is a fair amount of both, however. Some writers, such as Albrecht and Steve Solomon, would say that this soil seriously needs work. However, let's say that a host of fungi were encouraged by the addition of woody organic matter (driving Steve Solomon crazy in the process!) Wouldn't the fungi make sure that enough magnesium got to their plant allies, despite the fact that the magnesium is technically locked up? After all, fungi are able to dissolve rocks, break down lignin, and degrade pesticides, so this should be easy for them.
So no, Gilbert, you aren't missing anything, but you might be if you spend all your time lost in the virtual library of research journals. Speaking for myself, getting lost in the research library for days at a time can be a wonderful experience, although when I used to do that it was a real library of bound volumes, and I didn't have to worry about a garden that needed weeding.