Boron, in the form of borates, does originate in volcanic ash, and is a (rare) evaporite mineral usually found either in dry lake or shallow bay sediments.  Sulfates are found at rather low concentrations in volcanic gases, but in the form of calcium sulfate (epsom salt) are at really high concentrations in seawater and evaporated lake water, particularly in arid settings.

Now it turns out that the worst place for SJVF in the San Joaquin Valley is called Shark Tooth Hill, a volcanic siltstone that accumulated in a shallow bay. (And famous for shark tooth fossils, thus a geology class magnet.)  I forget now what the corresponding spot near Tucson was, but I recall it was volcanic ash-derived lake deposits.  In both cases it is the silty, not sandy  consistency that the fungus likes, and which blows around so readily. 

Now there is a dry lake in Mordor, around the Sea of Nurnen.  Could JRRT have known about SJVF?  I can hardly believe it, although VF has been known for awhile.  That would really be something.  SJVF popped up among soldiers during WWII, and it's possible JRRT might have heard about it, but it's a long shot.

Some who wander are lost.