Yeah, you eventually get good soil from volcanoes. Eventually. Initially, however, any ashfall greater than several inches will suffocate the living organisms in the soil and leave it sterile. So say goodbye to much of our agricultural production for a few generations if it blows.
Yellowstone magma/lava is generally rhyolitic. In other words, it’s made of the same stuff as granite, rich in silica. I think good soils only come from basaltic and possibly andesitic lavas. Andesitic is a combination of the two.