Slag in Gritty Mix
Has anyone ever used slag as a substitute for granite in the gritty mix? Here in NW Indiana we have lots of steel mills and, of course, lots of slag. Our town uses a gravelly-textured grade on the roads (instead of sand and/or salt) during the winter. It's probably in the 2-3mm range. I gathered up a bit last year and used it when I was re-potting a rosemary plant. The results were very encouraging. Wikipedia says that slag is used as a phosphorus fertilizer--I'm assuming this would be a finely ground powder, though.
I don't know that I'd use this on food crops but, aside from that, any reasons why this wouldn't be a good choice?