I would use compost, mushroom compost seems to be magical stuff according to what I see online.
Yes, use compost but what types of compost to give the potting soil mix what it needs?
I have studied this and Azomite compost (organic from Utah) gives it 67 trace minerals and the earthworm castings compost gives it what manure does (nitrogen, etc.) and if I add some mushroom compost with that, together it should be a very good organic compost replacement in that potting soil mix. I'm going to go with that unless someone on the thread shoots holes in this plan.