This year I want to try growing dahlias in pots -- 1 tuber per 12" pot. Is regular potting soil OK for this? What kind of fertilizer and fertilizing schedule would you recommend? any other advice?
From all that I've read, soil is soil, whether its inside or out. Dahlias like soil that's good for veg, and don't like anything that's been treated with a herbicide.
So get something you could start tomatoes in and you should be fine.
I have several tubers planted inside in pots intended to stay indoors. Several were planted in Miracle Grow seed starter, others in MG moisture control, and still others planted in a left over potting mix (don't know which kind, but its been sitting in the pot for a year or more.)
All seem to be doing fine.