Since this is unknown territory I would get a base line ph for your potting soil, as well as a baseline ph for pure ash (stir your ash until it's a consistent mix so you don't get any hot spots) Then start slowly adding measured amounts into your soil (so you can report your ratio here). As little as a table spoon at first, mix well, then ph your soil again, repeat the process until it starts to show alkalinity or acidity. If it's out of marijuana's happy range you should use the guide here:
GROWFAQ to get your soil back to a marijuana friendly ph. (5.5 to 6)