Advice needed on potting soil
I'm looking to redo my substrate AGAIN but I want to do it right this time , I am going with black diamond as the top layer but I want to use a potting soil as the under substrate , can I just use any kind of potting soil ? I just want to be able to walk into my local garden store and buy it (uk) . Does it have to be organic ? Which one is the best ? Which ones should I avoid ??