I'm exceedingly reluctant to dive into this thread, because my opinions and experience are contrary to almost _everything_ you're going to read on this forum. My opinion, just like everyone else, is like an asshole... and I've got one. But I have _much_ more experience than the other responders you're going to read in this thread. That being said...
I use whatever potting soil is on sale at Lowe's or Home Depot, frequently it's Miracle-Gro but sometimes it's a product called Sta Green, in addition to whatever peat based potting soil is on sale in Walmart. There's really not a whole lot of difference between 'em. First thing I do is empty out the bags and flush them with massive amounts of water. Then I put the soil in fabric bags, put clones or seedlings in, fertilize with Peters 20/20/20 when needed and watch the magic happen:
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