I like to buy a potting mix that feels lighter than it looks..... Peters brand is the last one I purchased that I really liked. When you go to pick up a big bag, you should think to yourself "wow- that's a lot lighter than it looks", not "man- this sh*t is heavy". Same with pots.
Course you could buy a heavy mix and add lots of perlite, but I like to buy as few bags of crap as possible. "
