FYi, here is a link to a recent Earthbox article about what to look for in potting media for Earthboxes. http://forum.earthbox.com/index.php?topic=9057.msg78857#msg78857
FYI, in defense of an earlier comment about 'styrofoam' in potting mix, the perlite looks similar to styrofoam in that it is white and seems to float to the surface of the potting mix if you water the mix from the top. It would not be a substitute for perlite, though. Perlite is volcanic in origin and functions to aerate the potting mix. It's found in just about every potting mix intended for containers but not in planting mixes intended for in-ground gardening.