You almost always need to add perlite to any commercial potting soils you'll find on the market. If the soil's prefertilized, I'd add about 25% perlite to 75% potting mix. Even if it's not prefertilized, which most are to some extent, the addition of perlite greatly improves aeration to soils that retain a good deal of moisture. You can pick up bags of it at your local Home Depot, Lowe's, etc.