Like Karen, I also use a soil-less mix for everything I grow in pots, regardless of the size of the pot. I grow about 800 plants per year for our Master Gardener plant sale. I use a very fine (soil-less) seed starting mix to get everything started, and then use a soil-less mix (Pro-Mix is my least expensive go-to option) for everything that I put into the transplant pots (3" - 5" pots). I also use a soil-less mix in my Earthboxes, Smartpots, and any other container I grow outdoors. It just seems to do better for developing good root systems. For my larger outdoor containers, Earthboxes and Smartpots, I use a mix of ProMix, horticultural grade perlite and coco-coir. I've had good results with this so far, but I'm sure a lot depends on what you are growing, your climate, etc.
Carol--there are many brands of soil-less potting mixes. ProMix is just one; Sunshine, MetroMix, and even Miracle Grow make some. A lot depends upon where you live what may be available, and it's WAY too heavy to justify shipping expenses. Just read the labels carefully....you will be fine.
