Grow bags are too shallow for potatoes. If grown in containers they should be grown in purpose bought bags (large plastic sacks with drainage holes will do as well) and general purpose compost.

Any peat-based or peat-free compost will do but you will have to give them lots of extra potato feed and water. Growbags, in my opinion are not suitable for potatoes as they will do better in deep compost bags with drainage and you will have to keep topping the compost up as they grow. You can, of course, buy the proper stout bags for growing potatoes.

Sorry Bulb, snap!