Doesn't really matter, since, quite honestly, they'll grow in pretty well anything! For example, it used to be common for people who'd moved into a newly built house to clear the builders debris from the garden area, then plant potatoes all over. Doing this helped clean up the soil from the mess left by the builders.
miraclegro do a potato planter bag. you just cut the top off the bag and put your potatoes in and off you go, costs �4.99 i think. and you can grow any root veg in it. im growing carrots and beets