Soil-less potting mix (not potting soil). Several brands available and it will use 'Mix' in the name.

Dave

Would this work on a balcony? What potato would you use?

I want to write a series of articles on growing potatoes in the city for the website www.potatoes-recipes.com (see below)

Here is a link that might be useful: Potato Recipes

flowergirl70ks, You & your grandson should have some fun! There is a straight-forward set of instructions, with pictures, in the link below. It can work in a 5 gallon pot, but you will get a much better yield in a 15 gallon pot. Even a 10 gallon pot would do well compared to a 5 gallon one. Since you are planting in summer, you do not have to worry about having the pots in a hoophouse. Potatoes like full sun. I use Gardner & Bloome Potting Soil and it works very well.

Good luck with the potatoes!

Here is a link that might be useful: growing potatoes in pots