Hi I have terrible clay/peat soil so have decided to grow onions , beans and peas in pots and have bought a growbag for my seed potatoes. When I have read about growing in bags only so much soil is used until shoots appear then keep topping up, how does this work with a veg growbag, I feel stupid here but do I empty some of it out and keep somewhere to then top up? Also how many potatoes do I put in, there are about 12 in my pack.