Just add to my earlier comment, I only grow in raised beds now and over the past couple of years I've grown most veg, potatoes, cabbage, brussels, carrots, swede, parsnips, peas, sweetcorn, beets, lettuce, onions, strawberries, beans, garlic & courgettes. My raised beds are made from scaffold boards which are 9" wide and have a good 8" of soil/compost in them. Last year one of the beds had a 2nd board on top which I grew my maincrop potatoes in and the soil/compost was around 12" deep.