You can go a little further if you want to. Dig down 2 shovels worth, add a wheel barrow full of compost, some lime, guano, manure or whatever you have that is good. It all depends on how many plants, how bad the soil is and how much effort you want to put into it. The better and looser the soil, the bigger the plants at harvest. Last year, my biggest plant was behind the compost pile. The stalk was as big as my forearm, due to all the goodies in the ground from many years of compost seeping down.