you've got another problem with the sand, but it's a lot easier to deal with than the clay. you're going in the right direction by digging down a couple feet and amending the soil. if you've got a lot of sand, organic matter is your best friend. find a local horse stable and trade some homebrew for a couple buckets of manure. sand will leach nutrients very easily where the organic matter will help you hold any available nutrients in that general area so the plants can get to them before they drizzle away. you don't have redo all of the plants at one time, unless you have a lot of free labor. do a few this year and next summer you can work on some new areas to be ready for the fall. who knows, maybe the 3rd year will be a charm. keep hoppin'!