Mike, I think maybe if you tilled the ground (or use a raised bed) to a depth of 16 inches, or till 8 inches and then rake loose soil up to another 8 inches, turnips as with most root crops need really loose and deep planting bed. you might want to think on incorporating some phosphorus in your soil, because phosphorus is needed to strengthen the root system, which in your case is the turnip itself. you can use rock phosphate, bone meal, or cottenseed meal. I use quiet abit of cottonseed meal, for it is pretty cheap usually found at your local farmers co-op, I get a 50 lb bag here in middle tn for around $11.00, and it will go a long way. that would be my suggestion for your turnips. your planting depth for the seed is fine, you just need more depth to let that turnip to grow, plus you will have excellent drainage
good luck with your turnips