We are doing this with tillage radish to break up the hard pan soil. It seemed to work pretty well. I don't know why turnips would not do the same thing if seeded a little light. We have problems with weedy grasses in the clover. To take up extra N that weedy grasses love, we are over seeding with rye/oats one year and radish/turnips the next. Right now we need to get a good grass kill though. Good luck. Let us know how it went. "D"
Give back twice that which you take.