There was no prep, we planted directly into spring killed sod (Round-up, and 2,4 D). We still do this, and is usually some of our best corn.
Now something you need to understand... that alfalfa, if still a good stand, will supply 150-200 lbs of N if killed. That means there is enough N fertilizer there for a crop of corn WITH NO FERTILIZER! Given this, and given that beans will no utilize this free N, why do you wish to plant beans?
I don't know where you live, but corn following alfalfa is probably the most lucrative crop there is, following marajuana and a couple others. Think about it.