In one of the studies they combined radishes, peas, and rye. I love this idea! I think I'll do a fall cover crop over existing lawn to prepare it for a spring vegetable garden. My soil tends rather heavily towards clay, so in my mind the radishes will break up the clay, the peas will provide nitrogen, and the rye much needed organic material and in the spring mulch.
Even better, my local library has radish seeds and pea seeds for free, so I will only have to purchase the rye seeds.