Don't know anything about your radish, but cowpeas, mung beans, Korean lespedeza are other options. They should give you 60-120 pounds of N per acre. Kauffman Seed in Haven should have them. They make good brood-rearing habitat and are also great deer attractors. It is a bit late, but I think there is still time. As for grain crops, proso millet has the shortest growing season. You will probably need to add some N if you're that low, but you could also mix the peas or beans with the millet and try that. The N will be all in the roots of the legume and unavailable to the millet this growing season, so again you may need to add some. There are plenty of other options as well.