The Daikon radish seed has arrived and it is cheap. $3.50 a kg will get you a share of this introductory offer. The 'tillage' style radish is potentially one of the best soil-building cover crops ever. Cattle (and presumably other grazers) love it and the large tap-root will turn to compost in the ground.
Check out the site at http://www.tillageradish.com/benefits/benefits.php