Ah, I see. Thanks So keep the plants off the ground using some sort of caging? I have some old metal grid shelves that might work.
Ive included a link that will show some metal grid benches designed for greenhouses. Its probably cheaper just to build your own though. Some good old fashioned wood, some 1/4th inch hardware cloth and Im sure you could whip something up, or try out the metal grid shelves. Either way, 12-24 inches off the ground with a fan below should provide plenty of circulation to prune those root tips.
http://www.greenhousemegastore.com/Benches-for-Commercial-Greenhouses/products/1083/