I agree with Herb & Suds, spraying them with a small amount of water at a time discourages root growth. When you water, you want to soak the entire medium until you see some slight runoff. Water it slowly, give it a little at a time to allow the media to fully moisten-if you water quickly, it will create channels through the dry media and quickly drain out of the cup while not fully wetting the media. Once you water it fully, you can allow it to dry out some, not bone dry though. The bottom of the cup will stay moist the longest, so as the top portion dries out, it encourages roots to spread out and seek moisture. Also, FFOF is quite rich and allowing it to dry out will cause the EC to skyrocket which is why probably why you are seeing some N toxicity.
