From what I've read the dirt isn't necessary. Seems most folks that grow fodder do so hydroponically. Maybe it's a seasonal issue but I'm having a heck of a time locating untreated seeds to use. I've got some BOSS going now, but would like to do barley, wheat, millet or pretty much any grain. Can't even find rye grass seed.
Does it really matter all that much if it's treated? With all the rinsing and soaking and whatnot you'd think the stuff would wash off. Esp. since I'm not re-using the same water.