Dirt is Yummy

So what exactly is it with eating dirt! Yuck!
He likes to eat it, roll in it, share it with his brother...
This too will pass, and I have to say that I am looking
forward to the not eating it part! I truly don't mind
him rolling around in it, but is it possible for him to
get worms or something? I once had a teacher who
truly believe that children ate dirt because their little
bodies needed it. Like a deficiency or something.
I admire her relaxed attitude about it, but I have a
hard time getting there.