Why is my foal eating dirt?

Even though your foal is getting the nutrition he needs, he's still responding to the natural instinct to supply his body with minerals. In the wild, they aren't given all the extra supplements, so he doesn't know that he's being well fed and given enough vitamins. The dirt around your barn is also likely high in minerals so to him, it's like a natural salt lick. This should not be a cause for concern unless there's a lot a sand in the dirt (horses can get sand colic if they inhale/ingest a lot of sand) I hope this helps!