Animals are so clever about finding minerals that they need instinctively. There is some woodland a few miles away that we occasionally ride through and there is a certain place in the woods where one of my horses will not pass unless you allow him to eat a few mouthfuls of dirt. I had never seen a horse eat dirt before, but he is quite adamant that he needs the dirt at that particular place. And my mare and filly used to stop and dig at a certain place on the track coming in from the field every day and then lick. Eventually after this repeated action, a huge boulder started to be visible at they dug the earth away and they licked and licked at that boulder every day until there was a smooth dimple in it from the erosion of their tingues. There was obviously some mineral in it that the girlies needed and they instinctively knew where to find it, even though it was buried a couple of inches under the surface. It is fascinating isn't it.