It's generally linked to sodium deficiency so if you put a large salt block out in your field you might find the horse stops. Personally I wouldn't worry too much; horses eat all sorts of weird things and they rarely do them any damage.
Unless the horse is seeking the dirt out and making an issue over it, I would not worry. Mine does it occasionally and also strips the bark off one of the trees and he is on an all round supplement...looks healthy...I just think that is what they do as part of their natural diet.