we had a herbilist come to see our horse, and i never knew but horse do self medicate, they know where their bodies are lacking something so if possible they will get it, so thats probably why they are eating the soil
One of my boys digs up the gravel then licks the dirt when I need to replace his mineral lick!!! Hes great as I never really take any notice of how much of the lick is left, and he never does the dirt thing untill the lick has all gone!!! He obviously needs someting in it!! My sisiter feeds seaweed and thinks its great, I just pop a mineral lick in all the boys mangers!!