my horse has been licking the dirt
My horse has been doing something strange lately. I noticed her lips had a grayish residue on them once or twice, and last night found her down by the pond, licking the ground that used to be underwater but is now dry because the pond has receded (due to lack of rain). At first I thought she was grazing, but she was actually licking the greyish clay that forms the bottom of the pond.
She's got a salt block, gets a complete feed (Horsemans Edge), has 24-hour access to nice green pasture and gets hay every day.
Why would she lick clay? Do I need to worry about this? I could close off that pasture if need be.