My horse has started doing this! Never have I seen him do it. Hes licking it mostly... in his run area where the rainwater drips into... its dried up but he will stand there and lick it and eat some... is he missing something important?? Hes got a full hay net 24/7 fed twice a day and gets turned out a few times a week on the pasture.. not a daily thing because he will get way too fat .. our pasture is being treated anyway for clover and other weeds so I dont want him on it often. I work him 4 to 5 days a week too. He looks great, has a good attitude and seems totally content but I've caught him several times over there licking that dried up dirt! 

My good mare has done this her whole life. Especially when we travel. She wants to sample the dirt in every stall, pen, fairgrounds we stop at. I used to give her psyllium first a week after every trip because I thought she was eating to much dirt. Lol. I asked the vet and she said some horses just do it so who knows.  She told me as long as she wasn't just biting and eating mouth fulls of dirt to just let her be. 

Hes got access to loose salt/minerals and a salt block but rarely and I do mean RARELY uses them. He drinks plenty of water as wel . Especially when it's so hot.. I refill his buckets 2 to 3 times a day. He never was a dirt licker/ eater up until about a few weeks ago. It's not everyday at least I dont think. 

He is lacking something in his system that the mineral is not providing.  Buy a good quality mineral and force feed it at the recommended daily rate in his daily feed and see if he quits the eating dirt.

I agree lacking some mineral. I would put him on Manna Sho-Glo, its priced right and you can get it at TSC or Wal-Mart, I really like this product and have had my horses up at the house on it for many years and I have never seen them eat dirt. 

This.

Don't remember my horses eating dirt but sometimes my dogs go crazy eating it by the mouthful, then they quit.  They especially like potting soil and that can't be good for them.  

Hes getting 1 scoop of platinum daily along with his grain ration. But I've been reading on other sites and hearing from other people that Platinum is not as good as they come off to be that they lack the needed levels of vitamins and minerals really required by horses. I dont know..just what I've been hearing thru the grapevine. Its plenty expensive for me to have any doubts in it... I have less than half a bucket to finish up and I may have to find something else. I had a free choice mineral out for him but he never ate it... I cleaned it out yesterday because it was literally covered up in dust from just sitting there for so long untouched.  I cant remember the name of it but it was designed to be fed free choice and I remember the bag saying do not put on feed