My horse is a 18 year old Warmblood eventer. I feed him good feed and joint supplements, hoof supplements, a fat supplement, and electrolytes. He also has a salt block in the pasture. I have been told he is getting too little salt, but he has all the salt he wants. Why does he still eat bark and sand? I am afriad he will sand colic in the future.

is he usually nervous when he does?

your horse probably has a upset stomach.
I know this lady and her horse eats sand and her vet said that her horse eats sand because she has an upset stomach.
If im wrong than look on google.com to see why your horse eats bark and sand.
I hope im not to late to reply!!!!!!!!