A horse normally requires 1-2 oz. of salt per day. This amount can increase to as much as 4-6 oz. per day with the onset of hot weather and exercise.
If your horse is salt deprived there are two behaviors to look for: The first is licking. They are trying to get salt from any source available. The second is decreased water consumption. A horse will naturally reduce water intake to keep from flushing salt out of its system.
There are also physical signs: Anhidrosis (which is the inability to sweat), tying up (which is the stiffening or trembling of muscles after exertion), increased heart rate, excessive potassium levels, and the lack of water consumption increases the risk of colic.