Salt is necessary for a variety of functions. It is the major electrolyte involved in maintaining osmotic pressure of body fluid, it is necessary for normal functions of the central nervous system and muscle contractions as well as transport glucose and waste products across the cell membranes. Salt will also encourage a horse to drink more. Typical horse feedstuff is very low in salt (.1%) so it needs to be supplemented. A horse will typically loose about 1 oz of salt per day in sweat, urine and feces under mild conditions and no work. It will increase dramatically with heat, humidity and workload.
If you feed concentrates, they will have roughly .5- 1% salt added to the mix. 4# will have .3-.6 oz of salt. At the lowest requirements, it is adequate with the additional salt the hay provides (.2-.3oz), but when you add heat and humdity, it falls short. Salt blocks are not the best option for getting additional salt into the horse. Many horses will just not sit there and lick it long enough to get what they need eventhough it is right there in front of them. Case in point. I have a group of 10 mares. They get both loose and block salt. A 50# block will last them over 4 months during the warmer weather. Not taking into account what dissolves, that's .7oz/head/day. In this hot spell, they're going through 3-4# per day or over 5oz/head/day of loose salt. Even with the salt block available, when they run out of loose there is a line at the feeder when I refill it. Sprinkling salt on her feed might actually be providing less than she needs. Hang a bucket with about 2 C of salt in her shed. Does she dive into it? If they're difficient, they will consume a large quantity than after 2-3 days drop down to a normal amount which would be around 4oz/day.
Do you ever see her licking metal? Salt will settle on it in coastal areas and that's another sign she needs more.