Everyone should know that when you sweat, you're losing electrolytes and all that medical stuff. Anywho, be sure to have a salt lick out for at will licking. I mean for the horse although you could use it in a pinch, I suppose...
I like sulphur blocks but it seems that a lot of horses nowadays don't like them. There are mineral blocks but make sure you get the one for horses and not beef cattle. The difference is urea and while I can't remember offhand why it's bad for a horse, it just isn't, so be careful.
You can buy holder things that look like a partial plastic box. These are handy if it rains as the water runs on through and keeps the block off the ground and melting too fast. I had a block one time that lasted about a whopping 1 1/2 weeks or so.
Table salt can be added but I've found that horses don't eat all of it as it's grains are too small so let the horse have at freestyle.