All horses should have access to free choice salt. Many horses do not, however, consume enough on their own. That is why I recommend adding loose salt to daily rations for every horse.
For most horses, 2 tablespoons a day will do the trick. The most cost effective way to do this is to buy a 50 pound bag of livestock salt. You can buy these bags at your local feed store!
This bag was only $8 and for my one horse will last close to a year. He is good about eating it in his meals, but some horses are pickier. For picky horses, I recommend purchasing salt from Redmond.
Also, sometimes these bags have an ingredient called yellow prussiate of soda. This is an anti-caking agent and perfectly safe for consumption!