Salt is usually added as one-half to one percent of the grain ration. One percent should be used for performance horses in the summer. For pleasure horses not being used often, a half-percent is okay. Block salt is commonly placed in horse pastures. But, salt blocks are designed for cattle, which are lickers.
Horses are nibblers.
So loose salt is better for them. Also, use trace mineralized salt rather than plain or white salt. Trace minerals are essential. Place loose trace mineralized salt in a dry place out of the rain. Make sure horses have a clean, fresh supply of water, too.