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. As salt intake increases, so does water intake. Do not feed too much salt. There is no need to add more than one percent salt to the grain ration.