Some people add salt to their horse’s grain or concentrate in the winter to encourage them to drink more water. There is no proof that this is effective.
Some people feel loose salt is easier for the horse to ingest and that a salt block may be irritating to the horse’s tongue. Loose salt will need to be kept under shelter in a container as it quickly melts away if rained on. Loose salt should be replenished often. Putting a large amount out may be wasteful if it gets dirty and the horses won’t eat the salt with grit in it. A block is more convenient and easier to move around from stable to pasture if necessary. Some of the block will be melted away in rain so for economy it should be kept under shelter. A block is easy to clean off with the water hose if it gets soiled.
(Salt is cheap, so I provide both a mineral salt block and loose plain salt and let the horses choose.)
Tip: Put your salt block in a sturdy plastic box like a milk carton so you can easily move it, moisture drains off it and it can be sprayed clean with the hose. "