Horses need salt all year as it is a very important electrolyte that is found in the blood and if they don't have enough it could cause tying up among other things. Salt helps with muscle contraction, nerve functions and it helps balance the water in tissues.
The best thing you can do for your horse is to provide them loose salt, don't put it in their feed because it can throw off their electrolytes. Don't give them blocks as horses don't have rough tongues and will have a hard time with salt blocks and it can cause their tongues to get sore. Salt blocks were designed for cattle with rough tongues.
If your horse is really salt hungry they will try to bite off a piece and it could mess up their temporomandibular joint mechanism or TMJ. They could also get choked on the piece they chew off. Keep in mind that salt blocks have chemical binders to hold them together and have been bleached.
Don't put salt in their food to get them to drink because it can throw off their electrolytes. Always make sure your horse has access to clean water. If you suspect your horse is not drinking enough water it could be because they don't like the taste of the water. You may find that your horse does not like the water because of heavy chlorination or if you have a water softener.
Salt is especially needed in the summer time because they sweat more. In the winter time it is also important because horses tend to drink less because of the cold weather. If you see your horse licking the ground, pipes, you or other objects this is probably a good indication they need salt.
If you are feeding grain don't use a mineralized loose salt because your horse is already getting vitamins and minerals in their feed. Be sure to keep the salt protected from the elements as you don't want it left out in the rain or snow.
Salt plays a very important role in your horses diet and if they don't have it they can get dehydrated and colic. Loose salt if great for horses and they will know the amount their body needs so don't be afraid to keep the salt container filled up. Salt can't be overfed, but you must make sure your horse has plenty of clean water.