horses need more than salt. Other compounds make up electrolytes.
There is some in grass and various feeds. Comercial horse elytes are not that expensive, look for ones without sodium bicarbonate, also on your lick get the ones made for horses, Those big solid red ones are made for cattle.
I have done a lot of research on the topic after posting on here, and the majority of articles I came across said that salt is the MAIN electrolyte horses need and that too many horse owners go over the top with giving electrolytes. There are other electrolytes that horses have readily available to them (phosphorus and calcium are the only ones I can remember now). They don't need as much of the other electroytes as they do sodium chloride (AKA salt, but technically phosphorous and calcium form salts too). However when horses don't have enough sodium chloride in their diet, then their body resorts to using the potassium and calcium for jobs that NaCl would do if the horse had enough of it. This is the time when horses need salt and additional electrolytes in their diet - at least according to the online research I have done.
I only use electrolytes when competing in the heat. Cross country in summer is not fun for either of us if we are dehydrated.
My horses have 24/7 access to a mineral salt lick for horses. One uses it more than the other - he is the heavier sweater.
I have never had a problem using that method. :)
I plan on purchasing a jar of powdered electrolytes for days where we do strenuous activity (i.e: cross country schooling) in the extreme heat. I just won't be giving him them on a daily or weekly basis. I've purchased a mineral (yes, one made for horses...according to the feed store anyways) lick for him but I never saw him actually using it! I'm interning at a vet clinic and I brought it up with the tech - she recommended either getting a vitamin/mineral supplement or using a grain that was fortified with essential vitamins and minerals (I think she recommended Strategy, but I'm still trying to figure out if which route will be more cost effective)
In all honesty this horse was my trainers horse, but now I am taking him with me to college. He was mainly used as a lesson horse, never anything too serious, and all he got was 4 flakes of alfalfa a day (no salt, no vitamins/minerals, nothing extra). I know that diet sounds cringe-worthy, but he has had issues requiring vet care only twice in the 10 years I have known him (for a sarcoid to be removed and the second time he got a deep cut on his coronary ban). But now that I am taking over his care and am going to be asking a lot more of him, I want to make sure his body has everything it needs so he can comfortably perform the best that he can.