Yep, sweat is very salty, and some horses will lose more salt than others through their sweat. If there are 'grains' of salt on your horse once it dries, she'd be losing quite a lot of salt so I would supply her with a mineral block at the very least, and as mom2pride suggested, an electrolyte supp.
As for general management, leaving sweat on a horse can irritate it's skin. Hose, or sponge her off after every ride that she gets a good sweat up. I an pedantic about getting rid of sweat, and give my horse a full sponge down, scrubbing all the sweat off him before allowing him to dry out and go back in his paddock.
Even if the weather is fairly cool, find a sheltered place, and just use a damp sponge, with warm water if you can. To work out the main areas of sweat.
Sponging/hosing the sweat off will leave her feeling much happier :) Liken it to you going for a big run, getting covered in sweat and then just putting your normal clothes back on and heading out without having a shower. You feel disgusting, and often your skin will get itchy. Same goes for horses