I still dont' understand....you're using super soft plastic, and adding salt, right?
Super soft plastic, is just that, super soft, for action. It's not super durable, or super lasting, it's soft.
Salt is for sinking, it doesn't toughen baits up, it makes them weaker, adding something else to the makeup of the plastic bait. It would have to be a ridgid object through the entire bait to shore it up.
If the baits don't last long enough, use a harder plastic, or, like a bunch of us, in the case of worms...catch a fish on a 6" worm, bite it...catch a fish on a 5" worm, bite it...catch a fish on a 4" worm, bite that, and so on