You need to talk with your vet, not a nutritionalist(??)
Our vet does NOT reccomend adding salt to a horses feed ...called force feeding.....unless the horse is in heavy training and a vet is consulted first.. He says it can cause serious chemical imbalances in the blood, and can cause severe kidney problems. Even death.
IMO, NO ONE should force feed salt unless a vet has been consulted first.
The fact that your horse's poop is watery means it is taking in TOO much water....i.e. it is thirsty because it is being fed more salt than normal and is drinking more water than normal. If this keeps up, you could be facing medical issues much more serious than watery poop.