Potassium deficiency is tough to correct. It takes time and patience. If you can get some, use Potassium-Magnesium Sulfate to minimize the chance of inducing a Magnesium deficiency when you supplement Potassium. I have to do that here in FL too but I'm less than thrilled with the results. I can't really tell you that I've ever seen it disappear. For what it's worth, Potassium deficiency is among the top causes of palm death in FL. Washingtonia is remarkably susceptible, and W. filifera is virtually impossible, especially if irrigated overhead with sprinklers.