I have never used it, but I have read banana peel ash has been used for centuries as a source of potassium in the days before synthetic fertilizers. Personally, I dry banana peels in the oven, smash them up, and then put them in as an ingredient in my tea. I'm not sure if that's the best way to go about that, but I feel like, and I will probably be corrected here, organics is a bit of a guessing game. You just gotta kinda go for it.
That being said, here is a PDF of an article on the presence of potassium in banana peel ashes.