Good question and a great way to recycle banana peels. Banana peels are 25% phosphorous and 42% potassium, so they are not only good for us, they are also good for plants. I found some instructions online that are pretty simple. If your peels are dark, cut them into small pieces and place them in a glass container. Add enough water to cover the peels and let sit for three or four days in a shaded area. You can put the peels in whole, but cutting them up shortens the soaking time and allows more peels to be put in one container. Then, pour the mixture through a sieve into another glass container and you have a liquid that can be poured on your plants. If your peels are still yellow, it will take closer to a week to make good fertilizer, particularly if you place the peels in the jar whole. I would love to hear from anyone who tries this. What were the results? Do you have any helpful hints for other readers?