Please turn your cap lock key off. Typing in all capital letters is the on line equivalent of shouting.
Unless you're growing super autos, 15 gallon pots are overkill. Three gallon are sufficient for most dwarf strains. If you're growing some of the larger, sativa dominant strains, then 4 or 5 gallon is enough. I always used 3 gallon for dwarfs, 10 gallon for supers and 15 gallon for photos.
With outdoor plants it's impossible to keep them on a regimented feeding schedule like you would indoors. You'll run into periods of rainy weather where the soil will be too wet to feed. So you should incorporate something into your soil mix so they can feed during those times. Composted manure, worm castings, guano, etc. all work well.