Chemical fertilizers seem to work just as well when applied as a top-dressing, so I apply them as-needed during the season.
I like to mix a little something organic in the hole, such as cottonseed meal or alfalfa pellets, maybe mix some worm castings, 1/4" ground bark or home-made compost. Theoretically, it's the soil that we feed, and if we keep it healthy, the plants are able to grow better in it.