Our crabapple had a lot more flowering this year than in previous years, probably due to pruning in Feb/Mar. We took off dead branches and suckers. I don't fertilize either, but I sweep up the leaves from fall to decompose naturally under the tree.

Unlike the previous years, our crabapple is loaded in blooms this year. I attribute this to the fertilizer stakes applied last summer. The stakes seem to have helped all of our ornamentals this year, and basically resusitated one very sad, non-blooming Kwanzan cherry- it is flowering beautifully right now. You might want to give the stakes a try. Good luck!