Probably not necessary, especially for large plants. A soil test is really the only way to know for sure if fertilizer is needed. Well-established rhododendrons, especially if kept mulched, usually don't need any unless they're showing symptoms of decline such as reduced bloom, paler foliage, etc.

None of that; no symptoms of decline, and this flowering season was the best ever. I've never done anything for these rhodies, yet they thrive. Lucky, I guess. Thanks for your response.