I have about 10 rose plants in clay soil in the San Francisco Bay Area. What would be a good fertilizer for these? I see Espoma has a "Rose Tone" product that is a 4-3-2 fertilizer. Why do roses want less potassium? I happen to have some Espoma "Holly Tone" already for rhododendrons, but that is a 4-3-4 mix. Is it usable for roses?