Q: How can I change the color of a pink hydrangea to blue, and how can I keep the plant healthy?--T.M., Montrose A: Not all hydrangeas will turn blue. Some varieties are better at it than others, and the word blue is usually part of their name. In acidic soils, those varieties turn blue naturally (in the Pacific Northwest and New Zealand, the plants are spectacular), but in our more alkaline soils, aluminum sulfate must be added. Apply it in late fall or winter, well before the plant's buds form; even then, it might not take effect until the next year. Hydrangeas require plenty of water in the summer, but they also need to be planted in well-drained soil. A clay or adobe soil, which causes foliage to turn yellow, can be corrected in part by adding iron sulfate or iron chelate. One classic California gardening book suggests the following: Every few years cut back the plant when it goes dormant, dig it up, shake some of the soil from the roots and then redo the soil--adding iron, aluminum and amendments to improve the drainage--and replant.