It's a perennial mystery of gardening: What makes a hydrangea in one person's yard produce deep blue flowers, while the one next door has pink blooms? Hydrangea macrophylla, a showy shrub with large heads of clustered flowers in late spring, is uncommon in the Tampa Bay area. But if you have a northern exposure in well-drained, rich soil in shade or partial shade, you might have luck. You can manipulate the color of flowers by changing the pH (acidity) of your soil. Blue flowers need a soil pH between 5 and 5.5. Pink prefer 6.5 to 7. The general rule is that you must make the soil more acid to produce blue flowers and more alkaline to produce pink blooms. That's why hydrangeas usually are blue under oak trees, where leaves fall and add acid to the soil. Hydrangeas grown in our sandy, alkaline soil usually are pink. Here's how to alter the soil pH artificially: For blue flowers, sprinkle about a tablespoon of chip sulfur (sold as soil acidifier) around the base of your hydrangea. For pink, sprinkle \-cup of lime. As the pH changes gradually, you may have lavender flowers for a season or two.