Answer: Soil acidity or alkalinity will change the color of hydrangea blooms. Nitrogen is a fertilizer and will promote growth but not color change in your hydrangea. You can change the color by treating the soil. Pink and red hydrangeas often turn blue or purple in acid soils, and plants can be made red by liming the soil or applying superphosphate. For blue flowers, treat the soil with 2 Tablespoons of aluminum sulphate per plant, making 4-6 applications at 2 week intervals, starting in the early spring before the plants bloom. (Sulfur will acidify the soil. The desired pH range would be between 5.5 and 6.) For green flowers, you'll need to adjust the pH to be more acidic, but not quite as acidic as for blue flowers. It will take a little experimentation to find just the right soil pH to produce the color you want. Be sure to test your soil to measure the pH range so you'll know what to add.
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