Hydrangea bushes

If gardeners want blue flowers, they often add aluminum sulfate [AI2(S04)3] or sulfur to the soil to reduce the pH to 5.5. For people who prefer pink, lime (a base) is used to bring the pH to 6.5 or above. Occasionally, a hydrangea bush will produce purple flowers. This color is difficult to obtain, however. Purple flowers only come about when the pH is between... 

Figure 3.2 A hydrangea bush s flowers turn blue if it is being grown in acidic soil the flowers are pink if the soil is alkaline, or basic. Purple flowers grow in soil that is in a narrow midrange between acidic and basic.

Why do some hydrangea bushes look red and others blue ... 

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