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Hydrangeas turn blue as a reaction to aluminum in the soil. The soil pH should be slightly acidic (5.2 - 5.5). Do a soil test first, to determine your starting pH. Adding aluminum sulfate to the soil will help lower its pH, if necessary. A word of caution: not all plants can tolerate acidic soil and neighboring plants may be adversely affected.
Answer #1 Â· John Heider's Answer Â· Hi Linda-There are a few things that can cause yellowing of leaves on your Hydrangea. Nitrogen, or iron deficiencies can cause yellowing. The hydranges likes a more acid soil and if the pH level goes up (more Alkaline) the amount of iron in the soil is less available to your plant.