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What element is needed to turn Hydrangeas blue? Is it sulfur or aluminum?

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You want to acidify your soil, so sulfur because that is used to make sulfuric acid.

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If you just want blue hydrangeas (Hydrangea macrophylla only) in pots, then a dose of aluminium sulfate will do the trick quite quickly. Either that or sequestered iron will do it, but in the ground, it's harder, because substances applied to the soil will only have a temporary effect - sulfur will need to be added frequently, and it takes a long time to make a difference; frequent use of aluminium sulfate is not recommended - used too often, aluminium can build up in the soil and phosphorus levels can be reduced. More general information re acidifying soil here https://www.rhs.org.uk/advice/profile?PID=82

  • couldn't you bury a few old batteries to lower the PH, or is that too quick? – black thumb Apr 4 at 16:17
  • @black thumb No idea, but I wouldn't recommend it - lead contamination over time is highly likely. – Bamboo Apr 4 at 18:26
  • who eats hydrangeas? – black thumb Apr 4 at 20:50
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    Soil contamination is not only a problem when you're growing food, its an environmental problem generally. other creatures that live on this planet, including the microscopic ones for instance. The general environment is far more important than human beings... – Bamboo Apr 4 at 22:29

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