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I am about to plant some new hydrangeas. What should I use to amend the soil with to make the hydrangeas blue?

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  • Which variety of Hydrangea - macrophylla (mophead/round flowers) or paniculata (cone shaped flowers) and what colour is it now?
    – Bamboo
    Apr 24 '18 at 20:32
  • Related, or possible duplicate: How do I change the color of my hydrangea flowers Apr 24 '18 at 23:14
  • Welcome Peggy! In addition to the excellent answer from andrewbuilder, would you please have a look at the question I linked above and see if it helps you with what you need? Apr 24 '18 at 23:16
  • Where are these hydrangeas planted? If they are near a concrete foundation or concrete walkway you will have a tough time keeping the pH acidic.
    – stormy
    Apr 25 '18 at 5:01
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To obtain a blue flowering hydrangea, the soil around the roots of the plant must contain aluminium.

For this aluminium to be available as a nutrient, the pH of the soil must be very low / acidic (around 4.5-5.5).

Note that an acidic soil may be detrimental to the growth of surrounding plants that may prefer a more neutral soil pH. It is therefore sensible to group acid loving plants together.

When added to the soil, aluminium sulphate can serve to provide a source of aluminium and create a more acidic soil. Aluminium sulfate is a water soluble chemical compound with the formula Al 2 (SO 4 ) 3 .

Creating suitable soil chemistry with aluminium sulphate may be a process that takes two or three or more years to achieve success.

Additionally, there are some important considerations:

  • aluminium sulphate used as an agricultural soil amendment, has the potential at stronger doses to burn the roots of young plants;
  • A fertiliser low in phosphorus and high in potassium is helpful in producing a strong blue color (superphosphates and bone meal should be avoided when trying to produce blue);
  • Rich compost will contribute to creating an acidic soil, however once the compost has broken down the immediate effects are significantly reduced.
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  • I thought I had already commented a moment ago so if there are two comments, that is why. Andrew, this is a fabulous answer. I also said something to the effect that decomposed organic matter BUFFERS the pH of the soil; making acidic soils less acidic, alkaline soils less alkaline. Nice answer...!
    – stormy
    Apr 25 '18 at 5:04

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