An easy and no cost way to get highly acidic soil is to simply save your coffee grounds. You don't have to do fancy stuff like mixing the grounds into the soil; just dump them on top in early to mid spring.
Only your pink hydrangea can turn blue. A white hydrangea or a red hydrangea will never turn blue as only the pink hydrangea contains the delphinidine pigment which can combined with aluminum; aluminum being the secret of the blue colour.
The blue hydrangeas keep their colour if they are planted in an acid soil (pH should be between 5 and 6) and provided with aluminum sulphate. Preferably, they should be planted in full shade (facing the North as Hydrangea do not like the heat)
If your soil is not acid, dig a hole 40 cm wide and 40 cm deep. Fill it with 2/3 of peat soil and 1/3 of potting soil. Add near the roots; a handful of rusty nails and plenty of pieces of crushed slates.
Each year in the spring you can spread at the foot of the hortensia 1 tablespoon of the following products : "France blue" or "Alun Salt" or "Aluminum Sulphate."
The hydrangea will turn blue after it's given time to settle in its new place, which means a year. To keep the soil acid you can put regularly at the foot of the Hortensia: coffee ground, oak leaves, pine needles and most of all pieces of crashed slate.
You should avoid fertilizers containing nitrogen, as nitrogen prevents the uptake of aluminum. Gravels around the foot of the hydrangea (as on the picture) should be removed as they provide the soil with calcium and calcium neutralizes active acidity in the soil.
I tried it one year, clay dirt on one side of plant, ashes on the other side. I can't remember which does which, but that year I had pink & blue on one bush I took pictures to prove it. But that's been so long ago I lost pics & the bush is gone now.
The website galmud listed is excellent. Just a further note, there are some bred now that will not change colors, no matter what you do.
Changing the PH of the soil. If you put fertilizer with more iron the color will get blue or purple, mean if you reduce the iron this will be pink.
I add coins, nails to the soil and the color of my flowers are really bright and beautiful.
Check out this website. http://www.hydrangeashydrangeas.com/colorchange.html
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