John Savage wrote:

Without consulting a reference, I'd say you have that 'round the wrong way.
I seem to remember hydrangeas being the opposite to litmus; hydrangeas are
blue in acid and pink in alkali I think.

(Actually it's not that straightforward, as I recall a science teacher
winning a science award for determining what it *really* is that changes
the hydrangea's colour from pink to blue or back again, but roughly
speaking using pH is good enough, but you probably know that.)

The acidic soil permits the aluminum in the soil to be taken up which is
what makes the flowers blue. Maybe with acidic soil and no aluminum
they would be white, I don' t know.

Aluminum is toxic to many plants such as azaleas and rhododendrons, so
don't think that aluminum sulfate is good for acidifying the soil for
all plants. But, it will make most mophead hydrangeas bloom blue.
--
Pardon my spam deterrent; send email to
Visit my Rhododendron and Azalea web pages at:
http://home.earthlink.net/~rhodyman/rhody.html
Also visit the Rhododendron and Azalea Bookstore at:
http://home.earthlink.net/~rhodyman/rhodybooks.html
Cheers, Steve Henning in Reading, PA USA Zone 6