Rob Halgren wrote:
Not sure what the crystals are, but color in hydrangeas is soil pH
dependent.

I can't remember where I read this, but the pH itself does not effect
the color of hydrangeas. The soil pH effects the plants ability to
take up aluminum (actually, I think aluminum sulfate). Plenty of
aluminum gives hydrangeas the blue color; absense results in pink. So,
pH is really an indirect effect.

--Matt