|MadSci Network: Botany
The classic example of a flower that changes color with soil pH is the florists' hydrangea (Hydrangea macrophylla). Its flowers are pink in alkaline soil and blue in acid soil. The blue color requires absorption of aluminum, which is more soluble at low soil pH. Hydrangea flower color is only a rough approximation of soil pH. For accurate soil pH measurement, use a pH meter or pH paper. Reference Larson, R.A. 1992. Introduction to Floriculture. New York: Academic Press.
Try the links in the MadSci Library for more information on Botany.