Nitrifying bacteria are sensitive to acidity, hence one reason for using
an indicator is to make sure that the pH is in the right region.
The other reason is that nitrification is, itself, an acidifying process
so that the generation of acidity is a fair indication that
nitrification is taking place. i.e. the phenol red can be seen as an
indicator of the process.
Soil Science department,
University of Reading, U.K.
AKA <P.J.Harris at reading.ac.uk>