measuring pH of roots

[MIKERICH]

I am not sure if this technique could be modified to measure pH, but we
have dripped water through a pure sand system to collected exudates from
plant roots such as allelopathic compounds.  You might consider a
circulating system where a sample can be collected over time as the water
is circulated through the root zone.  There were similar systems described
in the J of Plant Physiology.

Good luck !