I agree with Ray that pour-through is easiest method to look into the root environment and it is more likely to be more relevant than pH of water or fertilizer. I should add that it is probably better to standardize the amount of water (e.g. 1/4 cup etc) you apply for the measurement. Practically, if you are using dilute fertilizer most of the time, pH of fertigation water seems to be pretty much determined by the fertilizer, instead of the original pH of the water. But the pH of pour-through could be quite a bit different from the pH of the fertigation water. Pour-through pH seems to be influenced quite a bit by the root excretion, according to one scientific paper (with Phals). Researchers didn't find any ill effect of having really low pH (around 3.5-4) for these hybrid Phals.