if you are in soil then up need to test the run-off from the soil. I'll sometime take a handfull of my soil and moisten it well, then squeeze out the water and test.
I live in an area of high ph water. About 7.8 to 8.0 out of the tap. I add a little cider vinegar to a gallon of water shake and test. Shake and test. Shake and test. I add about a half teaspoon at a time.
Sure this would work for hydro also as I use it in my bubble cloaner.
Hope this helps.