My ro water is almost always right at 7.0 ph before adding the PH perfect, then it measure 5.8 after adding. I check it with a Eco 2 ph pen which seems to work fine. I am pretty sure you can use it with water that is 125 ppm or LESS. I can't help you with soil, never tried it with anything but promix. lol, Muddy will know, PM him if he or someone else doesn't chime in soon.