I never checked my run off until recently and noticed the same thing. I have not had any problems with plants growing due to the run off. Don't worry about it. At first the soil is always acidic which will bring your run off at the 5.0 range but it will gradually bring it back up to where your water ph has been set at a few waterings. Check into Dolomite Lime that would help it and you won't ever need to ph your water . Google it.well isnt it a way to tell whats going on in the soil? nothings wrong with my plant, perfectly healthy..i thought runoff mattered.