Are you growing in soil or in Hydro. The chart that zador posted is a good indication. If growing in Hydro you start with the base and work from there. In hydro I keep mine at 6. Each type of weed is a little different but can adjust the Ph by .2 one way or the other. I measure the PPM so know when the plant loves the ph as the PPM will move one way or other. Plants go through bit of a cycle either take in water or nut sometimes both. If Ph not right plant will not do anything.