I haven't tried it in soil, but if I was going to I wouldn't adjust it. But if you do decide to adjust, I wouldn't go past 6.3. If your soil is amended with some dolomite lime it will regulate the soil pH.
It all comes down to your water source, low PPM and a pH between 4-8. I have no doubt it wouldn't work with soil. Everyone I know is using it in soiless and hydro.