Quote Originally Posted by Gen
Don't worry about the ph of your water if you're doing a soil grow.
You should ALWAYS worry about the pH of water and soil. If you use tap water, check the pH of it. 'Soft water' with a low mineral content tends to be more acidic, and 'hard water' with a high mineral content tends to be more alkaline. If you live in a 'hard water' are (like me) then you should always try and either lower the pH with a few drops of lemon juice, as the soil will become more acidic and bacteria prefers areas with a high level of alkaline - making a healthier plant! By the way, add the lemon juice after the water has sat for 24 hours (makes the chlorine evaporate) as the pH will only be lowered for around 8 hours.
The way I water my plants is to fill three 2-litre bottles up with rainwater, adding ferts to two of them. I use up a ferted bottle, then the non-ferted, then the last ferted one. When they need watering again, I use tapwater that has 'sat' in a cool, dark place for 24 hours, then I add a half-dose of ferts and water with that to bring the pH level up. Making the pH fluctuate slightly allows the plant to absorb all the nutrients that are in the soil, thus reducing the buildup of salts - making a healthier plant! It also means you can flush the soil that little bit later, so that the plant will have time to take up more nutrients, meaning slightly bigger and more potent buds.

Oh, and to answer youre initial question - cannabis prefers soil with a pH of 6.3, although like I said earlier, let it fluctuate slightly from time to time so it can absorb all essential elements in the soil and water.