Marijuana enjoys a ph between 6.2 and 7 but should tolerate the range of 5.8 to 8. Higher or lower ph levels are harmful. If the soil becomes too acidic, nutrients get locked into the soil and cannot be absorbed. When levels too high, salt builds up to toxic levels and roots can't absorb water properly. Mix wood ash into your soil to raise it's ph, or pour it with water. Use vinegar to lower water ph or baking soda to raise it.

from High Times: Ready - Set - Grow