yeah you want to keep your pH below 7. what is most important to remember is CONSISTANCY. MJ plants like a constant environment, they don't like fluxtuations in things like temp, humidity and pH. So just try to keep a stable environment for them and they will love you back.

oh, and compost is simply broken down plant matter. you know, like when you have vegetables and you throw them in a pile of dirt and you let them rot and nourish the soil? haha that was kind of a stupid description but that's what compost is.