Or you can test test for free lime by putting a tablespoon of crumbled dry soil in a cup. Moisten it with vinegar. If the soil-vinegar mix bubbles, the soil has free lime.Just add one tablespoon of dolomite lime per gallon of medium, soil is an excellent pH buffer so no, you shouldn't have pH problems unless your water is either extremely acidic or alkaline
Refer to Richard Drysift's Most Excellent post above......p.s it pre mix bag brought canna terra professional.....