any fertilizer (chemical or organic) will lower the ph becuase they have buffers attached to them. if you dont want to use any fertilizer, or if youve already used it a lot, (which you havent becuase of your ph which is a good thing) and dont want to use chemicals, you can mix lemon with water, check ph of solution and then add to soil.
i use fertilizer once in veg and once in flower.
youll notice an increased rate of growth when the ph goes down a tad.