Organic PK booster?

Hi folks, I am starting up a soil grow again, because it requires much less maintenance than hydro, and I was wondering what a good PK booster would be for an organic soil grow. With coco and DWC I used PK 13/14, with fantastic results (once i figured out when to use it properly). This round, with soil though, I am at a loss when it comes to choosing a PK booster.

I am using: Subcools super soil, with the BMO line of nutrients.

Any advice would be appreciated!!!

Also on a side note, has anyone used citric acid for PH down, and if so, how do you measure out the dosages?