When using typical potting mixtures, I was taught in hort-school that you calibrate down one point so a plant that likes a pH of 6.5 in typical soil would do best with a 5.5 pH in the articficial medium we call potting soil. This is because the much higher level of organic matter in potting soil changes the chemistry of nutrient availability. A deeper explanation than that is beyond my present grasp.
[NAFEX] pH preference in pots,
Alan Haigh, 11/07/2008