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["Alan Haigh" <alandhaigh@gmail.com>]

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.