Growing media are packaged with relatively low moisture, so the limestone does not activate for some time; therefore, freshly packaged growing media may have a pH of 4.5-4.9 (saturated media extract test method or S.M.E.). Over time, the limited moisture within the package of an unused growing medium slowly dissolves the limestone, resulting in an older, unused growing medium having a higher pH than when it was first manufactured. This change in the pH will occur a little faster if the moisture content of the growing medium is higher. Typically the pH of an unused PRO-MIX product that is 4-9 months old is 5.2-5.8 (S.M.E. method), depending on the product and the moisture content. A growing medium should be used within nine to twelve months of its manufacturing date to ensure that the pH of the unused growing medium is not too high. However, the starter fertilizer charge and wetting agent degrade over time, which often dictates the best time to use the growing medium.
