>So, I suspect that calcium carbonate sand might change the pH, and that is >why it is undesirable for potting mixes? Probably, but my guess is that saying "silica sand" is a hedge against potential outrage from one or two customers who might have expected aragonite and gotten silica sand instead. I use sand (silica sand) to add weight to seed pots left outdoors, to prevent them from being blown away, and pure sand to grow cactus seedlings after initial germination in a regular mix (peat, perlite, sand, calcined clay, whatever). And I never use turkey grit without tasting it first .... Bob