Ray, something about the preparation of the Growstones results in some availability of calcium carbonate, if I am remembering correctly. I think this leads to the alkalinity. I haven't seen an analysis of how much silica might be available. The Growstones have substance and porosity but they don't float much for me, as compared to Perlite.
I grew Paphs and other genera in pure sphagnum moss, but the pour-through pH was always more acidic than I thought best and I couldn't water/fertilize more often than every 2+ weeks for many plants in order to have the media dry out enough. The mix of Growstones and milled moss has shortened this time up, but still leaves me able to leave the plants alone for a two week vacation without anything drying out too much.
So, in my environment indoors the GrowStone/sphagnum mix gives the right amount of moisture retention and a better pH result. I still use lemon juice (5 mL/gallon) with my feedings to further lower the pH and maybe to get benefits from citrate. I checked the pour-through pH today (after calibrating my pH meter) with a Phrag Robin Redbreast flavum currently in bloom and the pH was 6.5. I am in a fertilizer phase where I am using about 100 ppm N with K-Lite with each feeding so the pour through was about an hour after fertilization with this and the lemon juice. The fertilizer solution itself had a pH of 4.8.
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