This doesn't answer your question per se, but you might want to read Larry Jackel's book on Ponderosas. I believe he notes that collected ponderosas growing in soil pockets in between granite slabs are more moist than one would think. This is because the granite concentrates water into those soil pockets, and the soil pockets tend to consist of pine duff from the decomposing needles.
I keep my shimpakus in 100% inorganic soil and water daily.