nikv....
Do NOT think for even an instant that I have at any time recommended the use of limestone, or calcium, in the potting media for any paph species that I list in my book. Specific references in my book are strictly limited to the citations where particular species are found growing on limestone.
I have, on occassion, recommended the use of calcium to be incorporated into a potting medium for paphs, STRICTLY as a buffer against highly acidic water sources. If you wish for a list of species that would BENEFIT from calcium, you need to post the analysis of your water supply.
The rather common practice of using different forms of calcium, limestone, etc., in practically any potting mix used for Paphiopedilum orchids is nothing more than BS (Bogus Science). This ridiculous idea has been perpetrated by ignorant individuals who are not thinking.
If it were true, that some paph species REQUIRE calcium (common preception), then why is it that in habitat we often find the same species growing both on limestone rocks AND on trees?
Seems to me we'd have a lot more properly educated people if more of them would not base their education simply on what they read on the 'net.