many paphs grow on limestone cliffs in nature and therefore receive some amount of calcium, although one has to realize that limestone is much more dissolvable in cool water, as opposed to the typically warm rains that soak paphs in situ. a well-known commercial slipper grower from upstate NY has an excellent list on which paphs typically are exposed to calcium in nature and, therefore, might be expected to benefit from calcium supplementation. (PM for a link). personally, having used various types of lime, I've turned to crushed oyster shell purchased from a pet store, originally meant to be fed to birds (for what reason I am not familiar). this stuff doesn't dissolve too easily and sticks around, at least until it gets washed out of the pot.