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This is a discussion on Limestones for Paphs? within the Genus Specific forums, part of the Orchid Culture category; FWIW, I have used MSU RO formula (containing 8% Ca, 2% Mg) exclusively since about ...
FWIW, I have used MSU RO formula (containing 8% Ca, 2% Mg) exclusively since about April of 2003, switching to K-Lite (a low-K version of that, with 10% Ca and 3% Mg) since late November 2011, and apply it at 35 ppm N and a pH of about 5.5, and all of my plants - including a wide variety of paphs and phrags - are thriving without the need for added top dressings or Ca supplements.
A good example of getting to the same result in different ways...
If you have access to any of several specifically formulated water soluble fertilizers as Ray suggests, or if you can supplement your fertilizers in various ways like Zainal, you don't need to worry about getting calcium from limestone or oyster shell, etc. But orchids were thriving long before such formulations and supplements were available. If you only have access to simpler fertilizers or none at all, or you just prefer to be low tech or organic, natural sources of calcium and magnesium can get you there too. Especially for Paphs and various (semi-) terrestrial orchids, a little compost and some dolomite can go a long way.