Paphiopedilum sanderianum (not yet blooming sized)
Paph sanderianum is a calcicolous species; the term refers to plants which thrive in soil that is rich in lime. In nature, Paph sanderianum orchids are found growing on limestone cliffs in Borneo. Limestone is composed of different crystal forms of calcium carbonate. In practical terms, this means that Paph sanderianum can benefit from calcium supplementation when grown at home.
Since the media I use for my paphs does not contain any calcium, and New York City tap water contains rather low levels of soluble calcium, I wanted to try adding some calcium supplements. However, finding clear directions on how to do this proved difficult. There is plenty of references to some paphs benefitting from extra calcium, and the most common method of supplementation I could find involved adding crushed limestone or oyster shell directly to the potting media.
Using crushed egg shells to supplement calcium for paphiopedilum
Now I don't have any access to limestone or oystershells on hand, but I did have leftover egg shells, which are 95-97% Calcium carbonate crystals. Although CaCO3 has very low solubility in water under non-acidic conditions, a little can go a long way.
I boiled the empty egg shells to get rid of all remaining egg whites, and peeled off the membranes covering the inside of the shells. I then used a coffee grinder to crush the egg shells into small pieces, which I could sprinkle on top of my paph's potting media.
Unfortunately, I can't think of any way to make this more scientific. I can't readily measure soluble calcium levels before or after supplementation, and with one plant, I can't attribute any changes in growth as a benefit from adding egg shells. In addition, while writing this post, I remembered that my Paph Sanderianum had rocks in it's old media, which I had saved and mixed into fresh media when I repotted the orchid back in April. It is very possible that those rocks are limestone gravel. To avoid accidentally overloading on the calcium, I don't plan to do this more than twice a year at most.
In unrelated news, the Paph sanderianum is growing a new leaf. It also has a mysterious bulge at its base which may be either a new root or a new growth, either of which would be really great news.