Calcium nitrate
 

Does anyone use calcium nitrate on cattelyas or slippers?

If so, how much do you use as a normal supplement?
How much for treating a deficiency?

I make my own fertilizers that contain sufficient calcium so I do not need to supplement but if you wanted to use calcium ammonium nitrate as a standalone supplement then I would suggest 100 to 300 ppm.

Thanks for the replies. I only need a few tbs for these two sibs. Anyone know where I can get super small amounts of this? I don't have a local OS or gardening club to ask.

I have to ask "Why do you think you need Calcium nitrate" specifically?

Agreed.
If you use balanced fertilizer with calcium included in it, then you don't need it.

Do your cattleyas have black necrotic spots on their new growth? Then that's a sure sign of calcium deficiency unless if it's some kind of rot.

Remember if you add calcium, you'll likely need to add magnesium as well.

I've used calcium nitrate. It's good stuff because there isn't any phosphorus or potassium to interfere with the uptake of the calcium. Also the nitrogen is in nitrate form which is easy for orchids to utilize. I would recommend using it at 1/2 teaspoon per gallon, which is 102 ppm. It doesn't dissolve in water as well as other fertilizers. I dissolve it in a small amount of hot water and then add it to the water in my sprayer.

This might be off topic a little, but for my calcium loving plants i read in an old book to crush egg shells and sprinkle on top of the bark. Water like normal and the calcium will leach out slowly. I done it for years and seems to work great

Yes, any time releasing forms of calicium would be great to add in late winter or early spring so they get a chance to start breaking down by the time the plants need them. I'm not so sure egg shells would be of any use to orchids since the mix is so open (it even takes them forever to break down in my composter), but the leaves are dying now and any slow release amendment would be of zero use to them since they are already showing massive damage from deficiency. They need immediately available calcium in larger than normal doses to stop the damage, and from what I've read, calcium nitrate is the only water soluble form of calcium that plants can readily take up.