Lime with lava rock
 

I use lava rock and I love it. I have been doing a little more research and I seen that people use slow-release dolomite lime. My question is how much to use. A big portion my orchid collection stays outside in the summer time and gets fertilize weakly once or twice a week. They receive a lot of rain. I pretty much flush every time I water that I don't fertilize. I just recently started giving them Epsom salts for magnesium. Should I give them lime.

Lava rock is my medium, too. I would definitely add a source of calcium. I use eggshells and oyster shell throughout the year, powdered milk when they are outside and Cali-magic when they are inside. The calcium is important during growth to help build healthy cells. Keeping a steady supply for the orchids is important.

It depends on the water and fertilizer you use. Some water has plenty of calcium. The right pH can be an issue. You need to find out what you have before adding other stuff. Get your water provider's annual water quality report. They are required to provide it in the US.

For example, my water has very high amounts of calcium, but the pH is also high. This makes it difficult for orchids to use the calcium in the water. If I merely adjust the pH of my water they have plenty to take up.