Gordon, rock wool cubes

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I just add them to my mix dry. I never measure, but about 20-25%. I always pre-soak my mix for at least a day before using it. Really wet, I have to drain water out of the mix tub before I pot.

Only been adding them for a year or two, haven't noticed any adverse effects at that mix rate. 100% rockwool cubes never seemed to work as well as I hoped, so I am just using them as a moisture retentive component in my usual bark mix. I may or may not continue to use them after my (vast) supply runs out.
Is your bark mix Orchiata or other?
I believe the larger size are called Grow-Chunks.
We are moving all our Phrags into a mixture of Grodan cubes, diatomite, leca, and coarse perlite. See photo. The instructions say to mix plant food in water. For hard water areas add lemon juice. Then, soak the Grow-cubes in the solution.
I remember an article in Orchids Mag, I believe, that said diatomite and phrags were not a good combination. May want to investigate. I did a quick search on AOS.org in the Members section and found this from 2006 but the article I remember was much more recent. I think you have to be an AOS member to access it.
Thank you to all of you for posting. Now I'm trying to digest it all. I really would like to move away from organic media as much as possible, either because I'm lazy or don't want the waste of repotting (your choice). I especially appreciate the contributions of Ray, as he has a scientific bent that I like.

A few comments:

I see possibly the "best" mix to be 2/3 leca and 1/3 Grodan pre-soaked rockwool. Questions of algae and compacting still unanswered for me. May just have to try a few and see how it works for me. But I'm also quite aware that the medium is only part of the story--so are my local conditions (I'm on the northwest coast, so fairly cool and humid) and the container. That often gets left out of the discussion but is as much part of the root ecosystem as the medium. Do clay or net pots affect the outcome? It occurs to me that they do, with more rapid drying at least, and possibly more air at the roots (though with clay it's hard to know).

I believe Ray brought up another aspect, especially as it relates to the slippers. These are not true epiphytes, and are rather ground little or stream residents, and I'm with him in considering the native environment when potting. So shallow pots like with cacti? It used to be people used clay orchid pots for some genera, which appeared to give more air to the roots.

In the end, I suspect that successful root growing in the majority of the challenge of growing orchids in non-native environments. And getting the air/water balance right is at the heart of it, and that seems to vary by genus.

This is not a Catt list, but I learned an important lesson about them recently when I bought one from Tony Wells at letstalkplants grown in a wire hanging basket with leca only. Grew great, but I grow on a bench so have to contain the roots from going everywhere and attaching. So I put it in a larger clay pot, primarily to contain the roots, but lo and behold, it also became a medium, and now the roots are attaching to the clay in the airspace between the basket and the pot. I'll see if I can attach a picture. It loves it. I will never grow a Catt in a different way, but it's more a perch than a container and the plant can no longer be repotted or needs to be. When it gets too big just lop off a piece.


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Please let me know the gist of the reason, from the article, thanks.
I haven’t looked thoroughly, but I found this from Fred Clarke. AU diatomite is from freshwater sources and apparently okay/beneficial. Calif, and some others are sourced from salt water, so I’m thinking that might have been the issue (salt leaching). I thought I read it in AOS but can’t find it in a quick search. If I find it I’ll repost.
As an aside there’s a lot about using diatomaceous earth for bugs. It is very effective, as sharp particles kill insects by causing desiccation, however not practical for orchids as it is completely ineffective if it gets wet. I’ve used it in Bluebird and Purple Martin nests (under the nests) and it’s very effective against mites and blow flies as long as it remains dry.
Fred’s article