Sorry, Jacaranda - that's the same BS chart I was referring to earlier.

Something like that gets published, and over time, nobody knows the where the data came from or the details behind it, so it becomes "gospel".

I did tell you !
 

I did say where I got the chart from !.

It is a good chart in my opinion - but orchids are adaptable and they can make some adjustment in their own roots. So I would take away the green zone. My stage of chasing pH has ended to the point that I only rarely check it anymore. If I make fertilizer brand changes or media changes then I check. It is also a good check for media breakdown if you measure what comes out of the pot not what goes in. What I have observes is that roots rot more at lower pH (acid ) and much less at neutral (pH 7). Am using rocks (Stalite) so media breakdown is not a consideration.

I don't know exactly when the experiment was done that led to the generation of that chart, but it was based upon a single experiment with a single soil sample.

The availability of nutrients in soil is directly related to its cation exchange capacity, and CEC is definitely related to pH. But we don't grow in soil, and the CEC of most orchid media is essentially zero.

The fact that the graph has been used by "experts" for years - heck, I treated it as gospel until I learned better - does not mean it is valid and/or pertinent.

Sorry buys, but my own opinion is that there's WAAAAY too much worry about pH.

Very dilute solutions have little buffering and can vary wildly with slight changes. Even shaking a sample will dissolve more CO2 (which forms carbonic acid) and drop the pH. My own feeling is that in well aerated media this natural gas exchange will keep the pH the orchid sees in a good range.

Unless you live in an area with very hard and/or alkaline water, worrying about pH is probably more effort and the energy could better be spent repotting something, checking for pests, etc..