I measure the pH once and mix up the same formulation every time, but that's easier with RO, as it's always going to be the same. A periodic check is a good idea, just to be comfortable.
Orchids3: That pH range predates Bill Argo. Even Rebecca Tyson Northen talks in the 6.0-6.2 range 30 years ago.
Also, those so-called "nutrient availability" charts are relatively meaningless for us. They all come from one study involving the pour-through of solutions and SOIL, so shows how readily the ions can be entrapped or released at specific pH levels for that soil formulation (and its "cation exchange capacity") and one particular fertilizer.
For the most part, orchid media components interact very little - if at all - with nutrient solutions (i.e., the cation exchange capacity is near zero), so the simple fact is that if the mineral exists as an ion in solution, it is available for the plant to take up. Granted, under some pH conditions, molecules may become so complex in size or shape that it blocks absorption, but fertilizer producers are pretty savvy about such stuff, so any mineral in them will be available at pretty much any pH to which we expose our plants.
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