I study for long time all about pH management and plant nutrition and my "hungry wish to know more" bring me to ask something else...
what is with pH and plants without any medium like I grow my Cattleyas ;-)
They are in basket almost without medium, some do have little big orchiata, and some of they are without any medium. I give them water with fertilizer (50 ppm N) 2-3 times per week. I spray them like they receive rain and they are back to their place. Roots are dry very fast as they always have brezze at my 6 flor at opened balcony.
I know my RO water is 6.8 and after added fertilizer (MSU RO) solution is 6.6 pH. Sometimes I get 6.2 with vinegar but I must admit I am not very regular to do this ;-)))
While it is generally recommended to maintain the pH between 5.5 and 6.5 (+/- 0.5) for the sake of the plant, pH has very little impact on nutrition in plants that are not grown in soil.
pH affects the cation exchange capacity (CEC) of soils, that is, their ability to trap nutrient ions and how strongly they are held, which of course, affects their availability to the plants. Orchid media have almost no CEC, and certainly "air" doesn't.