The “word on the street” is that most orchids do best when the pH is between 5.5 & 6.5, but +/- one full unit is of no consequence. Based upon that “wisdom”, growers dedicate time, testing, and additives to make sure their irrigation solutions are in that range.
Unfortunately, there does not appear to be much knowledge of the fact that the pH of the applied solution generally plays very little role in the rhizosphere pH, unless the solution is high in alkalinity – the solution’s resistance to pH change up the addition of acids, which is different than being alkaline, i.e., pH > 7.
The plant itself, microbes living in the plant and medium, and the medium can all affect it more than the solution. LECA, often being pH neutral, may have less influence than do other media, but it is the sum of it all that controls things, and the plants’ various biological activities can make it fluctuate with the time of day.
If you want to really know what pH your plants are experiencing, do the “pour through” test method:
- Water the plant thoroughly, drenching the potting medium with the solution of your choice.
- Wait 60 minutes for the pot to fully drain and for the “pot chemistry” to come to equilibrium.
- Trickle enough pure water (distilled is best) evenly over the surface of the medium to collect about 50 ml (a shot glass) of the drainage.
- Test the pH of the collected liquid. That is representative of what the plant experiences.
If you repeat that using different solutions, you may be surprised how little difference it makes. If you grow in Semi-Hydroponics and want to assess the plant’s influence, do the test, then pour out a small amount of the reservoir contents and retest the pH at different times of the day – mid-morning, late afternoon, late at night, and just before dawn. You’ll be surprised at the variation resulting from the plant’s various chemical processes.
The bottom line is that, while in-pot pH may be important, that doesn’t mean that careful adjustment of your fertilizer solutions is worth doing.