I've seen several papers now that note pH not really being a factor in containers (soilless) for availability. At the very least, not in the same way or magnitude that occurs in soil.
As an example;
"A common belief is that pH of the container growth medium must be 6.5 to 7.0. This is fiction. In field soils, pH is important because as pH goes up complexes form and reduce solubility of micronutrients and likewise as pH goes down, micronutrients become more soluble and available to plants. But in a container you are not growing in soil. Components of a soilless container growth medium contain few micronutrients so pH change has little influence. In short, if you add Micromax to the mix the micronutrients are there. If you do not they are absent and lowering pH will not increase availability. A typical pine bark, peat, sand mix with nutrients added as noted above will typically have a pH of 4.5 to 5.5."
(Carl E. Whitcomb PhD - Growing Cannas in Containers)
The paper from RX points to an availability chart for soils when there are existing separate charts for hydro, so not sure why they didn't include the proper chart. However, they did at least mention exudates and their affect on pH or rhizosphere optimization. In soil, these exudates are critical for proper growth and conditioning of soil for micro-organisms - however many hydro setups negate or disclude exudates completely.
We often suplement CO2 for higher growth rates as that is one possible restricting factor. Not many consider that 30-50% of that assimilated Carbon is given off through roots as exudates and respiration. IMHO, this correlates to the importance (for the plant) in actively regulating the rhizome for nutrient and water use efficiency.
Plants can sort of "steer" the rhizosphere by altering exudate strength to better optimize uptake of elements its lacking. Primarily by altering the "bulk propensity for absorption" of either cations or anions. This has an ancillary effect of altering the soil pH and comes at the high cost of lost carbon.
If hand watering, the next watering will "reset" (or wash out) some percentage of the energy spent in optimizing the rhizhosphere. In high turnover recirculating systems the plant must technically expend enough energy to alter the entire water volume or otherwise get essentially no benefit from the carbon loss through exudates.
There are also compounding factors that can alter medium pH relative to the plants "intentions" too. CO2 given off by respiration is fairly large and is measured in Tons of carbon per acre per year.....yes Tons. Now in a soil situation, microbes utilize almost all of this CO2 and brings the net annual release per acre down to just pounds. However in soilless containers, since they lack microbe diversity, are much more likely to build up CO2 overtime and alter pH.
In addition, there is the potential for buildups of H+ or OH- as a result of cation or anion uptake and medium CEC. In a ground soil situation, diffusion is able to offset this to a large extent. However, again in containers (both soil and soilless) these 'uptake byproducts' can build up and alter pH too. Here is a case where recirculating hydro systems benefit by removing and aiding off-gassing of some of these compounds.
Overall the subject of medium pH is quite complex with all the variables and interactions, so it may be difficult to "blanketly" say one pH is better than another (for all environments or setups). We can say however that these relevant components are constantly in flux and that plants are actively and always attempting to optimize things for itself, irrespective of what we are adding ourselves.
I feel that in containers or closed systems, an optimization of this would be applying value to nutrient composition, medium, pH as well as root exudates and respiration equally - in order to balance variables relative to plant energy expenditure vs any pre-conceived notions of one aspect being "correct or better".
Having said all of this and just personally (in my situation, environment, medium etc) I tend to run pH lower in veg (low 5's) and tend to raise it after stretch phase of bloom. I've not been as happy with plants that fall into the 4's at all in veg, though IMHO that relates to the change in apparent surface charges of the medium particles and thus saturation ratios for cations / anions in the medium vs direct availability of X or Y.
