Just thought I'd post something here instead of starting a new thread about something which struck me recently.
pH is temperature dependent, and that means that when you're trying to have fine control over the kinetic properties of a solution (either your reservoir water--or what it is you're watering in)--which is essentially the point of adjusting pH--it matters BIG TIME what the temperature is of your water.
This is one of the reasons that soil growing is likely more forgiving in terms of feeding. As weird as it will sound the difference is that in soil the medium is the solvent, and the water and nutrients are solutes.
In hydro, that's reversed (and the medium, if any, tends to be insoluble).
I'm frankly shocked that this doesn't come up in hydro discussions more, though. This MATTERS a lot in terms of the kinetics of any reaction taking place. pH is talking about how many free H+ ions are in solution (technically it measures the Hydronium ion which is H30+).
The tendency of this ion to form and stabilize depends HEAVILY on temperature. In fact, as a chemist I look at "temperature" as a description of the translational, rotational, and vibrational movements of the molecules which I am measuring (and this includes solids, which are very much moving).
In the end, by making a solution and feeding it to the plants--we are attempting to control chemical kinetics. Temperature is at the very base of such considerations, it is a state function (and these are usually important :P ).
If you want to know whats important in a solution of just about anything (solids, gases, liquids--all three of which we're dealing with in hydro) you look at the gibbs free energy equation which is:
(delta)G = (delta)H - T(delta)S [for an isotherm]
Or, for the calculus inclined:
dG= dH - d(TS)
From this quantity we can find literally everything. Every function in this function is a state function (which means that the total change in the function between two points is the same irrespective of the path--a fancy way of saying that if you cool something from 20F to 0F it doesn't matter how you cool it, the thing is the same temp).
If we do a bunch of fancy integration we can find the work, the heat evolved, the enthalpy change (H), the entropy change (S), and perhaps most importantly for this discussion the temperature at which a reaction becomes spontaneous--and the degree to which it is favored.
Now, the systems we're looking at are way too complex to do this type of analysis on them (and we don't have the proper instrumentation). However, that doesn't mean that it isn't subject to the same rules--and that we shouldn't be looking at them with the same set of eyes.
This ultimately goes back to another of my gripes (and hopes) about the MJ community at large--we need to be collecting data better.
Checking TDS and pH is fine and good, and its at least half the battle to be sure--but without temp to go along with it, it is incredibly far from the full story.
Realistically pressure (both air and water) needs to be taken as well--and it would be helpful to know what the levels of dissolved oxygen are as well in the res, as this changes kinetics significantly as well.
It's definitely too hard of a thing to nail down--but I think failing to take rigorous temp/ph/TDS data and play around with the values in hydro is a mistake, and that opportunities will be missed as a result.