For hydro, what's an example of "available forms?"
I understand urea and worm castings, blood meal, etc., require some breakdown to be made available. I thought hydro was a little more direct and used more elemental forms?
Those "more elemental forms" are also more reactive. At high PH iron will bond to other elements, and low PH iron will break it's bond. Well add too much calcium and you would have to go even lower PH to get the iron out. These individual nutes/chemicals/minerals don't prefer to stay individual, they like to connect. The science and art of nutrients is getting them to behave with each-other so one doesn't block the other too much and everything is available as a reasonable level at a reasonable PH. This is also why I don't like people who use a bunch of bottle from a bunch of makers. If one sends you crap, he could have blocked everything. If you measure wrong, you create nutrient lockout. 1 part dry nutrients seem like the ideal to reduce this, and why I originally choose the Maxi-Series.
With all that said, we animals adapted to move to survive, plants and especially weeds adapted to change their environment through chemistry to survive. If you notice your PH start to swing wildly after first signs of a deficiency, don't correct it too fast, think first. If it's heading down, and looks iron deficient, the plant may be doing what it can to get iron, pushing acids into the soil/water to make it accessible.
For the above reasons I don't lock down a PH like some do, I wouldn't want a system to keep it exactly at 6.0 or whatever is consider ideal for your strain/nutes. If 6.0 is ideal I want my res to start at 6.0, but then slowly drift back and forth through the range from 5.5 to 6.5. Basically this lets half the week be awesome for high PH nutes, half the week be awesome for low PH nutes, and the rest of the time right in the middle at a happy medium. Plants are never lacking anything for long enough to waste energy trying to change it's environment.