Re: Water conductivity
KNO3, KH2PO4, K2SO4, MgSO4 etc are all salts.
I'll let you infer from there.
Conductivity is not very good for nutrient replacement, I'd considered it at one point.
Problem is: as the plants uptake nutrients, they expel counter ions (K+,H+'s etc) and the conductivity is typically the same in the water, but does change very close to the plant's surface as the uptake takes place.
If the plants did not do anything to the water, then yes, you might generalize it to some degree.
With hydroponics, as the solution becomes very concentrated, then it becomes more of a problem and Conduct meters are useful.
But we cannot tolerant that type of sludge in our tanks
Regards,
Tom Barr