How do you actually mesure the amount of your fertilizer? Is it so much fert in a spoon or by weight?
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No but you can use that information to give you a good idea as to how much salinity they have evolved with or are receiving.
Most big hydroponic systems have an in line salinity meter wired to an alarm to warn when EC goes too high or too low according to pre set parameters. Every plant crop has an established optimum salinity tolerance varying from very low to very high above which growth is reduced. But the actual fertilizer doesn't change that much.
The actual optimum combination and levels of individual nutrients is determined through other trials (which you are doing right now) but I say again that knowing the crop's tolerence to salinity is vital.
Also important is knowing that pot media water salinity will double when moisture levels reduce by half. You can use EC meters to determine the salinity of a pour through extraction and adjust accordingly. How else do you really know whats going on in the pot?
I measure fert by spoon. At a particular spoon/water volume concentration I check nitrate concentration by cadmium reduction method to verify the spoon measure.
The "salinity" of what the plants evolved to is not relevant to the "salinity" you are exposing them too with fertilizer.
An easy example is the reference toxicant tests we expose our water fleas to in the lab. We can use sodium chloride or potassium chloride. The EC of 2 gram/liter of either salt is 4000 usiemens/cm. However, the reproduction of the fleas will be suppressed at .8 to 1 g/L of NaCl, but down to .3 to .4 g/L of KCl. The most extreme difference would be between calcium sulfate and potassium nitrate. The water fleas will reproduce at just about the soluble limit of CaSO4 (3or so g/L), but will be impacted around .05 g/L of potassium nitrate. The EC at the toxic level is all over the board for the different salts. We observe water fleas thriving in waters with EC from 25 to 8,000 us/cm, but when we see them crash at EC of 500 we know we have a solution lopsided with K and N, and if they are thriving at EC of 4000 we know that there is mostly CaSO4 in the mixture.
We also have a different species of water flea that is more salt tolerant than its smaller counterpoint. For most of the salts (NaCl, KCl, CaCL) it can hand twice as much. But when it comes to the nitrogen salts it is just as sensitive as the wimpy one.
These observations are so predictable we actually have developed computer software to model the toxicity of salt combinations without having to even put organisms in the water to prove it.
Plants react in the same way as non photosynthetic organisms, but at different absolute levels. So going back to the kovachii example, even though the habitat EC is 400us/cm (not low by standard surface water levels), this is made up mostly by Calcium sulfate, and I bet you that if you continuously expose your PK to a solution of potassium nitrate at EC 400us/cm that you end up with big trouble.
The EC meters on hydroponic systems are "calibrated" for an optimized solution strength of known ingredients with known reaction concentrations based on observation of that particular system. It has nothing to do the the EC of the groundwater or surface irrigation water these plants may be exposed to in the wild.
So now when I get the urge to feed I go and look at the 50 ft poplar tree that I planted 12 years ago when it was only 1 ft high. It has never been fed or watered (other than rain). It's bloomed fantastically the last three springs. If the tree can grow 50 ft of biomass in 12 years, then why should I worry about feeding a 12" tall paph?
