You could buy the various salts and trace elements and totally make your own nutes but it's going to take some self-educating to be able to mix them all together in the right proportions to get the formulas that work best for the specific plants you want to grow. All the kind people at the various nutrient manufacturing facilities have plant scientists to do that for us and charge us plenty of coin for the privilege.
You could get analytical grade salts to make sure that you have the lowest levels of contamination possible then know you have the cleanest food for your plants thus the cleanest medicine for what ails you.
Your EC value is just a measure of how many ions are in your nutrient mix. It, like PPM, can't tell you what ions are present but only the concentration of all the ions present. A small electrical current passes between the two electrodes and the more ions that are present the less resistance there is to the current so more gets through and you see a higher number expressed in the display. Set up a small wet lab and you can find out how much of each is present.
Either accept that those that know better than you or I know what's best and mix up their concoctions properly to feed your plants correctly or be all anxious about every little thing and totally take the fun out of growing your own meds. I'm a chemist and don't want to go thru the hassle of mixing my own plant foods so after 16 years of using the same brand of nutes and always having lots of pot for more than my needs am happy to squirt some high priced bottled juice into a tub and sit back to watch the pretty girls grow.
Nutes are less than 10% of my growing costs. Saving a few dollars to mix my own and likely inferior product just does not compute.
Life is too short to waste time sweating the small stuff. Other than imminent death it's all small stuff.
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