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fertilizers, k2so4, etc

combing through the archives has not produced a satisfactory answer for me on this so i'm hoping TB (not tuberculosis) or Dr. morin will chime in. If i'm not mistaken the target range for potassium is around the same as that for nitrate or even more (so 10ppm or up to 20 or 30). although i haven't looked at flourish potassium it seems most fertilizers actually contain far too little - flourish as we know contains basically a trace of potassium but TMG for instance contains .79%K and Kent (the largest i've seen)contains 3%. these numbers are probably reported as K2O i think so its actually a little less. so it seems, for instance, 3% is 30g/L = 30,000 ppm.  even if
you add 10 mL to 100L (about 25g) which is a 1/10,000 dilution you only add 3ppm K. dosing KNO3 to 10ppm adds much more as KNO3 is something like 40%K i think so you get like 7ppm or so. that would seem to bring you into the lower range but only if you use KNO3.
   the situation seems to me to be the same with magnesium.  while its always listed as an ingredient (so you might think you're getting what you need) the amounts seem utterly insignificant compared to differences in GH. I don't have the numbers in front of me but i believe doses usually don't add 
more than 1ppm or so. medium hard water contains what, 30ppm mg? more? so if you have medium hard water what difference does 1ppm make, and if your water is too soft 1ppm isn't going to help.
are my calculations or assumptions off here? i know that's why we
sometimes add things like mgso4 and kno3 etc but why is commercial stuff formulated 
this way?