Originally Posted by Newt
Very interesting Ray and most likely correct.
I have tried doing something similar using the Fertilator and my digital scale. I mix a known amount of KNO3 into a gallon of distilled water and do a test. I use that to compare.
That is exactly right. You can do the same for phosphate as well.
I forgot to say that I use LaMotte kit reagents for K, P and N.
I have not found any test sensitive enough to test for Fe.
I use the APHA standard method for calcium and magnesium, which is similar to the LaMotte kit. I've tried the LaMotte kit and it works OK but the titration is not very precise (at least from my perspective). I think for getting an idea of where your water is relative to those substrates it works well.