Tom Barr says "DTPH last about 2-3 days in solution, ETDA, maybe a few hours, 1/2 day, the Fe Gluc maybe a few hours/minutes......." but what does that mean? Where does the iron go? It can be neither created nor destroyed, just changed form. So what does it mean that "it doesn't last"? Do the plants all use it or it converts to a form that can't be used?
I will definitely reduce testing eventually. For right now I need to understand the nutrient cycle in the tank so I know how best to manage it.
I'll leave it to the chemists to really answer that one. As we've established before, your desire to go down rabbit holes way exceed mine (and I don't mean that in a bad way!
) But from my limited understanding and past reading, it has changed form so that certain tests no longer register it and even more importantly, your plants won't be able to use it -- which is why I, myself, was a little curious of what happens to the buildup of this particular type. I'm remembering that I've read the buildup of the chelates of the other types can theoretically start doing weird things, but I thought this sort of problem was not true for gluconate.
This thread 'may' help or beget even more questions...