Combining iron with KH2PO4 would result in a precipitate from what I've read in the past.
Let's see!
These first two are a couple pics where we did dilution on another forum.
Some notes:
Doses are for ~2ppm PO4 and ~0 .5ppm Fe (and ~15ppm NO3) when diluted in my tank's volume.
CSM+B+Fe is Greg Watson's old (2004?) mix -- as I understand, he added Boron to CSM using the standard ratio (7% Fe by mass) and then added chelated Iron to bring the mix to 10% Fe by mass. Its been in a ziplock bag in a drawer for most of those years, and I'm not really sure what that does with chelated Fe.
"DPTA" should be "DTPA."
Macronutrients pic is maybe 10 minutes after dosing and doing a couple quick swirls with my tweasers. The KH2PO4 is fully diluted by this point. Micronutrient pic is less than that (30 seconds?).
I'm sorry for the cell pics. My camera is broken. Also, my tank is the best light I've got.
And here's our question tonight.
Notes:
In each case I dosed Fe first.
Same doses as above (~2ppm PO4, ~0.5ppm Fe).
This pic is maybe 5 minutes after the mix.
And this a little later.
Notice that there is some percipitate (FePO4) even in the 30 minute sample. Happy to take them an hour or so apart but, you know, it's water
(There should be no advantage to dosing K, P, or N at a particular time, assuming all are available, since all just float around anyway.)
For further clarification on daily iron supplement dosing. That only applies in non-Estimative Index (EI) dosing right?
No! I'm not sure Tom would agree (but I'm pretty sure he would), but it is important to understand that the concept of EI is to dose more than plants uptake, then use water changes to limit max concentrations. (If I put 20ppm NO3 into a tank with zero uptake and then do a 50% water change before the next 20ppm dose, I am only going to slowly approach 40ppm NO3.) That's it. Dosing macros and micros on alternating days and the commonly circulated dosing schedules are more a matter of convenience, and this is why the doses work for a range of tanks; You could dose once a week with EI if that dose is more than plant uptake before the next dose. or you could dose a bagillion times a day: your goal is no limiting nutrients, then water change because you can only estimate what's in the tank and a flush is effectively (estimatively?

) back to 0. If plants can use it, EI says we should add it, within reason, of course.
