Thanks for the info! And the link. Perfect. Here's a response by imeridian that I think is very logical:
PO4 and Fe will react if stored together, it is not a problem if dosed on the same day. The volume of the aquarium is so much greater and the dose is so much smaller. If small quantities in the aquarium were such a problem, then we'd not be able to dose them at all. It isn't as though your PO4 is zero on the days you dose micros... nor is your Fe level likely to be absolutely zero on days you dose macros. It isn't an instantaneous reaction anyway. PPS and EI 'liquid' both put Micros & Macros in the aquarium on the same day, there really is no problem with doing it that way.
[Here's what I wrote before checking the link you sent...]
Based on the variety of dosing methods that people employ, it seems like this is a foggy
issue overall. But they all sound like methods to simply avoid the iron phosphate from having an easy chance to form. Fair enough.
I was doing some additional thinking and came up with the idea that my soil probably could
capture the iron through natural bacterial processes (as Walstad explains), BUT it would have to get there in the first place. I can't think how the iron phosphate would penetrate the bio-layer on the surface of the substrate... and then, further more, penetrate an inch of fine gravel before reaching the soil.
I've only been dosing for three weeks and I haven't noticed anything that looks like an iron deficiency. But then again, my soil could have plenty of plant available iron by itself. Before my switch to dosing, it's very possible that my macro nutrient deficiencies outweighed any other problems. After three weeks of consistent and precise dosing, the tanks are superbly green, bubbling, and vibrant, and algae is receding.
But, still, I don't want to defeat the purpose of adding phosphate and iron only to create a substance that can't be used! I really want to know the particulars. It's difficult for me to dose every day, i.e. macros one day and then micros the next. So I prefer dosing every other day... Which means that phosphate and iron have to get added nearly at the same time. I want to know if it's possible at all.