Hi @_Maq_ can you elaborate a bit on this insight... I am reading it as: for a certain amount of x micros (i.e. say you're using 0.5 ppm/wk of Fe as a proxy) you will need a certain amount y of phosphorus... I assume to stimulate uptake... assuming my understanding is correct; is there a rule of thumb - say a ratio - that goes with this in your experience?
My way of dosing micros based on Marschner
Marschner's Mineral Nutrition of Higher Plants provides a chart indicating relative uptake of nutrients required for a healthy growth. You can see key numbers in red in column "
P eq.". Beware, the numbers are in
moles. So, if plants uptake one molar unit of phosphorus (or phosphate - it makes no difference in moles), they need 0.03 molar units of boron or iron, 0.015 molar units of manganese, etc.
But you, like most non-chemists, prefer weight units, like mg/L = ppm. Let's translate moles into mg/L. If you dose 100 µM phosphorus, which means 9.50 mg/L phosphates, then plants need to uptake 3.00 µM iron, which returns 0.1675 mg/L iron. And so on.
If you dose 1 mg/L phosphates, plants will require 0.0176 mg/L iron.
Alternatively, if you dose 0.1 mg/L iron, corresponding phosphorus uptake would be 5.67 mg/L phosphates.
I stress that these numbers refer to
net uptake by plants. To reach that, we need to overdose some nutrients. There's little worry about boron and molybdenum, they don't tend to get lost. On the other hand, iron losses are always significant because iron readily reacts with mere oxygenated water. The remaining metals (Mn, Zn, Cu, Ni) stand somewhere in the middle; they may or may not precipitate in various compounds. The risk is always higher in basic, alkaline, and generally lushly mineralized water.
To the very right I've added an example showing how I dose micros in my tanks. You can see, only 0.38 mg/L phosphates per week. Low-tech, lean dosing. Ideally, if there were no losses, I could dose corresponding 4.000 P.eq. "units" of iron and other micros. Yet my practice showed that that's not sufficient. So, I overdose iron (10 P.eq. units) and slightly overdose the other micros, too. (I never use Mo and Ni as it makes no sense, even in my RODI water, traces of these metals are present, and that's enough.)
I do not pretend that my numbers are valid for everyone, in any circumstances. It works well for me, but conditions in many tanks are very different. Still,
I consider Marschner's ratios a starting point for every tank. Like I said, iron regularly needs to be overdosed, the more so if bicarbonates (and phosphates) are abundant, which is often the case in hi-tech tanks with EI dosing. Yet overdosing non-toxic iron should NOT lead to overdosing other micros (which happen to be toxic). Their behaviour is different and risk of precipitation is remarkably lower.
in some states they are considered invasive... the Lysimachia nummularia , that is - not the Norwegians... 
