[JeffyFunk]

That's not quite how I interpreted the results... My interpretation of the results was that Fe(EDTA) was able to accumulate in the water column while Fe(glucconate) was not. It's really difficult to say what the cause of that difference is from. I can think of a couple of scenarios: (1) Fe(EDTA), being a Fe(III) species, is more difficult for the plants to assimilate than Fe(glucconate), which is a Fe(II) species. Similarly, the plants could just prefer the higher energy Fe(II) species over the oxidized (and lower chemical energy) Fe(III) species. (2) Fe(glucconate) is less stable than Fe(EDTA) so the Fe(glucconate) precipitates out of the water column faster. Maybe it reacts with phosphates faster? Without doing, say, isotopic labeling studies, it's not clear what mechanism is going on. (In that experiment, u would label the Fe species being dosed, then check the plants for signs that they took up the labelled Fe species. If the plants were not taking up the Isotopically labelled Fe, it would remain in the water column or precipitate out.)

My take home message would be that Fe analysis in the water column is complicated and u have to be knowledgeable of how it's chelated, what ur water chemistry is like, etc. Most importantly, u have to watch ur plants to c if they r showing signs of Fe deficiency - that's ur best indicator.