Re: Iron solubility

At pH values commonly seen in aquaria (6.5 to 8) the amount of FREE Fe+3 in 
solution is very low. Iron in oxygenated waters forms iron oxides and 
hydroxides that basically precipitate as rust. They coprecipitate 
phosphate and other metals. If you dont believe me, check a speciation 
chart for Fe in solution. Chelation is the only way to deliver adequate 
amounts of iron to plants via the water column. BTW EDTA complexes iron 
as the Ferric (+3) ion.
Biologically, the valence state of Fe makes all the difference in the 
world ... its precisely because of its redox properties that makes Fe one 
of the central atoms in photosynthesis and respiration.

dr. dave.