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Re: Fe+3

There is no evidence to suggest that plants are unable to take up ferric 
iron (+3)... its just that the solubility is so low. There is also no 
evidence to suggest that chelators such as EDTA sequester the reduced 
ferrous iron... EDTA can chelate ALL metals, regardless of charge. There 
are also abundant natural chelators in water that may enhance iron uptake.
The mechanism for plant uptake of chelated iron is also poorly 
understood... though it is interesting to note that while EDTA enhances 
the uptake of Fe, it reduces the uptake of Cd and other toxic metals... 
likely because the uptake rate of chelated iron is faster than the uptake 
of aqueous Fe+3 while the uptake rate of chelated Cd is slower relative 
to the uptake of aqueous Cd+2 (even though the uptake of FeEDTA and 
CdEDTA are likely identical). This suggests some sort of universal 
chelator receptor in plant membranes which is not sensitive to the 
chelated metal but rather to the chelator itself.