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I am finally moved to speak. What a load of drivel we have been hearing
about EDTA in the past few days! The purpose of using EDTA is to prevent
iron from precipitating out of solution as rust. This works because the
solubility of iron in oxygentated water is extremely low and EDTA
increases the amount of iron available to the plant.
When iron is complexed by EDTA in waters which contain oxygen, it is in
the form of Fe+3. The charge of iron when it is absorbed is irrelevant
to the plant ... there is no requirement for one form of iron over another.
EDTA will complex all cations regardless of their charge so again, there
is no requirement to supply Fe+2.
EDTA breaks down in the presence of light, no matter what cation is
complexed with it. This is because, like most organic compounds, EDTA
is degraded in the presence of UV light. To suggest that sodium EDTA is
somehow safe from this process is ridiculous, especially since Na will be
displaced from EDTA in the presence of Fe or other cations.
DTPA is more stable than EDTA and will stay in solution longer. The
trade-off is that DTPA is harder to find and needs to be heated for it to
dissolve in solution. Disodium EDTA is extremely easy to dissolve and
work with. Either chelator is effective and will result in iron being
more available to your plants.