You have to also include water hardness as ngrubich eluded to with his Ca+ example.
You won't be able to see a difference between DTPA and EDTA if you're dosing EI. Iron is a trace element so you don't need a lot and the difference(if there is one) between DTPA/EDTA will be masked by a lot of other variables that have greater effects on the plants (light, CO2, other traces, etc.).
Last edited by UDGags; 01-19-2013 at 10:17 PM.
Reason: updated spelling