Sorry everybody, I goofed. I add iron at 4.5 uM or about 0.25 ppm. This is
ten times more than I posted originally. The micronutrient mix I use is
based on work done by Elias Landolt who is (was, I think he recently
retired) the foremost expert on duckweeds in the scientific community.
A few more comments ... the mix also contains Na and Cl which are also
considered at times to be micronutrients ... the Co is probably not
necessary but I like to add it for completeness.
Again, thats 4.5 uM Fe twice a week.