Ferrous sulfate would be the chemical of choice since its water soluable, & the ferrous ion is used by the plant. Would consider the pH of the soil in the experment since a acid pH is essential for the absorption of iron by a plant.

There is a forum "Botany" & they may be able to better answer your question.

Ask at the Botany Forum. You will need to consider pH and chelation. Both are rather complicated. They will want to know which plants your are using, how you are controlling the environment i.e. light, moisture, carbon dioxide etc. and what is your time frame.

I had trouble growing Daphne until a friend who manages the plants at Luther Burbank Gardens gave me a hint. She suggests rusty nails in tne root zone, which solved my Daphne problem. Al