If it's iron, it would be an excess of iron not a lack of.
"Note that when adding iron to the solution, it is often necessary to not use fertilizer for that watering.
Iron has a tendency of reacting with many of the components of fertilizer solutions, and will cause nutrient lockup to occur. Read the labels of both the iron supplement and the fertilizer you are using before you attempt to combine the two.
- Iron Toxicity
Excess accumulation is rare but could cause bronzing or tiny brown
spots on leaf surface.