The additional micronutrient is not added because I pre-supposed it would need more based on the plant's reputation, it's based on what it needs to stay healthy

The additional micronutrient is not added because I pre-supposed it would need more based on the plant's reputation, it's based on what it needs to stay healthy. It is empirically determined by watching for chlorosis on the edges of the leaves. Not enough, and the leaves grow pale; too much and the plants are poisoned. Observations showed that brazilian spinach developed chlorosis if I did not give it a bump of iron compared to the other species. That being said, there are plenty of confounding factors so you're also correct that one may not simply conclude it is iron-hungry just because it needs more micronutrients to stay healthy. It could also be inefficient at uptake, could be a pH problem in the water, or it could be growing at a faster rate, etc. etc. A more accurate reporting of my experience would be to just say a "15uL per week bump for species like spinach" without opining on a causation. Readers should be aware that the article is riddled with problems like this, and should not be cited as any authority on botany. For example, I also haven't taken samples of the things growing around the roots and examined them to ensure they are mycorrhizae, although I assumed they are. They could also be bacterium or any other number of phenomenon.