People add extra Iron under several mistaken impressions:
1. They think that Iron is required by the plant in greater mass or is of more importance than Nitrogen, when in fact it's the other way around.
2. They think adding more Iron will turn their plants red.
On the one hand there is nothing wrong with adding more Iron. It's not toxic at these levels and there may be a genuine shortage in the tank due to poor distribution or excessive lighting. But on the other hand, Iron products costs a lot more than Nitrogen products, for example, and since the plant needs 500X more Nitrogen than Iron buying more Iron is not the best use of your resources. Of course, many people have the means and the cost of nutrient products is not a big deal. But many others have to watch their pennies, so this becomes an important point worth noting.
It's very easy to know. Iron should be used as a proxy for the other elements because the plant uses 100's of time more Fe than the other micronutrients. If you are low on the other micronutrients then there is a good chance you are also low on Iron, unless you are dosing only Iron and the tap water is low in micronutrients.
Iron or Magnesium shortages are characterized by yellowing of the green leaves or paleness/yellowing in non-green leaves. This occurs predominantly in new leaves and not so much in mature leaves. If you are already adding a trace mix and these symptoms appear, then you can just add twice or three times as much. After a few weeks the color of the new leaves will improve.
Again, if you are adding EI levels of the trace mix and are experiencing these symptoms then that is a strong indicator that your flow and distribution methods are suspect. Most likely, poor distribution will show an effect first in NPK because they are more important. If the plants are not showing deficiencies in N, P, or K but are showing these symptoms, then the cause is most likely an under-dosing of the trace mix.
Trace mixes vary in their composition and so it may be that a particular mix is low in Fe, say 5% or less, when normally we'd like it to be around the 13% mark. So people go out an spend more money for extra Iron when all they need to do is to add twice as much of the same trace mix, which will increase the concentration of the Iron.
Cheers,
