Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I think I would try it without iron for a while, see how the plants do. I can see a couple of possibilities, and you will have to wait for the plants to tell you what is going on.
a) Reduced dosing + reserves in the substrate = enough supply to last a LONG time, but at lower levels so toxicity symptoms go away. It may take a long time (months? longer?) for deficiency to show up. When the substrate reserves are used up, then the dosing might not be enough.
b) Similar to a, but a deficiency never shows up= the reduced dosing is more in line with how the plants are using the minerals.
c) Reduced levels mean that some mineral will drop below the thresh hold levels more quickly (especially if the substrate is sand or gravel, no cationic exchange capacity) and deficiency symptoms will appear sooner.
Part of the problem here is that there are no tests, short of laboratory testing, to show all the levels of all the micro nutrients, in the leaves and in the substrate. The only test available to the hobbyist is the iron test. This has been used as a proxy for all the micros, under the assumption that the fertilizer manufacturer made the micros formula correctly. So keep the iron at the right level, and all the other micros are assumed to be correct.
However, even several years ago I had heard that CSM+B was low in iron. Another way of saying this is that it was high in all or some of the others. So aquarists started dosing a little iron along with the CSM+B. If you dosed CSM+B in enough quantity to keep the iron high enough, it was over dosing the other minerals.
As long as sand or gravel was the substrate, this may not have been quite so much of a problem. Do a weekly big water change, and you remove the excess because it is still in the water, and can be removed. But if the substrate has any capacity to hold the nutrients (cationic exchange capacity) then any fertilizer that makes it into the substrate is bound to the particles in a way that plants can get them, but won't be removed with water changes, and won't show on a test of the water parameters. So, if you are using the iron test as a proxy for all the micros... and the iron is bound up so you keep testing negative.... you think the plants are using it all, so you keep up the dosing.
I KNOW this happened in my tanks- The substrate has high cationic exchange capacity, and every iron test I did seemed to show no iron within hours of dosing it. Even the sand substrate tanks had enough humus to bind the minerals. But when I stopped fertilizing (stopped macros and micros) the plants stayed healthy for many months. Granted it is a low tech system, but I am not even dosing Excel, and the plants are fine.