Simpte said:
That still doesn't account for nutrients that are immobile. Adding iron to your substrate is useless unless it leeches into the water column. And instead of adding root tabs, why not dose more in the water column?
I'm not sure but it sounds as though the term "immobile" may be adding to the confusion. First lets clear up that term, then go back to the original question.
First, iron is very much available via the substrate and taken in directly through the root system. Secondly, essentially immobile refers to the plant not being able to "relocate" or move that mineral element. Iron for example is immobile. Once the plant "takes in" iron and transports it the the leaf it becomes immobile. New leaves on plants in water without available iron won't get the benefit of the plant being able to move the immobile mineral element (iron in this example) to the new leaves and a deficiency becomes apparent in the new growth even though pleny of iron is present in other older leaves. This won't occur with mobile elements such as nitrogen. If the plant has stored amounts of nitrogen it can freely move the nitrogen to new leaves...this may continue to occur until the plants "relocates" too much nitrogen from the older leaves to the new leaves and the nitrogen deficiency would show in the older leaves. So the term immobile refers to the plants ability to move the nutrient internally.
There are several theories as to why some plants appear to be heavy feeders. One is that of nutrient pressures and its effect on the rate of uptake. Meaning, the lower the concentration of
NO3 in the water the greater the chance the plant is able to strip the Prandtl layer of nutrients before the needs of the plant are meet. Three options exist to overcome this from happening. One is to increase the concentration of
NO3 in the water column thereby increasing the pressure and satisfying the plants needs before the Prandtl layer is stripped. Second option is to supplement the substrate with nutrients. The problem with the second option is one of control, you can precisely control how much you add and when you add to the water column, but the same can't be done via the substrate. From my point of view and experience, the only mineral element that is beneficial to add to the substrate is iron. It's likely that more iron is taken in via the roots and iron is more readily available from the substrate. Iron is constantlyl being reduce in the anaerobic areas (root zone) of the substrate to make iron available to the plants. Constant additions of iron to the water column do precipitate and fall to the substrate where eventually these processes will make iron available again but it seems logical to me that adding iron directly to the substrate is a better and faster approach with less potential problems than dosing the water column heavily with iron. The third option is rapid circulation, nature often has this option in place. There are a couple other reasons/theories as to why some plants seem to grow better with substrate fertilization...meaning any plant with a heavy root system, or marsh plants that in nature will spend a large portion of their life emerged rather than submersed like we keep them. But, a complete and thorough answer would be beyond the scope of a simple forum response. It's my belief based on my aquatic growing experience that rooted plants such as swords and crypts are easier to grow for most people if they supplement iron in the root zone area of the substrate for those plants, but always take care of the nitrogen, potassium, and phosphorus needs via the water column combined with a comprehensive micronutrient provider to the water column.
