Why is everybody focused on adding ferts to the water column? Wouldnt it be better to fertilize the substrate like any other plant? If you can grow most of these plants emersed then it stands to reason that they a are perfectly capable of pulling nutrient that way. Of course floaters and moss and subwassertangy things and more to the point, algae, would probably need the nutrient in the water column. Why not stress the algae and only fert the substrate? assuming that you have a substrate with lowish water flow and high CEC.
There are a number of reasons why we focus on the water and not the substrate. But first let us go a little back and deconstruct this false assumption.
Wouldnt it be better to fertilize the substrate like any other plant?
It has long been known by plant scientist that terrestrial plants take up nutrients through leaves, also known as foliar uptake. This is not only the case for epiphytic plants. Take for example a table from Hudská 1976:
Nutrients Time at 50% absorption
Nitrogen in urea 1/2 to 2 hr.
magnesium 2-5 hr
potassium 10-24 hr.
calcium, manganese, zinc 1-2 days
phosphorus 5-10 days
iron, molybdenum 10-20 days
and this conclusion from
Fernández and Brown 2013:
"there is abundant evidence showing the beneficial effect of foliar fertilizers in terms of improving the metabolism, quality, and yields of crops"
The idea is, like any normal plant, aquatic plant take up nutrients trough the leaves as well.
Now back to why the water and not the substrate:
1. uniform distribution -fertilizing the water ( in an aquarium or small pond) allows you to get good nutrient levels to all the plants at the same time. IF you rely on the substrate alone, a nutrient demanding plant might use up the nutrients surrounding its roots before others and starve.
2. quick replacement - in terms of adjusting nutrient levels it is a lot easier to do a water change than to change the substrate
3. regulated amounts - a soil based substrate will leach into water, slower if capped but it will. There will be a gradient of concentrations of course but still some will get in the water column anyway. ( this is also the reason why in some lakes the top part of the water will be sampled and reported as nutrient poor. But nobody samples water from near the substrate where the plants are. And they you get books published showing X plant growing in nutrient poor conditions)
While ammonia can be bound into soil, NO3 cannot so it will all be quickly released into the water. PO4 the same, plus other forms of phosphorus found in soil would need an anaerobic soil depth of ~15cm to be brought in the PO4 form ( this is from a study on the cycle of PO4 in marshes, heavy anaerobic conditions). Iron and other metals also need anaerobic conditions to become plant available. So the soil will provide nutrients to the water column, thus nutrients will be available to algae as well. But at what rate will they become available to plants ? At what rate will they be released from the substrate ? Hard to tell, harder to reproduce.
4. the need to control - there are other reasons but I think it all sums up to our focus as a society and as scientific community to control all and to be in control of what happens. We like a success recipe and we like the guarantee the promise of success. At the same time we fear uncertainty. This all requires us to take an active role in our aquarium keeping instead of just hoping and relying on a soil that it has everything our plants need.
My conclusion is that plants don't really care where they take their nutrients from as long as they are available. It is not wrong to use fertile soil, but better to use it as a backup in case your column dosing is lacking something.