It's the reason I have always been a bit skeptical of these types of experiments. In a real planted tank there are a lot of forces at play. Light level, CO2 optimization, nutrients, flow, substrate type, substrate health/cleanliness, filtration, dissolved organics in the system, water change schedule, horticulture methods, plant mass in the tank, etc, etc, etc. And it's not to discount the interest and value of such experiments, but I think it's a mistake to extrapolate those results to every tank ...
I 100% agree with
@GreggZ that "it is a mistake to extrapolate these results to every tank". I even believe that one of the greatest enemy of true knowledge (and progress in any field) is subjective interpretation of results, where the researcher (or observer) is seduced into drawing speculative conclusions. I myself am often an example of this, unfortunately. I research something, something comes out of it, and I often interpret it incorrectly, exaggeratedly ... drawing conclusions that don't follow. This can then lead us to practice or promote incorrect practices, and the truth escapes us.
Even worse, however, is perhaps "selective blindness", where we try to convince ourselves and others that certain specific parameters (e.g. high concentrations of potassium, nitrates or microelements, or too low or unstable CO2 levels, etc.) are behind the successful or unsuccessful growth of a particular plant. The problem with this approach is that it ignores the context and various principles.
When someone complains on a forum that a certain plant is not growing, one political party starts advising them to add CO2 and increase all the nutrients, while the other political party starts advising them the opposite. The result is a completely pointless war that has no winner.
If someone says that they can grow Ammannia pedicellata 'Gold' or Rotala wallichii in a nutrient rich substrate and in water with zero alkalinity and low nutrient concentrations, absolutely nothing can be concluded from that, because we do not know, what characteristics the substrate has, how much nutrients it has, what its redox is (which is important in terms of accessibility of some nutrients), what bacterial composition it has (as
@sudiorca correctly points out), and last but not least what nutrients it has in the water and in what proportions. If I use a substrate with different properties, a different bacterial composition, a different nutrient content that can leach into the water, so that suddenly I also have a different water composition, then logically I can have a completely different result.
But that is why scientists are conducting controlled laboratory experiments to try to figure out (as challenging as it is) what factors are key and what factors are secondary => for example, when it comes to the bacterial composition of our substrates, perhaps it is not so much the exact species composition of the bacteria that matters, but rather their maximum diversity (if only one or two species overgrow, only then is there a problem). But if we are to figure this out, then we need to take all parameters into account (and not just the popular, "modern" ones). And in addition, we must try to understand the relationships between them. How nutrient availability affects pH and redox, what role bicarbonate or bacteria play, what happens to the bacteria when the substrate is disturbed during uprooting or sludging etc.
For example, if you look at my experiments with R. wallichii (a couple of years ago), a good observer will notice that in the first five experiments I used extremely low concentrations of chloride. And even though chloride is a micro-element, plants often use it in much higher amounts because of osmoregulation. So it is likely that the failure of these experiments was due to something as trivial as chloride deficiency. But nobody addresses chloride, and therefore virtually nobody noticed (myself not excluded). Similarly, in experiment #7, where R. wallichii grew poorly, I used extremely high doses of nitrate (54 ppm NO3), whereas in experiment #6, where R. wallichii thrived, I used only 10 ppm NO3. This in itself certainly means nothing, but in the context of the other parameters it certainly has some significance, even if we may not see it.
Bottom line: we should not consider individual parameters in isolation, but always in the context of other parameters. But to do this, we would have to start listing all relevant parameters (including Ca, Mg, Na, SO4, Cl, HCO3 ... pH, redox, substrate composition, bacterial composition, etc.) and not just the usual ones (NPK+Fe and CO2). Otherwise our discussions will be nothing more than pointless bickering that leads nowhere.
And a few more words on "Liebig's law of minimum" and "CO2 stability":
We are all familiar with Liebig's law of the minimum. According to this law, growth is not determined by the total amount of available resources, but by the amount of the scarcest resource (the limiting factor). Applying this to our aquariums, if a plant has 16 essential nutrients available in unlimited amounts, but 1 nutrient is in limited amounts, then its growth should be limited and it should grow as if all 17 of those essential nutrients were there in limited amounts. But that's not really the case. I would liken it to the fairy tale of Hansel and Gretel. Hansel and Gretel strayed from the evil Jezebel, who locked them up and fed them only sweets so they could get fat so she could bake them. So, we could say that Hansel ate an unbalanced diet with an excess of carbohydrates and fats. But this did not lead to limiting his growth (as he kept getting fatter), but to growth deformities => he was not developing properly. And I think it works the same way with plants with an unbalanced nutrient ratio. In my post #48 you can see, for example, if I increased the concentrations of all nutrients (except carbon, as
@Marwen very correctly points out) in the third aquarium, then according to this law there should simply be a reduction in growth. While there was some reduction in growth, there was also an immediate deformity. So I would say that the plants continued to gobble up the increased concentrations of other nutrients, but because they lacked carbon, they were unable to build healthy organs from it, only deformed organs. But the increased nutrients continued to force them to grow, only the growth was deformed. But this would suggest that the right nutrient ratio [at least in some cases] may indeed matter.
I think there are similar misconceptions about CO2 stability. I'm not denying that it is certainly challenging for plants to make enough of the enzyme they use to get CO2, and the less CO2 available to them, the more telling it is. But fluctuations in CO2 concentration are perfectly normal in nature and plants are used to it. So some normal (non-extreme) fluctuation in CO2 levels is perfectly natural. But one observation is important to mention here: it seems to be extremely difficult for plants to "switch" between CO2 and HCO3 uptake. This means that if you add CO2 to the aquarium and the CO2 concentration never drops to such a low level that the plants have to resort to taking up carbon from HCO3, then there is nothing to worry about. However, if you are adding a large amount of CO2 (40+ ppm) to the aquarium and suddenly there is a shortage of CO2, then it will probably be quite a shock to most plants. And while the plants can "switch" from HCO3 to CO2 (or from little CO2 to high CO2) quite quickly, the reverse is said to take days or weeks. So if we are talking about a kind of "CO2 stability", then this certainly does not mean the normal fluctuations of CO2 during the day or night.
I will give an example with my R. wallichii: I have grown R. wallichii in water with low nutrient concentrations, zero alkalinity and no substrate all the time ... and it did great. Then I suddenly increased the nutrient concentrations in two aquariums. But in only one of them did I add extra bicarbonate. If I hadn't done that, I would have found that the increased nutrients caused an immediate deterioration in her condition, and I would have immediately concluded, wrongly, that the high nutrient concentrations were harmful to her. But in the other tank with high nutrient concentrations, I had extra bicarbonate which "caused" her to suddenly do well. As
@Marwen aptly points out, the likely cause is that the higher nutrient concentrations sped up her metabolism, resulting in faster nutrient uptake. However, in an aquarium where the carbon concentration remained at its original (= low) level, this resulted in a carbon deficit. In the second aquarium, however, I increased the carbon concentration along with the other nutrients, and voilà, there was no deterioration. This beautifully illustrates how careful we should be in drawing various conclusions, especially when we are wounded by selective blindness, where we only see the parameters we want to and ignore the rest.
So until we start evaluating our aquariums holistically (i.e. really taking into account all parameters and not just the "popular" or "common" ones), then I think we will get nowhere and such a debate is meaningless for me personally.
so tifwiw
