In my early experiments, I have used nutrient solution essentially without the necessary expertise (theoretical background). The result was nutrient solutions that were designed without much regard for ion balance, and were based on the same unbalanced (and therefore suboptimal) methods commonly used in planted aquarium hobby today.

Based on a study of academic sources, and especially on reading Marschner's book "Mineral Nutrition of Higher Plants," my colleague Martin Langer (aka "Maq") proposed a recipe that better reflected the stoichiometry of the plant body (more precisely, the mutual ratios of individual nutrients in plant tissue). The unsuitable parameters of my original recipes may have been the main cause of the growth problems in Rotala wallichii, but also in other plant species which may have tolerated these imbalances better, but it was probably not optimal for them either. So the primary goal of this experiment was not to come up with some revolutionary formula, but to try to straighten out those imbalances towards a "normal" ratio of elementary ions.

We have read in the literature that optimum growth can usually be achieved for nutrients such as NPK and microelements at very low concentrations if these nutrients are supplied continuously to the plant, in other words, that the nutritional requirements of plants can be met by extremely low concentrations of ions in the external solution (assuming a continuous supply is maintained). Thus, under optimum conditions where a continuous supply of nutrients is maintained, only very low concentrations of nutrients in the external solution are required to achieve maximum plant growth (see Marschner 2.5.5). Based on this, we have formulated the hypothesis that it is probably better for plants to have lower concentrations of nutrients available, but in a balanced ratio (i.e., one that corresponds approximately to the ratio of nutrients in their tissue), rather than high concentrations of nutrients, which they then have to "rummage" through in a complex way, selecting only what they really need (which is probably much more energetically demanding for them).

Based on this initial hypothesis, we therefore used more or less "normal water" (i.e., water with a predominance of calcium and carbonate, which is the most common) as a base in the first test set, keeping NPK constant (at a relatively low but, in our opinion, sufficient concentration for good growth), while secondary ions (i.e. anions HCO₃^-, SO₄^2-, Cl^-, and the cations Ca²⁺, Mg²⁺, Na⁺) were scaled up from very small to relatively large amounts to check whether our initial assumption was correct. Thus, we sought to test the hypothesis that R. wallichii would be more likely to prefer soft water, whereas B. monnieri might prefer hard water.

Original formula:

As it turned out, our hypothesis seemed to be correct (at least in the case of R.wallichii), although we were unable to say exactly what (i.e., which specific ions) was responsible for this.

Thus, the balance (or concentration) of secondary ions is not equally important for all plants. Some plants (e.g. B. monnieri) are obviously much more tolerant of changing concentrations of secondary ions, and these [concentrations] only become harmful to them at much higher levels. On the other hand, this does not necessarily mean that higher concentrations of secondary ions suit them better. It may also be that they are simply better able to tolerate them. Further tests would be needed to verify the true preferences of these more tolerant plants. For now, it is sufficient to note that even at very low concentrations of secondary ions, these plants grew very well and without growth defects (as did R. wallichii).

Thus, the results of the previous experiment have given us the courage to focus strongly on the Marschner conditions as a basis in modelling the "right" water. Before this experiment we would not have dared to do such a thing; now we do. It is, of course, possible that we will find deviations between the ideal stoichiometry of the plant body and the composition of the optimal solution, but starting from the stoichiometry of the plant body is, in our opinion, the methodologically correct procedure. Marschner's stoichiometry (and not "normal water") is the fixed point from which we think it is correct to start and look around. In doing this looking back, we can then ask questions like "Why does a plant grow best with two atoms of calcium in solution when one atom is sufficient for its needs?"

This experiment is designed as two consecutive test sets (the first one "acclimatizing" and the second one "live"), lasting approximately 2-3 weeks.

We focused on creating two versions of recipes with a balanced nutrient composition (one less mineralized and the other more mineralized), while the other three recipes in this experiment were devoted to (1) checking genetic predispositions, (2) comparing our formula with the popular EI formula, and (3) finishing the test from the previous experiment (aquarium #4).

The primary objective here is to determine whether R. wallichii will grow better in a thinner or thicker solution mixed according to the content of elements in the plant dry matter (according to Marschner), and whether a formula based on Marschner's dry matter nutrient ratio is better, comparable or worse than a formula based on "normal water" from the previous set. Secondary objectives are then to determine if the emerse plants will suffer the same fate as the submerged plants (if they will grow in a solution where the submerged plants previously exhibited severe growth distortions), whether the plants will grow better in the EI solution (with an unbalanced nutrient ratio) or in its Marschnerian counterpart (with a balanced nutrient ratio), and whether the relatively dense solution of the fourth aquarium (which is a continuation of test 08-4 from the previous set) will eventually show similar deformations to those of the fifth aquarium (08-5) if the plants are left to grow there longer.

In this experiment I used the following plants:

1. Bacopa monnieri
2. Rotala wallichii

Lighting interval: 10h/day

Light intensity (PAR) in individual aquariums: