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The growth rate of plants based on the external amount of nutrients

Acknowledgement

This experiment would not have been possible without the contribution of the following people:

  • [Czech Republic]: Prof. RNDr. Hana Cizkova PhD., Jiri Novotny, Josef left, Jan Jaburek, Martin Styrak, Luke Manoušek, Petr Marek (akvarijni-hnojivo.cz), Martin Mithofer and Roman Soucek (ProfiPlants.cz)
  • [Slovakia]: Dusan Zervan, Anton Fuchs, James Vojtko
  • [United Kingdom]: Perran Trevan
  • [Malaysia]: Siang Kong Ting

Introduction

Every new aqua-hobbyist, who is growing aquatic plants in his/her aquarium will sooner or later begin to consider their nutrition (diet). On the Internet there are many articles on fertilization, and how he/she soon discovers there are also many different methods of fertilization. Unfortunately for him/her, some methods differ from each other diametrically, so soon a question comes to his/her mind as to what method is actually the right one, and how much nutrients are actually needed for plants to grow well.

On the one side of this extreme we have the Estimative Index (EI) fertilizer method, whos main advocate (Dr. Thomas Barr PhD.) claims that for a good growth of plants in his densely planted tank with relatively strong light (65-180 µmol PAR at the substrate level) and high CO2 levels (50 to 70 ppm) he has to (regardless of what kind of substrate he uses) add each week about 30 to 45 ppm NO3, 15 to 20 ppm PO4, 30 to 50 ppm K + 1.0 to 2.5 ppm Fe + other essential nutrients. This, with weekly 50% water change regime, leave us (him) finally with an average (permanent) concentration of nutrients as high as a whopping 60 to 90 ppm NO3, 30 to 40 ppm PO4, 60 to 100 ppm of K, 2.0 to 5.0 ppm Fe. On the other side of this extreme we have a folk's creativity in the form of conviction that aquatic plants don't actually need any fertilizing. And then there are many other methods such as PMDD, MCI, PPS and others that stand somewhere in between these two extremes.

Because many hobbyists have a hard time to understand this apparent contradiction among these different methods, I decided to get to the root of it using a controlled experiment in which I would verify how many nutrients are actually needed for a good growth by a selected group of aquarium plant species, or how their growth would vary under different concentrations of nutrients in water column.

The objective of this experiment

The aim of this experiment was to model a growth curve of selected plant species, depending on various external amounts of nutrients under conditions approaching parameters of the so-called "hi-tech" planted tanks (i.e. of tanks with relatively strong lighting and heavy CO2 supply), from which one could infer an amount of nutrients required to achieve:

  1. a minimal growth → minimum concentration required for the survival of plant
  2. a good growth → optimal concentrations of nutrients providing the most effective results (yield, gain, growth)
  3. a maximum growth → minimum concentration required to achieve maximum rate or "saturation" of photosynthesis

Experiment design



Suspended LED light; tanks with plants, filter and heater; cylinder with CO2; needle valves for precice gas regulation; divider (splitter)

Note: The needle valves in the picture are for illustration purposes only. In fact, I use another type (see the following description).

In the experiment I use five equally sized aquariums with the following dimensions: 8 x 8 x 16 inches [w-l-h] and a gross volume of 4 gallons (16L). In each tank there is an internal filter without filter media (JKA-MIF300) with a small spray bar, heater with thermostat (Eheim Jäger 25W), a glass thermometer, three small plastic pots with an inert substrate (black quartz gravel with a grain size of 0.7 to 1.2 mm). The plants used in the experiment were cultivated in 16G (60L) tank with nutrient-rich substrate (ADA Aqua Soil Amazonia), strong lighting (~100 µmol PAR at the bottom) and a constant CO2 supply (~30 to 40 ppm), combined with a decent fertilization using the recommended amount of Tropica Specialised Fertiliser. At the begining of each test I cut off equally long and equally large shoots of the same plant species which were weighed and planted into small plastic pots in the tanks. One or two shoots into each pot (according to the robustness of each plant species). The filter is used primarily for circulation of the water in the tank and for the dissolution of CO2 (it has no filter media inside). Stable temperature is maintained by the heaters at 77-80°F = ~25°C, and is continuously monitored by thermometers. The same intensity of light is ensured in each tank by the Bridgelux BXCD45 LED chips (6500 K, 9.2V, 9.2W, 950ℓ[email protected]) with a directional reflectors attached on an aluminum heatsink → one chip over each tank. For the light from one tank to not affect the lighting conditions in the neighboring tanks each individual aquarium is separated from each other by a black divider (ground pad). Irradiation values in each tank are 100 µmol PAR at the bottom and 150-200 µmol PAR at the water surface. The length of the photoperiod was set to 10 hours (10 hours light + 14 hours dark) corresponding to the regime used in most planted tanks. Carbon dioxide is supplied to the tanks from a cylinder. The amount of CO2 flowing into each tank is regulated by a fine Ideal Valve 52-1 needle valves (thereby ensuring the same CO2 concentration in each aquarium). The spray bar brings the water enriched with dissolved CO2 throughout the tank. The supply of CO2 into the tanks is on continuously (i.e. 24/7) for ensuring a stable CO2 concentration. In the tanks there is used a demineralized water → product of 5-stage reverse osmosis unit, whose output is water with virtually zero content of minerals and a minimum conductivity (1-3 µS/cm). This water (void of all salts) is then enriched by a specific quantity of nutrients prior to use in the tanks in order to achieve different concentrations of nutrients in each tank. Once a week all the water is poured out (changes), and 100% new water is used with a fresh dose of nutrients. All the nutrients are dosed right after the water change (i.e. once a week), except CO2 that is being supplied continuously.

Nutrients

In each test tank I used the following nutrient concentrations throughout the whole experiment:

1st tank: 2 ppm NO3, 0.2 ppm PO4, 1.25 ppm of K, 0.02 ppm Fe-DTPA
2nd tank: 4 ppm NO3, 0.4 ppm PO4, 2.5 ppm of K, 0.04 ppm Fe-DTPA
3rd tank: 8 ppm NO3, 0.8 ppm PO4, 5.0 ppm of K, 0.08 ppm Fe-DTPA
4th tank: 16 ppm NO3, 1.6 ppm PO4, 10.0 ppm of K, 0.16 ppm Fe-DTPA
5th tank: 32 ppm NO3, 3.2 ppm PO4, 20.0 ppm of K, 0.32 ppm Fe-DTPA

+ other essential nutrients (Mn, B, C, Cu, Zn, Mo)

In all tanks the same carbonate alkalinity (61 ppm HCO3 = 2.8°dKH), hardness (25 ppm Ca + 9 ppm Mg = 5.6°dGH), and concentrations of dissolved CO2 (35 to 45 ppm) was maintained. The pH was maintained in the range of 6.35 to 6.45.

Lighting

In all tanks there is the same light intensity, which is about 100 µmol/m2/s (PAR) at the bottom of the tank, and 200 µmol/m2/s at the water surface (see the picture), which can be called a "strong light".



Examined plants

1) Ludwigia palustris 'Red'
2) Rotala rotundifolia
3) Rotala macrandra 'Narrow leaf'
4) Didiplis diandra
5) Pogostemon erectus
6) Rotala wallichii
7) Rotala macrandra (classic variety)

Test results

1) Ludwigia palustris 'Red'


Fig. Tanks at baseline (Day #1)


Fig. Tanks at the end of the test (Day #28)


Fig. Growth curve
Y =
Weight gain (in grams of fresh weight)
X = Concentration of NO3 (ppm) + other essential nutrients


2) Rotala rotundifolia


Fig. Tanks at baseline (Day #1)


Fig. Tanks at the end of the test (Day #23)


Fig. Growth curve
Y =
Weight gain (in grams of fresh weight)
X = Concentration of NO3 (ppm) + other essential nutrients


3) Rotala macrandra 'Narrow leaf'


Fig. Tanks at baseline (Day #1)


Fig. Tanks at the end of the test (Day #20)


Fig. Growth curve
Y =
Weight gain (in grams of fresh weight)
X = Concentration of NO3 (ppm) + other essential nutrients


4) Didiplis diandra


Fig. Tanks at baseline (Day #1)


Fig. Tanks at the end of the test (Day #28)


Fig. Growth curve
Y =
Weight gain (in grams of fresh weight)
X = Concentration of NO3 (ppm) + other essential nutrients


5) Pogostemon erectus


Fig. Tanks at baseline (Day #1)


Fig. Tanks at the end of the test (Day #28)


Fig. Growth curve
Y =
Weight gain (in grams of fresh weight)
X = Concentration of NO3 (ppm) + other essential nutrients


6) Rotala wallichii


Fig. Tanks at baseline (Day #1)


Fig. Tanks at the end of the test (Day #18)


Fig. Growth curve
Y =
Weight gain (in grams of fresh weight)
X = Concentration of NO3 (ppm) + other essential nutrients



Detail of plants from individual tanks (day #10)


Detail of plants from individual tanks (day #18)

Second (duplicate) test:


Fig. Tanks at baseline (Day #1)


Fig. Tanks at the end of the test (Day #20)


Fig. Growth curve
Y =
Weight gain (in grams of fresh weight)
X = Concentration of NO3 (ppm) + other essential nutrients


7) Rotala macrandra


Fig. Tanks at baseline (Day #1)


Fig. Tanks at the end of the test (Day #25)


Fig. Growth curve
Y =
Weight gain (in grams of fresh weight)
X = Concentration of NO3 (ppm) + other essential nutrients


Evaluation of the tests

A summary table of the results:



Comments on the table:

  • The value "32" means 32 ppm NO3, 3.2 ppm PO4, 20 ppm K , 0.32 ppm Fe-DTPA + other essential nutrients.
  • The value "3.4" then by analogy means 3.4 ppm NO3, 0.34 ppm PO4, 2.1 ppm K, 0.034 ppm Fe-DTPA + other essential nutrients.

The above table (and graphs) implies a number of interesting things:

1) While some plants reach the maximum growth rate (and thus the maximum yield, i.e. gain) at concentrations above 30 ppm NO3 (+ other essential nutrients), other plants reach their maximum at much lower concentrations. So it logically follows that each plant species can have slightly different requirements for nutrients, and no fertilizing method, therefore, would be completely optimal for different plant species found together in one tank.

2) The growth rate is not linear but logarithmic. This means that a direct proportion such as: "30 ppm NO3 = 100% growth rate, 15 ppm NO3 (i.e. half concentration) = 50% growth rate" does not apply here. Unfortunately, it does not work this way (straightforwardly). It's a little more complicated, and we won't make do with simple math here (for correct calculations we need quite complicated logarithmic and exponential functions). Roughly one can say that plant growth is governed by the "law of diminishing returns". This law tells us that initially every small increase in investment (in this case "amount of nutrients") brings us relatively large revenue/profit (in this case "growth"). At a certain point, however, the card turns, and with each further investment our yield will be smaller and smaller, until eventually you reach a point where further investment brings us absolutely nothing (or our yield even decreases). Our investment (i.e. fertilizing) pays off the most when we supply the plants with around 5-10 ppm NO3 (+ other essential nutrients in appropriate proportions). Adding a greater quantity of nutrients into the water won't pay off too much, and in some cases it may even be "lossy" (i.e. won't lead to further increase in growth rate, but rather to its inhibition).

3) To reach half (50%) growth rate our plants need about 10 times less nutrients than in case we want a maximum (100%) growth rate. This means that if a plant needs say 30 ppm NO3 (+ other essential nutrients in suitable proportions) for maximum growth (100%), then for the half growth rate (50%) it will suffice with about 3 ppm NO3 only. It logically follows that if any visible nutrient-deficiency signs begin to show in our aquatic plants, it's more than enough to supply only a tiny amounts of fertilizer (maybe only 1 to 2 ppm NO3 + appropriate amount of other nutrients according to what plants are missing). Is by no means necessary (and often desirable) to immediately pump a huge amounts of nutrients into the tank.

4) Numerous aquarium plants will (at strong light and high CO2 concentration) grow pretty well even at concentrations below 2 ppm NO3 (+ adequate amounts of other nutrients).

5) The lower the nutrient concentration, the more effectively can they be used by plants. Plants are evolutionarily best adapted for uptake of small doses of nutrients. Average nutrient concentrations in most large rivers around the world is about 15 ppm Ca, 5 ppm Mg, 7 ppm Na, 1.5 ppm K, 8 ppm Cl. Concentrations of other nutrients fluctuate more. But for example, in the Amazon river (the largest river of the world) the average concentration of major nutrients is as follows: 6 and 8 ppm CO2 (main flow vs. tributaries), 0.7 and 0.5 ppm NO3 (main flow vs. tributaries), 0.06 and 0.05 ppm PO4 (main flow vs. tributaries). Calcium (Ca) is there in the range of only 1 to 3 ppm, and the concentration of magnesium (Mg) is in the range of 0.5 to 1 ppm. Yet it is sufficient for the local plants.

6) Some plants do not behave exactly "as prescribed". A typical representative of these plants in my experiment is Rotala wallichii, which does not behave "as expected" in the test tanks with higher concentrations of nutrients. While the plants in the first and second tank (where the concentration of nutrients were lowest) had short internodes with a large number of long leaves (which is good), the plants in the other tanks had long internodes with a small number of short (poorly developed) leaves, and growth tops were severely deformed (this was probably the case of more frequent branching also, although these side shoots suffered shortly from the same defects and deformations) → see the details of plants in the above pictures. Higher concentrations of certain nutrients can thus have apparently toxic effects on certain "sensitive" plants species (unless they are somehow protected from this toxicity). But since the toxicity of different elements depends on many factors (e.g. the concentration of humic substances in water, total hardness, pH, flow rates, temperature, redox, etc.) the toxic effects may not be so pronounced in some tanks, or under certain conditions they may not even occur at all. But certainly it can not be argued that the increased concentration of nutrients does not have any toxic effect on some sensitive plant species (as is clear from my experiments). If someone adds an extremely high doses of nutrients in his/her aquarium without observing any negative effects on these "problematic" plant species, it may mean that the tank has some "protective factors" (i.e. humic acids), or that some elements/compounds may quickly precipitate in the aquarium water (e.g. phosphate reacting with iron, and forming insoluble and plant-inaccessible precipitates), and thus their potentially toxic concentration is reduced to an acceptable levels. However, not all plants have such a low tolerance threshold. Some plants are able to grow without any visible signs of toxicity even at very high concentrations of nutrients (e.g. Rotala rotundifolia, and many others commonly called as "invasive").

Sources of nutrients

The nutrient concentrations shown in the table and graphs represent the total amount of nutrients, which is of course possible to cover by a variety of sources, whether it be a substrate, water, fertilizer, animal feed or mineralization of detritus.

So if you want to supply your plants by 10 ppm NO3, 1.0 ppm PO4, 6 ppm K, and 0.1 ppm Fe, while using a nutrient-rich substrate in your tank, you won't of course need to add the same (full) amount into the water column (it can be assumed that a large portion of these nutrients will be available to plants in the soil substrate). Some nutrients are also present in the tap water, and other nutrients got into the tank from fish feed, feces and mineralized debris. Therefore, there are many more sources of nutrients in the tank than just what we add into it in the form of a liquid or dry fertilizers. In some cases, therefore, it may not even be necessary to add any artificial fertilizer to the water.
 

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Discussion Starter · #2 · (Edited)
From some unknown reason the long posts can't be edited (even immediately after I submit/publish them), so I add my final note here:

A final note:

As with my previous thread (http://www.aquaticplantcentral.com/...28370-rotala-wallichii-growth-experiment.html) by this post I would like to share what I had found out in my controlled experiments. I describe it extensively on my Czech website with much more pictures, tables, charts, etc. If anyone is interested in more details, he/she can contact me through a personal message here or by email. I won't reply to any questions publically here in the forum, as I'm not compatible with this kind of discussion, and easily get banned.

PS: This experiment preceded the one on Rotala wallichii.

Marcel G

Typo correction:
Nutrients:
+ other essential nutrients (Mn, B, Cl, Cu, Zn, Mo)

[It's possible that other mistakes are present in the text as English is not my native language. Hopefully, the important data are free of errors.]
 

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Marcel, thank you! I haven't had time to read all of your study yet, but this is the type of empirical research that the hobby needs.
 
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