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Weekly nutrient consumption in planted aquarium

Other sources of error are:
- root mass...
- H from water to make carbohydrate...
- waste products...
- huge amount of excess organics...
I have to find out some relevant data to these potential errors, but as I already said, I did weighed the root mass in some cases, and in other cases we could use some average coefficient to account for this ... when we find out what's the average ration between stems and roots. Also consider this: If I trim some plants with huge root mass, do you think that the plant have to make a new roots for the new stems? I would say that once the plant makes its root system, the roots will not grow to infinity, so when you cut off the stem, (maybe) the roots will be the same and don't grow any further because you just cut the plant in half and it needs no such a big root system any more. But I'll try co ask these question to some experts.

As to the waste products I remember to read somewhere that the by-products of photosynthesis in macrophytes could be up to 10%. To be honest, this number doesn't seem to be so big. If I find out in my test that my plants will use up say 3 mg/L NO3 per week, then 110% (3.3 mg/L) will make nearly no difference.

But as I said, I'll try to find out some scientific data to account for these things.

Your supposed method of testing ferts consumtion is much more erroneous according to my opinion. Once I dosed ~30 mg/L PO4 into my tank. The second day I measured 5 mg/L PO4 in there, and the third day I measured 0,0 mg/L PO4. Do you really think that this amount of PO4 could be used up by plants over 48 hours? It's quite a known fact, that substrate can absorb huge amounts of nutrients. Also up to 60% on nitrigen can be used up (transformed) by bacteria, and escape the tank as N2 gas.
 
General question related to this topic. What is the "efficiency coefficient" for nutrient uptake in high light, high co2 situations for plants? Again, surely this is related in several different factors (species, which fert etc.) and is not linear, but if we have fixed co2 and lighting level and we assume that we give just the right amount of ferts for maximal growing in those conditions, how large part of the ferts in water column is used to build the plant mass? Are we generally talking about 90% or 50% or 10%?

The background for the question is that there usually are many biochemical processes where certain chemical compunds are needed, but they do not directly add plant mass. Also, organisms do not function with 100 % efficacy ever.
 
By PO4 you can control the growth rate very well. In my test, when I lowered the PO4 dosage to 0.1 mg/L per week, the biomass (live weight) was just around 5 g ... vs. 20 g when I added 3 mg/L PO4. So it's known fact that by limiting PO4 you can slow down the growth rate successfully without making plants suffer.
Ok, that's more or less what I have read in this forum as well in scientific literature regarding wetlands management. But my question is a bit different. Light is the most important driving force in planted tanks, then CO2 (let's say CO2 efficiency, thus including CO2 levels but also dissolution, distribution, etc.) and then nutrients (being PO4 a good controller of growth rate).

The question I was really asking is: in that limiting chain (light > CO2 > nutrients), if high amounts of PO4 are released to the water column and boosting plant growth, can this make my plants hungrier and more demanding regarding CO2? (thus making CO2 manageent in the tank more difficult?). Maybe I'm wrong but if PO4 boosts plant growth it will be also increasing the need of other nutrients uptake (N, K and micros which can be easily unlimited dosing EI, as well as Co2 BUT we all know is not that easy to have really unlimited levels of CO2). Or in other words, assuming that my CO2 is "good enough" for the lights I have: will lower PO4 levels giving me more safety margin for CO2 performance?

Jordi
 
I'm not expert, but as I understand it then plants have some general ratio of nutrient uptake. Light is a driving force of their photosynthesis rate. The nutrients are a building blocks. CO2 is also a nutrient = building block (it's not any different => the only difference is that plants need much more of this nutrient then any other). So look at the picture:
metody_stavebni_blok.jpg

For plants to grow they need a variety of different nutrients, which we can visualize in this example as building blocks (bricks). In order for plant to build one leaf, it needs 45 bricks of carbon, 45 bricks of oxygen, 6 bricks of hydrogen, 2 bricks of nitrogen, 1 bricks of potassium, 5 cubes of calcium, 3 cubes of magnesium, 2 cubes of phosphorus, 1 cube of sulfur, and 1 small cube of iron.
howFertilize_building_blocks.jpg

Now imagine that you have in your aquarium (in the water or substrate) the following number of building blocks (= nutrients): 360 bricks of carbon, 2925 bricks of oxygen, 1200 bricks of hydrogen, 40 bricks of nitrogen, 20 bricks of potassium, 10 cubes of calcium, 6 cubes of magnesium, 1 cube of phosphorus, 5 cubes of sulfur, and 3 small cubes of iron. How many leaves can the plant build of this?

The above table shows that plants in our hypothetical example have available in the water (or substrate) as much carbon that would be enough to create 8 leaves, the oxygen amount would suffice for 65 leaves, the amount of hydrogen to 200 leaves, the amount of nitrogen, potassium, calcium and magnesium, both for 20 leaves, the amount of phosphorus for five leaves, the amount of sulfur for 50 leaves, and the amount of iron to 15 leaves. How many leaves can therefore plants create in this environment? According to the law of minimum only 5 ! And this is true even though most of the other nutrients are available in much larger quantities.

On this example you can see that it is absolutely useless to add nutrients to the aquarium indiscriminately. If you would add more nitrogen (N) and potassium (K) into this hypothetical aquarium, it has little efect on the plant growth. Unless you increase the dose of elements which are in the shortest supply, the huge amounts of other nutrients will be absolutely to no good to you (or to plants).

If you add more phosphorus (P) into this aquarium, it ceases to be a limiting factor, and immediately the carbon (C) will become the most limiting factor in our example. Although the growth of plants will increase from 5 building blocks to 8 building blocks, but it will go no further. If you increase the quantity of carbon also (e.g. by adding liquid carbon or pressurized CO2), plant growth will immediately jump to the next level of the most limiting element in a row (which is iron) = 15 building blocks.

In most tanks, I would say that PO4 is the most limiting agent, and all other nutrients are there in much bigger volumes. So if you increase the PO4 level in your tank, your plants will began to grow like mad, because all other nutrients are in a rich supply ... so the real bottleneck will become light (not nutrients). I think if you have 30 mg/L CO2, you'll never be in a situation to limit plants with carbon. Also it's good to know that even plant growth has its limit (around 600-700 µmol PAR).
 
So to answer your question:
If high amounts of PO4 are released to the water column and boosting plant growth, can this make my plants hungrier and more demanding regarding CO2?
I would say, "No". If you add more PO4, then the plants will build a little more organic material for their growth. The only power controling their hunger being light.
 
Ok, understood...

I would say, "No". If you add more PO4, then the plants will build a little more organic material for their growth. The only power controling their hunger being light.
Unless in this high light tank, you add lots of PO4 (EI method is the only one that add 5-10x other systems) and your CO2 is not as good as you thought (not reaching 25 ppm and/or bad distribution)... in that case I guess CO2 would probably become the next limiting agent and a good chance to find problems.

Please correct me if I am doing a faulse assumption but beyond light PAR (which is always the most important driving force and obviously the easier solution to solve potential problems) it looks to me that fertilizing systems with high levels of PO4 require optimum CO2 performance (always in a high light scenario).
Can this fact be the reason why commecial ferts include such low levels of PO4?

Jordi
 
nice work ardjuna, keep going.

We also know that the law of minimum isnt always true though. Plenty of work has been done on allocation of resources by plants to combat shortages. google will give you the papers.
 
To be honest, I'm not sure if CO2 is needed in such a big amounts. According to my calculations, for plants to grow by 1 g/dm2 per week they would need ~165 mg CO2. In 110L tank (80x35x40cm) the consumption of CO2 by aquatic plants under high light should be 5-9 g CO2 per week. This amount is approx. a daily dosage. In other words, I put 7-times more CO2 into my tank (~35 mg/L), then my plants really need. So if my calculations are correct (or +- correct), then our plants need much less CO2 then we think. And this applies to nutrients also (see my chart).
The word "Rostliny" means "Plants" (my calculated average consumption of nutrients in high-light tank). All units are in mg/L.
average_consumption.jpg


So based on this, I would say that optimum/stable CO2 levels are maybe much more of a problem in regard to algae then plants. And the same apply of higher dosages of fertilizer. You all know probably that higher levels of PO4 can eliminate GSA or GDA. The similar apply for NO3 and BGA. So if you have algae problems, then it's maybe better to keep higher nutrient levels, although our plants will never make use of such a big amounts.
 
Very interesting discussion!

I am not a plant biologist but would like to add my 2 cents because I studied Life Science and Technology which has alot to do with cells, enzymes etc etc :)

I would first like to respond to the above reaction of CO2: CO2 is the carbon source used by plants. And although it is true that you could calculate the dry wait of your plant trimmings to get an estimate of how much CO2 your plants use, it's very difficult to relate this to how much your bubble count should be. This is because you are constantly loosing CO2 which "evaporates" from the surface of your tank. Therefore it's impossible to make an proper "element balance" (unfortunately I don't know the official English term for this) for the carbon element.

I few opinions/thoughts: as stated a couple posts above there is "nutrient" bottleneck. When you look at a plant from cellular level everything comes down to chemical reactions and their reaction speeds. Almost all of these reactions are in one form or another catalysed by enzymes and every enzyme known to men has a maximum reaction speed. This maximum reaction speed is only achieved when all conditions are optimal (pH, temperature, substrate concentration, etc). This substrate concentration (read: nutrient concentration) is where I think it get's interesting. Because concentration dictates whether the maximum reaction speed can be achieved (given all other variables are kept constant i.e. 25C and a pH of 7.0). However, the concentration does not have a direct correlation with the actual turnover from the substrate to a product (i.e. production of a protein which is needed for the plant to grow). For example: for enzyme A the reaction speed is the highest at a concentration of 100 mg/L, consumes 1 mg of substrate and produces 1 mg of product per hour (random numbers). Enzyme B could reach it's highest reaction speed at only 50 mg/L, consumes 2 mg of substrate and produces 2 mg of product per hour. To summarise: concentration of all nutrients could very well play a big role in the growth speed of plants although they don't use all these nutrients. At a lower concentration reaction speeds decrease and thus plants use less nutrients meaning slowing growth.

In addition to my above statement: plants need to absorb nutrients in their cells before they can be used. This absorption often happens either by diffusion or enzymes that transport these nutrients over the cell membrane into the cell. Both these processes run faster at higher concentrations.

Already mentioned is the fact that you can't weight the mass of the roots of the plans. But if you really want to know how much nutrients are used you need to look at the tank as a whole system. The amount of so called biomass, mostly in the form of bacteria in the soil and filter are HUGE. As with any living organism these bacteria also use up nutrients (sources of carbon, nitrogen, phosphor,hydrogen, oxygen and sulfur) and they will use part of the ferts intended for plant growth. This basically means it is very difficult if not impossible to get an accurate estimate of how much nutrients are used based on weekly plant trimmings.

However, there is a easy method of answering the question of how much nutrients you entire aquarium (plants, bacteria, etc) consumes each week (for a given light intensity, making the light the limiting factor). Unfortunately one would need access to a lab... What you need to do is make sure all other nutrients are available in excess. For this you should be fine using recommended EI dosing. After dosing you take a water sample which should be analysed for all added nutrients (except CO2, because it's pointless to measure as it quickly dissipates from the water). Let your tank run for a week and take another water sample. It's important that in the mean time no water changes or fertilisation has taken place (CO2 obviously must be running the whole time as normal). Have the second sample analysed and look at the difference in concentration of nutrients. Now you know EXACTLY how much nutrients you tank uses each week. To make sure your plant growth speed is maximum you need to repeat this experiment with higher concentrations of everything including CO2 (excluding your light). If the result of nutrient uptake are the same, your plants are growing as fast as they can with current lighting conditions, if they increased you are not yet at the max. If they decrease, well... maybe the pH of your tank changed so much due to more CO2 enzymes start to work less efficient.

In a perfectly economical setup you would never do water changes (meaning the concentrations of nutrients remain high) and you weekly add only what has been used. Obviously this isn't possible due to buildup of other chemicals/nutrients in your tank that need to be removed.

So basically I personally think the plants' nutrient uptake is higher at higher concentrations and thus growth speed is higher. Therefore it can be beneficial to have high concentrations of these nutrients present.
 
It would seem my random stab in the dark about high concentration is beneficial even for low uptake (makes uptake more efficient) was actually correct or at least partially correct! Slightly off topic but: yay!

Continue complicated scientific debate...
 
Hi DivZero, this is a really good point!

Also you say that if we want to know how much nutrients are used in our tanks we need to look at the tank as a whole system. As there are other organisms (besides plants) which also use up the nutrients. But I see a difference between what the plants need, and what the whole tank may need. So by weighing the new plant biomass we can find out what plants themselves need. Then using your suggested method we can find out what the whole system needs. And finally, by subtracting these two numbers we can come to know what part of the total amount of nutrients is alloted to plants, and what part to other organisms. So for me it means that we need both approaches to get to some relevant results.

Still, I see a couple of potential problems with your suggested method of finding out the total amount of nutrients needed for the whole system (i.e. measuring the amount of nutrients at the first day, and then at the last day of our test). The problem I see is called "substrate" (and maybe other media like filter). All clay substrates are able to absorb quite a big amount of nutrients from water column (especially phosphates). So how you know that the nutrients were really used up by the microbes? The nutrients could be just absorbed ("stored") without being used up. The next problem is financial, as lab analyses are costly, and we would need to make a lot of analyses to find anything relevant.

So I understand your point, and like it very much. But it means for me, that my task (of finding out the real average consumption of plants in a planted tank with some defined conditions) is probably beyond my financial capabilities. So probably I'll just give up.

One final point to a root biomass. I thought a lot of it, and I would say that its effect on the new biomass gain is not as big. Once the plants create a root system, this root system don't grow too much anymore (maybe except some invasive kind of plants). So even though the roots themselves can make more then 50% of the plant biomass, if you measure the new biomass gain throughout three months, do you really think that root system will grow any further from the day you began your test? I don't think so. Also, during the 3 months period, you trim your plants many times (maybe 10-times). So if you trim your plants in half in each trimming, then you get 5 whole plants in means of biomass (without roots) over this period. But the root biomass stays nearly the same. So I would say that the root biomass will not play so big role (maybe 20%).

Marcel
 
So basically I personally think the plants' nutrient uptake is higher at higher concentrations and thus growth speed is higher. Therefore it can be beneficial to have high concentrations of these nutrients present.
Not sure if you intended to say so but...why it should be beneficial to have high growth speed? As stated high concentrations of nutrients (especially PO4) gives you high growth, but you don't need this super growth (unless you sell plants). The point is to know what plants need.

I agree that the methodology can be probably improved (repetitions is the most interesting part to be included, as stated by the OP). But I don't agree with other aspects mentioned. For example you say it is important to understand the tank as a whole system but this would not give us any useful information regarding plants' needs (it would give us very interesting information for other things, I agree). We all know that part of the nutrients can be stored in the substrate, that microbes may be using some... But I think the point is trying to identify the minimum needs, not to say to folks 'you have to get adjusted to this limit' but again to play with a good safety margin. The interesting part of this thread IMO is to understand how large this safety margin is and (al least to me) if reducing some levels of nutrients (PO4) can indirectly help to reach easily an optimum CO2 performance.

I'm sorry to be a pain each time this issue is discussed (really need to learn more about this and there is plenty of question I still want to understand) but despite all this experiments, methdologies' discussions, etc. there is something quite obvious to me. There are thousands of hobbyist using successfully 'EI approach' but also 'lower nutrients approach'. In both sides we have failures for sure, but also success. EI is not the only method in which you don't use test kits and it is not the only one that gives you this safety margin so many times explained... It is probably the one that gives you MORE safety margin but I'd love to know how much (I recognize that it is probably useless to know how much, lots of people here will say 'don't worry about this, just be sure there is enough') because I want to know if there can be sides effects (sorry again to be a pain, but my concern is about high PO4 and optimum CO2)

Jordi
 
OK, my beloved wife give me an advise how to solve our problem (or at least come closer to its solution).
The obvious solution to this is to set up a tank full of water with an empty filter, put plants into it and use high light. I would attach the plants to some inert stones so that they stay on its place. This way we can know quite precisely how much nutrients the plants will use up in a set time (say one or two weeks). I'll try to do this kind of test. I'll set up an empty tank with empty filtration (to use it just for a water circulation), and after my plants will establish, then I begin my test. I'll put some known (unlimited) amount of nutrients inside this high-light tank and let it analyse in a lab on the first day, second day, third day and last day. I think it is important to analyse the water not only on the first and last day, but also more often in the early days to know if some nutrients degraded (e.g. chelates). What do you think about this kind of experiment?
 
Hmm, won't there will still be a significant amount of bacteria etc in the 'empty' aquarium (also on the plants) using up nutrients and creating by-products? And they will multiply exponentially until the nutrients run out.

I think it would therefore be better to measure the nutrient use of an aquarium with only hardscape, substrate and a filter and then compare the nutrient usage to an identical set-up but with plants as well.
P
 
Hmm, won't there will still be a significant amount of bacteria etc in the 'empty' aquarium (also on the plants) using up nutrients and creating by-products? And they will multiply exponentially until the nutrients run out.
Sure but I guess the system 'plants+microbes' has to go necessarily in the same pack, they do live together in any aquatic ecosystem you create. Therefpre the test would be to know 'the average nutrients uptake by plants and the minimum amount of bacteria needed to support this artificial ecosystem'. Anyway it looks like a good idea to get rid of the substrate which can probably stock part of the nutrients.

Jordi
 
OK, my beloved wife give me an advise how to solve our problem (or at least come closer to its solution).
The obvious solution to this is to set up a tank full of water with an empty filter, put plants into it and use high light. I would attach the plants to some inert stones so that they stay on its place. This way we can know quite precisely how much nutrients the plants will use up in a set time (say one or two weeks). I'll try to do this kind of test. I'll set up an empty tank with empty filtration (to use it just for a water circulation), and after my plants will establish, then I begin my test. I'll put some known (unlimited) amount of nutrients inside this high-light tank and let it analyse in a lab on the first day, second day, third day and last day. I think it is important to analyse the water not only on the first and last day, but also more often in the early days to know if some nutrients degraded (e.g. chelates). What do you think about this kind of experiment?

I think the basis of your experiment is sound. However it is of vital importance your biosystem (read: the tank and all it's content) are as close to equilibrium as possible. This is important because this is also the case in your "real" tanks. Because you have a fixed dosing schedual, fixed water changes amounts, constant lighting and CO2, feeding of fish etc. As a result of this both bacterial and plants growth are more or less the same over time. There growth speeds are always rate limited. The rate limiting factor can be many things, light, nutrients, pH, temperature but also physical limits that dictate maximum speed of enzymes etc. Once a tank and filter is properly cycled, all factors are kept constant, and amount of in this case plant trimming is similar each week a for of equilibrium is achieved. (note: it's important plants are mature and trimmed to keep them constant in size => bigger plants with bigger/more leaves have more surface area to uptake nutrients and a photons for photosynthesis. Once you feel confident the tank is stable and in equilibrium (I.e. looks the same from one week to another, after trimming) the testing can commence!

...let it analyse in a lab on the first day, second day, third day and last day. I think it is important to analyse the water not only on the first and last day, but also more often in the early days to know if some nutrients degraded (e.g. chelates)

One thing that is excellent from a scientific point of view is taking more than two samples. During my studies we actually had to perform experiments on how to determine at which concentration of a given substrate maximum product formation takes place. This is very important because with this knowledge industrial (bio) processes can be optimised for either speed of product formation and cost reductions. Because now you know at which concentration of a substrate it is just a pure waste to add more, because there is no benefit! But back to the experiment. How are these experiments performed? Without going into too much detail it is quite simple. Because your system is in steady state abd the limiting factor is either lighting, CO2 dosing all other nutrient uptakes are constant en linear over time. So if you take all data from the samples, and plot them against time, you you see straight lines in your plot. However, it is possible that one not the lighting or CO2 are the limiting factor, in which case one of the nutrient concentrations is limiting the growth speed. In this case you would see a decrease of substrate intake over time as the limiting substrate concentration slowly drops, slowing the growth and thus slowing substrate intake further (as substrate intake is dependent on concentration!).

Disclaimer: I do not know if the plants etc take up so much nutrients the data would be significant enough to conclude if the substrate intake is constant or decreasing over time!

If the experiment is performed well this would tell you two things (of this particular tank/bio system!!!):
1) an absolute nutrient uptake in a week giving a rough estimate of how much nutrients plants actually use.
2) it might give insights if all nutrients are available to the plants in excess and CO2 and lighting in this particular tank are the limiting growth factors (it can only be one of them btw). If it is shown that nutrients using recommend EI dosing are still limiting the growth speed it shows that that the EI dosing isn't as crazy as it seems

Things this experiment can't proof (with only one dataset):
1) if nutrients are proven to be in excess we have no idea HOW MUCH these are in excess. It could be as much as 100 mg/L or as little as 1 mg/L. To discover the amount of excess the experiment must be repeated multiple times for each single nutrient until growth speeds decrease. This would probably be a year long study :p
2) which factor (light, CO2, which nutrient etc) is the limiting factor in this system will remain unknown.

Not sure if you intended to say so but...why it should be beneficial to have high growth speed? As stated high concentrations of nutrients (especially PO4) gives you high growth, but you don't need this super growth (unless you sell plants). The point is to know what plants need.

From a biological standpoint plants need very little for their so called maintenance. Little light, little nutrients etc is needed for them to survice. Although I am not an expert on aquarium plants and all the water chemistry I think the important point is the competition aquatic plants have with algae as they need compete bother over light, CO2 and the other nutrients. Therefore I think many of the techniques in high tech aquaria are aimed to battle these algae. One of these techniques is frequent water changes, which removes nutrients (aka byproducts from bacteria such as ammonia) algae can use. It's important tot note that almost every chemical that can be found in the aquarium which is the product of one organism is the substraat thus food for another! And for some reason we only want plants, fish, and denitrifying bacteria (for the most part) in our aquarium and not algae and other organisms :p. The other "technique" against algae (I think!) are healthy plants that grow. When plants grow they use up nutrients and form biomass (i.e. more plant leaves) which remove these nutrients to be used by other organisms such as algae. One thing I do know is that plants are able to directly use ammonium for their growth, and algae spores need ammonia to germinate (please note that I can't find a reference for this to be true!! after looking most scientific articles state ammonia levels inhibit germination...). This could be also true for other nutrients we don't regularly add, and algae need. With rapid plant growth the plants can lower concentrations of these unknown (to me) nutrients and put a lid on algae growth.

Also while doing some research for this post I came across this: http://prirodni-akvarium.cz/clanky/Factors Affecting Spore Germination in Algae.pdf
There are many interesting things in this publication with this one standing out in particular:

The SG (spore gemination) was decreased not only by the lack of nitrogen, phosphorus or magnesium but also when their (and of calcium) concentration exceed certain levels; e.g., nitrate or phosphate at ≥5-fold level, or mag- nesium at 10-fold level of that present in the basal medium inhibited akinete GRM in Westiellopsis prolifica (Agrawal and Sharma 1994a). Magnesium at ≥5-fold level or calcium at ≥2-fold level also inhibited akinete GRM in Stigeoclonium pascheri (Agrawal and Sarma 1982a). This indicates that SG in algae is sensitive to high levels of inorganic nutrients. Omission of microelements (ZnSO4, MnCl2, MoO3, CuSO4, Co(NO3)2, H3BO3) from the basal medium increased SG in Stigeoclonium pascheri, and by increasing their concentrat- ion to ≥2-fold levels, the condition was reversed (Agrawal and Sarma 1982a). The presence of microele- ments in the basal medium therefore serves as a check in reaching maximum level of SG under control con- ditions. More study is needed to clear the role of micro- and macronutrients in SG.

This could very well mean that the crazy dosing of EI is having a large impact on germination of many algae species, keeping them in check. In this case the EI dosing would be a nice way to fight algae with plant growth as happy side effect ;)

Disclaimer: most of the above is either a personal theory and not based on personal experience or scientific research that could OR NOT be applicable to the aquarium ecosystem. Use what I said at own risk and if people think I'm calling out some crazy theories that are wrong, please let me know :)
 
This could very well mean that the crazy dosing of EI is having a large impact on germination of many algae species, keeping them in check. In this case the EI dosing would be a nice way to fight algae with plant growth as happy side effect ;)
This is very good point, DivZero! Because it's quite common experience among aquarist that high concentration of PO4 inhibits GSA (= green spot algae alias genus Chlamydomonas or Chlorococcum). Similar seems to be true with high NO3 concentration and BGA (= blue-green algae alias cyanobacteria). So there can be something on this speculation.
 
This is very good point, DivZero! Because it's quite common experience among aquarist that high concentration of PO4 inhibits GSA (= green spot algae alias genus Chlamydomonas or Chlorococcum). Similar seems to be true with high NO3 concentration and BGA (= blue-green algae alias cyanobacteria). So there can be something on this speculation.

Additionally I found an article that states that some (specific) species of cyanobacteria needs very low concentrations of PO4 to germinate, but standard aquarium sources that that BGA (not the species in the article!) need high PO4 to thrive. Maybe I will do some proper research on this topic some time ;)
 
I'm not really sure where this is headed and if it's still about answering the questions in the OP. But by its very nature the study lacks scientific rigor...for starters there are far too many confounding factors and you'd need to test the nutrient requirements of every individual species separately with laboratory spec equipment for it to be significantly meaningful...that's a hell of a lot of tests.
However, your study will give you a loose idea of the collective nutrient requirements of the plants in the unique conditions of your tank, and allow you to fine tune your fertz dosing. However as mentioned previously, it is possible to do the same using EI methodology simply by reducing the dosage to the point of negative plant growth response and then notching the fertz back up to the previous dose. This will give you results which are just as meaningful as the ones in your study...
 
Hi Troi,
if you're seeking scientific rigor, then I don't understand your suggestion to "simply reducing the dosage of EI to the point of negative plant growth response". Do you think that this will tell us anything meaningful? How can you be sure what nutrient was missing to cause plants to start exhibit some kind of negative response? How can you be sure that most nutrients don't end up in the substrate? And how can you be sure that EI method is based on any reasonable assumptions? You want scientific rigor but at the same time you propose absolutely dilettantish methods. My goal is to find out how much nutrients plants in an average planted tank can need for their growth. I don't need any precise numbers as I know very well that this is impossible to get without years of hard work and lab testing. Still I think we can find out a raw numbers to have at least some idea about the needs of our plants. If I find out the needs of some group of fast-growing aquatic plants under high light (~100 µmol PAR at the substrate) and recommended CO2 levels (~35 mg/L), then I can have a relative certainty that under lower light and not-so-fast-growing plants the demands won't be higher. And even if the plants CAN grow much faster even under lower light if we give them higher dosages of nutrients (e.g. PO4 which can be limiting in most tanks), my goal is to find out some reasonable range of nutrients which will ensure a good growth and vitality of my plants ... as I don't need them to grow like crazy (as with EI method); I want them to be fit and in a good shape. Why should I try to find out how much nutrients my plants will use up under some absolutely unrealistic conditions like 2000 µmol PAR, 70 mg/L CO2 and unlimited amount of nutrient supply? I'll NEVER have this situation in my tank, nor I want to have it. I don't want for my plants to grow like crazy. I want my plants to grow well (but rather slowly so that I need not to trim them each week).
 
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