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The problem with the Redfield ratio theory


2 Aug 2007
I've had a few discussion with folks, mostly in the UK over this one. I will not get into the issue with the difference between the mass ratio, which is what is used, versus what Redfield actually shown in his paper based on averages of marine phytoplankton, it was atomic ratios, atoms, not mass(eg ppm etc). So 106 carbon atoms to 16 nitrogen atoms to 1 P atoms and so forth. To convert to mass, you need to factor in molar weights, N/P will be 16N's* 14 g/mol/ 1P* 30.97 g/mol= 7.2 for a ratio for N:p for algae based on weight or about 10:1 for ratio for NO3/PO4. Likely N is higher in practice since NH4 is also used by algae and plants and that is rarely detected.

That aside, some seem to have fallen for the trap that because they went from bad to okay or even good, that the reasons must be due to the claim that it is the ratio, not that they simply have started to dose a strongly limiting nutrient.

The concept of limiting growth is based on Liebig's law of the minimum. Not ratios...........
Algae are not limited based ion ratios either. They are limited based on concentration, and light and CO2 play significant factors also.

Plants are doing better because they are not as limited now, not because of a "ratio".
You can have success but attribute it to the wrong reason and make the entirely incorrect conclusion.

That was what was done with PMDD in years past with PO4.
Same type of thing here and the same issue with most methods.

They take great liberty when it comes to explaining their results in the conclusion.
Even if the results do not support them :rolleyes:

I can have a ratio of 5ppom of PO5 and 5ppm of NO3 and still do pretty well.
Likewise, I can have 30ppm of NO3 and 1 ppm of PO4 and do well also.
Ratios are 1:1 and 30:1, far outside the RR, and yet no algae of any kind...........

So what explains these observations?
Simple: in both cases the tanks' plants are not limit for N or P.
The ratio with 1:1 will become limited faster for N, but at this ratio, it explains nothing. It's only when the nutrient become limiting, that growth is affected, so anything above a limiting value, just like Liebig states............is what determines growth.

Plant growth, not limiting algae(they are not limited in my ratio systems either, which is a wider range than RR), is what the issue is.

Generally, most have low light also, so that means/implies even more flexibility when it coems to dosing and ratios/lower values for limiting, easier CO2 dosing etc.

I also have not seen any of the so called algae issues based solely on the nutrient ratios as claimed either.
But then again, my CO2 levels are in good shape and demand for CO2 is not influenced by low PO4 etc. They seem to have non independent factors occuring(eg poor CO2 which is being propped up by semi limited PO4 and low light).

When you add more light and good CO2, these same results are no longer present and the model does not fulfill the logic: a "ratio" some how limits and controls algae and helps plants grow better.

If you I have a ratio of 0.01 PO4 and NO3 of 0.16, I can promise you, plants will not do well.
They are limited and the ratio falls apart.

Tom Barr

George Farmer

30 Jun 2007
Thanks, Tom.

I've tried to explain that folk shouldn't get too concerned with actual nutrient ratios on the PFK forum a few times.


Thread starter
2 Aug 2007
Well , the thing is that is distracts folks from the real issues that need to be considered to help solve the problem new folks have, regardless of the method you chose.

I do not poo poo on methods, I use several.

But when the method suggest something causes algae, or controls algae, I know they have not done their homework. EI does not address algae that way, nor do any of the suggested methods I offer. They never have.

They focus on plants.

PMDD used the same logic and RR is somewhat repackaged PMDD.
The underlying basis is false. If you want to learn to grow a culture algae, then that should be your focus, you learn why is grows, how and what germinates it. Each new reworked repackaged method has some hogwash about why algae does not grow. Some use sediments and claim it limits the water column and that's why it works well etc.

Neither is true.

Simply because you add some ferts and things work, is no surprise or if you do not allow one thing to get strongly limiting. But why fret and worry over that if that's not what is going on? Why misplace blame and cause for algae with each "new method"?

This does not help hobbyists.
Getting at the root trade offs and issues that cause algae or changes, no matter what method you chose, I think is a far better long term solution to help the hobby and increase % success rates for algae free healthy planted tanks.

Why do some folks fail at RR?
Why do some fail at ADA?
Why do some fail at EI?

they say they do everything right after all.............

Where are the places they will most likely mess up, what are symptoms and the cures for these mistakes? These issues are universal, not confirmed to one single method. So answering why and how something works that explains all/99% of the observations etc, is very useful.

Then we can focus on reducing that light intensity, which reduced CO2 demand which makes things much easie rto target, then the nutrients can be all over the place and very easy to deal with. You simply do not get this type of general universal approach stuck with one single method. You cannot connect the dots with non CO2 vs CO2 methods and why each works with seemingly opposing viewpoints.
Sure, anyone can say "X method works for me, see my tank full of plants, or my nice scape". No one argues with that part, it's only when they speculate why and then never follow it up with a decent test or compare those with other folks and see what differences their are. We have a large group in SF that does this and a few folks here and there. I know some are good at this in the UK, but I hope they see why I argue for this. It's not I like to argue and debate, I'd rather folks "get it", use their own critical thinking skills and teach themselves, prove it to their own eyes, but many get duped.

They buy into the diet pill sales pitch.
Even when you tell them, some will even say yes, but they still will go and use those pills anyway:)

I posted some interesting use of the RR for algae limitation estimates based on the CO2 critical concentration of several FW algae species. It shows why algae cannot be limited and why they are not CO2 limited either in water.

I can repost it here.


From this:

Originally Posted by detlef
So we're talking here about compensation points for freshwater algae of as low as 0,01 ppm CO2 or even less (at acid pH) which is incredibly low of course.

Thanks Tom!

Best regards,

Yes, this is why we cannot limit algae.
Think about how N, P etc relate to the ratio of carbon use.

Say the carbon is the same as the Redfield ratio, or even less, say 1/2 that for FW.
106/2 = 53 Carbon atoms for 16 N and 1 P atom.

Removing the O2/O3 etc
0.01 ppm C ~ 0.003N or 3 parts per billion and now let's go to P. => 0.00019 ppm P, which is over 2 orders of magnitude lower than any test kit sold for P.

Sub parts per Billion ranges...............fish likely leak more P than is required.............so do plant tissues..........

If you use RR that is.........I think FW algae are more in the range of 14: for N:p.
Carbon is about the same though, still, it's just an average.

But if you use 106 C, then these numbers drop even farther and support the idea that limiting the algae is very difficult unless you are in open oceans, large lakes, have lots of light, lots of algae and time. Then, they can and do become limited.

So what happens when plants are provided with stable CO2 which is their largest enzymatic and energy input(making sugars from carbon is what they do and what they are all about), and N, P etc?

They grow well.

Even in non CO2 planted aquariums, algae are still not CO2 limited.
Nor are they nutrient limited.

What is left? Light.............disturbances, limiting plants..........changes in concentrations(but not limitations) that may signal a good time for germination.


I've long argued this is not about limiting algae.
If you look at CO2 and other nutrients and algae growth, then use ratios like the Redfield of a FW version, then you can predict how much N and P are required, or Fe, to limit algae.

In deep northern lakes, adding P in the summer should increase algae as these are deep, good sized systems, lacking plants, lots of light over long days(Northern summers), and the P enrichment studies and rational for P limiting systems comes from such systems. At the lake edge, the algae are not limited, however, most of the open water, far from sources of P and lots of other algae under high light and long days will grow like mad.

If you go to Florida with shallow lakes, moderate light routines(still quite high intensity though if not more), lots of plants, this is not as common. In fact, there's no evidence of this eutrophication and we find a very high correlation between 30-50% plant infested lakes and clear water.

While this is not a direct way at figuring out how much P is required to limit algae, it does show an expected/predicted/modeled range.

Good old Liebig, which Paul and Kevin mentioned decades ago now with PMDD, still comes into play here.
But it's focus is for plants, and if you buy into ther whole RR thing, then show how it can possibly limit the FW algae based on the ratio model of the other nutrients.

How would they even begin to test such low levels?
That would be very very tough. Even as a researcher, that would scare me off right away.
whatever I did, would have to be indirect and extremely clean etc.

Planted Aquariums are so far removed from this.
When you go to Tropica, you will not find many who support those limited views.
you will find people who are focused on growing plants, the same is true here in the USA as far as growers and researchers

Tom Barr


Thread starter
2 Aug 2007
Well, still not getting through, but perhaps cracks will appear:)

If you think about controls and testing, the fact that you do not get algae in high nutrient tanks shows that the other parameters are truly independent. Since if you assume that excess NO3/PO4 = algae etc, you should have to see it.

So the test tank that does not get algae, is in fact a tank that is independent of other factors.
Thus it is the control tank, and you can add nutrients all over the place without issues.

In tanks where NO3/PO4 excess dosing "induces" algae, there are always some confounding factor, there might be 100's, who knows, they might be able to address some, most etc, but CO2 is the hardest one to rule out.

Tom Barr

Simon D

22 Sep 2008
Fascinating read.

I get the gist of of it, but will have to re-read and re-read to fully understand (maybe not even then will the hazy waters become clear). It's a bit over my head and I don't consider myself to be of below average intelligence. But well done to you for the time and thought you've put into this write-up.
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