If you are passionate about marine biology or simply interested in ocean life, you have probably encountered the concept of the “Redfield ratio.” This is one of those fundamental rules that are taught to students as basic truth. It describes the surprisingly constant ratio of the main elements in marine organisms and seawater itself.
Simply put, the rule states that carbon, nitrogen, and phosphorus atoms in oceanic biomass and dissolved nutrients are in a ratio of approximately 106:16:1. For a long time, this was considered the universal key to understanding ocean productivity, explaining how algae growth is limited by the availability of nutrients.
However, controversy has raged in the scientific world for several decades. New research shows that this ratio can vary significantly in different parts of the ocean. Scientists have discovered that marine ecosystems are much more diverse and complex than Redfield's simple formula suggested.
So does it really work? Yes and no. Its value lies in creating a convenient starting point, a general model. But today, it can no longer be blindly accepted as an immutable law of nature. There is ongoing debate about how universal this ratio is and what processes — biological, chemical, or physical — actually determine it.
In this article, we will try to understand where this rule came from, why it is still useful, and why it is the subject of heated debate in the scientific community. This is a story about how a simple and elegant explanation clashes with the chaotic and diverse reality of the ocean.
Background: What is the Redfield ratio and what does Liebig have to do with it?
In 1934, oceanographer Alfred Redfield described the atomic ratios of elements:
- carbon, nitrogen, and phosphorus in marine plankton – C:N:P = 106:16:1;
- nitrogen and phosphorus in seawater – N:P= 20:1 (first version), N:P= 16:1 (revised version).
At the same time, Redfield noted that a shift in the proportions of elements in water affects the growth and composition of algae. An increase in phosphorus content is favorable for blue-green algae, while an increase in nitrogen is favorable for green algae. Data was collected in the Atlantic, Indian, and Pacific Oceans and the Barents Sea. Accordingly, there can be no complaints about the representativeness of the sample, and the correlation found is not just a coincidence or statistical error.
The shift in the proportions of elements in water affects the growth and composition of algae
Skeptics will say, “That's great, Redfield did a great job and made an important scientific discovery, but how does that help us?” This is where we need to remember Liebig's limiting factor.
Note! In 1840, Justus von Liebig described one of the main principles of ecosystem development: the most significant factor for an organism is the one that deviates most from the norm.
Liebig's law in simple terms
Let's say we only have two elements: food and water. Then there are two unfavorable options:
- there is water, but no food – the organism dies of starvation, and quenching thirst cannot help here;
- there is food, there is no water – it dies of thirst, because food cannot replace water.
V. Trautschold. Liebig's laboratory in Giessen, 1840
In the first case, the limiting factor is water, which is in short supply; in the second, it is a lack of food, which is the blocking element. The principle applies to any number of factors.
Plant growth is limited by one main mineral, which is in relatively short supply. This concept of limitation can be represented by “Liebig's barrel,” a metaphorical barrel in which each slat represents an element
In relation to an aquarium, this means that if one element is lacking, the plant is physically unable to compensate for its deficiency with other elements. When there is a critical shortage of any component, the plant stops consuming other substances properly, eventually becomes diseased and dies. In addition, nutrients that cannot be consumed by the plant can become food for algae – blue-green “parasites” and green “dots” that spoil the appearance of the aquarium.
— Alexander Tkachuk, marine biologist, expert in aquarium and terrarium keeping
The Redfield ratio is the idea that in the ocean, carbon, nitrogen, and phosphorus in living organisms are almost always in a strict ratio of 106:16:1. For a long time, this was considered a law of nature, but now scientists are arguing: in some parts of the ocean, the proportions are indeed close to Redfield's, while in others they differ greatly. It turns out that the rule does not work everywhere and cannot be applied blindly. The debate is not because someone is wrong, but because nature is more complex than it seems: different organisms, conditions, and ecosystems produce different results. Instead of dogma, scientists now see it more as a guideline that helps them think, but does not always predict.
How it works in practice
There is a formula for calculating the atomic ratio by fertilizer mass. The optimal ratio is considered to be close to Redfield's data, the values for which are highlighted in white. Blue and green are fertilizer ratios that promote the growth of the corresponding algae.
Some UDO manufacturers use and recommend the Redfield ratio, others criticize it, and still others do not comment on it at all.
Two main arguments of criticism and responses to them
Reason 1. A scientist discovered a ratio in seawater, and it concerns phytoplankton. Its application in a regular freshwater aquarium seems far-fetched.
It doesn't matter how it looks, what matters is how it works. The de facto optimal ratio of nitrogen to phosphorus for plants is in the range from 10:1 to 20:1. This has been proven by many generations of aquarists.
What does Redfield have to do with it? Perhaps aquarists tried using his proportions as an experiment, the result was good, and it became widespread. Maybe they initially tried to get rid of algae using this ratio, looking for a way that would not harm the plants.
Please note! It is also possible that aquarists were using similar proportions long before Redfield wrote his scientific paper, and that his name was used for commercial purposes by UDO manufacturers. What actually happened is unlikely to be known.
Argument 2. In practice, it is impossible to strictly maintain the proportions because plants obtain nitrogen and phosphorus not only from fertilizers.
This is a fair point. Indeed, it is necessary not only to add elements in the specified proportions, but also to monitor the composition of the water in the aquarium. Plants also obtain nitrogen and phosphorus from the soil, water, and fish food.
It is necessary to monitor the composition of the water in the aquarium.
Accordingly, it is necessary to periodically analyze the water, be attentive to any changes within the ecosystem, and, if necessary, change the length of daylight hours and the schedule for adding fertilizers. However, no one has ever claimed that observing Redfield's ratio is all that is needed to maintain an aquarium.
The role of fertilizers in the life of aquarium plants
Answers to frequently asked questions
The proportions are more or less clear, but how much UDO is needed in terms of quantity?
It depends on the number of plants. The more plants per cubic centimeter, the more actively you need to fertilize.
I follow Redfield's advice, but blue-green algae still appeared. What should I do?
Apply less fertilizer while maintaining the ratio and/or reduce daylight hours to 6-8 hours.
How often should fertilizer be added to maintain the ratio?
It depends on the aquarium. You can add a little every day or once every three days and make sure there is neither a shortage nor an excess of elements.
Which element should be used for calculation: nitrogen or phosphorus?
Regarding phosphorus. If there is not enough of it, you need to add it and add 10-12 times the amount of nitrogen along with it. Otherwise, when saturated with phosphorus, nitrogen will become the blocking element.
If plants get nitrogen and phosphorus from fish, water, and soil, then maybe fertilizers are unnecessary?
Yes, this is possible. Therefore, in a new aquarium, do not rush to add fertilizer, but first see how the ecosystem develops. If there are many fish and few plants, fertilizer will not be needed soon.
So, what do we have in the end? The Redfield ratio is not a law of nature, but rather an interesting observation that often proves to be true for the open ocean on average. It shows remarkable consistency in the ratio of carbon, nitrogen, and phosphorus in plankton and water. It is as if marine organisms adhere to a universal recipe for life.
Does it always work? No, and this is the main reason for controversy. In coastal waters, lakes, or iron-rich areas of the ocean, this ratio is easily disrupted. Scientists argue not about whether this rule exists, but about how fundamental it is. Some see it as a profound principle, while others see it as a mere statistical coincidence that only works well under ideal conditions.
Therefore, today, Redfield's ratio should be considered a useful guideline rather than an absolute truth. It helps to build models and understand the general principles of the cycle of substances. But at the same time, it is always worth remembering that the real world, especially near the coast, is much more complex and diverse than any beautiful formula. Its value lies precisely in the fact that, despite its simplicity, it continues to stimulate new research and discussion in ocean science.
