Nitrogen and Phosphorus

Nitrogen and phosphorus in aquarium plants – this is the main essence of the article. What’s in the plants inside. How much nitrogen and phosphorus are in plants. What is their ratio of nitrogen and phosphorus (or nitrate and phosphate)? And most importantly – is the ratio the same for all plants, or not? These are very important questions since what they need is what kind of food they need, what fertilizers they need. Do all types of aquarium plants need the same fertilizer, or does each type need different fertilizers? Let’s figure it out.

When it comes to the ratio of nitrogen to phosphorus, many aquarists recall the well-known Redfield ratio. What is the Redfield ratio? These scientists took a huge number of different representatives of phytoplankton in the ocean and analyzed the ratio of carbon nitrogen and phosphorus in each of them. Then they deduced a certain average value for all these organisms. This is one number. Like the average temperature of patients in a hospital. Recently it was 16/1, now it is 14/1. This average may vary slightly for aquatic plants, or terrestrial. In-plant aquarium, this ratio for some reason has taken root very strongly and is taken as an undeniable truth for creating the perfect plant aquarium. It should also be noted that in plant aquariums, the converted value for nitrate / phosphate is usually used. Because in the aquarium, they often operate not with nitrogen and phosphorus, but with nitrates and phosphates. It is nitrates and phosphates that an aquarist can somehow calculate, test. Plants can consume nitrogen in another form (in the form of ammonium, for example), but this is a whole separate topic.
Redfield nitrate / phosphate ratio ~ 10/1.

The conversion factor is very simple:
(Nitrogen / Phosphorus atomic ratio) /1.5=NO3 / PO4 ratio

Does the NO3 / PO4 ratio affect algae growth in the aquarium?

It is believed that the appearance of certain algae is associated precisely with a deviation from the Redfield ratio. You can read on some sites that if the deviation is towards nitrates, then green algae appears. If the deviation is towards phosphate, blue-green algae will appear. If the Redfield ratio is respected, then you will have a clean aquarium with plants and without algae.
But there is an obvious contradiction. Different unwanted algae require different ratios of nitrates and phosphates. Some algae need more nitrate, some phosphates. But for all types of aquarium plants supposedly you need one single ratio of nitrate and phosphate.
Where is the logic? But there is no logic. In fact, different types of plants also need a different ratio of nitrate and phosphate, like algae.

Watch this video about Nitrogen and Phosphorus to get a good graphic knowledge of this issue: Be careful this video has sound!

Nitrogen & Phosphorus Cycles: Always Recycle! Part 2 - Crash Course Ecology #9
NO3 / PO4 – Blyxa
Specifically, to confirm this, we conducted a series of analyzes of different plants from aquariums. The first example – the Japanese favorite by many Blyxa aquarists, has in its composition the ratio of nitrate to phosphate 5/1. And the ratio of Redfield is 10/1. How do you like this deviation? Twice.
Now imagine you have an aquarium almost entirely planted with this beautiful plant. And so you read that on average all plants consume NO3 / PO4 in a ratio of 10/1 and begin to fertilize with this ratio. What will happen? Blyxa will be short of phosphates. She will grow smaller and slowly grow.

NO3 / PO4 Hemianthus Cube
Another no less interesting example. The most popular plant for creating a green rug is Hemianthus Cuba. We also did his analysis. The ratio of NO3 / PO4 in it is already 30/1. Deviation from Redfield’s ratio is three times, and the other way. See what a huge difference. 5/1 – Blyxa, 10/1 ratio of Redfield and 30/1 at Hemianthus of Cuba.
What fertilizer is needed for an aquarium in which only Hemianthus Cuba grows?
Obviously, it should not have a lot of phosphates, as is the case with Blyxa. And even using fertilizer with a ratio of NO3 / PO4 according to Redfield, a lot of excess phosphates will remain in such an aquarium, which algae will use with pleasure. Often this is a green coating on the stones, because such aquariums are usually decorated in the Iwagumi style.

If we take long-stemmed species, among them there is also a large spread in preferences. Here Ludwigia brevipes likes phosphates more. She consumes nitrates and phosphates in a ratio of 7/1. But Didiplis likes nitrates more. He has a ratio of nitrates to phosphates of 16/1.

The example of Fissidence shows that he just likes Redfield’s ratio. It has very close to its ratio of nitrates and phosphates of 11/1.

Cryptocoryne beckettii, almost like a cube, needs only a small amount of phosphate. She has a ratio of 23/1.

We analyzed a whole list of plants. According to preferences from each other, they can be very different. Some plant likes nitrates more, some phosphates more. The difference may be at times. But the average ratio of nitrates to phosphates is very close to the ratio of Redfield. According to our list, this is 14/1.
What does this affect? For illustration, I want to give one example from life.

What really affects the ratio of nitrate to phosphate?


Test for aquarium PO4

This incident, which occurred in 2012, just prompted us to conduct a study of plant compositions.
At that time, we were reorganizing our experimental aquarium plant nursery. And they decided to keep order in it in each aquarium to contain only one or two types of plants. In one Blyxa aquarium, in the second – Lilaeopsis, in the third – Water pennyworts, in the fourth a different view and so on. And of course, we all used fertilizer AQUAYER Udo Ermolaeva MACRO + and MICRO + in them all. One set of fertilizers at all. And what do you think? Many plants did not grow well. How so? This is fertilizer, which was tested on the mass of different aquariums and it never failed. And here is such a problem. Why?

They started to understand. What have we done? We took all these species and mixed them back so that in one aquarium there would be not one or two species, but several species. The bigger, the better. Nothing else changed. The same fertilizers, also lighting, soil, all the same. Just made a wide variety of species in one aquarium. And everything became wonderful. All plants grew and had an excellent appearance, even those that had not previously grown alone.

Why did it happen? Because the 10/1 ratio works when there are several or many different types of plants in the aquarium. When there are many different ones in one aquarium, they compensate each other. Blyxa and Lilaeopsis took a lot of phosphates and left a lot of nitrate in the water, and this excess of nitrate took a cube with Cryptocoryne. But when in the aquarium there are only one or two types of plants, there is no one to compensate for and you need to select the ratio of nitrate and phosphate for these plants. And it can be both 5/1 and 20/1.

Phosphates need a little more

Another very important point. In the course of our observations, it turned out that in the conditions of an aquarium, phosphates need a little more than plants require them. What for? Who else needs phosphate in the tank besides plants?
The fact is that phosphates can be bound by precipitated iron. It is known that iron is very unstable in an aquarium and quickly precipitates in silt even if it is introduced with very strong chelators. Therefore, iron is poured a lot for plants. By the way, we also conducted an analysis of plants and iron content. So, the average ratio of nitrate / iron in plants according to the mentioned list is 650/1!
And the ratio of nitrate / iron in fertilizer systems of various manufacturers ranges from 100/1 to 20/1. The excess iron in fertilizers compared to how much iron reaches plant cells is simply huge. And all this excess falls into the sludge. Just don’t think that if you pour iron in the ratio nitrate / phosphate 650/1, you will grow plants. Of course not. They will simply die, because the loss of a huge fraction of iron in the sludge is inevitable in an aquarium

So, silt with iron is retained in the soil, or in an external filter, and partially absorbs phosphates. Only if bound phosphates are available in the soil to plant roots, are they not available in the filter. Then the filter is cleaned and, together with silt, phosphate with iron irrevocably leaves the aquarium.


How to look for the perfect nitrate / phosphate ratio
aquarium phosphate nitrate

Now a few recommendations on how to conduct experiments correctly and how to correctly evaluate whether this or that ratio is suitable for your aquarium.
The most important indicator of whether the selected ratio of nitrate and phosphate is suitable for your plants is the appearance of the plants and their growth. And in this assessment, you can’t rush. Fertilizer must be applied for at least two weeks. And then see if you like how plants grow or not. If something is wrong, then change the ratio.

In no case should you evaluate the effect of the fertilizer you have prepared using tests for nitrate and phosphate. This is a very common mistake. And many do indeed. For some reason, it is generally accepted in the online community of aquarists that the ratio of nitrate / phosphate 10/1 should be in aquarium water. Wrong! 10/1 is the average ratio of nitrate / phosphate in the fertilizers you apply, not in the aquarium water.
After all, this is contrary to logic. It is impossible to make fertilizers in a ratio of 10/1 and have water in the aquarium constantly with this ratio. Because phosphates, remember, will be partially bound by sludge.
In practice, aquarists who seek to obtain a 10/1 nitrate / phosphate ratio in water are forced to add a lot of phosphates additionally. At the same time, they often forget about what kind of ratio nitrate / phosphate is ultimately brought into the aquarium, because what the tests show is put in the forefront.

And the right approach is when you initially know the ratio of nitrate to phosphate in the fertilizer, be sure to watch the plants when you use this ratio and evaluate in two weeks whether you need to change this ratio or use it further. Tests are not needed here. They do not give a complete picture of the availability of nutrients for plants in the aquarium, as they show only what is in the water. What is in the ground, they will not show.

That’s right – the ratio of nitrate / phosphate in fertilizers is constant, and the ratio of nitrate / phosphate in water is a variable.
Wrong – the ratio of nitrate / phosphate in the aquarium water is constant, and the ratio of nitrate / phosphate in fertilizers is a variable.
Changing the ratio of nitrate to phosphate in the fertilizer is necessary only when you are looking for the perfect ratio.