The Redfield ratio is often used as a reference in feeding aquatic plants. What exactly is this nutrient ratio? Is it still applicable by modern standards of planted aquariums? We explain.

Definition

The Redfield ratio describes the proportional composition of chemical elements in marine phytoplankton. It therefore refers to microorganisms in salt water. The ratio was discovered by Redfield in 1934 through empirical studies and later published in 1963 with two colleagues. These values, which are still widely used today, are:

1 mole phosphorus : 16 moles nitrogen : 106 moles carbon

This means that, with sufficient availability of nutrients, plankton contains 16 moles of nitrogen and 106 moles of carbon per mole of phosphorus. The significance of the Redfield ratio for oceanography and marine biology lies in the fact that plankton forms the base of the marine food pyramid.

Application in the aquarium hobby

It was found that a ratio of 1:16 for phosphorus to nitrogen is beneficial for the growth of higher aquarium plants. A deviation from this ratio means that one of the two substances is used up first and plant growth then stagnates. Since aquarists generally do not measure the atomic proportions of phosphorus and nitrogen directly, but phosphate and nitrate, an adjusted ratio is calculated using the molar mass. This leads to the so-called Buddy ratio of 1:10.33 for phosphate to nitrate. Among planted-tank aquarists, the Redfield ratio is therefore often given as a ratio of 1:10 between PO₄ and NO₃.

In the early 2000s, the Redfield ratio was used in planted aquariums to combat an infestation of green algae or cyanobacteria. Here, the ratio of nitrate to phosphate in the water is measured, which should be within a certain "green" range. Then the factor that is present least is added through appropriate fertilization.

Evaluation

First of all, it should be noted that the Redfield ratio refers to the composition of phosphorus, nitrogen (and carbon) in marine plankton. There is therefore no scientific basis for transferring the ratio directly to the required fertilizer amounts for aquarium plants, especially since Redfield determined the ratios not in the water but in the biomass of green algae and cyanobacteria. The insight that macronutrients such as phosphorus and nitrogen are important for healthy growth of aquatic plants, and that nitrate should also be present in higher amounts, is quite accurate. However, you should refrain from adhering very strictly to the 1:10 ratio of phosphate to nitrate. Rather, this ratio should only be accepted as a rough guideline. After all, nutrient consumption in an aquarium should also be considered individually.

Experience in planted aquariums has shown that values of 15-25 mg/l NO₃ are much more effective. Since phosphate is quite reactive and, for example, iron phosphate precipitation can occur, values below 1 mg/l are much more workable. A ratio of 1:100 (PO₄:NO₃) corresponds more closely to today's understanding of how to treat aquarium plants. For example, 20 mg/l NO₃ and 0.2 mg/l PO₄ would be good nutrient concentrations. But here too, you should not interpret this ratio too strictly but adjust it individually. It should also be mentioned that besides phosphorus and nitrogen, other macronutrients such as potassium and magnesium, but also carbon (CO₂), light and micronutrients (iron and trace elements) are important for the growth of aquarium plants. Only a complete supply produces vital growth that is as free of deficiency symptoms as possible.