NPK fertilizers are among the most commonly used fertilizers in aquaristics. NPK fertilizers are available from various suppliers. Here we look at what the NPK fertilizer is all about and why it is so relevant for aquariums.
Liebig’s law of the minimum
Liebig’s law of the minimum describes the principle that it is not the sum of the available growth factors, but the scarcest resource as the limiting factor that determines the influence on plant growth.
Justus Freiherr von Liebig, the «father of the fertilizer industry», generalized an agricultural scientific knowledge of Carl Sprengel and expanded it to include the perspective of nitrogen and trace elements as essential plant nutrients.
This made it clear that an uninterrupted supply of nutrients is essential for plant growth.
Plants need many different nutrients, called essential nutrients, to grow healthily. If even one of these nutrients is missing, plant growth will be stunted, even if all other essential nutrients are in abundance.
This also applies to all other resources, such as e.g. B. Light and CO2 in an amount suitable for the plant species. The scarcest resource always limits plant growth and is therefore called the limiting factor.
Growth factors and plant nutrients in the aquarium
The four most important factors for good growth of aquarium plants are listed here:
- Light
- carbon
- macronutrients
- micronutrients
It depends on how the plant nutrients are available for the aquatic plants. For example, carbon in the form of dissolved CO2 must be present in sufficient quantity to act as a growth factor.
If aquatic plants show signs of deficiency even with intensive algae growth, this is usually due to an imbalance in the nutrient supply.
So more light and CO2 will not lead to better plant growth if there is a deficiency of another specific nutrient such as nitrogen or iron.
What is NPK fertilizer?
NPK fertilizer contains proportions of nitrogen, phosphorus and potassium for the purpose of supplying the aquarium plants. These elements are generally added to the aquarium water as a liquid fertiliser.
The functions of NPK in aquarium plants
- Nitrogen: found in chlorophyll, nucleic acids and amino acids; Component of proteins and enzymes.
- Phosphorus: an essential component of DNA, RNA and phospholipids, which play an important role in cell membranes; also plays an important role in the energy system (ATP) of plants.
- Potassium: plays an important role in plant metabolism and is involved in photosynthesis, drought tolerance, improved hardiness and protein synthesis.
nitrogen (N)
It is a component of chlorophyll. With a lack of nitrogen, the leaves turn yellow in the middle of the leaf (chlorotic). The growth of new plants is then also reduced and can appear red or red-brown.
Because nitrogen is an essential component of amino acids and proteins, nitrogen-deficient plants and grains are low in protein.
An excess of nitrogen leads to extremely dark green leaves and promotes vegetative plant growth. Nitrogen is mobile in both soil and plants.
Phosphorus (P)
Phosphorus is another essential macronutrient whose deficiency is a major problem in growing systems. It is an essential part of the components of DNA and RNA and is involved in the function and integrity of cell membranes.
It is also a component of the ATP system, the «energy currency» of plants and animals. A lack of phosphorus is noticeable by purple or reddish discoloration of the plant leaves and is associated with poor plant and root growth, lower yield and delayed ripening.
An excess of phosphorus can also cause problems, but it is not that common. Zinc deficiency can occur in soils with extremely high levels of phosphorus. Phosphorus is generally immobile in soil and somewhat mobile in plants.
Potassium (K)
Potassium is the third macronutrient that is most commonly supplemented. It has important functions in plant metabolism, is involved in regulating water loss and is required for adaptation to stress.
Plants that are potassium deficient may show reduced yields before any visible symptoms are noticed. These symptoms include yellowing of the edges and veins, and curling or curling of the leaves.
An excess, on the other hand, leads to a reduced absorption of magnesium by the plant due to chemical interactions.
Macronutrients in the planted aquarium
macronutrients
Macronutrients are nutrients that plants need in large amounts. In a planted aquarium, these primarily include elements such as nitrogen (N), phosphorus (P) and potassium (K).
Below we look at these important macronutrients and the most common deficiency symptoms.
nitrogen deficiency
Nitrogen (N) can be taken up by aquatic plants in different forms: ammonia, urea and nitrate.
To determine the nitrogen content of the water, there are usually only water tests available that can display nitrogen in the form of nitrate (NO3). Optimum concentrations are between 10 and 25 mg/l NO3.
A typical symptom of a nitrogen deficiency is a general yellowing of the plant, especially on the older leaves. A reduction in the size of the new leaves or stunted growth can also indicate a nitrogen deficiency.
Some plant species take on a distinctly reddish tint. An increased occurrence of green thread algae, brush algae or black beard is quite common when there is a lack of nitrogen in the aquarium.
With suitable liquid fertilizers, for example, the supply of nitrogen can be increased without significantly affecting other nutrients.
phosphate deficiency
The concentration of this element in water is usually determined by measuring the concentration of phosphate (PO4).
This is done using a commercially available standard water test. The symptoms of a phosphorus (P) deficiency are easily recognizable in fast-growing stem plants.
The very slow growth and the reduction of the shoot tips are the most common biological indicators.
Some aquatic plants can take on a darker or even purple color. Often a phosphate deficiency can cause an increase in green spot algae. In a planted aquarium, a phosphate concentration of around 0.1 to 1 mg/l PO4 is recommended.
This substance does not have to be measurable and must be kept permanently in the water. Phosphate is quite reactive and can interact with other nutrients such as iron.
Plants are also very good at storing phosphate. An excess of phosphates in the aquarium can lead to the appearance of filamentous green algae.
potassium deficiency
Often for potassium deficiency (K) are the leaves with holes or dying leaf tissue (necrosis). When potassium is deficient, they first show up as small black dots, but then they become visible holes that are partially outlined in yellow or black, similar to nitrogen deficiency.
These bioindicators can occur in Anubias, Hygrophylas and ferns, and the leaves can turn yellow and show reduced growth.
An excess of potassium in aquarium plants can block the plants’ uptake of other nutrients, and the new leaves will grow twisted and stunted. Optimal potassium concentrations are around 5 to 10 mg / l in the water.
Supplying the aquarium with NPK macronutrients
NPK macronutrient delivery is one aspect of the planted aquarium that unfortunately tends to be neglected. NPK stands for the elements nitrogen (N), phosphorus (P) and potassium (K) that plants need, and in relatively large amounts.
The nitrogen and phosphorus compounds that plants can absorb are created in the aquarium by the activity of microorganisms that break down fish droppings, dead plant parts and leftover fish food.
However, these sources are usually insufficient, especially in aquariums with many plants, with very strong light or with CO2 supply. The plants need more than they get this way.
And the most important macro element, potassium, is not even part of the waste cycle in an aquarium. Nevertheless, the plants need it.
Fertilizer is mainly supplied to the aquarium in the form of liquid fertiliser. It is desirable to add the NPK macronutrients at the levels required by the plants. With the help of water tests and aquaristic experience, we can estimate the necessary quantities.
In this way we can identify possible deficits in the aquarium plants. As a countermeasure, nutrient gaps must be recognized and closed.
Two things are important for this: On the one hand, the water parameters have to be checked with appropriate water tests, on the other hand, the symptoms of visible deficiencies in the plants have to be interpreted.
This can be done with the help of guides and explanatory illustrations on the subject. However, it is better to avoid nutrient deficiencies from the outset.
However, if more fertilizer is used than the plants need, the excess is used by the algae. If you over-fertilize, there is a risk of excessive algae growth.
Concentrations of NPK macronutrients in the aquarium
It is not possible to determine standard amounts that should be present in an aquarium. The necessary quantities depend rather on the individual conditions of the respective aquarium.
However, specific concentrations of NPK macronutrients in aquarium water can be described as guide values within the following ranges:
- 10 to 25 mg/l nitrate (NO3)
- 0.1 to 1 mg/l phosphate (PO4)
- 5 to 10 mg/l potassium (K)
In general, a distinction must be made between nutrients whose concentration must always be kept at a certain level (eg CO2, nitrate, potassium and magnesium) and nutrients which do not require a constant level (eg iron and phosphate).
The concentration of the latter substances can no longer be detected in the water tests after a few days. This does not have to have negative consequences, as plants are quite good at storing some nutrients, such as phosphate.
If there are no deficiencies and stunted growth in the plants, there is no need to constantly maintain measurable levels of iron and phosphate.
Calculate nutrient consumption
Aquatic plants consume nutrients, but at very different rates and amounts, depending on the nutrient.
In our experience, nitrogen consumption is quite high in a densely planted aquarium with many fast-growing stem plants, while other…