In order to better understand the needs of your aquarium plants, we'd like to take you on a short plant journey. Like all living creatures, aquatic plants need certain conditions in order to live and grow. There are three key factors:
The physical environment, including light and temperature
The biological environment, including fungi, herbivorous fish or digging fish
The chemical environment, including macro- and micronutrients
When it comes to fertilising plants, let's take a look at the macro- and micronutrients. Plants make use of biologically available compounds as appropriate in order to absorb a wide variety of elements. Carbon dioxide (CO2) is used as a source of carbon (C), for example. In the table below, we explain the different elements that should be added when fertilising.
Macronutrients
Element
Biologically available forms
Effect
Deficiency symptoms
Note
Carbon, C
CO2
HCO3–
Basic element of all life
No carbon source means no photosynthesis → Depleting energy reserves → Plant death
Ensure sufficient carbonate hardness (> 5 °dKH) in combination with pH values between 6.5 and 8.5; fertilisation with CO2 possible
Nitrogen, N
NH4+
NO2−
NO3−
Component of chlorophyll
The most important nutrient for the formation of amino acids and protein
Metabolism disorder → Protein depletion → Poor root growth → Discolouration and death of leaves
Fertilisation depends on stocking density and feeding behaviour: minor supplement recommended
Important for cell division, cell differentiation and cell stretching, cell wall stabilisation, tissue formation
Prevents cell building and cell development → Reduced root growth, changes to young leaves
Supplement required
Copper, Cu
Cu2+
Component of protein synthesis and photosynthesis
Responsible for stem stabilisation
Activates enzymes
Prevents cell division and inhibits photosynthesis → Youngest leaves roll up and die
Supplement required
Zinc, Zn
Zn2+
Increases disease resistance
Component of enzymes and influences enzyme reactions
Various secondary diseases such as inhibited growth, leaf discolouration or leaf deformation
Supplement required
Cobalt, Co
Co2+
Nitrogen binding
Enzyme activator
Central element in vitamin B12
Decreased nitrogen uptake → Leaf colour change, reduced growth (symptoms as with nitrogen deficiency)
Minor supplement
Nickel, Ni
Ni2+
Central element in nitrogen conversion
Reduced nitrogen utilisation → Darkening and death of the leaf tips
Minor supplement
Iodine, I
I−
Stimulates growth
A component of defence mechanisms
No known signs of poisoning at very high concentrations
Minor supplement
Many more elements could be added to the list. Titanium (Ti) and chromium (Cr), for example, are not essential, but their effect is astonishing. They significantly affect the colour intensity of both emersed (air) and submerged (water) plants.
In terms of fertilising technique, you should be familiar with the law of minimum and the law of optimum in order to provide your plants with the best possible care and prevent algae growth. Don't forget: any nutrients that are not absorbed by the plants are available to potential algae.
The law of minimum
This law states that it is the scarcest nutrient which determines overall nutrient intake and conversion by any living being. For example, if iron is proportionally the least available nutrient, the absorption of all other nutrients is reduced/limited accordingly.
The following graphics will help illustrate the law of minimum. The first graphic shows the initial situation. You can see various elements (x-axis), the light factor (orange) as well as the corresponding requirements (y-axis) of the plant. The optimum demand (blue) and the quantity available to the plant (yellow) are shown. In our example, the difference between optimum and available quantity is proportionally highest for CO2.
Illustration 1:
Figure two shows the effects of the initial situation. The absorption of all essential elements is restricted in relation to the least available element (CO2). The actual intake decreases proportionally (purple).
The range between purple and yellow is available to potential algae, as this cannot be absorbed by the plant.
Illustration 2:
The law of optimum
The third figure illustrates the optimum law and shows how it can be exploited in the domestic aquarium. A plant can only convert a limited amount of nutrients (the optimum, shown in blue in the example) in order to achieve its highest growth rate. Everything else remains in the water.
This condition is not practically feasible in community aquariums, so the goal should be a uniform "throttling" of all nutrients. When this is achieved, there are no deficiency symptoms and algae growth is greatly reduced. All elements are proportionally throttled (the ratios between optimum and available demand are the same).
Illustration 3:
Composition of an aquatic plant
The "Redfield" ratio, named after the man who discovered it, provides a guide to the needs of aquatic plants, in that it describes the atomic composition of phytoplankton (C, H, O, N and P). Over time, it has been supplemented by further elements:
This composition allows us to infer several different facts:
CO2 and H2O are the most important "foods" for plants. These are the most common. → (C106H263O110N16P1)1000
The ratio of N to P is 16 : 1. However, since the water contains both nitrate (NO3) and phosphate (PO4) compounds, a ratio of 11 : 1 must be maintained.
Although the trace elements are required in very low concentrations, they are still an important component. → Fe8Mn14Zn0.8Cu0.4Co0.2Cd0.2[xy]
The elements above are found in the natural habitat of aquatic plants. It is therefore vital to add trace elements such as copper or zinc in an artificial system such as an aquarium.
