Distinction of the elements
Macro and microelements
Excluding the trace elements, requested in very small quantities, the dry weight of a plant is made by 16 chemical elements.
The three principal elements are oxygen, hydrogen and carbon. Together, they represent 96-97% of the total weight: the carbon represents among other things is effectively the 45%.
They are not minerals, so they are usually excluded from the articles related to fertilizers, however the carbon is most of the times added artificially in the aquarium by introducing CO2. So we will talk about it in this article.
The word «macroelements» refers to a list of 6 very important elements for the plant, that can be measured in decimals of percentage.
In order of percentage of dry weight, they are:
This order is just related to the percentage of dry weight, while the phosphorus would be for sure the first on the list based on the usefulness of these elements for the plant.
Sometimes it is possible to hear about primary and secondary macroelements, as indicated in the table.
Some other times, the elements in the right column are called mesoelements. This distinction is not concern the quantity of this element in the plant, but merely to fertilization, that is, the artificial introduction of nutrients.
In agriculture is very rare to artificially add calcium, magnesium and sulfur, they are usually largely present in the soil.
On the other side, it is very common to add nitrogen, phosphorus and potassium (NPK).
Even in the aquarium it’s the same, but nitrogen and phosphorus depend on the organic load of the aquarium and by polluting elements like fish food, while potassium is the element that lacks more often.
The microelements are requested in far lower quantities, they are measured in hundredths or thousandths of percentage of the dry weight of the plant.
On some texts, especially in Europe, they are indicated in ppm (parts per million).
As for the macroelements, we can make a list of the microelements in order of percentage of dry weight:
This list is not universal, because it may depends on the species of plant. Furthermore, there are plants where the zinc is placed third, surpassing manganese, or for example the demand for boron can be larger than the demand for clorine during the flowering phase.
In the end, it is important to say something about iron, first of the list.
Traditionally it is considered as a microelement. However, in aquaria with strong lights and with many plants its demand can raise so much that it can be considered largely as a macroelement.
Mobile and immobile elements
Another criterion to distinguish the elements is based on their mobility. Some of them can be moved by the plant, if needed, from the old to the new parts. These elements are defined as mobile.
Others have a structural function, once established in the tissues they can hardly be moved again. They are called immobile elements.
The mobility of an element is’t conceivable like «black and white». In fact it would be better to define them as «very mobile» or «slightly mobile»… but the exact description of this characteristic is very complicated, for sure too much for the needs of an aquarium owner.
On the other side, this sharp division between mobile and immobile elements leads sometimes to discrepancies and contradictions between different sources in bibliography.
So don’t be surprised if you will find opposite indications, for example about nitrogen and iron.
Without any doubt, the following elements can be considered mobile:
While the following 4 elements can be considered surely immobile:
All the other elements present hybrid characteristics, with a degree of mobility that can give a certain margin for different interpretations.
This distinction between mobile and immobile is actually quite important because it will give us precious indications and clues about the elements that are lacking in our aquarium.
If our plant is beautiful and lush in the top part, while we can see some problems in the lower part, then we can be sure that there is a shortage of one of the mobile elements. The few quantity of the mobile elements available is moved to the new leaves, sacrificing the old ones.
On the other side, when the new leaves are ugly and present some problems, then we are missing a plastic or hardly mobile element.
The plant cannot take it from the old leaves, so the shortage is visible on the new ones.
In the next chapters, we will talk about the single elements. We will start from the most requested one by the plants.