Why does a plant need potassium?

​Potassium has a guiding role in the development of the crop. His presence ensures that a large number of processes run smoothly. The plant needs potassium for photosynthesis, production and transport of carbohydrates, energy supply and moisture management.

Because it plays a role in so many processes, potassium is called the quality element. It improves quality properties such as taste, color, smell and shelf life. In a potassium deficiency, all functions are performed to a lesser extent. An excess can cause salt damage.

Share of potassium in the plant
The proportion of potassium in the plant is the highest after nitrogen. Yet much less is known about it. This is because potassium is not built into anything, such as nitrogen in proteins and enzymes. Potassium only exists in dissolved form in the plant as potassium ions (K+). The dry weight of a plant consists of 1,0% potassium. This average varies from 0,5 to 5,0% depending on the crop. Potatoes, onions and cabbage crops, for example, have a strong need for potassium.

Moisture balance and cell tension
Potassium regulates water absorption (osmosis), because it is present in the cell fluid as a K+ ion. Cell tension is built up by the absorption of water into the cell. This increases the firmness of the crop. Due to the strong cell walls, the crop is less sensitive to pests and damage, especially by fungi. Osmosis also plays an important role in opening and closing the stomata of the plant. This mechanism is important not only for evaporation, but also for the inlet of CO2 (carbon dioxide) and thus for photosynthesis.

Potassium makes crops more resistant to the cold. This is especially important for winter crops (hardiness). The effect is twofold: on the one hand, potassium as a salt in the cell fluid prevents rapid freezing of the cell fluid. On the other hand, potassium ensures good water absorption. Frost damage is often the drying out of the plant.

Transport of nitrogen and sugars
Potassium is absorbed by the roots as K+ (positively charged ion). Potassium is therefore used by the plant to compensate for negatively charged ions such as nitrate, phosphate and amino acids. This enables the plant to provide transport, for example of sugars and starch from the leaves to the other parts of the plant.

Potassium as a cofactor
Potassium makes processes, such as metabolism and energy supply, possible through its role as a so-called co-factor. Most reactions in the plant take place with the help of enzymes. Those are long complicated molecules. They have to be folded in a certain way if they want to do their job well. Only in that folded state can other molecules bind to it. Potassium ensures the correct folding.

Potassium temporarily binds to the enzyme and this puts it in the right position. The reaction then takes place – for example the formation of a protein or sugar – and then potassium is released again. It can then be used elsewhere. In fact, it is always reused.

Potassium causes enzymes (blue) to fold in a certain way. Only in that folded state can two substances (red and green) attach to the enzyme.

How do I recognize potassium deficiency and potassium excess?
Despite the great importance of potassium, the element does not receive much attention when fertilizing. That's because real problems almost never arise. Potassium is very mobile. If there is a threat of a shortage, the plant itself ensures that the places where the element is most needed are provided.

A lack of potassium creates thinner cell walls, making the crop less sturdy. In addition, a deficiency manifests itself in weak growth and desiccation symptoms. Very specific are the dead edges on the older leaves. The potassium has been removed there, so that the fluid balance does not function properly. With an absolute potassium deficiency, just about everything stagnates. The plant will then die.

An excess of potassium can have a major influence on osmosis. Potassium fertilizers dissolve well, increase the ion concentration in the soil moisture and make it difficult for the plant to absorb water. This phenomenon is called salt damage.

In addition to salt damage, the excess of K+ ions around the plant root has another adverse effect. The potassium ions tend to oppose other positively charged ions in the absorption. Elements such as magnesium, sodium and boron are affected by this. This phenomenon is called potassium antagonism. Plants are not well capable of limiting their potassium absorption. If you offer more, the plant will also absorb more. This is called luxury consumption and can lead to a loss of quality.

source: Krikke, R. Potassium plays a guiding role; Kierkels, T. & Heuvelink, E. (2005). Potassium, the main regulator in many processes in the plant. Wageningen UR; Broekhuizen, J. (2013). Soil, fertilization and cultivation plan.