Spotlight on potassium

25 January 2021

Eurofins Agro is publishing a series of articles on important nutrients for horticultural crops. The first two articles covered nitrogen and phosphate. This time, the spotlight is on potassium.

Like nitrogen (N) and phosphorus (P), potassium (K) is an important nutrient. The element plays a leading role in the development of a crop.

Potassium in the plant

After nitrogen, potassium makes up the largest portion of the plant. Potassium is mainly stored in the vacuole and the cytoplasm of the plant. It fulfils several functions there.

Firstly, potassium works as a salt and thus provides osmotic value. As a result, it plays an important role in the plant’s water balance. The cell tension is built up by the absorption of water in the vacuoles, which ensures the plant's rigidity. By contributing to osmotic regulation, potassium plays a significant part in the opening and closing of stomata. This allows the plant to respond to drought and allows carbon dioxide (CO2) - formed during photosynthesis - to be released and oxygen to be absorbed.

In addition, potassium is absorbed as a positively charged ion and remains present in the plant in that form. In this way, the plant compensates for absorption of negative ions (phosphate, nitrate, sulfate), organic acids and amino acids. This, in turn, allows the plant to control the transport of sugars and starch. The K+ ion also plays a role in pH regulation in the plant sap, through exchange with H+. This is important for allowing enzyme reactions to take place.

Potassium therefore plays an important role in the plant’s metabolism. The element is involved in the work of more than fifty enzymes and contributes to building proteins and cell walls.

Potassium in the soil

Large amounts of potassium may be present in clay soil. They are strongly bound to the clay-humus complex, the CEC. In river clay, it is in fact so strong that there is fixation; in that case, the soil supply may be high, while the availability to the crop is low. The potassium concentration in the soil moisture must be high, or the plant will not be able to absorb it.

In sand and peat soil, the potassium content is naturally low because potassium is not bound and is easily washed out. It moves in the soil via diffusion; due to the water film around soil particles, potassium moves from places with a high concentration to places with a low concentration.

Potassium and fertilization

A lack of potassium is visible as poor growth, a weak crop with dehydration symptoms. The edges and leaf tips become chlorotic. This occurs primarily in the oldest leaves because potassium is mobile in the plant. When potassium is lacking, the flower buds open more slowly and the fruit quality is compromised.

There is a wide choice of fertilizers containing potassium. Potassium can be given as artificial or organic fertilizer. In artificial fertilizers it is often from mineral sources, together with chloride, phosphate or sulfate. See also the overview in the Nutrient Solutions Manual. They are all rapidly soluble salts. A commonly used potassium fertilizer is potassium nitrate, which is mostly chlorine free. Applying a coating delays the dissolving.

Potassium and nitrogen are often combined in organic fertilizer, and it is made from vinasse (a by-product of sugar beet processing), cocoa shells, oil cake flour (soy, coconut) and dried coffee husks. But there are also biological fertilizers, in which it is processed with calcium, magnesium and sulfate.

Analyzing potassium

Eurofins can analyze potassium in any substrate. In coconut-based organic substrates, we measure how much potassium reserve there is in the substrate (as well as calcium, magnesium and sodium) in a 1:1.5 volume extract with barium chloride, before crop cultivation. In the 1:1.5 volume extract with water we measure how much is released (from the basic fertilization at the potting soil supplier) and is therefore immediately available to the plant.The potassium content can also be analyzed in (coated) mineral fertilizers, both solid and liquid ones. It is expressed in weight percentage or in mmol. 

When potassium in a crop sample is analyzed, it is expressed in mg per kg, or in mmol/kg of dry matter if measured by the dry matter analysis method. It is expressed in mg per liter, or mmol per liter when analyzed by the plant sap method.