The Importance of Potassium
Figure 1 (right). Potassium cations are present in the soil in three forms – dissolved in the soil solution, adsorbed onto the surfaces of soil particles, or fixed within the clay structure. Picture courtesy of Better Crops (2011, No. 4)
Potassium is required in relatively large quantities by the plant. It regulates the uptake of water by the roots, turgor pressure within the cells, and transpiration by the leaves. It is involved in the production and transport of carbohydrates within the plant. As a result, it can improve the color, flavor, size and shelf life of fruits, nuts, grains and vegetables. Potassium builds strong cell walls which can increase the plant’s tolerance to insect attack, disease incidence, drought and frost.
Up to 80% of the movement of potassium in the soil is by diffusion in the soil solution. Diffusion is the movement of a nutrient from a high concentration to a low concentration. Potassium is present in the soil as a monovalent cation K+. This means that it has a single positive charge. Since it is a cation, available potassium can be adsorbed or attached to the surfaces of clay particles, organic matter and humic acids in the cation exchange capacity (CEC). Potassium can also be held inside of the clay particles. This form of K cannot move by diffusion in the soil and is not available for absorption by the roots.
Potassium fertilizers are applied to maintain an optimal level of K in the soil and to supply the ongoing requirement by the crop. The table below lists some of the common potassium fertilizers available in the market.
Commodity Potassium Fertilizers
Typical Nutrient Analysis
Potassium chloride (MOP)
Potassium sulfate (SOP)
0-0-22 + 11% Mg + 21% S
Mono potassium phosphate (MKP)
Potassium chloride (MOP)
Potassium thiosulfate (KTS)
0-0-25 + 17% S
When potassium fertilizers dissolve in the soil, the potassium ions directly enter into the soil solution. Most of these cations will then attach to the surfaces of the soil particles. Some will be leached deeper into the soil profile. The few potassium ions that remain in the soil solution surrounding the roots are immediately available for absorption by the plant.
Nucleus 0-0-21 and Nucleus 0-0-15 provide superior performance because they contain unique fertilizer additives. These additives maintain K in an available form, release tied up K in the soil, and enhance the movement of K into the root.
Figure 2 (right). Potassium ions diffuse from a high concentration on the clay particle into soil solution which has a low concentration of K. The roots then absorb the K from the soil solution and pump it upward into the plant.
Potassium fertilizer applications should be based on recommendations from soil test results. Basic fertilizer applications may be required to bring the soil up to the optimal level for the crop and soil type.
Nucleus 0-0-21 or Nucleus 0-0-15 can be applied throughout the season at critical growth stages. Crop stages, such as nut fill, sugar accumulation or fruit sizing, can set up a demand for K that is greater than the soil can supply. Applications of Nucleus potassium fertilizers can help to fill this demand at these critical stages to improve crop yield and quality.
Figure 3 (left). Nucleus 0-0-21 and Nucleus 0-0-15 allows for more potassium to move to the roots by diffusion. The organic acids in the Nucleus products prevent K tie up and release additional K exchanged on the clay particles. The roots can then absorb a much greater amount of available K to supply vital needs within the plant.
By Kevin Dickinson, CPAg - Technical Service Manager