Potassium fractions in the soil
The concentration of potassium in soils ranges from around 0.3 to 3%. This amount is almost exclusively present in an inorganic form. The dynamic exchange processes of potassium in soils are represented schematically in the figure below.
Potassium is present in the different K pools as follows:
- Available in the soil solution as K+-ions.
- The potassium ions in the exchangeable fraction are adsorbed to the negatively charged surfaces of cation exchange particles (clay minerals and humus particles). These bonds are weak (simple ionic bonding) and this means that the potassium can be easily released in the soil solution if needed.
- Potassium ions in the non-exchangeable fraction are fixed into holes present in the sheets of clay minerals. These sheets, through contraction and expansion and through weathering processes may release some ‘fixed’ K over a long period of time
- Potassium located in the lattice is a direct building block of soil silicates. Because this is a very strong bond this potassium is not available for plants. As the silicates weather the potassium ions are released from the lattice to another fraction. This is an ongoing but very slow process.
Soil fertility status
The fraction of potassium that is easily available from the soil solution is the important fraction for plant nutrition. Soil analysis for its plant available nutrients can be used to calculate the fertiliser requirements.
Most countries have a scale of K index with a recommended value for the different types of crop depending on their K requirement. These indices are also greatly affected by the soil.
Potassium in plants
Potassium is taken up from the soil solution by plants only as potassium ions. It is very mobile in the plant and is vital for the plant as it affects in many ways the various metabolic processes.
Functions of potassium in plants:
- Affects photosynthesis directly through an effect on the chloroplasts and indirectly through an effect on the closing mechanism of stomata.
- Involved in the metabolism of plants through its involvement in activating more than 50 enzymes.
- Improves the efficiency of water utilisation and thereby reduces drought stress.
- Improves synthesis of carbohydrates such as sugar and starch.
- Supports transport and storage of carbohydrates from the leaves to the storage organs (tubers, grains, beets etc.).
- Improves the qualitative value of products through higher protein and vitamin content.
- Positively increases the content of organic anions and improves, mainly in connection with sulphate ions, the taste of fruit and vegetables.
- Enhances the development of supporting tissue. This reduces lodging (i.e. in cereals) and supports a vigorous bushy growth in broad-leaved crops for maximum interception of sunlight
- Increases the natural resistance of plants against disease, insects and frost.
- Results in a decrease of black spot in potatoes.
- Initially visible on older leaves as K is easily transported via the phloem.
- Leaves become flaccid and droop.
- The entire plant looks limp and wilted
- Starting from the margin leaves become increasingly bright green.
- Later, necrotic patches develop on the margins and leaf tips
- Delayed growth occurs.
- Leaves stay small and fruits remain tightly attached to the plant.
- The decreased lignification of cell walls increases the risk of lodging in cereals and the susceptibility to fungal infections.
Potassium-deficiency symptoms of rape, maize and vine