Potassium in soil is generally classified into four categories:
The basis for the above classification is the availability of potassium to plants. Depending on the type of soil and environmental conditions, potassium availability may vary.
Unavailable Potassium – found in crystalline structure of feldspars, clay minerals and micas which are part of the soil. Plants cannot use the potassium in these insoluble forms. However, with time, these minerals eventually break down, and small quantities of potassium are released to the soil solution.
Fixed potassium – potassium that becomes slowly available to plants over the growing season. Clay minerals have the ability to fix potassium. During wetting and drying of the soil, potassium becomes trapped in-between the mineral layers (clay minerals have a layer structure). Once the soil gets wet, some of the trapped potassium ions are released to the soil solution. The slowly available potassium is not usually measured in regular soil testing.
Exchangeable potassium – is readily available potassium, which plants can easily absorb. This fraction of Potassium is held on the surface of clay particles and organic matter in soil. It is found in equilibrium with the soil solution and is easily released when plants absorb potassium from the soil solution. Exchangeable potassium is measured in most soil testing.
Soil solution potassium – potassium dissolved in the soil solution and is readily available to plants. It is the smallest pool of potassium. Therefore, testing only potassium in the soil solution, does not represent the total amount of potassium available to plants.
Potassium fractions in soil
Several factors can affect the ability of plant to absorb potassium from soil: