Potassium (K) is one of the three major nutrients needed by plants, the others being nitrogen (N) and phosphorus (P). In the soil environment, potassium exists in three different forms. Plants only absorb potassium ions (K+) in one form called exchangeable potassium, provided by moist clay particles or organic matter. Fixed potassium is "locked up" in parent materials like mica or feldspar minerals, accounting for over 90 percent of all potassium reserves in soil. Non-exchangeable potassium is an intermediate form between fixed and exchangeable forms.
Soil Parent Material
The principle sources of potassium in soils are the minerals called feldspar and mica, according to the authors of "Soils and Soil Fertility." If natural soils have been made over ions from these minerals, potassium levels will be high. Regardless if the garden soil is sandy, loamy or clay, the release of potassium from fixed to intermediate non-exchangeable forms is faster when the soil particles are smaller. Clay soil particles are much smaller than sand particles.
Weathering of feldspar and mica in the soil causes the breakdown of potassium from fixed to non-exchangeable to exchangeable (plant available) forms. Mica releases potassium more easily than feldspar. Although leaching of potassium is slow or minimal, regions where the climate is humid and wet tend to have lower amounts of plant available potassium. Leaching is faster where natural moisture is prevalent. In the eastern United States, farmers add potassium fertilizers to soil whereas in the western United States where rainfall and humidity are lower, potassium often remains in much higher levels in the soil in all forms. Only in heavily irrigated fields in the American West are potassium levels diminished.
Abundance of Organic Material
When discussing only plant-available potassium, the potassium ion is positively charged and is most easily absorbed into roots when the soil is not dry or pH not alkaline. Exceptions always exist; some wet, poorly draining alkaline clay soils can bind potassium, making it difficult for plants to utilize. Decaying plant material replenishes soils with a lot of potassium ions. Therefore, if a soil is amended with copious amounts of compost, for example, and plants grow and naturally die and decay, available potassium levels will be high. Conversely, if organic matter is lacking, or no plants grow in the soil, the amount of available potassium may be quite low even if the fixed potassium levels are high. If soil moisture is lacking, uptake of potassium by roots is diminished and the amount of potassium ions that remain in the soil's profile is higher.
Since leaching of potassium is slow in soil, regular applications of potassium-rich fertilizers over long periods can increase amounts of exchangeable potassium in the soil. Research in 1966, cited in "Soils and Soil Fertility," revealed that 77 years of crop fertilization in a loam soil in New York increased potassium available to plants in the top 2 feet of top soil. This was noted in crop fields where both animal manure (organic matter) and synthetic (man-made) fertilizers were applied.