The decomposition of litter (leaves, bark, seeds etc.) added from tree species in the forest ecosystem can release K present in the litter. Generally, potassium is not bound in the organic combinations, so it can release during early stage of decomposition. The decomposition products (such as H+) of litter or tree roots can release K present in the structure of minerals, K present in exchangeable or non-exchangeable forms in the rocks/soil.   

Generally the majority will be derived from the parent material (underlying rocks or transported materials) and cycled through the soil plant system as Baljit has described. There may be inputs from dust/rainfall/aerosols, but these will not usually cause localized high concentrations. 

Bio-geochemical weathering of litter deposited via annual cycles of litter fall in forests aids a lot  in maintaining  the high levels of K in both soil solution as well as exchangeable soil. Find enclosed a good study in this regard...

Patterns in potassium dynamics in forest ecosystems.

Tripler CE1, Kaushal SS, Likens GE, Walter MT.

Source : Ecol Lett. 2006 Apr;9(4):451-66.

 Abstract: The biotic cycling of potassium (K) in forest systems has been relatively understudied in comparison with nitrogen (N) and phosphorus (P) despite its critical roles in maintaining the nutrition of primary production in forests. We investigated the ecological significance of K in forests from a literature review and data synthesis. We focused on (1) describing patterns of the effects of K availability on aboveground growth and change in foliar tissue of tree species from a variety of forests; and (2) documenting previously unreported relationships between hydrologic losses of K and N in forested watersheds from the Americas. In a review of studies examining tree growth under K manipulations/fertilizations, a high percentage (69% of studies) showed a positive response to increases in K availability in forest soils. In addition, 76% of the tree studies reviewed showed a positive and significant increase in K concentrations in plant tissue after soil K manipulation/fertilization. A meta-analysis on a subset of the reviewed studies was found to provide further evidence that potassium effects tree growth and increased tissue [K] with an effect size of 0.709 for growth and an overall effect size of 0.56. In our review of watershed studies, we observed that concentrations of K typically decreased during growing seasons in streams draining forested areas in the Temperate Zones and were responsive to vegetation disturbance in both temperate and tropical regions. We found a strong relationship (r2 = 0.42-0.99) between concentrations of K and N (another critical plant nutrient) in stream water, suggesting that similar mechanisms of biotic retention may control the flow of these nutrients. Furthermore, K dynamics appear to be unique among the base cations, e.g. calcium, magnesium, and sodium, because the others do not show similar seasonal patterns to K. We suggest that K may be important to the productivity and sustenance of many forests, and its dynamics and ecological significance warrant further study. We suggest that knowledge about the dynamics of this understudied element is imperative for our understanding patterns and processes in forest ecosystems.

Normally, the decomposition of leaves etc.. will cycle nutrients absorbed by the plants back to the soil.  Potassium released by the process of decomposition is in the "available form". Potassium that is dissolved in soil water (water soluble) plus that held on the exchange sites on clay particles (exchangeable K) is considered readily available for plant growth. The exchange sites are found on the surface of clay particles. This is the form of K measured by the routine soil testing procedure.  

This said, the potassium could have been carried of leached to the point under study and deposited there, which results in a high concentration of potassium.

The high level of potassium in forest soils can be due to decomposition of leaves which increased the soil organic matter content and supplies more cations such as K, Ca Mg to improve soil cation  exchange capacity .besides, the organic matter under forest does not experience breakdown easily bacause of forest layer unlike soils that are exposed to sunlight

Inputs of potassium through leaf litter and organic matter accumulation dont explain why a system would have high K (sure in a mature forest much of the nutrient budget is in the biomass/litter) - it must have come from somewhere before it got into the leaves and the organic matter - hence my comment about the underlying parent material. Increasing organic matter and increasing CEC does not by itself increase the overall budget of K in the system if the K is low to begin with. It may prevent it from being leached and hold it in a readily available form, 

Very interesting feedback Andrew...appreciate your response..

Nutrient limitation in plant communities is often operationally defined as the requirement of a single nutrient necessary to stimulate an increase in production of biomass , usual phenomenon in forest ecosystem.  Recent work has shown, however, that there can be substantial variation in the nutrient demands and relative proportions of essential nutrients at the species level . Results further  suggested that multiple elements, including K, may also co-limit tree growth in forests systems. Therefore,  plant species may actually vary in their resource demands for N, P, and K based on their stoichiometric requirements, and the relativeand absolute abundance of N, P, and K in soils may influence plant community composition in addition to growth. Conceptual extension of nutrient limitation to colimitation has some precedent in the terrestrial ecological

literature , and has been widely accepted in the agricultural literature. Comparative work has shown that fertilization with N alone and combinations of N, P, K and other base cations  can have differential effects on above ground and belowground processes within forest ecosystems . Quantification of the fluxes and internal cycling of multiple nutrients together may lead to a more complete view of the influence of limiting nutrients on patterns and processes in forest ecosystems.

Furthermore, K dynamics appear to be unique among the base cations, e.g. calcium, magnesium, and sodium, because the others do not show similar seasonal patterns to K.  K may be important to the productivity and sustenance of many forests, and its dynamics and ecological significance(The ecological importance of K in forest ecosystems)  warrant further study. Source ; Ecology Letters, (2006) 9: 451–466 doi: 10.1111/j.1461-0248.2006.00891.x

PDF enclosed for further reference , its an interesting study...

Forest trees make less demand on the soil for K than annual crops because a large proportion of absorbed nutrients are returned annually to the soil in leaf and fine root litter and are reabsorbed after biological breakdown of litter materials. Also, a large portion of nutrient K requirement of trees are met through internal cycling as compared with agricultural crops.

Dear Abubkari,

When you say " soil K" which soil K type you are referring to?. The reason I raised this question is that in soil there are Five types of K which are: 1. Water soluble K, 2. Exchangeable K, 3. Slowly available K, 4. Unavailable K and 5. Total K. If you agree with this and if you question is related to plant available K,  you are referring to  the water soluble and exchangeable K. The water soluble K is very small and it is the amount of exchangeable K that determines the amount of K available to plants in the soil. As you said plant available or exchangeable K is high in the forest soils than soils of cultivated and agroforestry lands. I agree with most of the answers as to why high soil K in forest systems by scholars above. But one most important reason for high K content in forest soils is due to the fact that forest is a closed system where there is no loss of K by leaching, through crop harvest etc. Moreover, roots of trees in forest go deep soil profile and bring K to their system where it is released when their liters decompose which is not a character in cropping systems.  So these are additional reasons for high K content in soils of forests. 

Agree with Dr AK Srivastava Sir,