Potassium in natural waters comes mainly from the weathering of potassium-containing rocks such as mica and feldspar. So the natural concentration will vary somewhat with the local geology, but both of these minerals are very insoluble anyway, so almost all natural systems are low in K.
Rainforest soil is notoriusly poor in nutrients (hence the need for rotating, slash-and-burn agriculture) because, in a tropical system there is no need for storage of nutrients during the winter. Nutrients are constantly cycled through the biosphere, and dead material is recycled back into the system very quickly. It is quite likely that K could be a limiting nutrient in these systems. Interestingly, in marine systems, K is never considered to be a limiting nutrient, usually N, sometimes P or Fe are limiting.