Potassium and Ammonium Fixation

Both K & NH4 fixation result in removal of available (ionic) forms of these 2 nutrients from the soil solution. K+ and NH4+ ions have exactly the same ionic radius and their radius also happens to match the interlayer spacing of some phyllosilicates (ie fine grained mica). So in soils with abundance of that type of phyllosilicate numerous K+ and NH4+ ions are trapped (or fixed) in unavailable form. The only way how these ions can become available is through weathering of that phyllosilicate mineral and this takes a long time