Principle factors governing K fixation in clay lattice are

Principle factors governing K fixation in clay lattice are Both Ionic radii as well as Hydration energy. Potassium ions and other cations are readily fixed because they have relatively low energies of hydration.
Fixation occurs when K is held in highly charged sites in the interlayer region of vermiculite and mica minerals. This differs from exchangeable K which is held at lower charged sites, on the surface and edges of clay minerals.