Remember the lyotropic series for cation adsorption strength on clay surfaces:
\[ \text{Al}^{3+} > \text{Ca}^{2+} > \text{Mg}^{2+} > \text{K}^+ \approx \text{NH}_4^+ > \text{Na}^+ \]
Because trivalent aluminum has the highest charge among these common cations, it is adsorbed most strongly and dominates the exchange complex in acidic environments.