Q62: Which ion is the dominant ion adsorbed on clay surfaces... - Soil Physics, Soil Chemistry, Evapotranspiration & Conservation - ICAR AIEEA PG 2024 - Water Science and Technology | Agrigyan

Exchangeable cations in acidic soils:
When soil pH falls below 5.0 to 5.5, octahedral aluminum within clay crystal sheets undergoes dissolution.
Trivalent aluminum cations (\(\text{Al}^{3+}\)) and its monomeric hydroxy-aluminum species [\(\text{Al(OH)}^{2+}, \text{Al(OH)}_2^+\)] strongly adsorb onto the negative exchange sites of clay surfaces due to their high charge density and valency (replacing \(\text{Ca}^{2+}, \text{Mg}^{2+}\), and \(\text{K}^+\)).
While \(\text{H}^+\) ions are present in solution, they readily attack the clay crystal lattice, releasing more \(\text{Al}^{3+}\). Thus, \(\text{Al}^{3+}\) is overwhelmingly the dominant exchangeable cation on clay surfaces in strongly acid soils.