Sorbed cations and anions Sorbed cations and anionsA stronger type of interaction of cations and anions with soil minerals occurs through the sorption process, often termed inner-sphere complexation . In this case, the cation or anion forms a bond with the mineral surface in which the ion actually enters the coordination shell of the mineral structure. This bond is much stronger than electrostatic bonds associated with exchangeable ions. Reaction kinetics are on the order of minutes to hours to reach sorption equilibrium and the reaction is often not completely reversible. The dominant sites for sorption reactions are hydroxylated surfaces associated with broken mineral edges, noncrystalline materials (e.g., allophane, imogolite, ferrihydrite), and oxides/hydroxides.
The pool of sorbed ions is dominated by phosphate and several transition metal nutrients (e.g., Cu, Zn, Mn), and to a lesser extent sulphate. Anion sorption capacity increases with decreasing pH values. While sorbed ions are generally near sorption equilibrium with the soil solution, the strong affinity (high bonding energy) of the ion for the solid-phase often results in sorbed ions being sparingly to non-available to plants. The non-availability of strongly-sorbed phosphate often leads to P deficiencies in tropical soils dominated by iron and aluminum oxides/hydroxides and in volcanic soils dominated by poorly crystalline minerals (e.g., allophane and imogolite). The hydroxylated surfaces of these later minerals form strong bonds with phosphate through ligand exchange type reactions.
