Influence of Associated Ions
The second important factor influencing the plant uptake of a given cation is the effects of the ions held in association with it. For instance, assume that suitable amounts of exchangeable calcium are held by two soils and that the accompanying cation in one case is dominantly hydrogen and in a second case mostly sodium. Since the strength of adsorption is in general of the order H > Ca > Mg > K > Na, it is obvious that in the first case Ca ions will be present in the soil solution in more quantity because they are held less tenaciously than the associated H ions. In the second case, the concentration of Ca ions in the soil solution will be relatively low since they are more tightly held by the soil colloids than are the sodium ions.