This is similarly the case with the divalent cations of calcium and magnesium. Because the hydrated magnesium ion is larger than that of calcium, the magnesium ion is held more weakly and behaves in some instances in soil (i.e. when calcium is low) like sodium.
The charge of the cation and the size of the hydrated cation essentially govern the preferences of cation exchange equilibria. In summary, highly charged cations tend to be held more tightly than cations with less charge and secondly, cations with a small hydrated radius are bound more tightly and are less likely to be removed from the exchange complex. The combined influence of these two criteria can be summarized generally by the lysotrpoic series.
Aluminium > Calcium > Magnesium > Potassium, Ammonium-NH4+ > Sodium > Hydrogen
It indicates, from left to right, the decreasing strength of adsorption of the various cations. As such, the less tightly held cations are located furthest from the surface of colloids and are most likely to be leached away or further down the profile most quickly. Conversely, the most strongly adsorbed cations will tend to move the slowest down through the profile.
The proportion and kinds of cations adsorbed on soil mineral particles and organic colloids is also a function of the concentration of cations in the soil solution. If the concentration of a cation in soil solution is high, there is an increased chance or tendency for that cation to be adsorbed.
This is the reason that dissolved gypsum (CaSO4) is added to ameliorate sodic soil. In this case, the addition of dissolved gypsum increases the concentration of calcium in the soil solution and this leads to an increase in calcium ions on the exchange complex at the expense of exchangeable sodium.
The major source of cations in soil solution are from mineral weathering (i.e. primary minerals), mineralization of organic matter and addition of soil ameliorants (i.e. lime, gypsum, etc).