The ranking system is based on the cation charge and strength of attraction of the the cation to the exchange sites. Al has the highest charge and highest attraction while single charged ions (K, NH4, Na) have the lowest charge and attraction. This is somewhat modified when the hydrated ionic diameter is taken into consideration. H is usually placed after Al because it is essentially a very small point charge-like ion that is strongly attracted to exchange sites.
Dr. Larry Cihacek has offered a good way to define the lyotropic series. It is very correctly is based on the strength of adsorption. Although the hydrated ionic size and charge on an ion indirectly determine the place an ion in such series. That why scientists have such series for monovalents, divalents and trivalent ions separately. recently some work is in progress to assess the effect of CEC of exchange materials on these lyotropic series, i.e. the place of an ion in the series, but still is inconclusive.
The criteria are : charge of cation and radius of cation buf the ratio between them (Charge/radius)affects on bindig energy of cations with the soil particles.Increase in the mentioned ratio causes increase in binding energy between cations and -ve charge of soil particles.
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