Empirical SAR-ESP Relationship

The equation most commonly used to describe heterovalent cation exchange, such as Na+-Ca2+ exchange, is the Gapon exchange equation. For example, 

An experimental plot of SAR versus ESR, for only a fraction of the CEC of the soil or soils involved in the study, could produce a straight line with slope Kc (Fig. 11 A). Substituting the regression equations from the experimental SAR versus ESR plot into Equation E produces Equation 11.4, which for convenience is given below  

The y intercept (-0.0126) of the SAR-ESR relationship represents an empirical system constant and the slope [0.01474 (mmol L-,)-1/2] represents the average KG of a number of western U. S. soils (U. S. Salinity Laboratory Staff, 1954). Note that this empirical equation (Eq. G) does not necessarily have universal application. It appears that the K0 varies depending on soil type and the weather conditions under which a particular soil was formed. For additional information, see Chapter 4. 

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