Relationship Between SAR and ExNa

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

The limits of Equations B and C at zero and infinity imply that a plot of Equation B in terms of SAR versus ExNa (with ExNa being the dependent variable) at a constant CEC and Ka will give a curvilinear function approaching the CEC 

---

A plot of SAR versus ExNa (see Eq- 4.40) will produce a curvilinear line, asymptotically approaching CEC (see section entitled Relationship Between SAR and ExNa). 

Figure 4C. Theoretical relationships between SAR and ExNa representing various hypothetical soils with different CEC and Kq values (from Evangelou and Phillips, 1987, with permission). ...

---