Fig. 4. Adsorbed K⁺ (E_K) vs. solution K⁺ compared to the nonpreference line for Ca²⁺–K⁺ exchange. Data are for the <2-mm fraction (filled circle) of the nontreated soil and for the 2Bt3 clay fraction (open triangle) that was treated with H₂O₂ to remove organic matter and citrate-dithionite to remove free Fe oxides before dispersion and particle-size fractionation. Nonpreference line (dashed line) calculated following Sposito et al. (1983) (i.e., K_V = 1; see Eq. [2])