Not just any molecule. CEC is a measure of the "Positive" (cations) molecule, and how they are Adsorbed and not Absorbed by the "Negative" (anion) nutrient molecules. Adsorbed being that the molecules are attached on the outside of the negative molecules where thay can be easily detached, rather than being absorded, where s they are surrounded by negative molecules and rather hard to dislodged for plant uptake. To many negative results in a absorbsion of Cations, rather than adsorbsion and to many Cations, will result in absorbsion of Anions, whereas a correct balance of Cations and Anions will result in more readily availability of both types of nutrient molecules. Hydrogen being the major acid causeing Cation where the positive charge of the hydrogen molecule is used to dislodge the other positive charged molecules from the negative molecules. Further, as hydrogen is found in abundance in water solutions, this same hydrogen molecule is used to exchange negative nutrient molecules with the plants. Ph being one measure of how much hydrogen is in the soil.