Acidity, exchange The total amount of the Cation Exchange Capacity (CEC) of a soil that is due to H+ and Al3+ ions. It is a proportion of the total acidity and it is dependent on the type of soil and the percentage of the CEC that is composed of exchangeable bases (Ca2+, Mg2+, K+). Originally from the CUAHSI HIS VariableNameCV. See: http://his.cuahsi.org/mastercvreg/edit_cv11.aspx?tbl=VariableNameCV. ODM2 Variable Name Controlled Vocabulary ODM2 Working Group A vocabulary for describing the name of Variables.