High Quality Content by WIKIPEDIA articles! In soil science, cation-exchange capacity (CEC) is the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, for exchanging with the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect groundwater from cation contamination. It is expressed as milliequivalent of hydrogen per 100 g (meq+/100g), or centi mol per kg (cmol+/kg). The numeric expression is coincident in both units.