What Is Cation Exchange Capacity?
Because of the negative charges in their chemical structure, most clay minerals and soil organic matter have the ability to attract or retain positively charged ions (cations) of calcium (Ca++), magnesium (Mg++), potassium (K+), aluminum (Al+++), hydrogen (H+), and others. Attraction between the clay minerals and these ions is weak enough that an exchange between ions can occur; those ions most strongly attracted or occurring at higher concentrations in the soil solution may displace other ions from exchange “sites” on the clays. The capacity of a soil to retain cations under specific conditions is called the “cation exchange capacity” (CEC). This property affects the availability of potassium, calcium, and magnesium to plants. The term used to report CEC is milliequivalents/100 grams (me/100g) of soil. A recent addition to the Kentucky soil test results reports a “calculated CEC” that uses results from the current extractant in the Kentucky Soil Test Lab (Mehlich III). This extractant
