Cation Exchange Capacity (CEC) and Anion Exchange Capacity (AEC) of soil

CEC AND LESS WEATHERED SOILS OF HAWAII

Less weathered soils, that contain minerals such as montmorillonite, are said to have a ‘cation exchange capacity,’ or CEC, under acidic, neutral, and alkaline conditions. CEC is the soil’s ability to attract, retain, and supply nutrients, such as calcium, potassium, ammonium, and magnesium. These nutrients are positively charged atoms, known as cations. The surfaces of less-weathered clay minerals (such as montmorrillonite) generally have a negative charged. Much like a magnet, negatively charged soil surfaces attract positively charged cations. However, under acidic conditions, the soil will also have a tendency to attract aluminum and hydrogen cations. The presence of aluminum and hydrogen contributes to soil acidity. In contrast, under alkaline conditions, the soil attracts sodium which contributes to soil alkalinity.

Less-weathered soils that are founding in Maui County include Vertisols, Mollisols, Aridisols, and Inceptisols.

AEC AND HIGHLY WEATHERED SOILS OF HAWAII

Although the majority of the world’s soils have CEC, the highly weathered soils of the tropics are an exception. In addition to the having CEC, many tropical soils also have an ‘anion exchange capacity,’ or AEC, depending upon the pH of the soil. Under neutral and alkaline conditions, the soil has CEC, like the less weathered soils. However, under acidic conditions, these soils generate AEC. This means that the soil becomes positively charged and attracts, retains, and supplies negatively charged anions, such as sulfate, phosphate, nitrate, and chloride. For soils with AEC, proper management of pH is crucial in order to provide sufficient amounts of the nutrient cations (calcium, magnesium, ammonium, and potassium).

Minerals that exhibit AEC are highly weathered kaolinite, aluminum and iron oxides, organic matter, and the allophanes and imogolites of volcanic soils. Highly weathered Ultisols and Oxisols, volcanic Andisols, and organic Histosols all have AEC under acidic conditions. The pH at which these soils develop AEC differs depending upon the minerals within the soil. Since organic matter only generates AEC at a very low pH, it is still a good source of CEC.

IMPORTANCE OF CEC AND AEC

CEC and AEC values are important measurements that provide us with important information regarding the soil’s ability to retain and supply certain nutrients to the plant. In addition to nutrient retention, CEC and AEC helps us predict the leaching potential of certain nutrients in areas with high rainfall. When the soil has a very high CEC, negatively charged nutrients such as nitrate are not be retained by the soil. Instead, nitrate leaches through the soil profile in areas with high amounts of precipitation. Likewise, soils with high AEC experience leaching of positively charged nutrients, such as calcium and potassium.

CEC is often expressed in centimoles of charge per kg of soil. By sending a sample of your soil to a soil testing laboratory, you can determine your soil’s CEC. The value of knowing a soil’s CEC cannot be underestimated in nutrient management. Without it, your soil would not be able to provide your plants with sufficient amounts of nutrients. However, one must be cautious when interpreting the laboratory results for CEC depending on the soil type being analyzed. You can obtain accurate results for most laboratory methods that measure CEC in less weathered soils with permanently negative charge. On the other hand, these laboratory methods can overestimate the CEC of highly weathered soils that have an AEC. This is because the pH of highly weathered soils affects the CEC of the soil. As the pH of highly weathered soils with AEC increases, the CEC of the soil also increases. Thus when the method used to determine the CEC uses solutions that raise the pH of the soil, the reported CEC is higher than its actual value in the field.