Cation Exchange Capacity

Most soil characterization reports leaving AGVISE Laboratories have a value on them called Cation Exchange Capacity (CEC). Unless you have a background in soil science or chemistry, this number is often a mystery to many individuals. I have spent many hours trying to explain to individuals what this CEC value means. I am sure there are many more individuals who may be too embarrassed to ask.

cecpic┬áLet’s start with the basics. Many elements in the soil (the fertilizer industry will call them nutrients) are positively charged. Some of these positively charged elements are potassium, calcium, magnesium, zinc, iron, manganese, copper and ammonium. These elements are called cations. Soil particles themselves generally have a negative charge inside and on their edges. Because the soil particles have a negative charge and the cations have a positive charge they are attached together just like a magnet would attract a metal. Since cations are held to the soil by their positive charge, they cannot be leached downward in the soil when water moves through the soil profile. The measurement of the soil’s ability to hold cations is called the Cation Exchange Capacity (CEC) of the soil.

The units used by soil testing laboratories to report the amount of cations a soil can hold (CEC) is milliequivalents per 100 grams of oven-dry soil (meq/100g) or centimoles of positive charge per kilogram of oven-dry soil [cmol(+)/kg]. These units are equivalent, so a soil with a CEC of 20 meq/100g is equal to a soil with a CEC of 20 cmol(+)/kg. Soils with a high CEC tend to be more fertile and are capable of providing more nutrients to crops. A high CEC value(>25) is a good indicator that a soil has a high clay and/or organic matter content and can hold a lot of cations. A soil with a low CEC value (<5) is a good indication that a soil is sandy with little or no organic matter that cannot hold many cations.

pict6 The CEC of the soil will influence the ability of the soil to hold and interact with pesticides too. Pesticides that have a positive charge will be held more tightly to the soil when the CEC of the soil is high. Soils with a low CEC are often of sandy nature and have a poorer ability to hold cations than high CEC soils. This can lead to potential leaching and movement of some pesticides on soils with a low CEC.

There are many laboratory methods used to measure the CEC of the soil. The most common method used in commercial soil testing laboratories is to remove the common cations from the soil with ammonium and measure the cations removed. The cations removed from the soil are then determined in the laboratory by an Atomic Absorption Spectrophotometer (see picture). The total of cations is then reported as CEC. This method works good on most soils, however it does not work on saline soils or some soils with pH greater than 7.3. The ammonia solution used to remove the cations from the soil in this procedure, also dissolves the excessive salts such as calcium sulfate found in saline soils. These dissolved salts which are not held on the soil, cause the CEC level to be inflated and inaccurate. To obtain an accurate CEC on saline soils, the excessive salts have to be removed with an alcohol wash prior to determination of the CEC. When working with saline soils it is advisable to discuss the type of soil samples you are submitting with the staff at the laboratory so the proper CEC method of analysis is used.