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Understanding the Importance of Cation Exchange Capacity


I was recently asked to provide a simplified explanation on the importance of Cation Exchange Capacity (CEC) values.


My Response: 
  • CEC is the ability of a soil to hold onto plant nutrients.  
  • The finer the particle size the higher CEC value, generally speaking. For example sand particles are course and visible to the naked eye, where as clay particles are fine and are not visible to the naked eye. So clay will have a higher CEC value than sand. It is obvious to most of us that sand cannot hold onto water or nutrients as well as a soil with a finer texture. Therefore, soils rich in Clay and Loam size particles are universally recognized as being better for farming - CEC explains this. 
  • The numerical value for CEC represents how much nutrition can be held by a given amount of soil. For example one pound of a clay loam soil with a CEC value of 20 will hold 4 times as many nutrients as a sandy soil with a CEC value of 5. 20/4 = 5. 
  • It's all about math, for example Nitrogen in a cation form (not all nitrogen fertilizers are in a cation form) can be stored in a soil. Here is a simplified calculation - A CEC value of 20 can hold 200 pounds of cation nitrogen, 200/20 = 10. Where a sand with a CEC value of 5 can only hold 50 pounds of cation nitrogen,  50/5 = 10.  
  • Well decomposed organic matter may have a higher CEC value than sand or clay, but it is insignificant when organic matter represents a tiny percentage of the soil.  See the attached chart on the potential CEC values of various substances!  For example, if your soil is only 1% organic matter it does not contribute significantly to CEC values, when compared to the massive volume of the soil itself.  This is often called the buffering capacity of the soil and is also related to any attempt to change the pH of the soil!   On the chart you will notice that Humus and the organic matter of compost are not the same thing, as Humus is referring to the recalcitrant (persistent and resistant to decay) soil carbon substances that are powerful biologic chemicals containing the active ingredient of Humic Acids.  These substances have a huge CEC value and are very significant when it comes to plant nutrient availability.   If the Humic Acid is Supramolecular, the molecules are capable of amazing characteristics, including how they perform as a CEC factor.    The best way to increase CEC is to increase Humic Acid levels in your soil. This is good because Humic Acids are part of the recalcitrant soil carbon pool, which is resistant to decay; meaning Humic Acids are capable of lasting thousands of years in an undisturbed soil. Where as organic matter is labile carbon, which is subject to rapid changes; because labile carbons are unstable they decay easily and disappear from the soil quickly. 
  • CEC may be the limiting factor for having a fertile soil.  In order to improve the fertility of your soil when CEC is limiting you, add Humic Acids as the most effective tool.  However, don't be fooled by "New and Improved" formulations of Humic Acids that claim you only need a few gallons or pounds per acre, as that's not how the math works and CEC and the ability to hold onto the hundreds of pounds of nutrients needed by a crop is all about math.  


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