What is Cation Exchange Capacity?
Photo Credit: Jim Isleib, MSU Extension.
Soil Cation Exchange Capacity (CEC) refers to the negative charges of a particular soil to adsorb and exchange positively charged chemical ions such as Calcium (Ca2+), Magnesium (Mg2+), Sodium (Na+), Hydrogen (H+) and Potassium (K+). These negative charges are provided by clay and humus (most decomposed form of organic matter) particles, so as soil clay and organic matter contents increase, CEC will also increase. CEC ranges from 1.00 to 100 with sandy soils having the lowest values, whereas, clays will have the highest values. CEC measurement units are milliequivalents/100 grams of soil (Meq/100 g) or cmol (+)/kg (S.I. unit). Each Meq/100 grams of soil is equal to each cmol (+)/kg.
Since soil CEC measurements vary with varying soil pH (CEC is lowest at pH levels of 3.5 to 4.0 and increases with an increase in pH), commonly it is measured at a pH of 7.0. There are two methods to measure soil CEC: by using “Summation or Addition method” or by measuring it through “Sodium (Na+) Saturation and Ammonium (NH+4) Extraction method”. The summation or addition method adds the concentration of Ca2+, Mg2+, Na+, H+ and K+ from a soil extract and will generally result in artificially higher CEC values, especially when the levels of water soluble salts are in excess (reflected by a higher EC). The Na+ saturation and NH+4 extraction method is considered more accurate and CEC value measured by using this method is considered as the “True Soil CEC”. In this method, first soil negative charges are saturated with NH+4 ions and then NH+4 is replaced by Na+. CEC is then determined by calculating the amount of NH+4 that was replaced by Na+. CEC is a very important chemical property that reflects soil functions such as ability of a soil to retain and exchange essential plant nutrients and help calculate rates of soil amendments to remediate sodicity.
Soil base saturation or percent base saturation is the measurement of the proportion of soil CEC (negatively charges exchange sites) that is occupied (adsorbed) by the basic cations such as Ca2+, Mg2+, Na+ and K+. The unit for measuring base saturation is percent. An alkaline soil pH of 7.0 or more is indicative of a high base saturation, whereas, a highly acidic soil pH is indicative of a higher percentage of soil CEC occupied by acidic cations like H+ and Aluminum (Al3+).
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| Title | Source | Resource type and Date | Short Summary / Preview |
|---|---|---|---|
| Calculating Cation Exchange Capacity, Base Saturation, and Calcium Saturation | Ohio State University | Extension Publication
Aug 2019 |
The purpose of this fact sheet is to define soil cation exchange capacity, base saturation and calcium saturation, and demonstrate how these values are calculated in soil test reports. |
| What is your soil cation exchange capacity? | Michigan State University | Extension Webpage
Nov 2011 |
The basic aspect to remember is CEC indicates how well soil holds on to anything applied to it (emphasis on anything) and how difficult it is for plants to take it away. Soils with low CEC grab hold of very little. |
| Understanding base saturation ratios on your MSU soil test report | Michigan State University | Extension Webpage
May 2019 |
The “% of Exchangeable Bases” on your MSU soil test report gives insight into nutrient balances in your soil. |
| Cation Exchange Capacity Management | SARE | Book Excerpt
2010 |
The CEC in soils is due to well-humified (“very dead”) organic matter and clay minerals. The total CEC in a soil is the sum of the CEC due to organic matter and due to clays. |
| The Basic Cation Saturation Ratio System | SARE | Book Excerpt
2010 |
This article deals with a somewhat complicated topic and is intended to clarify the issues for those interested in soil chemistry and a more in-depth look at the BCSR (or base ratio) system. |
| Title | Source | Resource type and date | Short Summary |
|---|---|---|---|
| Cation Exchange Capacity and Base Saturation | University of Massachusetts Amherst | University Webpage
Not Dated |
Cation exchange capacity (CEC) is a measure of the soil’s ability to retain and supply nutrients, specifically the positively charged nutrients called cations. |
| Cation Exchange Capacity and Base Saturation | University of Georgia | Extension Publication
Mar 2017 |
Basics of CEC and base saturation ratios, how to calculate, and their significance. |
| Cation Exchange | New Mexico State University | Video
Sep 2016 |
This video explains how soils (often negatively charged) interact with nutrients (often positively charged) through cation exchange. The cation exchange capacity (CEC) of a soil determines how frequently and in what doses it should receive fertilizer. |
| Cation exchange capacity | New South Wales Government | Government webpage
Not Dated |
Cation exchange capacity (CEC) is a useful indicator of soil fertility because it shows the soil’s ability to supply three important plant nutrients: calcium, magnesium and potassium. |
| Cations and Cation Exchange Capacity | Soilquality.org – Australia | University Factsheet (Australia)
Not Dated |
Cation exchange capacity (CEC) is a measure of the soil’s ability to hold positively charged ions. It is a very important soil property influencing soil structure stability, nutrient availability, soil pH and the soil’s reaction to fertilizers and other ameliorants. |
| Cation Exchange Capacity and Base Saturation | University of Georgia | Extension Publication
Mar 2017 |
Basics of CEC and base saturation ratios, how to calculate, and their significance. |
| Cation Exchange Capacity (CEC) | Cornell University | Extension Publication
Not Dated |
Cations are positively charged ions such as calcium (Ca2+), magnesium (Mg2+), and potassium (K+), sodium (Na+) hydrogen (H+), aluminum (Al3+), iron (Fe2+), manganese (Mn2+), zinc (Zn2+) and copper (Cu2+). The capacity of the soil to hold on to these cations called the cation exchange capacity (CEC). |
| Title | Source | Resource type and Date | Short Summary |
|---|---|---|---|
| Cation Exchange Capacity, Exchangeable Cations, and Sodicity (Chapter 9 of Soil Analysis: An Interpretation Manual) | Csiro Publishing | Book chapter
2001 |
This chapter explores charges associated with soils, and provides definitions and measurement methods for both cation exchange capacity (CEC) and exchangeable cations. Chapter also provides in-depth analysis of sodicity, including causes, effects, and management of sodic conditions in Australian soils.
Note: This links to a preview of the book. You may need to contact your library to request a copy of this book as a free electronic version is not available. |
| Clay Dispersion (Chapter 14 of Soil Physical Measurement and Interpretation for Land Evaluation) | Csiro Publishing | Book chapter
2002 |
This chapter examines the topic of clay dispersion with emphasis on sodic conditions, and provides methods to measure and analyze dispersion-related soil characteristics.
Note: This links to a preview of the book. You may need to contact your library to request a copy of this book as a free electronic version is not available. |
| Exchangeable Ions, pH, and Cation Exchange Capacity (Chapter 6 of Standard Soil Methods for Long-term Ecological Research) | Oxford University Press | Book chapter
1999 |
This book chapter provides the definition of Exchangeable Ions, pH and Cation Exchange Capacity. It also lists materials needed and describes the procedure, calculations and special considerations in determining these soil properties. |
Technical review: March 31, 2020