The Soil Cation Exchange Capacity (CEC) and % Soil Base Saturation

Introduction to soil Cation Exchange Capacity (CEC).

Cation exchange capacity (CEC) gives an insight into the fertility and nutrient retention capacity of the soil. 

Certain soil minerals, such as clay, particularly in combination with organic matter, possess a number of electrically charged sites, which can attract and hold oppositely charged ions.  CEC is a powerful data point for a soil test. 

This single number tells us a lot about a soil’s ability to hold water and nutrients. It’s also a good indicator of what to expect in terms of a soil’s structure. 

The negatively charged sites make up the CEC, the ability to hold H+, Ca+2, Mg+2, Na+1, and NH4+1, etc., and the positively charged sites, which hold OH−1, SO4-2, NO3−1, PO4-2, etc., make up the anion exchange capacity. Ions held at these sites can be exchanged with others of similar charge. 

CEC is an important index of nutrient status because exchangeable cations are the most important source of immediately available plant nutrients. Over a course of time, CEC was found to increase, due to increases in the nutrient storage capacity of mine spoils.   

Soil cation exchange capacity (CEC), as reported by nearly all soil testing laboratories, is a calculated value and is an estimate of the soil’s ability to attract, retain, and exchange cation elements. It is reported in milliequivalents per 100 grams of soil (meq/100g). 

Few soil laboratories in Latin America and Europe continue to report the soil cation exchange capacity in cmolc/kg (centimoles of charge per kg soil), rather than meq/100 g. 

That means, cmolc/kg: “c” subscript before the slash in cmolc/kg denotes “charge” and the magnitude of the numbers is the same. While cubic decimeters (dm3) = a volume unit equivalent to a kg, as you recognized.

Therefore:

1. 1 mole = 10 centimole = 1000 millimole:  1 centimole = 10 millimole

2.  Thus, 10 meq/100 g = 10 cmolc/kg:  10 cmolc/dm3= meq/100cm3.

When nutrients are dissolved in soil solution, they become in a form called "ions". Ions of positive charges are called cations, and those of negative charges are called anions. Table salt is an example, composite is sodium chloride (NaCl), and when it dissolves in water it becomes two ions; one of a sodium cation (Na+1) and one of a chloride anion (Cl-1).

However, nutrients, like anions, can be attracted to any opposite positive charges present in the soil. Such "opposites attract" leads to what is known as “the soil chemistry” a combination of attract and repel. Like in our life, negative charges find positives, and those positive charges find negatives. 

Cations are positively charged ions in the soil solution (Ca+2, Mg+2, K+1, NH4+1, Na+1, and H+1, etc.). CEC is defined as the total amount of cations, in milliequivalents (meq), held to soil components through an electrostatic attraction, which can be exchanged with cations in soil solution. A specific soil’s CEC is dependent upon three main factors: 1. The amount of clay (soil texture), 2. The type of clay, and 3. The amount of soil organic matter (SOM).

For this reason, the CEC of a given soil can vary from 0 to 50 meq/100g soil. Soils with a low CEC typically have a high sand fraction and low SOM content, whereas soils with a high CEC have a relatively high clay fraction and/or SOM content.

More commonly, the soil testing labs estimate CEC by summing the calcium, magnesium, and potassium measured in the soil testing procedure with an estimate of exchangeable hydrogen obtained from the buffer pH. Generally, CEC values arrived at by this summation method will be slightly lower than those obtained by direct measures. The table below shows the levels of CEC, ranging from very low to very high.