Unit 12 Chapter 2 -Cation Exchange & Cation Exchange Capacity

Soil Cation Exchange - The interchange between a cation in solution and another cation on the surface of any negatively charged material such as clay or organic matter. What determines if a cation is on the exchange complex or in the soil solution?
Cation exchange is influenced by:
1) strength of adsorption--->Strong adsorption » Al+3 > Ca2+ > Mg2+ > K+=NH₄+ > Na+ >H+ »Weak adsorption
2) the relative concentration of the cations in the soil solution.
At any one time the quantity of ions on the exchange compared to what is in the soil solution is determined by the kind of ions present and the quantity of ions present in the soil. Take a look at the diagram for cation exchange CE Diagram

Cation Exchange Capacity (CEC) is the ability of the soil to hold onto nutrients and prevent them from leaching beyond the roots. The more cation exchange capacity a soil has, the more likely the soil will have a higher fertility level. When combined with other measures of soil fertility, CEC is a good indicator of soil quality and productivity.

The cation exchange capacity of a soil is simply a measure of the quantity of sites on soil surfaces that can retain positively charged ions by electrostatic forces. Cations retained electrostatically are easily exchangeable with other cations in the soil solution and are thus readily available for plant uptake. Thus, CEC is important for maintaining adequate quantities of plant available calcium (Ca++), magnesium (Mg++) and potassium (K+) in soils. Other cations include Al+++( when pH < 5.5) , Na+, and H+.

Cation Exchange Capacity can be expressed two ways:
1) the number of cation adsorption sites per unit weight of soil or,
2) the sum total of exchangeable cations that a soil can adsorb.
Soil CEC is normally expressed in units of charge per weight of soil. Two different, but numerically equivalent sets of units are used: meq/100 g (milliequivalents of element per 100 g of dry soil) or cmolc/kg (centimoles of charge per kilogram of dry soil).

The unit of milliequivalents (meq) per 100 g of oven dry soil is used to better reflect it is the charge in the soil that determines how many cations can be attracted. The equivalent weight of an element is the molecular or atomic wt (g) ÷ valence; or charges per formula milliequivalent (MEQ). One meq wt. of CEC has 6.02 x 10 ²⁰ adsorption sites. Cation exchange sites are found primarily on clay and organic matter (OM) surfaces.

Normal CEC ranges in soils would be from < 1 meq/100 g, for sandy soils low in OM, to >25 meq/100 g for soils high in certain types of clay or OM. Soil OM will develop a greater CEC at near-neutral pH than under acidic conditions. Additions of an organic material will likely increase a soil's CEC. Soil CEC may also decrease with time through acidification and OM decomposition.

History of CEC Determination

Researchers in the 1920's and 30's did not recognize either the existence of exchangeable Al³⁺ or the increase in CEC with increasing pH (pH dependent charge). It was believed that the basic cations (Ca²⁺, Mg²⁺, and K+) could be easily extracted and measured but that a significant portion of the CEC was occupied by hydrogen (H+) and this portion was difficult to fully extract. For this reason, initial CEC tests used extractants buffered at high pH. For example pH 7 ammonium acetate (Schollenberger, 1927) and pH 8.2 barium chloride-triethanolamine (Mehlich, 1938) were used. "Exchangeable acidity" measured in this fashion was really a measure of pH buffering capacity.

Now it is accepted that to measure the actual CEC of a soil, pH must not be changed during the procedure. In interpreting results, it is important to know what type of CEC measurement was performed. Older, buffered methods will give higher results, especially for acid soils.

Soil testing laboratories do not usually provide a direct measure of CEC but, rather, an estimate based on the quantities of Ca2+, Mg2+, and K+ extracted by their normal soil test solution, (e.g. Mehlich 3, Morgan's). If soil pH is <5.5, significant quantities of exchangeable Al3+ may be present but not able to be accurately measured with these extractants, causing an underestimate of the CEC. To overcome this, many labs add exchangeable acidity to soil test K, Ca and Mg.

These types of measurements, termed "effective" CEC and often abbreviated as CECe, will be reasonably accurate when soil values are below 7.5 (or unless the soil has been recently limed). Above this pH, significant quantities of unreacted lime (CaCO₃) or other free salts may be dissolved in the extracting solution, causing an overestimate of the CEC.

Because of the differing methods to estimate CEC, it is important to know the intended use of the data. If a pH-buffered CEC measurement is needed (e.g. for regulatory purposes), the pH 7.0 ammonium acetate procedure of Chapman (1965) is recommended. To estimate the actual CEC (at the pH of the soil), the sum of cations extracted by a routine soil test (CECe) should suffice. For a very precise measure of CEC, the BaCl₂-compulsive exchange procedure is suggested (Gillman, 1979, Gillman and Sumpter, 1986; Rhoades, 1982).

Predicting CEC
CEC estimates can be made based on the texture and amount of organic matter of the soil.
1) Estimation based on texture:

2) Calculation of CEC with % clay and % OM. With this formula assume avg. CEC for % OM=200 meq/100g and the avg. CEC for % clay=50 meq/100g .
Therefore: CEC=(% OM x 200) + (% Clay x 50)
From soil data: soil with 2% (.02)OM and 10%(.1) Clay
& CEC=(200 x .02 + (50 x .1)=(4 + 5)==9 meq/100 g

Determining CEC
To see this animation of determining the CEC Animation of CEC Determination you will need to be "on-line".

The determination of CEC in a soil testing laboratory can be summarized with the following two procedures:
1) sum of cations : remove all cations and total the amount of all the cations removed from the soil exchange sites; and
2) NH₄+ saturation:the soil is saturated with NH₄+, then the NH₄+ is replaced by Ca++, and lastly the NH₄+ removed is measured to determine the number of exchange sites that were occupied by ammonium.

Laboratory Procedures for CEC
Calculation of Effective CEC (CECe: With or without Exchangeable Acidity). CECe is a simple, rapid means to estimate the CEC of most soils at the current soil pH. It does not require additional tests beyond the routine soil test hence it can be easily done for large numbers of soils at low cost. However CECe is not a direct measurement of a soil's CEC, rather it is an estimate based on soil test extractable Ca, K, and Mg and some rapid measure of exchangeable acidity. Direct measures of CEC may be preferred because they will be more accurate.
CEC Determination by the BaCl₂ Compulsive Exchange Method (Gillman and Sumpter, 1986) Determining CEC by compulsive exchange is the method recommended by the Soil Science Society of America (Sumner and Miller, 1996) because it is a highly repeatable, precise, direct measure of a soil's CEC. However the disadvantages of the compulsive exchange method for CEC include a very time-consuming approach that can require specialized equipment, it is not well suited for most routine soil testing laboratories where rapid estimates of CEC for many soils are required and this method also generates a hazardous waste
Determination of CEC at pH 7 with Ammonium Acetate (Chapman, 1965). The pH 7.0 ammonium acetate CEC method has been widely used in the U.S.. for decades. Consequently, a large data base exists for soil CEC by this method. Many state agencies have traditionally required CEC to be measured by this procedure.

The pH 7.0 ammonium acetate CEC method is more time-consuming than effective CEC but can be readily adapted by most soil testing laboratories. The main problem with this method is that it buffers soil pH at 7.0 causing large overestimates of CEC for many of the acid soils. The Univ. of Minnesota Soil Testing Laboratory & Soil Analytical Laboratory will do CEC determinations for around $15.00/sample. For more information go to Uof M Soil Analytical Laboratory

References for CEC: CEC References

Chapter 3 Base Saturation

Lab Units

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