The ability of a soil to retain cations (positively charged ions) in a form that is available to plants is known as cation exchange capacity (CEC). A soil's CEC depends on the amount and kind(s) of colloid(s) present. Although type of clay is important, in general, the more clay or organic matter present, the higher the CEC. The CEC of a soil might be compared to the size of a fuel tank on a gasoline engine. The larger the fuel tank, the longer the engine can operate and the more work it can do before a refill is necessary. For soils, the larger the CEC, the more nutrients the soil can supply. Although CEC is only one component of soil fertility, all other factors being equal, the higher the CEC, the higher the potential yield of that soil before nutrients must be replenished with fertilizers or manures. When a soil is tested for CEC, the results are expressed in milliequivalents per 100 grams (meq/100 g) of air-dried soil. For practical purposes, the relative numerical size of the CEC is more important than trying to understand the technical meaning of the units. In general, soils in the southern United States, where physical and chemical weathering have been more intense, have lower CEC's (1-3 meq/100 g) than soils in the northern United States, where higher CEC's are common (15-25 meq/100 g) because weathering has not been as intense. Soils in warmer climates also tend to have lower organic matter levels, and thus lower CEC's than their northern counterparts.
Dear Mr. Fadhil:
Cation exchange capacity abbreviated to CEC measures the amount of cations (positively charged ions) which can be absorbed by, e.g. organic matter or clay minerals and which can be exchanged for cations dissolved in a solution, e.g., pore solution or meteoric fluids. CEC is used in sedimentary petrography/ chemistry , technical mineralogy and pedology to characterize the interaction between mineral and organic matter and solutions . It used to increase with increasing pH. Determination of the cation exchange capacity:
The cation exchange capacity (CEC) of kaolin is measured using the Cu-triethylenetetramine method (Meier and Kahr, 1999) with and without Hofmann-Klemen-treatment (Hofmann and Klemen, 1950). The method was applied to the 1 – 2 mm fraction with different reaction times. Standard reaction time for the powders is 2 h shaking in an end-over-end shaker. The 1 – 2 mm samples were additionally investigated with respect to the CEC after 48 h shaking followed by 10 min ultrasonic treatment and finally after additional 72 h shaking followed by additional 10 min ultrasonic treatment. The method is a good tool to get an idea of the quantity of smectite-group minerals in kaolin. It is measured in meq/ 100 gr (milligram equivalent per 100 gram). It is measure and reported as meq/ 100 gr, e.g., halloysite , a kaolinite-group mineral, has a cation exchange capacity which stands at 5 meq/ 100 gr, whereas that of smectite ranges from 75 to 100 meq/100 gr. The low CEC is caused by OH-groups (=edge-effects). For comparison only, its closest relative kaolinite has a CEC of 1 to 2 meq/100 g. Micaceous and chloritic phyllosilicates are between kaolinite and smectite. Organic matter may have a much higher CEC depending on the degree of alteration.
Meier, L.P., Kahr, G., 1999. Determination of the cation exchange capacity (CEC) of clay minerals using the complexes of Copper (II) ion with triethylenetetramine and tretraethylenepentamine. Clay Clay Miner. 47, 386–388.
Hofmann, U., Klemen, R., 1950. Verlust der Austauschfähigkeit yon Lithiumionen an Bentonit durch Erhitzung. Z. Anorg. Chem. 262, 95–99
I wish you much success
H.G.Dill
Basics of CEC definition is nicely described in short in the internet. As H. G. Dill pointed out the intensity of CEC is controlled mainly by the presence of soil constituents with active surfaces (e.g. clay minerals) and / or the potential for cation complexation (e.g. humic substance). If you have to render an official expert opinion on soil CEC there exist two international standards DIN ISO (DIN: German Institute for Standardization). An older one (1997) describes the determination of the potential CEC: “DIN ISO 13536; Soil quality — Determination of the potential cation exchange capacity and exchangeable cations using barium chloride solution buffered at pH = 8,1 (1995)“. In this procedure the extracting solutions’ pH is buffered to pH = 8.1 (to maintain, hopefully, the comparability between different soils). The determination of the effective cation exchange capacity is described in ISO 23470:2018(en): „Soil quality — Determination of effective cation exchange capacity (CEC) and exchangeable cations using a hexamminecobalt(III)chloride solution (2018)”; (ISO: International Organization for Standardization). In this case the CEC is determined at the particular soil pH. Prior to determination of effective CEC the soil pH is determined using ISO 10390: “Soil quality — Determination of pH“; (2005). DIN recommends to use air dried soil, which to my opinion is not correct; fresh pristine soil samples should be used. Drying soil samples will cause agglomeration of fine particles, especially at high amounts of clay components. Following disaggregation is very difficult and, above, will create new active surface sites. Not to disaggregate will yield too low CECs (reduced surface area). Also, samples have to be brought thoroughly in finest suspension with extraction solution before starting determination procedure. Shaking of sample with extraction solutions must be done slowly (end-over-end or rotationary shaker) to avoid attrition, by this falsifying CEC to higher values.
D. Zachmann
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