I came across a confusing concept in a reference and was wondering wether increasing CEC, increases the preference of adsorb sites to bind with cations with higher valence ? For example divalents over monovalents. If yes, does this mean that dry soils (with their double layer and pKa of OH groups decreased and their CEC increased)adsorb more divalent cations?
The charge and size of cations influences their distribution between the double layer and bulk solution.
CEC is usually measured at a defined pH, typically alkaline. For many soils this condition is not at all like that in the field.
In the even of enrichment of divalent cations is much more than mono valent ones in soil solution , we can expect soil is not acidic and will show higher CEC.
@ Kamyar, many minerals in soils are negatively charged and, as a consequence, can attract and retain cations such as calcium , magnesium, sodium, potassium and ammonium etc. Cation exchange is a reversible process, in which elements or nutrients that can held in the soil and lost through leaching can subsequently be released for crop uptake. Certain organic compounds also contribute to cation exchange capacity. Additionally CEC is influenced by soil pH. A certain portion of the total negative charge is permanent, while a variable portion is pH dependent. Values of cation exchange capacity are in the range of 1.0 to 100 cmol (+)/kg, least for sandy soil and high for clay soil. Similarly, higher CEC value reflects the dominance of 2:1 clay minerals, and lower value reflect the presence of 1:1 clay minerals.
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