Positive ions that are available in soils absorb on grain surfaces. The total sum of cations that can be absorbed bij a soil/sediment at a certain PH is defined by the cation-exchange capacity (CEC, in meq g-1: mol equivalents per gram). The uptake of cations is an important parameter in agriculture and the larger the CEC, the more cations can be absorbed to the soil. The CEC depends highly on the pH of soil and sediments, where the CEC decreases with decreasing PH (increasing acidity). The exchange of ions on sediments occurs commonly fast on geological time scales, but the kinetics of adsorption in natural environments is still poorly understood. The strength of the bonding between the cations and the sediments varies from weak Van der Waals bondings (physical adsorption) to strong chemical bonds.

The CEC is widely used for agricultural assessment because it is a measure of general soil fertility as well as an indicator of structural stability because CED is capabel of enhancing development of shrinkage cracks.

The list below shows the CEC for different types of minerals. The data indicate that the CEC can vary over 2 orders of magnitude for various types of ,minerals and can vary one order of magnitude within one soil type.

Cation exchange capacity for different types of sediment (Eisma, 1992; Locher and de Bakker, 1990):

Type of sediment/mineralCation exchange capacity (mol eq. kg-1)
Quartz sand0.01-0.02
Coastal plain soils0.06-0.02
Kaolinite0.03-0.15
Illite0.15-0.4
Montmorillonite0.8-1
sedimentary clays0.6-2.21

Since the CEC is determined by the mineral composition of soils, the concentrations of radionuclides can be used to determine the CEC of soils.

Huang et al. used (amongst other methods) an airborne gamma-ray spectrometer with windows analysis to map the CEC of a test site and they developed an error budget procedure.

Huang, J., Bishop, T.F.A., Triantafilis, J., 2017. An error budget for digital soil mapping of cation exchange capacity using proximally sensed electromagnetic induction and remotely sensed γ -ray spectrometer data. Soil Use Manag. 33, 397–412. doi:10.1111/sum.12347