Cation-exchange capacity

Cation-exchange capacity (CEC) is a measure of how many cations can be retained on soil particle surfaces.

Cation-exchange capacity is defined as the amount of positive charge that can be exchanged per mass of soil, usually measured in cmolc/kg.

[1] In addition, the edges of these sheets expose many acidic hydroxyl groups that are deprotonated to leave negative charges at the pH levels in many soils.

Organic matter also makes a very significant contribution to cation exchange, due to its large number of charged functional groups.

CEC is typically higher near the soil surface, where organic matter content is highest, and declines with depth.

The amount of negative charge from deprotonation of clay hydroxy groups or organic matter depends on the pH of the surrounding solution.

[1][5] The cation-exchange capacity of a soil is determined by its constituent materials, which can vary greatly in their individual CEC values.

[1] Base saturation provides an index of soil weathering[4] and reflects the availability of exchangeable cationic nutrients to plants.