Measurement of the cation exchange capacity of clays, kaolins, bentonites and soils in general. The test is carried out using the methylene blue adsorption method. This is a titration, under controlled conditions, where the saturation point is determined, in which the cations of the methylene blue molecule replace the exchangeable cations present in clay minerals. The analysis, as it relates to the surface area of the particles and the nature of the clay minerals, establishes an important correlation with the plasticity and ease of deflocculation of clays. Furthermore, it can be used to assess soil fertility.