Introductory soil science courses contain many chemical and physical concepts that are new to most students. Cation exchange capacity is an example of a basic soil property that may be difficult to transfer from a conceptual idea to solid student understanding. The objective of this project was to develop a scale model of cation exchange that could be used for classroom demonstration. To do this a kaolinite clay surface 0.002 mm in diameter with 5 cmol c was used as the exchange surface. The quantity of exchangeable cations was typical for an Ultisol located in the southeastern USA. A scale model of a 1‐ηm 2 section of clay particle was created using a 1 m 2 and 0.64 cm thick piece of plywood. A clay surface of this area would contain 99 charges. To represent the charges, 99 nails (7.6 cm length) were placed in randomly oriented holes in the plywood. Exchangeable cations, scaled to the appropriate hydrated radius, were developed from foam balls. By using the nails to represent charges and the foam balls to represent exchangeable cations, the instructor can actively demonstrate the concepts of cation exchange capacity of a clay, basic cation saturation, and replacement of exchangeable cations. The model can be used in the laboratory or classroom to provide an active and interesting introduction to the concept of cation exchange, leading students to a better understanding of the exchange complex of soil clays.
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Guertal et al. (1996) studied this question.