Título en español.

Catalina and Cialitos soils have different cation exchange capacities, even though their organic-matter and clay contents are similar. X-ray and differential thermal analyses were made on the clay fraction to determine whether this variation in exchange capacity could be attributed to differences in clay mineral composition. The analyses revealed that, in both cases, kaolinite is the predominant clay mineral. However, it was found that, in Cialitos clay the kaolinite is poorly crystallized, whereas crystals are well defined in Catalina clay. Since the poorer the crystallinity, the higher the exchange capacity, this could account for the higher cation exchange capacity of Cialitos clay. Another factor that may also partly account for the difference in cation exchange capacity is the higher content of free iron oxides in Catalina clay. It is well known that free iron oxides block exchange positions in kaolinite clay minerals.