The clay mineralogy of soil samples from the Morrow Plot Experiment (University of Illinois, Urbana Campus) was investigated. Analysis of soil samples taken at various intervals between 1913 and 1996 indicates that there is a significant influence of cropping method on the clay minerals in the soils. Curve decomposition methods were used to identify and follow the evolution of the different clay minerals: mica, illite and two randomly mixed-layered illite-smectite phases. The most striking difference is seen for continuous corn and corn-oats-hay rotations. Little change in clay mineralogy is seen in the rotation plot while a significant loss of illitic material from different phases was noted for the continuous-corn cultivation plots. Use of NPK fertilizer since 1955 appears to restore the clay mineralogy in continuous-corn cropping compared to that of the 1913 samples. From these data it appears that the I-S minerals play the role of a K buffer, becoming K-poor when the soil cannot furnish enough K from mineral reserves of detrital phases and K-rich when the soil is able to release enough K to enter into the I-S minerals, where it is available during a growing season, for plant growth.