Field and laboratory observations based on X-ray diffraction techniques suggest that 2:1 clay minerals behave as a K reservoir. However, 2:1 soil clay assemblages are most often composed of a mixture of different 2:1 clay mineral populations and the role of these different clay mineral populations in K absorption or K release are not fully understood. This present work addressed this question, through laboratory experiments, by using an X-ray pattern decomposition program to quantify peak area modifications of different clay mineral populations induced by K addition and K removal through plant action. This study was performed on three typical temperate grassland soil clay assemblages containing illite, interstratified illite/smectite and soil vermiculite. Our results revealed that the quantities of well crystallized illite increased at the expense of soil vermiculite as a result of K absorption whereas the opposite trend was observed following K depletion. This particular relationship between well crystallized: illite and soil vermiculite suggests that they constitute a subgroup among 2:1 clay minerals. Poorly crystallized illite and interstratified clay minerals were also significantly affected by K absorption or removal indicating that they are also involved in soil K dynamics. Moreover, we observed that these short term clay mineral modifications (occurring at the day scale) induced by root K extraction are reversible. We also observed that the linear relationship between clay K content and the centre of gravity position of X-ray diffraction patterns in the 2:1 clay mineral range seem to be a general property for soil clay assemblages dominated by illite and interstratified illite/smectite materials. Our results support the view of 2:1 clay minerals as a K reservoir and specify the role of each of the 2:1 clay mineral populations in K absorption and release. All the 2:1 clay minerals commonly found in temperate grassland soil clay assemblages take part in the soil K reservoir.