Abstract Analysis of new data and reinter- pretation of published information for clay minerals found in temperate climate soil profiles indicates that there is often a gradient of ‘‘illite- like’’ clay minerals with depth. We used the term ‘‘illite-like’’ because these observations are based on X-Ray Diffractogram patterns and not on layer charge measurements which allow to define properly illite. It appears that ‘‘illite-like’’ layers are concentrated in the upper, organic - rich portion of the soil profile both under grassland and forest vegetation. ‘‘Illite-like’’ layer quantity seems directly related to soil potassium status. Indeed, intensive agriculture practises without potassium fertilization reduce ‘‘illite-like’’ content in surface soils, whereas several years of potassic fertilization without plant growth can increase ‘‘illite-like’’ content. The potassic soil clay mineral, illite, is particu- larly important in that it can be the major source of readily available potassium for plants. Spatial and temporal dynamics of clay minerals should be related to the potassium cycle. We propose that the frequently observed general trend of increasing exchangeable potassium in the top soil can be correlated with an increase in ‘‘illite-like’’ in the clays and that the decrease of potassium caused by intensive agricultural practices leads to ‘‘illite-like’’ layer destabiliza- tion. This vision of ‘‘illite-like’’ layer as a potassium reservoir refueled by plants and emptied by intensive cropping renews the con- cept of potassium availability and indicates a need to be discussed as well in natural ecosys- tems as in cultivated ecosystems. Keywords Clay minerals Æ Illite Æ Potassium availability Æ Potassium cycle Æ Plant–soil interactions Æ Soil Abbreviations HI hydroxy-interlayered clay minerals IS mixed layered illite-smectite clay minerals PCI poorly crystallized illite WCI well crystallized illite XRD X-Ray diffractometer Introduction Potassium is an important element for plant nutrition. According to Knecht and Go ¨ ransson P. Barre ´(&) Æ L. Abbadie Laboratoire Bioge ´ ochimie et Ecologie des milieux continentaux, UMR 7618, Universite ´ Pierre et Marie Curie-Paris 6, ENS, CNRS, 46 rue d’Ulm, 75230 Paris cedex 05, France P. Barre ´ Æ B. Velde Laboratoire de Ge ´ ologie, UMR 8538, ENS, CNRS, 24 rue Lhomond, 75231 Paris cedex 05, France Biogeochemistry (2007) 82:77–88 DOI 10.1007/s10533-006-9054-2 123 REVIEW Dynamic role of ‘‘illite-like’’ clay minerals in temperate soils: facts and hypotheses P. Barre ´ Æ B. Velde Æ L. Abbadie Received: 15 December 2005 / Accepted: 17 August 2006 / Published online: 3 October 2006 Ó Springer Science+Business Media B.V. 2006