SUMMARYA mica was inserted into the soil at four sites to try to identify,in situ, the physico‐chemical changes in the soil caused by the introduction of different tree species, and especially conifers. Changes in the chemistry and mineralogy of the mixed‐layered mica, mica‐chlorite, vermiculite, after 11/2and 31/2years in the soil, showed good differentiation between soil horizons and between soil types. Differences due to tree species were present, but small. The mechanisms of mineral evolution can be defined with reference to experimental data obtained in the laboratory. In litter layers, the mica‐chlorite sheet was transformed into a mica‐vermiculite, and exchangeable cations predominated on the exchange sites. In the organo‐mineral horizons, changes varied depending on the soil type: in the podzol, transformation was similar to that in the litter layers, but in thesol brun acide, changes were dominated by the Al fixation on the exchange sites and its rapid hydroxylation which blocks these sites, leading to the formation of Al intergrade minerals.Different tree species produced deviations linked to their ability to cycle cations, and also to the different organic materials produced which can attack the mineral.The method has the potential to define the present soil environment and weathering processes, but it needs a more complete experimental design and the use of statistics to clarify the effects of the parameters and their i