The effect of plant species on soil-clay mineralogy has not been studied widely. In the present study, the clay mineralogy of top soils under Sequoia gigantia and grass regimes, maintained side by side for up to 150 y in the parks of several French châteaux, were investigated using X-ray diffraction (XRD) methods and chemical analyses of the clay fractions. The seven paired samples that were studied originated from soils developed on calcareous, granitic, and loess substrates. The XRD data indicated the presence of a trioctahedral chlorite with a trioctahedral hydroxy-Mg sheet in sequoia soils observed in four of seven of the sites whereas it was absent from the adjacent prairie-soil samples. Parent materials influenced the formation of magnesian chlorite as it was observed in all soils developed on granite and in none of the soils developed on limestone. The exchangeability of the interlayer hydroxy-Mg sheet replaced by K+ from newly formed chlorite in a 14 y old sequoia-influenced soil suggests that the mineral was initially a hydroxy-interlayered mineral. Increased stability was observed in the older (100 and 150 y) soil chlorites, indicating a progression of polymerization of the Mg hydroxy-interlayered material. The small amount of chlorite in the whole clay assemblage impeded the observation of changes in Mg content by direct chemical measurements of the clay fractions but the systematically greater amount of exchanged Mg2+ ion measured under sequoia compared with adjacent prairie supports the formation of Mg magnesian chlorite.

The results presented indicate, on the one hand, the importance of plant regimes in controlling the soil chemistry and hence the clay mineralogy of surface soil horizons (magnesian chlorites were observed only under sequoia), and, on the other hand, that parent material modulates this plant influence (chlorite formation was observed on granite-derived soils).