Quantitative clay mineralogy of a Vertic Planosol in southwestern Ethiopia: Impact on soil formation hypotheses

Publication date: February 2014
Source:Geoderma, Volumes 214–215
Author(s): Mathijs Dumon , Alemayehu Regassa Tolossa , Boris Capon , Christophe Detavernier , Eric Van Ranst
Planosols, characterised by a bleached, silt-textured surface horizon abruptly overlying a dense, clayey subsoil, are a very common soil type in Ethiopia. The origin of the abrupt textural change is still often debated in literature. One of the processes frequently put forward to explain the coarse textured material in the topsoil is ‘ferrolysis’: an oxidation–reduction sequence driven by bacterial decomposition of soil organic matter, resulting in the destruction of open 2:1 clay minerals. Recent studies of representative profiles of Vertic Planosols in south-western Ethiopia indicate that these soils are composed of a weathered volcanic ash layer deposited on top of a deflated vertic subsoil, which refutes the ferrolysis hypothesis. To strengthen the geogenetic origin of these profiles, a quantitative mineralogical analysis of the clay fraction was undertaken. Results of a sequential fractionation revealed a strong aggregation of clay particles in the bleached horizon, while the effect of aggregation was far more limited in the vertic horizon. This is believed to be related to the dispersed, impregnative nature of iron oxides in the bleached horizon, compared to the segregated nature of the sharp, nodular concretions found in the vertic horizon. The annealing XRD analysis revealed only minor changes in dehydroxylation temperatures of kaolinites and 2:1 minerals between untreated and DCB-treated samples, indicating that the pretreatment did not significantly alter the mineral lattices. Multi-specimen, full-profile fitting of XRD patterns revealed no large quantitative differences between sub-fractions of the bleached and vertic horizons, although a net increase of 1:1 layers over 2:1 layers towards the top of the profile can be observed in the bleached horizon. This could be interpreted as the result of neo-formation of kaolinite. The main mineralogical differences between the bleached and vertic horizons of the <2μm fraction are mainly a result of the different proportions of sub-fractions. Interestingly, the <0.05μm fraction seems to be dominated by a complex assemblage of kaolinite and smectite mixed-layer minerals. The obtained detailed view on the mineralogical composition of the clay fraction of a typical Vertic Planosol has provided new insights in the complex nature of these duplex soils, confirming ferrolysis not to be at the origin of the abrupt textural change.