Revisiting nitic horizon properties of Nitisols in SW Ethiopia

Nitisols are a soil group whose nature and genesis are relatively poorly documented. In order to better understand the development of nitic horizons, characterized by a blocky structure and shiny ped faces, soil profiles in a major Nitisol region in Ethiopia were analysed, using physico-chemical, mineralogical and micromorphological methods, as well as Mössbauer spectroscopy. Factors that are identified as contributing to Nitisol development are (i) the presence of open 2:1 clays, besides dominant kaolinite, (ii) seasonal cycles of wetting and drying, contributing to strong blocky soil structure development, and (iii) exceptional Fe oxide characteristics, particularly extremely small particle size. Together with the presence of abundant clay coatings, these Fe oxide characteristics may explain the shiny aspect of ped faces that is characteristic of nitic horizons. The profiles are affected by an admixture of surface-derived volcanic ash components, which might be common in Nitisol regions and which puts into question the use of silt/clay ratio as a requirement for this WRB reference soil group. •New data and interpretations for Nitisols, a poorly known reference soil group•Exceptionally fine grain size of iron oxides, based on Mössbauer data•Characteristic shiny ped faces explained by clay coatings and Fe oxide properties•No confirmation of Ferralsol–Nitisol relationship suggested in earlier studies•Volcanic ash admixture invalidating silt/clay ratio as classification requirement