Revisiting nitic horizon properties of Nitisols in SW Ethiopia L. De Wispelaere a , V. Marcelino a , Alemayehu Regassa a,b , E. De Grave c , M. Dumon a , F. Mees d , E. Van Ranst a, ⁎ a Department of Geology and Soil Science (WE13), Ghent University, Krijgslaan 281/S8, B-9000 Gent, Belgium b Department of Natural Resources Management, Jimma University, College of Agriculture and Veterinary Medicine, Jimma, Ethiopia c Department of Physics and Astronomy, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgium d Department of Geology and Mineralogy, Royal Museum for Central Africa, Leuvensesteenweg 13, B-1080 Tervuren, Belgium abstract article info Article history: Received 27 October 2014 Received in revised form 15 December 2014 Accepted 23 December 2014 Available online xxxx Keywords: Nitisols Micromorphology Mineralogy Fe oxides Soil classification Ethiopia Nitisols are a soil group whose nature and genesis are relatively poorly documented. In order to better under- stand 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 micromorpholog- ical methods, as well as Mössbauer spectroscopy. Factors that are identified as contributing to Nitisol develop- ment 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Nitisols are defined as deep, well-drained, red, tropical soils with dif- fuse horizon boundaries and a clay-rich ‘nitic’ subsurface horizon that has typical ‘nutty’, polyhedric, blocky structure elements with shiny ped faces, as expressed by its name (L. nitidus, shiny) (Driessen et al., 2001; IUSS Working Group WRB, 2006). These soils are predominantly derived from basic parent rocks by strong weathering, but they are far more fertile than most other red tropical soils. More than half of the area occupied by Nitisols worldwide is found in tropical Africa, and Ethiopia is one of the mainstays of these soils (IUSS Working Group WRB, 2006). Although Nitisols cover an estimated 200 million ha world-wide, the properties of these soils are not well documented in the international literature. The exact origin of the shiny faces of the polyhedral, flat- sided or nutty elements typical for the nitic horizon is still under debate. The development of these typical nitic properties, or the nitidization process, is assumed to be related to alternating micro-swelling and shrinking (Driessen et al., 2001), but it cannot or only partially be attrib- uted to clay illuviation (IUSS Working Group WRB, 2014). Sombroek and Siderius (1977) used the term ‘metalization’ to indi- cate the accumulation of metal oxides as thin coatings on ped faces, giv- ing rise to their shiny appearance. Creutzberg and Sombroek (1987) suggested that the common thin illuvial clay–iron oxide coatings ob- served in thin sections are responsible for the shiny ped faces. Some studies on the genesis of Nitisols, in particular the origin of the typical moderate to strong blocky structure and the thin clay coatings, have been carried out by Brazilian soil scientists on Brazilian “Nitossolos” (e.g. Cooper and Vidal-Torrado, 2000, 2005; Oliveira et al., 2004; Cooper et al., 2010). For this region, it has been suggested that nitic horizons originate from structural transformations of ferralic horizons as a result of climate change (Cooper et al., 2010). In the area covered by the present study, the Gilgel Gibe catchment in the Jimma zone in SW Ethiopia, Nitisols represent a major reference soil group in this region, besides Acrisols, Ferralsols, Vertisols and Plano- sols (FAO-Unesco, 1974; Alemayehu Regassa, 2009). The Nitisols are in- tensively used for agricultural cropping and thus very important for food production. Despite their importance, published analytical data for these soils is still scarce. The general aim of this study is to investigate the origin of nitic properties, in particular the blocky structure, the shiny faces of structural units, and the presence of clay coatings, in Nitisols from the Gilgel Gibe catchment, using various laboratory procedures. Physico-chemical analyses are complemented by a mineralogical study of the silt, clay and fine clay fractions, by a micromorphological study and by a Mössbauer spectroscopy study for iron oxide characterization. 2. Materials and methods 2.1. Environmental setting The Gilgel Gibe catchment is located between 7°22′72″ to 7°34′84″ N and 37°21′05″ to 37°28′80″ E. It is part of the Omo-Gibe River basin, the Geoderma 243–244 (2015) 69–79 ⁎ Corresponding author. E-mail address: eric.vanranst@ugent.be (E. Van Ranst). http://dx.doi.org/10.1016/j.geoderma.2014.12.021 0016-7061/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma