Argillic horizon development in calcareous soils of arid and semiarid regions of southern Iran

Ten representative pedons with argillic horizons from southern Iran were selected for micromorphological and mineralogical study. They were classified as Haploxeralfs, Haplustalfs, Argixerolls, Argiustolls and Argigypsids with thermic, hyperthermic and mesic temperature regimes. Some of the argillic horizons were probably formed in a less arid previous climate, during which carbonate migrated through the profile, thus allowing clay to illuviate into deeper horizons. Thin sections showed few to no clear clay coatings in argillic horizons with greater clay content, where shrink/swell has disrupted them. Clay coatings superimposed on carbonate coatings, and in turn covered by calcite, indicate recalcification. Calcite depletion pedofeatures were the most striking features in these soils. Sodium may have played an important role in the dispersion of clay and formation of argillic horizons within two of the pedons studied. A micromorphological index of soil evolution in highly calcareous arid and semiarid conditions is proposed. It correlates with the content of free Fe2O3 (FeDCB in fine earth), the colour index, the clay illuviation index and the smectite/(chlorite+illite) ratio. The index can be useful for correlation of soils with argillic horizons formed on highly calcareous parent material in different landscape units. New subgroups of Haploxeralfs and Argiustolls are suggested for inclusion in Soil Taxonomy based on the properties of some of the studied profiles.