Genesis of clay skins in tropical eutric soils: A case study from NE-Brazil

Clay skins are morphological features that manifest in soils as a waxy shine, mainly on the surface of aggregates. Their genesis is related to the formation of pellicles or films of clay and micro features of stress on ped faces. Clay skins provide pedogenetic evidence and may indicate previous or current occurrence of flocculation and dispersion, as well as the nature of constituent minerals of the soil. Thus, we aimed to study the genesis of clay skins, testing the hypothesis that they are formed as a result of alternating micro-swelling and shrinking processes and that the argilluviation is currently inexpressive. For this, a toposequence of eutric soils from metamorphic rocks (gneiss-migmatite) in a tropical forest environment was studied. Soil sampling based on a structural model of the pedological coverage, as well as physical and chemical analyses; monitoring of clay dispersion in water; coefficient of linear expansivity; x-ray diffractometry; selective dissolution of Fe oxides using sodium citrate-bicarbonate-dithionite and acid ammonium oxalate; optical and electron microscopy analyses; and modelling of x-ray diffractometry patterns using ClaySIM software were carried out. The results indicated that clay dispersion in the studied eutric soils seems to be of little significance during the rainy period since the greater dissociation of bivalent cations such as Ca2+ and Mg2+ favors the forces of attraction forming cationic bridges, resulting in the union between negatively charged particles. Furthermore, the illite-vermiculite phase may have little influence on the macro-morphological properties of the soils; although on a microscopic scale, it may favor the formation of granostriated b-fabrics. Clay skin formation in soils of a hot, humid tropical climate is essentially controlled by the argilluviation process and by wetting and drying cycles, the expression of this feature being proportional to the quantity of intra-aggregate voids (smaller size voids). Nevertheless, the orientation and metallization process (hematization) of the coatings and infillings seem to determine the expressiveness of the clay skins, and the type of b-fabrics and their development variations are not related, or are insignificant, in regard to the occurrence and degree of development of clay skins.