Genesis and evolution of the fragipan in Albeluvisols in the Precarpathians in Ukraine

The fragipan (Bx) is a subsurface soil horizon restricting the penetration of roots and infiltration of water due to its high bulk density, low total porosity, and discontinuous voids (so-called closed box system). Additionally, the pan is characterized by a hard or very hard consistence when dry, but brittle when moist. The genesis and evolution of the fragipan have been the subject of many research studies, and numerous theories and models concerning these issues can be found in the literature. The principal aims of this study were to: 1) explain the genesis of the fragipan in Albeluvisols in the Precarpathians in Ukraine, basing on micromorphological studies and quantitative mineral composition analyses and 2) propose a model of pan formation and evolution under moderately humid climate conditions. The abundance of clay coatings and clay infillings as well as iron–clay cutans within the studied fragipan indicates that the formation of the pan is related to the translocation of colloids (clay minerals and iron hydroxides) from upper soil horizons. This leads to the filling of voids in the Btx horizon and a decrease in the porosity and hydraulic conductivity of the pan. The enrichment of the fragipan in clay minerals (especially in swelling clay minerals) is responsible for the subsequent degradation of the pan due to wetting and drying, leading to the swelling and shrinking of soil material. The occurrence of many Fe–Mn nodules and a mottled color indicates cyclical wetting and drying within the studied fragipan. In effect, vertical cracks are formed, which serve as pathways for water infiltrating down the soil profile. The water washes out weathering products (iron oxides and clay minerals) from soil material adjacent to the cracks. This leads to the formation and development of bleached tongues along the vertical cracks, which penetrate the fragipan.