Origin of reversible cementation and brittleness of the fragipan horizon in Albeluvisols of the Carpathian Foothills, Poland

Fragipan horizon is a natural, subsurface soil layer exhibiting higher bulk density and lower porosity in comparison with the overlying horizons significantly restricting infiltration of water and root penetration. In contrast to hard and cemented horizons (e.g. duripan), clods of the fragipan horizon (Btx) slake or fracture immediately when placed in water. The most interesting issue concerning genesis and properties of the fragipan horizon is its reversible cementation, i.e. very hard consistence in dry state and brittle consistence in moist state. The main objective of the present study was to explain and improve the understanding of the specific consistence of the fragipan horizon in moderately humid climate on the example of Albeluvisols of the Carpathian Foothills in Poland basing on mineral and chemical composition of bridges linking coarse soil material. Quantitative X-ray diffraction shows that the studied fragipan horizon contains high concentration of swelling clay minerals. Soil material from the fragipan horizon exhibits numerous clay and iron–clay coatings and clay infillings indicating translocation of colloidal clay and iron oxides from overlying horizons. In addition, bridges between coarse grains of the soil material occur. The fragipan cementing material is composed mainly of SiO2, Al2O3, and Fe2O3 with lower concentration of K2O, MgO, CaO, and Na2O. The chemical composition of bridges within the pan is similar to that of clay coatings and clay infillings suggesting that clay minerals are major bonding agents and are responsible for brittleness and more friable consistence of the pan in the moist state. Additionally, free iron, aluminium, and silica may play some role in specific cementation of the investigated fragipan horizon but these elements are not main bonding agents.