Transformation of rocks to soil material mostly involves two steps: 1° weathering of primary minerals with formation of secondary products and 2° transformation of the lithic fabric to a soil fabric (so called pedoplasmation process). Overlapping of both steps commonly occurs. Moreover, there are increasing evidences that the soil skeleton, or rock fragments or clasts, have chemical characteristics able to affect other soil properties such as cation exchange capacity, content of organic matter, N and available P, adsorption and release of heavy metals, adsorption and degradation of pesticides, and may provide a suitable environment for biological activity or may improve quantity or quality of some cultivation. On the other hand pedogenic (hard) rocks are commonly registered in the stratigraphic sequences, especially of continental sediments. Examples are calcretes, gypcretes, silcretes, and a variety of ferricretes (including laterites and bauxites), but also less universal layers such as tepetate and cangua play locally an important role.
The aim of this session is to contribute to the characterization and the understanding of processes related to transformation of rocks to soil, of the active role of the soil sekleton, and of the formation of pedogenic hard rock, based on field observations, chemical and mineralogical properties and microscopic fabric analyses.