Genesis and role of the skeleton water-extractable fines in volcanic soils.

Little is known about the characteristics and genesis of particles <2 mm in size that adhere to the surface of soil rock fragments, termed skeleton water-extractable fines (SWEF). To focus on the origin and role of SWEF in soil genesis, we investigated physical, chemical, and mineralogical properties of SWEF, fine earth, and rock fragments (ranked according to their size as 2–4 mm, 4–10 mm, and >10 mm) of volcanic Entisols. These soils were chosen because they formed from highly porous rock fragments whose anfractuosities and vesicles may be niches in which formation, accumulation, and modification of fine particles is favored. Results suggested that in the soil profiles studied, the SWEF, being more weathered than the fine earth, was not a transitional material between rock fragments and fine earth, as found in other cases. The genesis of the SWEF seemed to happen inside the vesicles of the rock fragments, where fine materials produced by weathering of the vesicle walls, derived from roots colonizing vesicles and carried through illuviation, can accumulate. In the vesicles, mineral and organic matter underwent alteration and evolution, probably favored by the presence of an active microbial community. Conversely, the evolution of the fine earth occurred outside the clasts, where it experienced different weathering conditions. With time, the weathering processes inside the vesicles caused the breakdown of the vesicle walls, facilitating the progressive reduction of the clasts size and the consequent mixing of SWEF with the fine earth. The processes that were responsible for the formation and evolution of the SWEF could be considered a pedogenic driving force.