Properties and geographic relevance of fragipan and other close-packed horizons in a non-glaciated Mediterranean Region

Abstract

To investigate the properties of different close-packed horizons present in soils evolved from mainly siliceous colluvia and alluvia in a Mediterranean environment several profiles were selected and analysed, Selection of the profiles occurred after a geomorphological and soil survey in an area known as «Montagnola Senese» in central Tuscany. Close-packed horizons were classed according to their morphology as: () fragipan, (i) glossic horizon, (iii) densipan and (iv) pedal dense subsurface horizon; they all showed the following properties: high bulk and packing density, very low hydraulic conductivity of the matrix and a porosity dominated by micropores. They all were free of carbonates and extracted silica did not occur in significant quantities. The great abundance of silt and very fine sand, derived from the alteration of schist and quartzite present in the parent material, was the main common characteristic of all close-packed horizons, although densipan and pedal dense subsurface horizon could be more clayey. Geomorphological evidences showed that the area was undergone, since the late Pliocene, through several uplifting episodes that have resulted in slope erosion, colluviation and formation of extensive and thick deposits, followed by the draining of the saturated material. The origin of the high density of the examined horizons could be related to this particular geomorphological evolution of the surfaces during the Pleistocene, as well as to the lithology of the materials, having a high degree of sorting in the fine-earth fraction. Differences in morphology between fragipan, densipan and glossic close-packed horizon call for distinct stages of a disintegration process, depending on the tree root penetration into the firm soil mass and on the distance between the close-packed horizon and the surface. The noticing of their position in the profile can allow the acquisition of useful information about the Quaternary geomorphological evolution of the areas where soils with this kind of close-packed horizon are present.