MINERALOGICAL, PHYSICAL, AND CHEMICAL PROPERTIES OF ROCK FRAGMENTS IN SOIL

Abstract

During sample preparation, the coarse fraction (> 2 mm) of soils is commonly excluded from further analytical determinations. Our results demonstrated, however, that the coarse fragments of soils derived from sandstone in Tuscany, Italy, are not chemically inert. From an intensive study of the clasts from three profiles in the Vallombrosa Forest 50 km east of Florence, we have established that the clasts of this fraction, especially those altered and partially altered, manifest properties that in some cases equal or surpass those of the more reactive fine earth. The profiles are underlain by Arenaria del Falterona, a sandstone of the Oligocene time intercalated with siltstone. Our research shows that the coarse fraction participates in a sequence of weathering events that involves dissolution of carbonates and formation and transformation of secondary minerals. Although fresh sandstone and siltstone display distinct mineralogical composition, these differences are not maintained once they undergo accelerated weathering in the soil. Dissolution of carbonate brings indirect enrichment of the noncarbonatic components. Formation of HIV (hydroxy interlayered vermiculite) and HIS (hydroxy interlayered smectite) is the dominant process in the fine earth and clasts. This process becomes particularly important in stabilizing the micropores of the clasts and maintaining a porosity that allows the soil solution to be adsorbed and circulate. Weathering processes thus create voids, release nutrient cations, and render the most weathered clasts similar to fine earth. The progression of weathering into the clasts is demonstrated by the levels of extractable Fe and Al obtained by selective dissolution. Furthermore, our research shows that the rock fragments contain C and N in considerable quantities and have a higher pH and a mineral assemblage less weathered than the fine earth. In addition, clasts constitute a reservoir for nutrients and possess a capacity for proton consumption. Moreover, although the fine earth, including organic matter, dominates the effective cation exchange capacity (ECEC) in the upper horizons, in the B and BC horizons the rock fragments represent from 20 to 55% of the total ECEC. In conclusion, our findings show that the clasts of these reactive lithologies are not inert but play all important role in soils as: (i) reservoirs of nutrients, including N, (ii) sources of cation exchange capacity, (iii) water supplies, (iv) adsorbers of organic pollutants, and in the consumption of protons.