Volumetric Surface Area and Clay Saturation Indexes for the Prediction of Soil Hydraulic Properties.

Pedotransfer functions (PTFs) for soil hydraulic properties are traditionally based on gravimetric fractions of soil solids. Recent studies suggest the use of volumetric fractions and surface area indexes since they are presumably directly related to e.g. volumetric water contents at given soil-water matric potentials and soil-water retention curve parameters. Also, it has been suggested (Dexter et al., 2008, Geoderma 144:620-627) that different PTFs are needed depending on if soils are clay saturated (containing more than ~10 g clay per g organic carbon) or not. In this study we suggest five volumetric surface area indexes (VSAI) and apply them to a Danish database comprising 40 soils (22 clay-saturated (CS), and 18 non-clay-saturated (NCS)). The five VSAI are (i) volumetric surface area measured by Ethylene Glycol Monoethyl Ether method (SAvol(EGME)), (ii) volumetric surface area based on N2/BET method (SAvol(N2/BET)), (iii) volumetric content of fines (clay and organic matter) (FINESvol), (iv) volumetric cation exchange capacity (CECvol), and (v) volumetric soil-water content at air-dry (20 C and ~20% relative humidity) (ADWvol), from which a monolayer water coverage area can be calculated. Inter-correlating the five VSAI, SAvol(EGME) versus SAvol(N2/BET) exhibited different relations for  CS and NCS soils and only CS soils showed high correlations between SAvol(EGME or N2/BET) versus FINESvol, CECvol, or ADWvol. The latter three VSAI exhibited minimal effects of clay saturation and high correlations with soil-water contents at the dry end of the soil-water retention curve (pF >= 3.2). Thus, they could be used to accurately predict water contents within that pF range and across the Dexter clay saturation index. FINESvol was also highly correlated to the Campbell soil-water rentention parameter (b) for the wetter part (1.2 <= pF = 3.2). Further promising relations between the VSAI and Dexter clay saturation indexes versus soil-water fractions of the water retention curve, saturated soil-water permeability.