The Soil Characteristic Curve at Low Water Contents: Relations to Specific Surface Area and Texture.

Accurate description of the soil-water retention curve (SWRC) at low water contents is important for simulating water dynamics, plant-water relations, and microbial processes in surface soil.   Soil-water retention at soil-water matric potential of less than -10 MPa, where adsorptive forces dominate over capillary forces, has also been used to estimate the soil specific surface area (SA, m² kg^-1). In the present study, the dry part of the SWRC was measured by dewpoint potentiameter on 41 Danish soils covering a wide range of clay (CL, kg kg^-1) and organic carbon content (OC, kg kg^-1). It was found that measurements by traditional pressure plate apparatus generally overestimated water contents at -1.5 MPa (plant wilting point).  The 41 soils were classified into four textural classes based on the so-called Dexter index n (= CL/OC), and the Tuller-Or (TO) general scaling model describing the water film thickness at a given soil-water matric potential (< -10 MPa) was evaluated. Only for low organic soils with n > 10, the estimated SA from the dry soil-water retention was in good agreement with the SA measured using ethylene glycol monoethyl ether (SA_EGME). A strong relationship between the ratio of surface area estimates and the Dexter index n was observed and applied as an additional scaling function in the TO model to rescale the soil-water retention curve at low water contents across soil textural classes. However, the TO model still overestimated the water film thickness at potentials approaching oven dry condition (~ -800 MPa). Alternatively, the linear Campbell-Rossi-Nimmo (CRN) model for pF (where pF = log (negative of matric potential; cm H₂O)) versus volumetric water content (m³ m^-3) showed accurate fits to data for all 41 soils from -10 to -800 MPa, and yielded high correlations between CRN model slope (1/B) and SA and CL. We, therefore, recommend to use the empirical CRN model for predicting the dry part of the water retention curve (-10 to -800 MPa) from measured soil texture or surface area and, in perspective, to modify the more conceptual TO model to obtain better descriptions in the very dry range (-300 to -800 MPa).