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Summary

We present a new model for the soil-water retention curve, θ(hm), which, in contrast to earlier models, anchors the curve at zero water content and does away with the unspecified residual water content. The proposed equation covers the complete retention curve, with the pressure head, hm, stretching over approximately seven orders of magnitude. We review the concept of pF from its origin in the papers of Schofield and discuss what Schofield meant by the ‘free energy, F ’. We deal with (historical) criticisms regarding the use of the log scale of the pressure head, which, unfortunately, led to the apparent demise of the pF. We espouse the advantages of using the log scale in a model for which the pF is the independent variable, and we present a method to deal with the problem of the saturated water content on the semi-log graph being located at a pF of minus infinity. Where a smaller range of the water retention is being considered, the model also gives an excellent fit on a linear scale using the pressure head, hm, itself as the independent variable. We applied the model to pF curves found in the literature for a great variety of soil textures ranging from dune-sand to river-basin clay. We found the equation for the model to be capable of fitting the pF curves with remarkable success over the complete range from saturation to oven dryness. However, because interest generally lies in the plant-available water range (i.e. saturation, θs, to wilting point, θwp), the following relation, which can be plotted on a linear scale, is sufficient for most purposes: inline image, where k0, k1 and n are adjustable fitting parameters.