Analysis of physical quality of soil using the water retention curve: Validity of the S-index

Among the various soil indicators established in order to discuss physical properties of soils is the S-index, derived from the slope of the soil water retention curve at its inflection point, used by a number of authors. In this publication we discuss the value of the slope at the inflection point of the soil water retention curve according to the independent variable used to plot it. We show that a representation of the water content according to the arithmetic rather than logarithmic expression of the suction for the S-index yields a different result for the soil selected. More generally, our results show that examining the physical properties of soil using a water retention curve plotted with an arithmetic expression of suction offers greater potential than when plotted with its natural or decimal logarithm as is often found in the literature.