Nowadays the concept of soil physical quality is well-established, based mainly on the indices that quantify soil quality. The S index is a new indicator of soil physical quality related to important soil characteristics. The purpose of this study was to evaluate the method of obtaining the S index and the derivative of the function of the of water retention curve, using the transformation ln(h) or not, and assess the problem of units and the restriction m = 1 - 1/n for the index calculation. It was concluded that the S index value is higher when the soil moisture used to adjust the water retention curve of van Genuchten (1980) is expressed as volumetric than as gravimetric moisture. For the same unit of soil moisture, no difference was observed in the S index for any tension unit. The S index value differs according to the use or not use of the restriction m = 1 -1/n to estimate the parameters m and n of the van Genuchten retention curve.
water retention curve; soil quality; modeling
