The effect of textural groups on the fitting capability of soil particle size distribution curve models

The important step in using the soil particle size distribution (PSD) to predict soil moisture characteristic curve is the determination of accurate and continuous curve for PSD. Many models have been introduced to describe PSD curve and fitting to the experimental data، but their fitting capability in different textural groups have been rarely investigated. The aim of this study was to evaluate the ability of 15 models to fit the experimental data and determine the best model among them for the PSD of all soil samples and for each soil textural groups. In this study، 160 soil samples were collected from subsoil and top soils of Gilan، Hamadan and Kermanshah provinces، and after determining the PSD and soil texture، they were partitioned into three soil textural groups of sandy، loamy and clayey. Fifteen PSD models were fitted to the experimental data of all soil samples. The most accurate model was determined for the total data and for each of three groups based on error criteria. Results showed that the Weibull model was the most accurate model for all soil samples as well as for the clayey and loamy groups. After the Weibull model، the models of Rossini Ramlr، Fredland and van Genuchten were the most accurate ones; however، their differences were not significant at 95% level. The S-Curve model showed the better fit than the other models for the coarse texture group. The results indicated that the different models have higher accuracy in different textural groups.