Soil particle-size analysis up to 250 μm by X-ray granulometer: Device set-up and regressions for data conversion into pipette-equivalent values

The particle-size distribution is a fundamental soil physical property commonly used for attributing the textural class and, given its strict linkage with both several soil edaphyc properties and geomorphology processes, it represents a routine lab determination. Over recent decades, various new methods for grain-size analysis have been developed, among them, the X-ray granulometer represents the unique innovative technology based on the Stokesʹ law, directly comparable with the standard methods (pipette and hydrometer). The authors illustrate the possibility of employing the Micromeritics SediGraph 5210 device for analyzing soil particles up to 250 μm. This aim involves some modifications to the soil sample preparation along with a proper set up of the apparatus in order to assure the conformity with the Stokesʹ law. A data set of 180 Italian soils, distributed over the entire soil textural triangle, was analyzed by both the SediGraph and pipette methods. Two-thirds of samples were employed for the calibration phase while the remaining part was used for validation. A set of six multilinear regressions, one for each analyzed grain-size class, was developed to convert the SediGraph data into pipette-equivalent values. Regardless of the high significance level (p < 0.001) of all the regressions, the coefficient of determination was always larger than 0.87, with the only exception of very fine sand (50–100 μm) fraction (R2 = 0.64). No regressions were needed when SediGraph clay content is ≥ 68%; in such a case no conversion was required because SediGraph results match with the pipette data.