Rubidium measured by XRF as a predictor of soil particle size in limestone and siliceous parent materials

Purpose: Information about particle size distribution (PSD) and soil texture is essential for understanding soil drainage,porosity, nutrient availability, and trafficability. The sieve-pipette/gravimetric method traditionally used for particle sizeanalysis is labour-intensive and resource-intensive. X-ray fluorescence (XRF) spectrometry may provide a rapid alternative.The study’s aim was to examine the use of XRF for rapid determination of PSD in Irish soils.
Methods: Soils (n = 355) from existing archives in Ireland were analysed with a benchtop energy-dispersive XRF (EDXRF).Correlation and regression analyses were determined to compare Rb, Fe, Al, and Si concentrations to % clay, % silt, and% sand. Also, linear regression models were developed to compare % clay, % sand, and % silt measured by the gravimetricmethod to values predicted by EDXRF.
Results: The relationship between element concentration and PSD was dependent on parent material. Rb, Al, and Fe showeda significant (p < 0.05) correlation (r > 0.50) with % clay and % sand in soils derived from limestone and siliceous stone parentmaterials. Rb was the best predictor for % clay (R2 = 0.49, RMSE = 10.20) in soils derived from limestone and siliceousstone-derived soils.
Conclusion: Geochemistry and clay mineralogy of the soils’ parent material strongly influenced the EDXRF’s ability topredict particle size. The EDXRF could predict % clay in soils from parent materials which weather easily, but the oppositewas true for soils with parent material recalcitrant to weathering. In conclusion, this study has shown that the EDXRF canscreen % clay in soils derived from limestone and siliceous stone parent materials.