Direct soil gypsum quantification via portable x-ray fluorescence spectrometry

Soil scientists have long struggled with an effective method for quantifying gypsum content in soils. Several methods exist, but each is fraught with problems. Recent enhancements to portable X-ray fluorescence (PXRF) spectrometers have provided better detection limits especially for lighter elements such as S, a key component of gypsum. Thus, this research aimed to test the effectiveness of PXRF as a means of directly quantifying gypsum in soils. A total of 102 soil samples containing a wide variety of gypsum (~2-95%) were subjected to both traditional laboratory analysis (thermogravimetry) and elemental analysis via PXRF. Simple linear regression and multiple linear regression were used to establish the relationship between the two datasets. Log transformation of some datasets was necessary to normalize the data. Using simple linear regression for Ca, laboratory data and PXRF Ca data produced an R₂ of 0.8794. Similarly, simple linear regression for laboratory-quantified gypsum vs. PXRF S produced an R₂ of 0.9120. Multiple linear regression of laboratory quantified gypsum vs. both PXRF S and Ca produced an R₂ of 0.9127. No significant differences were observed between model generation and validation datasets. Overall, PXRF shows great promise for the direct quantification of gypsum in soils.