Characterizing soils via portable X-ray fluorescence spectrometer: 3. Soil reaction (pH)

Soil reaction (pH) is one of the most commonly measured soil characteristics. Traditionally, soil pH measurements have been made either colorimetrically or electrometrically. These approaches, while accurate, are destructive in nature and require sample modification to form soil pastes or extracts. The use of portable X-ray fluorescence (PXRF) spectrometry for pH determination was investigated using elemental data as a proxy for soil pH. Two datasets representing a wide variety of soil pH (4.17-8.70) were first evaluated via PXRF followed by standard laboratory techniques. Datasets were divided into modeling and validation datasets. Simple and multiple linear regressions were used to develop models associating both pure elemental data from PXRF as well as PXRF elemental data with auxiliary input data (clay content, sand content, organic matter content). Multiple linear regression with auxiliary input data provided the best predictive model (R²: 0.825; RMSE: 0.541); multiple linear regression with pure PXRF elemental data provided a quality model (R²: 0.772; RMSE: 0.685); simple linear regression was ineffective as producing significant model predictions. Validation via correlation analysis confirmed the significance of the developed models. Summarily, PXRF shows considerable promise in providing rapid pH predictions in soils with reasonable accuracy, especially for unusual circumstances where non-destructive sampling is required.