Penetrometer-mounted VisNIR spectroscopy: Application of EPO-PLS to in situ VisNIR spectra

Visible near-infrared spectroscopy (VisNIR) has been used to measure many soil properties. Typically, VisNIR is used on air-dried and ground soils in the laboratory. Recent developments in VisNIR instrumentation have allowed for the collection of VisNIR spectra from in situ soils. In this study, we demonstrate the viability of an in situ VisNIR system. VisNIR spectra were collected using a penetrometer-mounted VisNIR probe. The penetrometer-mounted VisNIR system requires no field calibration or pre-drilling of access holes. This system has several advantages in that it: 1) allows for measurement of soil properties without sample collection, preparation, and laboratory analysis and 2) can provide rapid soil measurement at high-depth-resolutions (2 cm). We applied an external parameter orthogonalization (EPO) to the in situ spectra to remove the effects of soil moisture and other in situ effects from the spectra. We calibrated partial least-squares (PLS) models using spectra from an existing library of air-dried and ground spectra. PLS models were then used to predict clay content of the EPO-transformed in situ spectra. Model results showed good predictive ability for in situ spectra with RMSE, bias, and R² of 88 g kg^− 1, − 15 g kg^− 1, and 0.76, respectively. A site-wise hold–out validation of EPO calibration demonstrated that EPO calibrations were robust to changes in soil characteristics and parent materials between study areas. These results show that by using the EPO-PLS method, in situ VisNIR is a viable tool for rapid, minimally invasive collection of soil data.