A Vis-NIR penetrometer system for field measurement of soil organic carbon

The evaluation of soil properties is labour-intensive and time-consuming. The cost of laboratory analyses thus limits the sampling density achievable in soil surveys. Consequently, in-situ estimation of soil properties such as soil organic carbon (SOC) is urgently needed. Visible and near-infrared (Vis-NIR) spectroscopy is a feasible alternative for field measurements; however, most applications focus on surface soils rather than the full soil profile.

This study integrates Vis–NIR spectrometers with a penetrometer-based sampling system to enable rapid acquisition of soil spectra along the soil profile, increasing both vertical and horizontal sampling density. The system consists of two low-cost Vis–NIR spectrometers covering the 350–2500 nm range, mounted on a penetrometer attached to a tractor-mounted hydraulic sampler. Soil spectra were acquired in situ at 2-cm depth intervals, and spectra from five consecutive depths were averaged to represent a 10-cm depth interval, corresponding to the thickness of core samples used as ground-truth data. The system allows spectral acquisition to depths of up to 100-cm.

Field measurements were conducted at 18 locations in NSW, Australia, and spectral data were calibrated using 130 soil samples collected from these sites. SOC was estimated from Vis–NIR spectra using a Cubist regression tree model with ten-fold cross-validation, resulting in an R² of 0.74 and a root mean squared error of 0.21%. These results demonstrate that SOC can be reliably estimated from field-acquired Vis–NIR spectra. The proposed system enables rapid spectral acquisition of soil profiles, supporting field-based SOC estimation with accuracies comparable to laboratory analyses.