Near infrared reflectance spectroscopy for estimating soil characteristics valuable in the diagnosis of soil fertility

Soil fertility diagnostics rely not only upon measurement of available nutrients but also upon the ability of the soil to retain these nutrients. Near infrared (NIR) reflectance spectroscopy is a rapid and non-destructive analytical technique which allows the simultaneous estimation of standard soil characteristics and does not require the use of chemicals. Previous studies showed that NIR spectroscopy could be used in local contexts to predict soil properties. The main goal of our research was to build a methodological framework for the use of NIR spectroscopy on a more global scale. The specific goals of this study were (i) to identify the best spectral treatment and processing-LOCAL versus GLOBAL-regression methods, (ii) to compare the performance of NIR to standard chemical protocols and (iii) to evaluate the ability of NIR spectroscopy to predict soil total organic carbon (TOC), total nitrogen (TN), clay content and cationic exchange capacity (CEC) for a wide range of soil conditions. We scanned 1300 samples representative of the main soil types of Wallonia under crop, grassland or forest. Various sample preparations were tested prior to NIR measurement. The most appropriate options were selected according to analysis of variance and multiple means comparisons of the spectra principal components. Fifteen pre-treatments were applied to a calibration set and the prediction accuracy was evaluated for GLOBAL and LOCAL modified partial least square (MPLS) regression models. The LOCAL MPLS calibrations showed very encouraging results for all the characteristics investigated. On average, for crop soil samples, the prediction coefficient of variation (CVp) was close to 15% for TOC content, 7% for TN content and 10% for clay content and CEC. The comparisons of repeatability and reproducibility of both NIR and standard methods showed that NIR spectroscopy is as reliable as reference methods. Prediction accuracy and technique repeatability will allow the use of NIR spectroscopy within the framework of the soil fertility evaluation and its replacement of standard protocols. LOCAL MPLS can be applied within global datasets, such as the International global soil spectral library. However, the performance of LOCAL MPLS is linked to the number of similar spectra in the dataset and more standard measurements are needed to characterise the least widespread soils.