Prediction of Soil Fertility Properties from a Globally Distributed Soil Mid‐Infrared Spectral Library

Abstract

Globally applicable calibrations to predict standard soil properties based on infrared spectra may increase the use of this reliable technique. The objective of this study was to evaluate the ability of mid‐infrared diffuse reflectance spectroscopy (4000–602 cm−1) to predict chemical and textural properties for a globally distributed soil spectral library. We scanned 971 soil samples selected from the International Soil Reference and Information Centre database. A high‐throughput diffuse reflectance accessory was used with optics that exclude specular reflectance as a potential source of error. Archived data on soil chemical and physical properties were calibrated to first derivative spectra using partial least‐squares regression. Good predictions for the spatially independent validation set were achieved for pH value, organic C content, and cation exchange capacity (CEC) (n = 291, r2 of linear regression of predicted against measured values ≥0.75 and ratio of standard deviation of measured values to root mean square error of prediction (RPD) ≥2.0). The root mean square errors of prediction (RMSEP) were 0.75 pH units, 9.1 g organic C kg−1 and 5.5 cmolc CEC kg−1 Satisfactory predictions (r2 = 0.65–0.75, RPD = 1.4–2.0) were obtained for exchangeable Mg concentration and clay content. The respective RMSEPs were 4.3 cmolc kg−1 and 126 g kg−1 Poorer predictions (r2 = 0.61 and 0.64) were achieved for sand and exchangeable Ca contents. Although RMSEP values are large relative to laboratory analytical errors, our results suggest a marked potential for the global spectral library as a tool for advice on land management, such as the classification of new samples into basic soil fertility classes based on organic C and clay contents, CEC, and pH. Further research is needed to test the stability of this global calibration on new data sets.