Evaluation of a miniaturized portable NIR spectrometer for the prediction of soil properties in Mediterranean central Chile

Abstract:

Near/infrared (NIR) spectrometry to estimate soil properties has been evolving toward more portable and accessible equipment in recent years for the prediction of indicators of soil fertility in agroecosystems. There were two objectives of this study. First, to calibrate and validate a miniaturized portable NIR spectrometer NeoSpectra/Module 2.5 with a limited spectral range (1350-2500 nm) for the prediction of total nitrogen (TN), soil organic carbon (SOC), soil pH, clay content and exchangeable cations (Ca2+, Mg2+, K+ and Na+) in dry conditions. And second, to evaluate the use of an external orthogonalization parameter (EPO) to reduce the effect of soil moisture in samples with field moisture conditions collected from Mediterranean central Chile (33° SL to 36° SL). Soil classified as Mollisols, Alfisols and Inceptisols was sampled (n = 170) from 0 to 30 cm depth and scanned with the NeoSpectra spectrometer. The spectrometer showed good performance (R2 > 0.5 and/or RPIQ>1.4) for predictions of SOC and exchangeable cations. When the spectra data without transformation was used (raw spectra) versus transformed spectra, better performance in the predictions was obtained for TN, SOC, pH, clay content and Ca2+. The cubist model (CM) showed better performance than partial least squares (PLSR) for most assessed soil properties. The EPO procedure allowed us to decrease the error due to soil moisture in the prediction of some soil properties. Although the use of a miniaturized portable NIR spectrometer showed promising results for prediction of soil properties, future testing should increase the number of soil samples, vary the spectral data processing, and/or test new miniaturized portable NIR spectrometers with a wider spectral range.