Classification of Vineyard Soils Using Portable and Benchtop Near-Infrared Spectrometers: A Comparative Study

Soils characterization is often accomplished by means of extensive field observations followed by laboratory analysis, which is extremely time consuming and can be prohibitively expensive. Trying to address this issue, there is growing interest in using near-infrared (NIR) spectroscopy as a rapid and cost-effective tool for the prediction of a soil's physical, biological, and chemical properties. This method is nondestructive and provides spectra highly characteristic of soil properties and composition, enabling the analysis of many soil properties with a single measurement. The purpose of this study was to compare the quality of in situ measurements between a dispersive NIR (portable) spectrometer with a benchtop Fourier-transform NIR instrument (laboratory), thus investigating the potential of NIRS as a rapid and low-cost technique to map vineyard soils both in the field and in the laboratory. Soil samples collected from different areas of a vineyard in the center of Portugal with fully characterized soils were analyzed by NIRS, and the spectra were modeled by principal component analysis and partial least squares discriminant analysis. Both instruments proved to be able to differentiate the analyzed soil types. When samples were collected from nearby locations (e.g., within the same vineyard block), 75 to 100% successful soil identification rates were achieved depending on the soil type. Lower prediction percentages (around 70-75%) were obtained when soils from the entire vineyard were analyzed simultaneously. Results obtained with the portable instrument were, to some degree unexpectedly, equivalent to those obtained with the laboratory instrument.