Accessible Soil Spectroscopy: Evaluating Low-Cost Vis–NIR Spectrometers for Resource-Constrained Environments

Abstract

Visible and near-infrared (Vis–NIR) spectroscopy provides a rapid approach to assess soil properties in situ, reducing the need for labour-intensive analyses of large-scale soil surveys. However, research-grade spectrometers may present financial and logistical challenges for widespread deployment, particularly in field applications. This study evaluated the performance of two emerging low-cost spectrometers from OtO Photonics: a visible-range spectrometer (HummingBird, HB) operating at 350–1020 nm and a NIR-range spectrometer (SideWinder, SW) operating at 900–2500 nm. These spectrometers cost less than half the price of the research-grade Vis–NIR spectrometers. A total of 386 soil samples from Eastern Australia were used in the study to build models for pH, total carbon, clay, sand, and cation exchange capacity. HB, SW, and two Vis–NIR spectrometers (AgriSpec from Malvern Panalytical and PSR+ from Spectral Evolution) were tested in this study. The spectra acquired by different spectrometers were characterized by identical shapes, and the features of organic matter and iron oxides were clear. When constructing models for soil properties, the combination of HB and SW achieved performance comparable to research-grade spectrometers. Together, HB and SW provided the best predictions for clay content (R2 = 0.82, RMSE = 6.93%) and sand (R2 = 0.72, RMSE = 8.62%). The variable importance in projection scores indicated that the models recognized the same features for HB, SW, and standard Vis–NIR spectrometers. Given their capacity to predict soil properties, the low-cost spectrometers were expected to undertake versatile tasks, such as mapping of soil organic carbon and high-resolution monitoring of soil conditions.