Digital RGB photography and visible-range spectroscopy for soil composition analysis

Abstract We demonstrate calibration models developed for a number of soil variables, based on colour information derived from standard digital photography and a prototype visible-wavelength spectroscopy device. The samples used were taken from the National Soils Inventory of Scotland and included topsoil samples taken either from the upper horizon or bulked from the top 20 cm. A total of 31 variables were investigated, including organic matter content (Loss On Ignition), carbon, nitrogen, pH, sand/silt/clay and a number of elemental concentrations. Model calibrations were developed using visible-wavelength colour information alone and in combination with spatial covariates (topography, climate, soil, vegetation, parent material). A set of thresholds for characterising quality of model calibration was defined using r-squared value, with Good (> 0.8), Fair (0.6–0.8), Moderate (0.4–0.6) and Poor (