Instrumental color determination of local soils and its variation with elemental profiles

Forensic soil comparison typically relies on qualitative color description through visual observation and side-by-side comparison with standard color chips from the Munsell Soil Color Charts to determine a color notation. Soil color distinctions are then made according to the US National Soil Survey color contrast classes. However, this approach is inherently subjective due to the variations in human examiner’s color perception, which is further influenced by factors such as light intensity, viewing angle, and background contrast. Conversely, color-measuring instruments like spectrophotometers can detect subtle color variations reliably and with greater precision, including colors falling between the chips in Munsell Soil Color Charts. Color differences between a soil pair can be expressed numerically as $\Delta E_{00}$ and a threshold based on $\Delta E_{00}$ to distinguish soil colors can be established for objective color comparison. In this study, the colors of soils collected from Singapore parks (soils sampled from areas within 1 m, within the same park, and across different parks) were measured quantitatively using a spectrophotometer. The occurrences of false positives and negatives were calculated and evaluated for the visual and instrumental color comparison methods. We further investigated correlations between the color discrimination threshold based on $\Delta E_{00}$ and a previous survey conducted on local soil’s elemental variability. The outcome of this study will help streamline soil analysis workflows in laboratories and strengthen the interpretation of forensic comparison of local soils.