Universal quadratic soil spectral reflectance line and its deviation patterns' relationships with chemical and textural properties: A global data base analysis

Quantitative evidence for the fundamental spectral shape similarity existing between different soils is presented utilizing Pearson's correlation and SAM (Spectral Angle Mapper) analysis in-between signatures representing thousands of soil samples collected from different environments around the world. A universal quadratic soil line (UQSL) was found to highly fit these representative signatures as well as spectra from the ISRIC (International Soil Reference and Information Center) global database including soil samples collected from 58 countries. K-means clustering of ISRIC spectral reflectance deviations from the UQSL facilitated identification of 5 characteristic deviation patterns. Significant correlation was found for a set of 1000 soil samples between changes in multiple chemical properties and corresponding reflectance changes between these clusters. Future availability of large global soil data bases may allow improved determination of the UQSL and generalization of its deviation patterns relationships with PBC (Physical, Biological and Chemical) properties for wide areas monitoring of soil conditions.