On the soil information content of visible-near infrared reflectance spectra

We describe the information content of soil visible-near infrared (vis-NIR) reflectance spectra and map their spatial distribution across Australia. The spectra of 4030 surface soil samples from across the country were measured with a vis-NIR spectrometer in the range 350 to 2500 nm. The spectra were compressed by a principal component analysis (PCA) and the resulting scores were mapped by ordinary point kriging. The largely dominant and common feature in the maps was the difference between the more expansive, older and more weathered landscapes in the centre and west of Australia and the generally younger, more complex landscapes in the east. A surface soil class map derived from the clustering of the principal components was similar to an existing soil classification map. We show that vis-NIR reflectance spectra (i) provide a rapid and efficient integrative measure of the composition of the soil, (ii) can replace the use of traditional soil properties to describe the soil and make pedological interpretations of its spatial distribution and (iii) can be used to classify soil objectively.