Defining soil sample preparation requirements for MIR spectroscopic analysis using principal components

Diffuse reflectance MIR (mid-infrared) spectroscopy when combined with PLS (partial lest squares) statistical analyses can be used to quantify various soil chemical properties. Of particular interest is the application of MIR/PLS analysis to provide a rapid and cost-effective methodology for predicting soil organic carbon content and its allocation to component fractions for use in soil carbon scenario modelling. The technique applies a beam of approximately 3.14 mm 2 (2 mm diameter circle) to the surface of a powdered soil sample to define chemical composition. Given the small area sampled, soil sample homogeneity and particle size become important issues to examine when interpreting the spectral results obtained. MIR analysis was completed on three different soils with variable texture that had been ground for durations ranging from 0 – 180s. Four aliquots of four replicate samples prepared for each soil by grinding time combination were analysed. Principal components analysis (PCA) was applied to the acquired MIR spectra to define to spectral variability and PLS analysis of the spectra was used to quantify the variability in predicted values for soil carbon and carbon fraction contents. PCA provided an excellent means of assessing the impact of grinding on the spectral homogeneity for each of the soils examined. The sandy soil required significantly more grinding time to reduce the variability of acquired MIR spectra. However the variability in predicted organic carbon and carbon fraction contents stabilised at shorter grinding times.