Mid- Versus Near-Infrared Spectroscopy for On-Site Analysis of Soil

Abstract

Research has demonstrated that for the determination of soil carbon, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is often more accurate and produces more robust calibrations than near-infrared (NIR) reflectance spectroscopy (NIRS) when analysing ground, dry soils. But DRIFTS is not considered feasible on non-dried samples due to the strong water absorptions in the mid-infrared. Also, mid-infrared spectrometers are generally either sealed from the ambient atmosphere or purged with CO2-free dry air. While DRIFTS has been shown to be advantageous in the laboratory, if samples need to be ground and dried and instruments purged, it may not be practical for on-site analysis. The objective of this research was to determine the effect of ambient atmospheric conditions and soil state (ground, dried, etc.) on DRIFTS and NIRS calibrations for soil C and N. Results using a portable DRIFTS spectrometer over a wide range of ambient conditions demonstrated that purging is not necessary to obtain calibrations for C in soils equal to those obtained in the lab with dry or dried–ground samples. However, efforts with field-moist samples have demonstrated that while calibrations using DRIFTS can be developed with an accuracy equal to those obtained using NIRS, the calibrations may not be as robust and more research is needed. Comparing these results with results obtained using large-scale scanning such as remote imaging or in situ NIR (scanning on the fly with a tractor-drawn NIR unit) indicates that soil analysis in which the sample is analysed by both traditional analysis (combustion) and spectroscopic analysis as performed here results in better calibrations. However, a much smaller area of the total field is sampled. An important question that emerges is Which is more valuable, lots of lower quality information or much less information, but of perhaps much higher quality?