Wet or dry? The effect of sample characteristics on the determination of soil properties by near infrared spectroscopy

Most of the published articles related to the application of near infrared (NIR) spectroscopy for the quantitative analysis of soils parameters, have reported experiments or studies that are largely laboratory based where soil properties such as soil organic carbon (SOC) content are assessed on dry and sieved samples. The recent arrival of hand-held and portable analytical equipment allowing for the quantitative analysis of samples in situ have provided new possibilities for the analysis of soils. The implementation of this type of instrumentation will result in reduced time and cost of analysis, and will have a profound effect on the associated method steps (e.g. storing, drying, and sieving) currently used when soils are analysed in the laboratory. The aim of this short review is to highlight the gaps in the research related to the analysis of soil samples by NIR spectroscopy with high moisture (HM) content. Inconsistent results were found in the scientific literature in relation with the ability of NIR spectroscopy to measure soil chemical properties with HM content. The use of pre-processing methods did not improve the calibrations nor the selection of samples with different moisture levels. The re-wetting (hydration) of the sample and the conversion of units into volumetric ones led to optimism for the measurement of in field samples by NIR spectroscopy. However, in field applications are still in their infancy.