Quantitative analysis of soil chemical properties with diffuse reflectance spectrometry and partial least-square regression: A feasibility study

Soil chemical properties from different locations in the Trier region, Rhineland-Palatinate, SW Germany were evaluated using VIS/NIR reflectance spectrometry (ASD FieldSpec-II spectrometer, 0.4-2.5 m) and partial least-square regression (PLS). Generally, laboratory spectrometry performed better than field spectrometry probably due to strong interferences of soil structure. In a plot experiment reliable estimations were obtained for total amounts of Ca, Mg, Fe, Mn and K but not for organic carbon and nitrogen. In the landscape-scale context the estimations for organic carbon could be significantly improved but it was also concluded that the development of statistical prediction models is limited to geologically homogeneous areas. In both experiments CAL extractable nutrients could not be satisfactorily estimated. This excludes diffuse VIS/NIR spectrometry as a diagnosis tool of short- or medium-term changes of the soil's nutrient status. However, the method can be used as a quick screening method in questions where the spatial distribution of organic carbon and total metal contents is addressed, as in soil development and soil degradation monitoring, and when time or laboratory costs are critical factors.