Visible‐Near‐Infrared Spectroscopy Prediction of Soil Characteristics as Affected by Soil‐Water Content

Core Ideas The soil–water retention characteristics govern the water effect on soil vis‐NIR spectra. Soil clay, silt, sand content, and the water content at pF3 can be predicted with high degree of accuracy using spectroscopy at any field moisture condition. The OC and the derived properties of OC showed poorer prediction when the soil–water content exceeds field capacity (pF<2). Soil physical characteristics are important drivers for soil functions and productivity. Field applications of near‐infrared spectroscopy (NIRS) are already deployed for in situ mapping of soil characteristics and therefore, fast and precise in situ measurements of the basic soil physical characteristics are needed at any given water content. Visible‐near‐infrared spectroscopy (vis–NIRS) is a fast, low‐cost technology for determination of basic soil properties. However, the predictive ability of vis–NIRS may be affected by soil‐water content. This study was conducted to quantify the effects of six different soil‐water contents (full saturation, pF 1, pF 1.5, pF 2.5, pF 3, and air‐dry) on the vis–NIRS predictions of six soil physical properties: clay, silt, sand, water content at pF 3, organic carbon (OC), and the clay/OC ratio. The effect of soil‐water content on the vis–NIR spectra was also assessed. Seventy soil samples were collected from five sites in Denmark and Germany with clay and OC contents ranging from 0.116 to 0.459 and 0.009 to 0.024 kg kg‐¹, respectively. The soil rings were saturated and successively drained/dried to obtain different soil–water potentials at which they were measured with vis–NIRS. Partial least squares regression (PLSR) with leave‐one‐out cross‐validation was used for estimating the soil properties using vis–NIR spectra. Results showed that the effects of water on vis–NIR spectra were dependent on the soil–water retention characteristics. Contents of clay, silt, and sand, and the water content at pF 3 were well predicted at the different soil moisture levels. Predictions of OC and the clay/OC ratio were good at air‐dry soil condition, but markedly weaker in wet soils, especially at saturation, at pF 1 and pF 1.5. The results suggest that in situ measurements of spectroscopy are precise when soil‐water content is below field capacity.