Changes in soil-water content are known to affect soil reflectance. Even though the phenomenon of increased forward scattering due to the presence of water in the soil was long ago suggested to be related to water film thickness and matric potential, no studies have investigated this in detail. We assessed the effects of moisture conditions on the visible near-infrared (vis-NIR) spectra of four Danish soils as a function of both water film thickness (expressed as number of molecular layers) and matric potential. Complete water retention curves, from wet (pF 0.3, pF = log(|φ|), where φ is the matric potential in cm) to the hyper dry end (oven and freeze-dry soil), were obtained by initial wetting followed by successive draining, and drying of soil samples, while performing NIR measurements at each step.

Soil reflectance was found to decrease systematically, yet not proportionally, to decreasing matric potential and increasing molecular layers. The changes in molecular layers were best captured by soil reflectance of the clay-rich soils. Here the largest increase in reflectance occurred between pF 3 and 4, caused by the shift from capillary to adsorptive surface forces. Supporting this, the smallest changes in reflectance were seen for the sandiest soil. Freeze-drying the soil highest in organic carbon increased reflectance, possibly due to alteration of organic matter during freezing. The different reflectance behavior for the higher-organic soil may be linked to differences in both the amount, the quality (higher hydrophobicity), and the state (more free, non-clay-complexed) of the organic matter. This, however, needs to be confirmed with further studies.