Chapter 20: Application of Fast Field-cycling NMR Relaxometry to Soil Material

Nuclear magnetic relaxation dispersion (NMRD) curves were determined by the fast field-cycling technique for different soil materials with textural compositions ranging from pure sand to silt loam. Special attention was paid to the investigation of the effect of desiccation on the steepness and shape of the dispersion curves to draw conclusions on possible changes in the local relaxation mechanism. The relaxation time spectra of the saturated soil materials reflect their textural composition. Pure sand has a slow, monomodal distribution, sandy loam is broad and multimodal and a clay-rich silt-loam has a moderately broad distribution, but comparably fast. With decreasing saturation, the shapes of the NMRD curves persist but become steeper inversely proportional to the water content. Finally, they were fitted by the biphasic exchange model of Korb et al., modified by a factor to describe the behaviour over a wide range of water contents with one set of only four parameters. For all soil materials, the surface affinity index, defined as the ratio of surface residence time to surface correlation time, remained constant from saturation down to water contents near the permanent wilting point. It is concluded that the surfaces are still completely wetted with a sufficiently large film thickness so that the water mobility is not restricted.