NMR Fast Field Cycling Relaxometry of Unsaturated Soils

The bioavailability of water for plant nutrition in natural soils is controlled by the pore system structure and the interaction of water with the pore walls at variable degrees of saturation. For the characterization of these processes T (1) relaxometry is particularly suitable because it is not influenced by internal gradients and the frequency dependence of T (1) includes detailed information about the local dynamics at the pore walls. Using Fast Field Cycling Relaxometry, we have determined T (1) relaxation dispersion curves of unsaturated soil materials which cover a broad range of textures between pure sand and silt-loam. The mean relaxation rates scale inversely with the water content, as expected according to the Brownstein-Tarr model, which means that the effective pore volume is the only water-contributing fraction. By further analysis of the relaxation dispersion curves we find a bi-logarithmic behavior which is describable by a model of two-dimensional diffusion at the liquid-solid interface in the neighborhood of paramagnetic impurities at the surface. The microscopic wettability, as expressed by the ratio of surface residence time and correlation time, is identical for the soil material but decreases by a factor of two for the sand. This relaxation mechanism is unique for all textures and water contents and proves that the water mobility at the surface does not decrease even at the lowest water contents.