Determination of quantitative pore-size distribution of soils with 1H NMR relaxometry

The determination of pore-size distribution (PSD) in soil by conventional methods can be time- and effort-consuming. Therefore, several attempts have been made recently to assess and implement proton nuclear magnetic resonance relaxometry (H-1 NMR relaxometry) to determine PSDs quantitatively. The H-1 NMR relaxometry promises not only to be more time- and effort-saving but also offers a broader range for the estimation of pore size, better resolved results and non-destructive measurements. To evaluate these possibilities, we tested the extent to which a single calibration curve for the estimation of surface relaxivity of soil, derived from seven soil samples, can be applied to unknown soils. For validation, the quantitative PSDs of seven additional soil samples were determined from the distributions of NMR relaxation times of water saturated samples using this calibration curve; they were compared with their classical water retention curves (WRCs). In general, the quantitative PSDs determined by this approach described the classical WRCs quite well. The best results were obtained for sandy, silty and loam soils. The largest differences were for soils with large clay (47%) and SOM contents (10 and 21%). Our results suggest that these differences derive from swelling processes.