Evaluation of 1H NMR relaxometry for the assessment of pore‐size distribution in soil samples

Abstract

Summary 1H NMR relaxometry is used in earth science as a non‐destructive and time‐saving method to determine pore size distributions (PSD) in porous media with pore sizes ranging from nm to mm. This is a broader range than generally reported for results from X‐ray computed tomography (X‐ray CT) scanning, which is a slower method. For successful application of 1H NMR relaxometry in soil science, it is necessary to compare PSD results with those determined from conventional methods. The PSD of six disturbed soil samples with various textures and soil organic matter (SOM) content were determined by conventional soil water retention at matric potentials between −3 and −390 kPa (pF 1.5–3.6). These PSD were compared with those estimated from transverse relaxation time (T2) distributions of water in soil samples at pF 1.5 using two different approaches. In the first, pore sizes were estimated using a mean surface relaxivity of each soil sample determined from the specific surface area. In the second and new approach, two surface relaxivities for each soil sample, determined from the T2 distributions of the soil samples at different matric potentials, were used. The T2 distributions of water in the samples changed with increasing soil matric potential and consisted of two peaks at pF 1.5 and one at pF 3.6. The shape of the T2 distributions at pF 1.5 was strongly affected by soil texture and SOM content (R2 = 0.51 − 0.95). The second approach (R2 = 0.98) resulted in good consistency between PSD, determined by soil water retention, and 1H NMR relaxometry, whereas the first approach resulted in poor consistency. Pore sizes calculated from the NMR data ranged from 100 μm to 10 nm. Therefore, the new approach allows 1H NMR relaxometry to be applied for the determination of PSD in soil samples and for studying swelling of SOM and clay and its effects on pore size in a fast and non‐destructive way. This is not, or only partly, possible by conventional soil water retention or X‐ray CT.