Determination of soil pore size distribution and water retention curve by internal magnetic field modulation at low field 1H NMR

The determination of the soil pore size distribution, water retention curve, and derived parameters that control important processes in soils, such as water supply for plants; infiltration; water and solute movement in soils; erosion; plant nutrients and contaminants transport, etc, are challenging and the available methods are expensive, time-consuming and prone to bias and errors. The use of 1H Nuclear Magnetic Resonance (NMR) relaxometry to characterise the soil porosity and hydraulic properties through spin-lattice and spin-spin relaxometry results in an ill-posed problem with two correlated unknown quantities: the pore length scales, and surface relaxivity. To overcome this limitation of NMR relaxometry, we propose the use of a method that directly accesses the NMR diffusion modes governed only by the pore size, and therefore, independent of the unknown surface relaxivity. The manuscript describes an unprecedent application in Soil Science of the Decay due to Diffusion in Internal Field (DDIF) method to successfully determine the pore size distribution of undisturbed soil samples, as well as to estimate the water retention curves from the pore size distribution.