Soil Water Measurement by a Low‐resolution Nuclear Magnetic Resonance Technique

Summary A low‐resolution nuclear magnetic resonance analyser was used to measure soil water. The physical principles of the method are outlined to enable the non‐linearity of the response to water to be discussed. Two sands, five soils, and vermiculite were tested in the Laboratory. The instrument readings for all were depressed below the values that might have been expected from absolute water contents. In sands the depression increases with water content, that is, as pores of progressively larger size fill with water. In clay soils an initial depression is attributable to the rigidity with which the first small amounts of water are adsorbed on to the clay lattice: with more water the depression, as in sand, reflects the size distribution of the macropores.‘Relaxation’, a condition needed to ensure accuracy, can be induced by adding iron compounds and salts, but there is evidence that where these substances are present naturally in the soil they may cause different degrees of relaxation from soil to soil. When many soil samples of known type, structural state, and chemical status have to be analysed, a good calibration curve can be obtained and used to give water contents to a fraction of a per cent. A moisture characteristic was measured on a sand to assess the ability of the instrument to scan moisture profiles in long columns.