Estimating molecular diffusion coefficients of urea in unsaturated soil

A correct value for the molecular diffusion coefficient of urea in soil (Ds) is required to accurately predict urea movement in soil by molecular diffusion. In previous work, to estimate Ds in a simulation model of urea diffusion, we used an empirical equation of Papendick and Campbell that describes Ds for a dissolved species in soil to be a product of a tortuosity factor squared, the species diffusion coefficient in water (Dw), and the volumetric water content. Comparisons of measured and computed urea concentration with depth indicated that this equation was not adequately general over a wide range of soils. The objective of this study was to modify the parameters in the equation and, if necessary, develop a new relationship to estimate the value of Ds in soils. Laboratory studies were conducted on seven soils in which the clay content ranged from 10 to 51%. Urea concentrations with depth at 48 h following surface-application were measured and also computed using numerical techniques with an initial estimate for Ds instead of computing it using Papendick and Campbells̀ equation. The Ds was modified incrementally, until the difference between computed and measured concentrations was minimized. In all seven soils, good agreement was obtained between measured and computed urea concentrations with depth. The maximum depth of urea movement occurred in Kahola soil (approx. 3.5-cm deep), whereas least movement occurred in Crete soil (approx. 2.6-cm deep). Nonlinear regression analysis gave a better relationship (Ds=0.18 X Dw (volumetric soil water content/porosity)2.98, R2=0.88) when relative water content (volumetric soil water content/porosity) of the seven soils was substituted for the volumetric water content (Ds=0.73 X Dw (volumetric soil water content)2.58, R2=0.66) in Papendick and Campbell's equation.