Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title><jats:p>Urea fertilizer is often applied at the soil surface, where it hydrolyzes and can form NH<jats:sub>3</jats:sub>. To quantify the volatilization of NH<jats:sub>3</jats:sub>, the molecular diffusion of urea into the soil must be described. The diffusion coefficient of urea in soil is related to its diffusion coefficient in water, which varies with temperature. We initially regressed the value of the urea diffusion coefficient in water from the international critical tables (ICT) on temperature for the range of 10 to 20 °C. Since surface soil temperatures often fall outside this range, additional values for the urea diffusion coefficients were needed. The capillary tube method of Phillips and Ellis was used to measure the diffusion coefficient of urea in water at temperatures ranging from 0 to 50 °C. The laboratory values obtained for the diffusion coefficient of urea in water were higher, by a factor of 1.5, than the ICT values. A comparison was made between measured values of urea concentration in columns of Richfield and Haynie soils and simulated values using regression equations based on laboratory results (new) and ICT data (old). The new regression equation allowed a better agreement between actual and simulated urea concentrations in soil.</jats:p>

<jats:title>Abstract</jats:title><jats:p>A correct value for the molecular diffusion coefficient of urea in soil (<jats:italic>D<jats:sub>s</jats:sub></jats:italic>) is required to accurately predict urea movement in soil by molecular diffusion. In previous work, to estimate <jats:italic>D<jats:sub>s</jats:sub></jats:italic> in a simulation model of urea diffusion, we used an empirical equation of Papendick and Campbell that describes <jats:italic>D<jats:sub>s</jats:sub></jats:italic> for a dissolved species in soil to be a product of a tortuosity factor squared, the species diffusion coefficient in water (<jats:italic>D<jats:sub>w</jats:sub></jats:italic>), 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 <jats:italic>D<jats:sub>s</jats:sub></jats:italic> 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 <jats:italic>D<jats:sub>s</jats:sub></jats:italic> instead of computing it using Papendick and Campbell's equation. The <jats:italic>D<jats:sub>s</jats:sub></jats:italic> 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 (<jats:italic>D<jats:sub>s</jats:sub></jats:italic> = 0.18 × <jats:italic>D<jats:sub>w</jats:sub></jats:italic> (Θ<jats:sub><jats:italic>v</jats:italic></jats:sub>/porosity)<jats:sup>2.98</jats:sup>, <jats:italic>R</jats:italic><jats:sup>2</jats:sup> = 0.88) when relative water content (Θ<jats:sub><jats:italic>v</jats:italic></jats:sub>/porosity) of the seven soils was substituted for the volumetric water content (<jats:italic>D<jats:sub>s</jats:sub></jats:italic> = 0.73 × <jats:italic>D<jats:sub>w</jats:sub></jats:italic> (Θ<jats:sub><jats:italic>v</jats:italic></jats:sub>)<jats:sup>2.58</jats:sup>, <jats:italic>R</jats:italic><jats:sup>2</jats:sup> = 0.66) in Papendick and Campbell's equation.</jats:p>