Computer Simulation of Soil Processes Relevant to Irrigation and Fertilizer Management

Efficient use of water and nitrogen fertilizer depends on the method of application and also on the soil processes governing the transformations of nitrogen and its movement with water. Studies of these dynamic processes under various water and fertilizer management practices can greatly help in improving water and fertilizer utilization efficiency. A mathematical model was developed to describe the distributions of water, urea, NH4, and N03 in soil as functions of soil depth and time. The model assumes one-dimensional transient water flow, convective-dispersive transport of solute, first order transformations of nitrogen, linear adsorption of urea and NH4, and plant uptake of NH4 and NO3. The solute dispersion coefficient in a Moloki soil under unsaturated field conditions was determined on an in-situ column of soil with steady flow maintained at the desired flux by a porous gypsum-sand layer. Nitrate movement obeyed very closely the convective-dispersive transport equation. A computer program was developed to determine the optimum value of the dispersion coefficient from experimental data. Nitrogen transformation data, both from published reports and from laboratory experiments conducted during the course of this study, were examined to assess the applicability of first-order kinetics to describe mathematically the processes of urea hydrolysis and nitrification. In general, the first-order model was acceptable, but inclusion of a lag phase in the model improved computed results in some cases. A least-squares optimization technique was programmed for the computer to obtain kinetic coefficients for urea hydrolysis and nitrification from the experimental data. A numerical computer simulation model was developed which utilizes the explicit-implicit finite difference approximations of the dimensionless form of the partial differential equations. The model was used to conduct simulation experiments designed to evaluate the effects of various system parameters and management variables on the efficiency of water and nitrogen fertilizer utilization. It was found that under intermittent irrigation the fluctuations in soil water contents damped out some distance below the soil surface. This distance increased with decrease in the frequency of irrigation. Under high-frequency irrigation the waterholding capacity of the soil was found to be unimportant in water management. However, the waterholding capacity proved to be an important consideration in efficient fertilizer management. Urea hydrolysis, nitrification, and NH4 adsorption all were found to be important factors in determining; the distributions of various N species in the soil profile, leaching losses, and plant uptake of N. But the movement of water played a more important role than any other process in determining the fate of N in the plant root zone. The crop rooting depth and the value of the solute dispersion coefficient had very little impact on fertilizer management. Thus, the computer simulation model was shown to be a powerful tool in evaluating various water and fertilizer management practices