Modification, Calibration and Validation of APSIM to Suit Maize (Zeamays L.) Production System: A Case of Nkango Irrigation Scheme in Malawi

Nitrogen (N) is the most important nutrient in maize production and its availability can affect the production potential of maize. Availability of nitrogen in soil largely varies with place and time. Models are some of practical methods used to evaluate and monitor availability and impact of nitrogen on maize production; APSIM is one of such models. APSIM has several modules that have different functionalities and one of such modules is SoilWat module. The study modified SoilWat module by incorporating Nitrogen Distribution model. Trial and error method was used in the calibration of the nitrogen distribution model that was incorporated in the APSIM model as subroutine. The initial values of nitrogen distribution were obtained from literature and these values formed the basis for development of the model. After development of model using parameters obtained from literature review, field experiment was conducted to collect data to be used in redefining the model. The simulated nitrogen distribution was compared with values obtained from the field experiment and their mean differences were initially high but the process was repeated until the mean difference was small. In field experiment, the study had two factor, each with four regimes. The Triscan Sensor (EnviroScan, Sentek Pty Ltd, Stepney, Australia) was used to measure total nitrogen concentration at lateral distances and vertical depths. Primary soil samples were collected and analysed at Bunda College Laboratory. The study inferred that Soil water percolates down to underlying layer only when proceeded layers are satisfied i.e. has reached its field capacity, above which excess water is left free to percolates down the soil profile. Before water arriving in last layer it had to satisfy the above-lying soil profiles. The study has shown that increase of nitrogen contents in underlying layers corresponds with decrease of the same in top layers due to advection movement. Consequently, the increases of soil water in a specific layer correspond to decrease of nitrogen content in that particular layer. The study has shown that APSIM under predicted during the latter stage of the maize growing season and over predicted in the early stage of the growing season, and it overestimates soil water contents in soil profile.