In tropical regions, organic materials are often more important than fertilizers in maintaining soil fertility, yet fertilizer recommendations and most crop models are unable to take account of the level and quality of organic inputs that farmers use. Computer simulation models, such as the Agricultural Production Systems Simulator (APSIM) developed by CSIRO and the Queensland Department of Primary Industries, have proven their value in many cropping environments. These proceedings report the results of an ACIAR-supported project to test and improve the capability of APSIM to predict the decomposition of various organic inputs, the dynamics of nitrogen and phosphorus in soil, and crop yields. They document the achievements of the project and show the benefits of linking laboratory, field and modeling studies. Another activity of the project was to train and support national collaborators in East and southern Africa in the use of APSIM for integrated, nutrient-management practices.