Optimal fertiliser application involves trading off between the costs of buying and spreading fertiliser and the benefits of increased production. This is complicated because fertiliser nutrients carry over in the soil from one year to the next, so that investment in fertiliser in the present contributes a stream of benefit through future years. We are developing a commercial decision support computer application, named Outlook(TM), to assist New Zealand farmers in planning their fertiliser expenditure on a 10-year time horizon. The biological basis for this software is a dynamic nutrient carryover model which predicts the changes in soil nutrient status from year to year under a given application of fertiliser. The original model considered only phosphate dynamics (Metherell et al 1995) and the economic optimisation of that model is described in Woodward (in press). This model is currently being updated to consider the coupled dynamics of both phosphorus and sulphur, the two most important nutrients in New Zealand pastoral farming.