Issues of long term soil fertility decline and sustainability are becoming more important for cropping industries in Australia. Helping to manage the level of soil fertility in this context is an aim of economic response analysis. This paper reviews the theory and methods used by economists to derive the optimal level of an input to be used in a production process. In particular, response functions generated by a crop simulation model are used as a basis for the analysis. The use of such models is becoming widespread in the research and extension community. A variety of methods are presented, in increasing order of complexity, to account for the real world characteristics of the production environment in this context.