A model of fertilizer response is outlined which makes a distinction between conventional fertilizer response curves, and the relationship between product yield and maintenance application of fertilizer. The derivation of optimal fertilizer rates for two enterprises on three soil types is used to illustrate the model. A simple rule-of-thumb, which can be used to avoid some computations, is also discussed. In the Australian context, the model has implications for the derivation of optimal super-phosphate rates, and also has important implications for the type of applied super-phosphate research which should be conducted in the future.