Assessment of fertilizer efficacy utilizing the substitution rate methodology

The substitution rate method describes the relative effectiveness of fertilizers according to the amount of nutrient required to produce a given crop yield. The two main reasons in which this method has had restricted use are: a) the model needs to be solved iteratively, thus, it is necessary an optimization program, and b) methods considering only a single point from response curves, such as the agronomic efficiency index, have been adopted for the sake of simplicity. However, these simple measures cause loss of data, and the interpretation of results may be impaired. We compared goodness-of fit measures from a published work with those generated by an optimization spreadsheet developed in Excel. We proposed changes in the equation from the mentioned work that rendered better goodness of fit. The Excel spreadsheet, with embedded statistical decision support is made available for download to encourage the testing of additional fertilizers by using the substitution rate method.