Evaluation of small site-specific N fertilization trials using uniformly shaped response curves

The framework was tested on a data-base of a two-year site-specific N fertilization trial in northern Germany. Three different types of N response curves were evaluated individually for each subarea: Linear plus plateau (LP), quadratic (Q) and quadratic plus plateau (QP). In a second step an alternative parameterization that includes a shape parameter was introduced. For each type of response curve, this shape parameter was set to a common value for all subareas within a field and year, thus reducing the number of parameters that need to be estimated. Results show that the reduction of parameters did not reduce the goodness of fit when describing yield response, since the yield response of each subarea still depends on different maximum yields and different N amounts to attain their maximum yield. It turned out that the models using a common shape parameter allowed to evaluate even subareas with separately indetermined parameter values. Furthermore, agronomically meaningful correlations of the estimated response curves to the empirically observed characteristics of their respective subareas were improved.