Nonlinear models in the comparison of nitrogen fertilizers applied to coffee

ABSTRACT Nitrogen (N) is essential to the fertilization of coffee. The fertilizer most used to meet this demand is urea, although it has high rates of N loss due mainly to ammonia volatilization. To reduce this loss, fertilizers with increased efficiency have emerged. Thus, the aim was to select the most appropriate nonlinear regression model to describe N loss attributable to ammonia volatilization in slow- and controlled-release fertilizers applied to coffee plants and to compare the different fertilizers based on the parameters of the selected model. The data studied are controlled-release fertilizers: urea + sulfur + polymer (U+S+P), urea + plastic resin (U+PR), urea + polymer insoluble in water (U+PIW) and slow-release fertilizer: urea formaldehyde (UF) applied to coffee. The Gompertz, Brody, and von Bertalanffy Logistics models were fit by the least squares method. The goodness of fit was assessed using the adjusted coefficient of determination, mean absolute deviation, and Akaike's information criterion. The von Bertalanffy model was the most appropriate for describing the data in most cases. After selecting the best model for the means, the parameters of the von Bertalanffy model were estimated again for each repetition of treatments. With the repetitions of the parameter estimates of these models, the F test of analysis of variance (ANOVA) and Tukey's test were applied. In the F test, p < 0.05 for all parameters. In the Tukey's test, the UF fertilizer reached the asymptote more quickly and presented a lower accumulated loss of N in the coffee tree. The fertilizers U+S+P and U+PIW have later inflection points (IP).