Determination of optimum N-supply in sugar beet by different mathematical methods

Various mathematical models used to predict optimum N-supply (fertilizer-N + N-min) for maximum root yield, sugar yield and white sugar yield were compared by fitting data from 39 field trials in Northwest Germany (1980-1992). Quadratic, modified exponential and square root models resulted in higher estimates of optimum N-supply than did 2-regressions and plateau type spline models with differences as high as 130 kg N/ha between models. Residual variance was lower with the latter two models. Data were grouped at 40 kg/ha intervals of N-supply for analysis of variance and resulted in the lowest estimates for optimum N-supply. Based on the overall average, the optimum fertilizer-N dressings were 80 kg N/ha for root and sugar yield and 70 kg N/ha for white sugar yield while mineral soil-N in spring was 70 kg N/ha.