NITROGEN UPTAKE OF KOHLRABI, ESTIMATED BY GROWTH-STAGES AND AN EMPIRICAL GROWTH-MODEL

Some fertilizer recommendation systems for field vegetables are based on estimated nitrogen uptake curves, The estimation of nitrogen uptake should be as accurate as possible. Therefore 3 estimation methods and their accuracy (standard error of difference between measured and est imated values, sd) are compared. Comparisons were carried out with mea sured nitrogen uptake curves of kohlrabi (Brassica oleracea var. gongy lodes L.) from 16 field experiments. The use of observed growth stages resulted in the smallest estimation error (sd = 14 kg N.ha(-1)). This method is recommendable because it is accurate and easy to use. Estim ations by applying an empirical nitrogen uptake model (sd = 16 kg N.ha (-1)) are more complicated. The estimation based on time after plantin g is easy to use but showed the largest estimation error (sd = 28 kg N .ha(-1)).