Autumn-sown sugarbeet (Beta vulgaris L.) responses (sugar yield, plant N-uptake and juice quality) were studied in relation to the residual NO3−-N in a soil of southwestern Spain which, for the previous five years (1989–93), had received high N rates, in accordance with conventional fertilization schedules used by farmers in the area. Three different combinations of fertilizers, supplying equal amounts of N, were used during the fertilization period (1989–93): a mineral fertilization treatment (MF, a complex 15N-15P2O5-15K2O) and two organo-mineral fertilization treatments (an olive mill wastewater sludge compost, AC, and a depotassified concentrated beet vinasse, V). All these treatments also received a top-dressing with urea (46% N). A control treatment (C), without fertilization was included for comparison.

During the major part of the beet growing season, the presence of almost four times as much mineral N in the 0·100 cm soil layer of previously fertilized plots (AC, V and MF) than in the unfertilized one (C), led to a significant increase (P<0·05) in total fresh weight yield and N-uptake, but also to a significant decrease (P<0·05) in sugar content and beet processing quality. The time course of NO3−-N concentration in sugarbeet petioles and the evolution of the nutritional state of leaf-blades gave advance information about the final response of the crop to the different fertilization treatments. Besides N, Na was the element which, due to the repeated and high fertilization rates applied, had a major effect in reducing the technological quality of the sugarbeet.