MARKETABLE AND NUTRITIONAL QUALITY OF LEEKS AS AFFECTED BY WATER AND NITROGEN SUPPLY AND PLANT-AGE AT HARVEST

The marketable and nutritional quality of leeks as affected by nitroge n fertilisation, water supply, and plant age at harvest was investigat ed during 1991 and 1992. The yield of marketable leeks increased with increasing nitrogen supply, although only small differences were obser ved between 160 and 280 kg total N ha(-1). The total nitrogen supply i ncludes soil mineral nitrogen within the 0-50 cm depth. The yield was lower when leeks were grown with less frequent irrigation, ie irrigati on at a soil water potential of -0.09 MPa at 20 cm depth. On average, the yield of leeks increased until harvest in the middle of October, 2 00 days after sowing. The concentration of nitrate increased and the c oncentrations of dry matter and potassium decreased with increased nit rogen supply. Leeks grown with less frequent irrigation showed increas ed concentrations of dietary fibre, vitamin C, nitrate, protein, calci um, magnesium, and manganese. The nutritional quality of leeks improve d with increased plant age at harvest because the concentration of pot assium increased and the concentration of nitrate decreased.