NITROGEN UPTAKE BY AUTUMN SOWN SUGAR-BEET

The influence of N fertilizer rate on uptake and distribution of N in the plant, N-15 labelled fertilizer uptake and sugar yield were studie d for 3 years on autumn sown sugar beet (Beta vulgaris L.) under Medit erranean (Southern Spain) rain-fed and irrigated conditions. Available average soil N prior to sowing was 69 kg N ha-1, and net mineralisati on in the soil during the growth period was 130 kg N ha-1. Maximum N u ptake occurred in the spring and increased with increasing fertilizer rates in the irrigated crop. There was no increase in N uptake in the sugar beet cropped under rain-fed conditions because of water shortage . Maximum average N uptake both by roots and tops was between 200 and 250 kg N ha-1. When N fertilizer was not applied, average uptake from the soil was between 130 and 140 kg N ha-1. At the end of the growth p eriod there was a marked translocation of N from the leaves to the roo t which increased with the N fertilizer rate. The N ratio top/roots at harvest was 0.45-0.5 and 0.8- - 1 in the irrigated and rain-fed sugar beet, respectively. Maximum N-15 labelled fertilizer uptake took plac e in May-June, being larger in irrigated sugar beet or when spring rai nfall was more abundant. Fertilizer use efficiency varied between 30% and 68%. Sugar yield response to N fertilizer rates depended on the N available in the soil and on the total water input to the crop, partic ularly in spring. The response was more constant in the irrigated crop , where optimum yield was obtained with a fertilizer rate of 160 kg N ha-1. In the rain-fed crop, the optimum dose proved more erratic, with an estimated mean of 100 kg N ha-1. The amount of N required to produ ce 1 t of root and of sugar ranged between 1.5 and 3.8 kg N and betwee n 11.1 and 22.4 kg N respectively, and varied according to the N ferti lizer rates applied.