Nitrogen fertilizer and the efficiency of the sugar beet crop in late summer

This study investigated whether nitrogen (N) fertilizer is necessary to maintain the efficiency of sugar beet foliage in late summer or whether the crop can continue to operate effectively on N mineralized from the soil. Three field experiments were carried out at Broom's Barn in 2000, 2001 and 2002. The treatments were early doses of N fertilizer (0, 80 or 160 kg N ha−1) without and with a late N fertilizer application (60 kg N ha−1) made as soon as the foliage reached 85% cover of the ground. The late N fertilizer dose increased the N concentration in the plants and canopy size in late summer, but canopy size still declined throughout autumn. The late N application increased chlorophyll concentration in the leaves but had no significant effect on radiation use efficiency (RUE) in late summer and autumn. In consequence, late N application increased foliage dry weight at final harvest but failed to have a positive effect on sugar yield. This occurred on a soil which should mineralise no more N during summer than any other soil used for growing beet in England. Therefore, nitrogen from fertilizer is not necessary to maintain canopy efficiency and seems to be necessary solely to stimulate the rapid growth of the canopy in early summer.