Nitrogen fertilization to broccoli cultivars at different planting times: Yield and nitrogen use

Abstract

The effects of three nitrogen (N) fertilizer rates (0, 120, and 240 kg N ha−1) and two planting times (May or late June/July) on yield and N use of the early cultivar ‘Milady’ and the late cultivar ‘Marathon’ of broccoli (Brassica oleracea var. italica) were investigated on three silty loam soils varying in soil mineral N (Nmin) in the southernmost part of Norway during 1999 and 2001. In all crops receiving fertilizer, rapid uptake of N started about three weeks after planting. The relative yield of broccoli heads increased with increasing soil available N (fertilizer N plus Nmin) at planting to 200–250 kg N ha−1 and then levelled off. The two lower fertilizer rates were more restrictive to yields in early-planted than in late-planted crops. A general increase in harvest index with increasing N rate reflected a stronger effect of N on the head yield than on the total above ground biomass production. The apparent recovery of fertilizer N decreased with increasing N rate and was on average 74% in total above ground biomass and 25% in broccoli heads. Despite a higher N uptake, the average soil mineral N level at harvest increased from 12 kg N ha−1 on unfertilized plots to 27 and 78 kg N ha−1 on plots receiving 120 and 240 kg N ha−1, respectively; this increase was stronger in early than in late plantings and stronger in ‘Milady’ than in ‘Marathon’. The yield of broccoli heads was similar in the two cultivars, but ‘Milady’ had a lower total biomass production and thus a higher harvest index, presumably due to earlier head initiation.