Growth analysis of broccoli in relation to fertilizer nitrogen application

SummarySix field experiments with different planting dates over three years in the South of Norway were used to analyse biomass production of broccoli. N was applied at rates of 0, 120, and 240 kg ha–1. Mineralized N levels in soil before planting differed among the experiments. Crop biomass, leaf area index (LAI), and accumulated intercepted photosynthetically active radiation (PAR) increased with enhanced N rate. So did also radiation use efficiency (RUE), but not significantly from 120 to 240 kg ha–1. Thus, crop biomass increased to the highest N rate rather by more intercepted radiation than by enhanced RUE. RUE reached saturation level when the sum of applied N and Nmin in soil before planting was about 200 kg ha–1. There was a strong effect of N application on early plantings, when low temperature and possibly oversaturation of light were the most likely reasons for low LAI, RUE, and RGR at low or no N application. The plant N concentration decreased with increasing biomass over time. Of two alternative relationships of critical N concentration (Nc) suggested by Greenwood et al., the linear one, which gave the higher estimates, fitted better. Both equations were evaluated on the correspondence between a relative N concentration N/Nc = 1 and maximum relative values for: biomass, LAI, accumulated intercepted PAR, and RUE. However, a curvilinear function for Nc might produce a higher correlation over the whole range of plant N concentrations. RUE and accumulated intercepted PAR approached a saturation level at a lower relative N concentration than did biomass and LAI, suggesting that N was more limiting for biomass production than for PAR interception and RUE at N rates below the highest application rate. Incident radiation was fully intercepted at a low LAI, resulting in a low RUE.