A canopy approach to nitrogen fertilizer recommendations for the sugar beet crop

This study investigated whether the main role of nitrogen (N) fertilizer in the sugar beet crop is to allow a full canopy to grow quickly, and whether the minimum amount to achieve this is the optimum dose of N fertilizer.Five field experiments were carried out at four sites within the UK to determine the smallest N fertilizer rate which is the first to produce 85% canopy cover and to compare this with the optimum N fertilizer rate for maximum sugar yield. The experiments tested fertilizer treatments, which ranged from 0 to 160 kg N ha−1. Canopy cover was assessed throughout the season, N uptake measured and sugar yield determined. A canopy model fitted the data well and was used to determine the smallest amount of N to reach 85% cover soonest. These crops needed ca. 900 °Cd from drilling and an uptake of ca. 100 kg N ha−1 to reach 85% cover. Until July, the N uptake was hardly affected by variations in soil N, so canopy cover at 900 °Cd could not be predicted from measurement of soil mineral N and N fertilizer applied across sites. The yield response to N fertilizer was a split line response. Across sites, the smallest dose of N fertilizer which was the first to produce 85% canopy cover was no better a predictor of the optimum dose of N than published recommendations. Rather, the results suggest that in mineral soils, soil mineral N assessments do not improve accuracy of fertilizer recommendations for sugar beet and that, in the absence of organic manure, 100 kg ha−1 should be applied for maximum yield.