Relationship between subsoil nitrogen availability and sugar beet processing quality

The loss of sugar content and processing quality during the harvest period happens frequently in sugarbeet (Beta vulgaris L. ssp. vulgaris) grown in the Po Valley of Italy. Th e aim of this study was to assess the consequences on sugar content and quality of excess mineral N in the deep soil explored by the roots. Soil mineral N concentration and other chemical properties to a 3-m depth were correlated with sugar content and processing quality of sugarbeet in 27 sites sampled in 2000–2003. At each site, 12 soil samples with 0.25-m depth increments were collected and analyzed separately. Organic matter as high as 10% and mineral N as much as 100 mg kg−1 frequently were found between 2 and 3 m, corresponding to the maximum depth of the sugarbeet root system. Significant negative relationships were observed between mineral N at the 2.5- to 3-m depth and sugar content (r = –0.63) and quality (r = –0.72). This study indicates both the utility of soil sampling to the depth reached by the roots, and the need of more complete analyses of mineral N, which should include not only nitrate N, but also ammonium N and organic matter. To reduce the losses of sugar content and quality, the presence of organic layers in the rooting zone should be avoided, or adequately considered in the fertilizer management of the crop.