| Abstract: | In 2‐years field experiments near Nienstädt (60 km west of Hannover, northern Germany), the effects of rate and timing of nitrogen (N) application on leaf N‐concentration, leaf greenness (SPAD chlorophyll meter readings), canopy greenness (canopy light reflectance), leaf area development, photosynthetic activity of leaves, and yield and quality of sugar beet were studied. In 1999 (pre‐planting soil mineral N: 15 kg ha—1), N fertilizer was applied at rates of 0, 105, 125, 145, 165 and 205 kg N ha—1. In 2000 (pre‐planting soil mineral N: 60 kg ha—1), an N rate of 100 kg ha—1 was applied at planting (100/0/0/0) or split applied at planting and 8 (60/40/0/0), 12 (60/0/40/0), and 16 (60/0/0/40) weeks after planting (WAP), respectively. In both years, canopy greenness as indicated by ”︁sensor values” (a combination of the reflectance of visible and near infrared light) changed with crop age. However, at each time of measurement, sensor values precisely reflected the different N application treatments and were significantly correlated with leaf N‐concentrations and SPAD chlorophyll meter readings. Beet yield and processed white sugar yield increased up to an N supply (fertilizer N + pre‐planting soil mineral N) of 160 kg ha—1. Split N application slightly retarded leaf growth but had no effect on photosynthetic activity per unit leaf area. Beet yield and beet quality were not systematically affected by the timing of N application. Certain application schemes tended to favor either beet yield or beet quality, resulting in similar processed white sugar yields. Our data suggest that moderate N topdressing can be integrated in site‐specific N management systems in sugar beet production. Canopy light reflectance might serve as a useful diagnostic tool to assess the N status and sidedress N demand of sugar beets. However, due to changing sensor values over time, on‐site calibration (using established standard methods or reference plots receiving extra N at planting) will be necessary. The applicability of this approach has to be tested in further field studies. |
