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Carmen Feller and Matthias Fink

The objective was to provide results to optimize the production of table beet (Beta vulgaris L.) with respect to yield and quality. Field experiments were carried out over 2 years, where the effects of nitrogen (N) supply, sowing date, and cultivar were tested in a block design with four replications. In addition to yield, soluble solids and nitrate N contents of roots were measured to assess quality. Sowing date was an important factor for determining yield and quality of table beet. Sowing dates later than June at the experimental site are not recommended because they resulted in an increase in nitrate N content in fresh weight of up to 3027 mg·kg-1 and an average yield loss of 46% compared to sowings in April. Soluble solids content (SSC) was only slightly affected by planting date. Nitrogen supply did not affect SSC, but increasing N supply led to a major increase in nitrate N content, especially if combined with late sowing dates. It was concluded for early sowing dates that N supply be determined to achieve the maximum yield. With an early sowing date, nitrate N content in fresh weight at harvest was <563 mg·kg-1, even with a high N supply of 250 kg·ha-1. Late sowing dates required a reduced N supply to keep harvest nitrate contents below the 2500 mg·kg-1 required by the processing industry. Recommendations for optimizing N supply, sowing date, and cultivars for table beet should always take into account strong interactions between these factors.

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Carmen Feller and Matthias Fink

The objective of our study was to determine how accurately refractometry can quantify soluble carbohydrates in the storage roots of asparagus (Asparagus officinalis L.). Fructose, glucose, sucrose, and fructans as well as refraction were measured in 51 root samples that were taken from commercial fields. There was substantial variation in refraction both within roots of the same plant (cv, 6%) and within plants in the same field (cv, 20%). Samples of asparagus root sap contained fructose, glucose, sucrose, and fructans in varying fractions and, in addition, significant amounts of other solubles, which contributed considerably to refraction. Therefore, refraction readings are no direct measure of fructose, glucose, sucrose, and fructans in asparagus root sap. However, the concentration of these carbohydrates can be well estimated by a regression function, which uses refraction readings as input (r = 0.89).

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Carmen Feller and Matthias Fink

To reduce nitrogen (N) losses from vegetable fields, fertilizer recommendations should be adjusted according to the large range in yield and thus in N uptake of vegetable crops. Therefore, a model was used to predict total N uptake based on expected yield. The model has been validated successfully in a series of studies for Brussels sprouts (Brassica oleracea L. var. gemmifera), white cabbage (Brassica oleracea L. var. capitata) and kohlrabi (Brassica oleracea L. var. gongylodes). The objective of this study was to validate the model for table beet (Beta vulgaris L. var. conditiva), a crop with a considerable variability in N uptake, which is caused by a large potential range of selecting sowing dates, plant densities and cultivars. Field experiments were carried out over two years. Fifty-five combinations of N fertilizer levels, plant densities, cultivars and sowing dates were tested. Plants were sampled at 2- or 3-week intervals, and fresh matter, dry matter and N content of leaves and roots were measured. Crop specific model parameters for table beets were determined from independent data. The model wverestimated N uptake for N-limiting conditions, but for optimally fertilized table beets measured and estimated N uptake showed a close correlation (R 2 = 0.93) when total yield was used as an input parameter for the model. Although the error of estimation (35 kg·ha-1) was considerable, studies with other vegetable crops using the model found the error even higher if other tools, such as look-up tables, were used for predicting N uptake.

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Carmen Feller and Matthias Fink

The nitrogen requirement of broccoli (Brassica oleracea var. italica) ranges from 300 to 465 kg·ha–1. Recommendations for N fertilization are accordingly high. High fertilizer rates applied at planting result in a high soil mineral N content that remains high for weeks because the N requirement of the crop is low at early growth stages. Therefore, the risk of leaching is high for several weeks until the available N is finally taken up by the crop. Our study had two objectives: 1) to quantify yield responses to preplant fertilization, and 2) to test our hypothesis that the preplant fertilization rate could be reduced without yield losses by increasing the N content in the transplants and improving crop establishment. Field experiments were carried out on transplants with four levels of N content in dry matter (0.018 to 0.038 g·g–1 dry weight), which were tested in all combinations with four fertilization timings. All treatments received the same amount of N fertilizer (270 and 272 kg·ha–1 in 2001 and 2002, respectively), but with different rates of supply at the time of planting (0 to 90 kg·ha–1 N fertilizer plus 30 and 28 kg·ha–1 soil mineral N in 2001 and 2002, respectively). Total and marketable yields increased significantly with an increasing N supply at time of planting. In our experiments, in which topdressing was applied 25 days after planting, an N supply at planting of 80 to 118 kg·ha–1 was required to obtain maximum marketable yields. The N content in transplants had little effect on growth and yield, and there were no significant interactions between the N content in the transplant and fertilizer timing.