Nitrogen uptake by table beet - Validation of a model

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 were estimated 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.