The research aim was to determine the influence of different organic and mineral fertilization treatments and post-harvest treatments on the content of nitrogen and crude proteins in the edible part of beetroot (Beta vulgaris var. conditiva). A field trial (2003-2005) was set up in a hilly part of Croatia according to the Latin square method with four types of fertilization (control, 50 t ha-1 stable manure, 500 and 1000 kg ha-1 NPK 5-20-30), while treatments involved harvested fresh beetroot and stored fresh beetroot. The highest dry weight (DW) content was determined in climatologically favourable 2004 (average 14.8% DW) and in the treatment with 1000 kg ha-1 NPK 5-20-30 (15.6% DW) in harvested beetroot. In 2004 and 2005, the highest levels of nitrogen and crude proteins in harvested beetroot were determined in the treatment with 1000 kg ha-1 NPK 5-20-30 (2.41 and 2.43 g N kg-1 in fresh weight and 15.07 and 15.21 g crude proteins kg-1 in fresh weight, respectively). Regardless of fertilization treatment or studied year, nitrogen and crude protein contents were higher in stored than in harvested beetroot, by 12% on average. The lowest crude protein content was determined in treatment with stable manure what confirmed that protein content decreased by organic fertilization. It can be concluded that beetroot lost some water during the storage period, which increased its content of nitrogen and crude proteins in fresh weight and thus increased the nutritional quality of beetroot as a functional food.
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