Carrots cultivated in Poland are either sold as fresh vegetables or preserved. The quality of processed products does not depend only on the properly implemented technological process, but, also on the biological value of the initial raw material. The objective of the research study was to determine the impact of foliar fertilization of plants with a 3 % solution of magnesium amounting to: 0, 45, and 90 kg of MgO ha^-1 on the quality of roots of five carrot cultivars (‘Berio’, ‘Flacoro’, ‘Karotan’, ‘Koral’, and ‘Perfekcja’). Moreover, the effect of processing operations of carrot roots was determined as regards the quality of frozen and dried products as well as the quality of preserves in jars. The highest amounts of total carotenoids and vitamin C were found in the products produced from storage roots of carrot cultivated on small fields that were fertilized with a magnesium dose of 90 kg ha^-1. Irrespective of the applied processing operation, there were reported losses of carotenoids: 36.1 %, of vitamin C: 59.2 %, and of magnesium: 23.3 %. Under the freezing and preserving processes, the losses of carotenoids and vitamin C were the lowest, whereas under the drying process, the losses of those two components were the highest. The levels of those losses were, respectively: 28.6 and 23.6 % in the frozen products; 27.7 and 64.4 % in the preserves, and 51.9 and 89.6 % in the dried products. As for magnesium, the losses reported were quite different to those of the two previous compounds: the highest losses of 52.3 % occurred under the preservation process, whereas the lowest of 0.14 % under the drying process. The preserves produced from roots of carrots fertilized by the highest dose of magnesium were characterized by the highest nutritional value. Of the investigated carrot cultivars, it was found that the order of suitability for processing was ‘Karotan’, ‘Koral’, and ‘Perfekcja’.