This investigation was conducted to determine the effect of organic fertilizers on the content of chlorophyll a, chlorophyll b, antioxidant activity, crude fibre, and zinc in two onion cultivars, Stuttgarter Riesen and Rote Laaer, during 2016, 2017, and 2018. In this research, the following treatments were used: B-Stimul (contains Azospirillum Tarrand et al., Azotobacter Beij., Bacillus Cohn, Chlorella vulgaris Beij., and Herbaspirillum Baldani et al.), EkoBooster 2 (contains biostimulators and mineral salts of nitrogen, phosphorus, and potassium) and Vermifit A (extract of compost of Californian earthworm). The results showed that the application of biofertilizers to onion resulted in the highest chlorophyll b content in 2017 in Stuttgarter Riesen. EkoBooster 2 positively affected crude fibre content in both cultivars, but only in 2016. Antioxidant activity was not significantly affected by the interaction of experimental factors. The application of the biostimulating fertilizers can have a positive impact on the quality parameters of onion, but the kind of fertilizer must be suited to seasonal conditions and the cultivar.