Health—Promoting Properties of Highbush Blueberries Depending on Type of Fertilization

The purpose of the experiment was to demonstrate a comparison of fertilization with and without biostimulation. A study was carried out in an experimental blueberry field in central Poland (51°55′42.7″ N 20°59′28.7″ E) during the three growing seasons of 2019, 2020 and 2021, on ‘Bluecrop’ shrubs growing at a distance of 1 × 3 m. The plants were re-planted in the spring of each year and irrigated using drip irrigation. The experiment was conducted using a random block design (four fertilizer treatments × five replications × six bushes). The fruits were tested for antioxidant activity and amount of total polyphenols. Additionally, anthocyanin quantitative and qualitative analysis was performed. The results indicated a significant effect of fertilizer combinations on the values of the evaluated parameters. The positive effect of biostimulants on the content of antioxidant compounds in highbush blueberry fruit was significant. In most of the combinations in which additional biostimulants were used, higher values of the analyzed indicators (antioxidant activity and polyphenol content) were observed. The most noteworthy was the T4 fertilization program, where during treatment, soil and foliar fertilization were carried out with preparations that contained biostimulants.