Integrating Boron and Zinc Foliar Fertilization with Mineral Nutrition Improves Growth, Nutrient Acquisition, and Productivity of Sugar Beets

Sugar beet (Beta vulgaris L.) is an economically important industrial crop, and its productivity and sugar accumulation depend heavily on effective nutrient management. This study investigated how mineral fertilization combined with foliar applications of boron (B) and zinc (Zn) influences plant growth, photosynthetic performance, nutrient uptake, yield, and sugar production under irrigated alkaline soil conditions in southeastern Kazakhstan during the 2024 and 2025 growing seasons. The field experiment was conducted using a randomized complete block design with three replications and five treatments: (1) control (CK), (2) N300P150K300 (NPK), (3) NPK + B, (4) NPK + Zn, and (5) NPK + B + Zn. Mineral fertilization substantially enhanced plant growth and productivity compared with the unfertilized control, while foliar supplementation with B and Zn provided further improvements. Across both growing seasons, the combined NPK + B + Zn treatment consistently produced the greatest leaf area (55.6 × 103 m2 ha−1), dry biomass (4.9 t ha−1), photosynthetic potential (1631.5 × 103 m2 day ha−1), and net photosynthetic productivity (7.7 g m−2 day−1). It also resulted in the highest total uptake of nitrogen, phosphorus, and potassium, reaching 431.5, 64.7, and 761.4 kg ha−1, respectively. Foliar application of the combined micronutrients markedly increased their accumulation in plant tissues, with total B and Zn uptake rising to 874 and 1005 g ha−1, respectively. Sugar content improved from 17.1–17.2% in the control to 17.9–18.1% under the combined micronutrient treatment. As a result, the NPK + B + Zn treatment achieved the highest mean sugar yield (15.12 t ha−1), representing a 76.4% increase over the control and a 12.5% increase compared with NPK fertilization alone. The results demonstrate that foliar application of boron and zinc effectively complements mineral fertilization by enhancing photosynthetic activity, nutrient acquisition, and sugar accumulation. The combined application of NPK with foliar B and Zn can therefore be recommended as an effective nutrient management strategy for improving sugar beet productivity and sugar yield under alkaline soil conditions.