Optimizing onion growth, yield, and quality with foliar and soil applications of zinc and boron in salt-stressed soils

Soil salinity is a major constraint to onion production, reducing yield and limiting the availability of essential micronutrients such as zinc (Zn) and boron (B). Deficiency of these elements not only impairs growth and bulb development but also lowers nutritional quality, which is important for human health. Therefore, this study evaluated foliar and soil applications of Zn and B for improving onion growth, yield, quality, and biofortification. Field experiments were conducted during two consecutive winter seasons using a randomized complete block design with three replicates. Onion seedlings (Allium cepa L. cv. Giza 20) were cultivated in saline soil (ECe = 5.09 ± 0.11 dS m−1) and supplied with Zn (Zn-EDTA, 13% Zn) and B (borax, 11% B) as either foliar sprays or soil applications. Foliar sprays were applied at 30 and 60 days after transplanting, while soil applications were split between land preparation and bulb initiation. Results revealed that foliar application of 100 mg L−1 Zn + 50 mg L−1 B was the most effective treatment, producing the highest chlorophyll content (69 SPAD units), total soluble solids (17.4°Brix), bulb Zn (55.3 mg kg−1), B concentration (46.7 mg kg−1), and total yield. Soil application at 2.5 kg ha−1 Zn + 1.0 kg ha−1 B also improved growth and yield, though less effectively. In conclusion, Zn and B supplementation enhanced onion growth, yield, and biofortification under salt stress. These findings support micronutrient management as a sustainable strategy for onion cultivation in arid regions, with implications for salinity management and postharvest quality.