Synergistic effects of organic fertilizer and copper-calcium nanoparticles on onion growth and storage under arid conditions.

Onion (Allium cepa L.) production in hyper-arid environments is constrained by poor soil fertility, limited water availability, and post-harvest losses that reduce market viability. The interactive effects of foliar application of copper oxide (CuO) and calcium oxide (CaO) nanoparticles (NPs), combined with sugarcane filter mud cake (FMC), on onion growth and storage quality in calcareous hyper-arid soils remain to be characterized. A two-season field study (2023/24-2024/25) evaluated the effects of FMC (30 t ha⁻¹) combined with foliar CuO NPs (10, 20, and 30 mg L⁻¹) and CaO NPs (50, 100, and 150 mg L⁻¹), applied singly and in combination, on onion cv. Sabeeni in Upper Egypt. Vegetative growth, yield traits, nutrient uptake, and storage performance were quantified. Organic fertilization and nanoparticle treatments interacted significantly: yields of 42-45 t ha⁻¹ (15-21% above control) were achieved with low-to-moderate CuO NPs (10-20 mg L⁻¹) and high CaO NPs (150 mg L⁻¹); high CaO NPs alone maintained yields close to control values (< 3% increase); and excessive CuO NPs (30 mg L⁻¹) with insufficient Ca was associated with phytotoxicity and yield reductions (21.29-28.90 t ha⁻¹). Tissue Cu concentrations ranged from 6.17 to 39.33 mg kg⁻¹ and Ca from 2,556 to 4,750 mg kg⁻¹. These results suggest that combining organic amendment with appropriately balanced nanoparticle nutrition may represent a scalable approach to improving onion yield by 15-21% and extending post-harvest shelf life under hyper-arid conditions.