Background Among abiotic stresses, salinity and socidity are the major productivity-limiting factors for cultivated crop plants. In two seasons (2022 and 2023), two field-level experimental attempts were made to study the impacts of urea–phosphate (UP) and magnesium oxide nanoparticles (MgNPs) and their combination on the nutritional status, physiological attributes, and yields of Glycine max (L) plants growing under saline-sodic conditions. At 30, 45, and 60 days, UP was applied to the soil at rates of 85.0, 107.0, 127.0, and 150.0 kg ha-1, corresponding to UP1, UP2, UP3, and UP4, respectively, as well as MgONPs via foliar application at doses of 0.0, 50.0, and 100.0 mg L-1, corresponding to MgONP0, MgONP1, and MgONP2, respectively. This study was conducted in a split-plot structure according to a randomized compete block design with three replicates. Results Our results showed that UP3 and UP4 had the strongest effects on most of the measured traits. Both application rates produced the maximum growth–physiological attribute values, except for the leaf dry matter percentage and leaf nutrient levels in both seasons and the leaf iron and zinc contents in the first season (the highest values in these characteristics were achieved in the untreated plants). In addition, the UP4-fertilized plants produced the highest 100-seed weight (HSW), total seed yield (TSY), and seed manganese contents in the two growing seasons. The highest seed oil contents in both seasons, as well as the highest seed phosphorus, calcium, and copper contents in 2023, were recorded in the UP3-treated plants. Regarding MgONPs, our results revealed the significant superiority of the MgONP-treated plants in terms of all the aforementioned growth–physiological parameters and leaf macro- and micronutrient contents, irrespective of the applied dose, except for the LMnCs in 2022 and 2023. The integrative application revealed the clearly superior influence of applying UP3 or UP 4 with MgONP 2 and MgONP 3 on most studied traits. Specifically, for TSY, Model 3 in the 2022 season (adjusted R22 = 0.931) and Model 2 in LPC, LA, and LCaC as the most influential attributes in 2022, and LNC and LKC in 2023. For seed oil content (SOC), Model 3 in 2022 (adjusted R22 = 0.344) and Model 2 in 2023 (adjusted R22 = 0.278) were selected, with SPAD readings and LMnC in 2022, and LMnC and HSW in 2023 as key predictors. Conclusions The use of high rates of a highly soluble and acidic-impact fertilizer, such as UP supplemented with magnesium oxide nanoparticles, may be recommended in saline environments.
