Saline-calcareous soils induce many stresses in plants when grown on them, including salinity, high carbonate, and limitation of water and nutrients, all of which severely damage plants that may die. Attenuating these stresses is a challenge for human and animal feeding sustainability. During 2020 and 2021, the potential promoting impacts of treating saline-calcareous soil with an acidified leguminous compost (AcC) on plant growth, physiological and biochemical traits, antioxidant systems, and nutritional value of Atriplex nummularia seedlings were investigated. Treating the tested saline-calcareous soil with AcC at a rate of 2 kg per m² significantly improved A. nummularia growth, root activity, photosynthetic efficiency, leaf cell integrity, soluble protein, osmo-regulators, different antioxidants, and nutritional status, while markers of oxidative stress (H₂O₂ and O₂^•‒) and oxidative damage (lipid peroxidation and electrolyte leakage), total phenols, and Na⁺ levels were suppressed. The results of this investigation recommended the utilization of AcC for producing satisfactory plant productivities under saline-calcareous soil conditions.