Nutrient depletion caused by heavy rainfall and excessive irrigation significantly affects soil fertility and plant productivity. The present study investigated the impact of nutrient loss induced by soil washing on the growth and biochemical performance of Spinacia oleracea (spinach plant) and evaluated the potential of magnesium ferrite nanoparticles to mitigate nutrient deficiency stress. Normal soil (C1) and washed soil (C2) were used as control groups, while washed soil supplemented with magnesium ferrite nanoparticles at 10, 20, 40, and 80 mg per 100 g soil constituted the treatment groups (E1–E4). Plant growth parameters, including germination rate, vigour index, and fresh biomass, along with biochemical parameters such as protein, reducing sugar, chlorophyll, and proline contents, were assessed at 15, 30, and 60 days after germination. Plants grown in washed soil exhibited reduced biomass, chlorophyll, protein, and reducing sugar levels, accompanied by elevated proline accumulation, indicating nutrient-deficiency induced stress. Supplementation with magnesium ferrite nanoparticles significantly improved growth and biochemical parameters in a concentration-dependent manner and reduced proline accumulation compared to washed-soil controls. The findings demonstrate that magnesium ferrite nanoparticles can partially restore plant growth and metabolic performance under nutrient depleted soil conditions, highlighting their potential as a nano-amendment for improving crop productivity in leached soils.