Freshwater Algae Biostimulant in Mitigating Impacts of Saline Irrigation on Onions

Salinity poses a significant challenge in modern agriculture, often inhibiting growth and yield, especially in sensitive crops like onions (Allium cepa L.). This study evaluated the effectiveness of a freshwater-algae-based biostimulant on two onion cultivars, Franciscana IPA-10 and Vale Ouro IPA-11, to mitigate saline irrigation’s adverse effects. Five biostimulant concentrations (0, 1, 2, 3, and 4 mL L−1, applied to the soil) were tested, along with two foliar treatments at 2 mL L−1 as controls. Our findings showed that applying 4 mL L−1 to the soil boosted growth rates to 1.0 cm per day (1), increased the potassium-to-sodium ratio in bulbs, and improved both average bulb weight by 25.11% and overall productivity by 24.28%, relative to untreated conditions. These results suggest that the biostimulant at 4 mL L−1 is an effective method to enhance resilience to saline stress and increase productivity in the IPA-10 and IPA-11 cultivars. However, while the biostimulant improved plant performance, it did not counteract the accumulation of salts in the soil. Therefore, additional management practices such as leaching and drainage are recommended to ensure sustainable onion production under saline water irrigation.