Sustainable Fertilization with an Organic By-product Compost Tea Produced Using the SEFEL System Improves Nutrient Uptake and Drought Tolerance in Solanum lycopersicum and Lactuca sativa

Improving crop performance under water scarcity requires fertilization strategies that not only supply nutrients but also strengthen soil–plant interactions. In this work, we evaluated a compost tea obtained from recycled organic by-products using the “Sistema de Elaboración de Fertilizante Ecológico Líquido” (SEFEL) system, as a multifunctional biofertilizer in tomato (Solanum lycopersicum) and lettuce (Lactuca sativa) grown under well-watered (WW) and deficit irrigation conditions (WD). We explored its capacity to modulate physiological responses, metabolic adjustments and soil microbiota under WW and WD. SEFEL compost tea application consistently promoted plant growth. Tomato plants showed a 63.9% increase in leaf dry weight under optimal irrigation and a 32.6% increase under WD, while root biomass rose by 30.3% and 53.3%, respectively. In lettuce, leaf dry weight increased by 34% under WW and by 23% under WD. Although drought reduced photosynthetic rates in both species, SEFEL compost tea-treated plants maintained higher carbon assimilation and stomatal conductance during prolonged stress, indicating improved physiological adjustment. These responses were accompanied by a greater accumulation of proline and soluble carbohydrates under water deficit. Interestingly, a different activation of the raffinose family oligosaccharides (RFOs) pathway was detected in tomato and lettuce, pointing to species-specific metabolic strategies. Additionally, SEFEL compost tea improved nutrient uptake under both irrigation regimes and promoted shifts in soil microbial communities, suggesting reinforced soil–plant connectivity. Our results indicate that SEFEL compost tea acts as an integrative soil–plant biostimulant, enhancing nutrient acquisition and drought tolerance while contributing to organic waste valorisation within sustainable production systems.