Emergence of Autotoxicity in Closed Hydroponic Cultivation of Basil and Its Recovery by Compost Tea Application

Hydroponic systems enable constant and high-quality crop yields while avoiding soil-borne diseases and significant pedoclimatic limitations. Recycling nutrient solutions (NSs) makes these systems more environmentally friendly, but long-term cultivation often leads to a decline in the quality and quantity of final products. Biochar and compost tea (CT) are an emerging nature-based solution known to improve both soil and plant health. This study investigates whether biochar or CT treatments can counteract the physiological and productive decline observed in recycled hydroponic systems. We established a closed floating raft system in a controlled-environment greenhouse, cultivated basil (Ocimum basilicum L. cv. Eleonora) over five cycles (conditioning phase), and then performed a last cycle (recovery phase) with the application of either compost tea or biochar filtration. Plant physiology and growth parameters were monitored. As expected, basil plants grown in untreated recycled NS showed significantly lower yields and dry matter content and reduced physiological values compared to controls (fresh NS). Among the applied treatments, biochar did not show any recovery function, whereas CT treatments fully restored physiological parameters and growth performance in a concentration-dependent manner. Recycled hydroponic systems often lead to physiological decline in plants, which can be effectively counteracted by CT treatments.