A three-level cascade hydroponic system was designed to enhance resource efficiency by reusing drainage solutions across sequential crops: tomato (primary-donor crop), herbs (mint, peppermint; secondary receivers from primary), and halophytes (lemon balm, sea fennel; tertiary receivers from secondary). The aim was to address salinity, a common challenge in hydroponics limiting plant growth and resource use. Two fertigation strategies were applied to secondary and tertiary crops to simulate salinity, with electrical conductivity (EC) increasing weekly by 1 dS m−1 to reach 9 dS m−1 for secondary and 11 dS m−1 for tertiary crops. Control (S1) used fresh nutrient solution (FS), while the recycling treatment (S2) used tomato drainage with added NaCl. For tertiary crops, the control (S3) received a salinity-enriched FS, and the recycling treatment (S4) reused 70% of secondary crop drainage combined with 30% of its own, plus NaCl to reach target EC. Under moderate salinity (9 dS m−1), mint produced 2.5 kg m−2, whereas lemon balm dropped 16.7%, showing sensitivity; peppermint was more tolerant. Sea fennel showed resilience under high salinity (11 dS m−1), with high chlorophyll (97.2) and improved ion uptake. The system reduced nutrient and fertilizer use by 86–88%, highlighting potential for sustainable nutrient recycling and efficient crop production.
Karatsivou et al. (Wed,) studied this question.