Light Intensity and Production System Influence on Nutrient Solution Refill Strategies: A Case Study for Spinach

Hydroponic spinach ( Spinacia oleracea ) production requires diligent nutrient solution management. Nutrient imbalances may compound over time in recirculating systems, which affects quality and yield. There is a lack of studies comparing nutrient solution refill (NSR) strategies on crop yield, quality, and resource use across diverse production settings. This study aimed to assess six NSR strategies for spinach production: weekly drain and refill with fresh nutrient solution, and the remaining NSR included full and half strength topping off, controlling electrical conductivity (EC), refill strength from water use efficiency, and individual nutrient replacement. Each strategy was studied under photosynthetic photon flux density of 250, 500, and 700 µmol·m −2 ·s −1 and in a greenhouse (ambient CO 2 ) or a vertical farm (elevated CO 2 ). Strategies that used diluted refill solutions, replenished based on plant water use efficiency, or nutrient-specific fertilizer additions resulted in a 91% reduction in fertilizer application compared with full-strength refilling, EC maintenance, and drain/refill methods. We found no change in yield, leaf tipburn incidence, or chlorosis among all light intensities and production systems. These results demonstrate that spinach yield and quality can be maintained with NSR strategies that reduce water and fertilizer use in greenhouses and vertical farms.