Impact of Light Intensity and the Production System on Nutrient Solution Refill Strategies for Lettuce

Management of hydroponic nutrient solution is a critical factor to consider when producing leafy greens using a recirculating system because they are prone to nutrient imbalances that can impact yield and quality. Producers use several strategies for nutrient solution refill (NSR), but there have been no comparisons of the performance of different NSR strategies and their impact on crop yield, quality, and resource use in a wide range of production environments. Our objective was to compare the performance of six NSR strategies when growing lettuce ( Lactuca sativa ‘Green Forest’) under three lighting intensities within two production systems. The standard control system was drained and refilled with fresh nutrient solution weekly, whereas alternative NSR included full-strength and half-strength topping-off, controlling electrical conductivity (EC), refill strength based on water use efficiency, and individual nutrient replacement. Experiments were conducted in greenhouse and vertical farm production systems using deep water culture trays with light intensity levels of 250, 500, and 700 µmol·m −2 ·s −1 . The NSR strategies that used diluted refill solutions or replenished solutions based on plant water use efficiency or nutrient-specific fertilizer additions reduced fertilizer application by 94% compared with full-strength refilling, EC maintenance, and drain/refill NSR with no reduction in yield or differences in leaf tipburn or chlorosis incidence. Net nutrient use efficiency was also increased by using diluted or targeted replenishments, and producers can easily implement diluted solutions at a relatively low risk and cost compared with those of other NSR methods. These findings show that yield and quality of lettuce can be maintained using NSR strategies that minimize water and fertilizer application across a wide range of environmental conditions.