Nutrient solution dynamics and yield of lettuce (Lactuca sativa) in an EC-controlled recirculating hydroponic system

In this study, the nutrient dynamic and growth performance of lettuce in a closed recirculating hydroponic system were investigated. Lettuce was grown in three parallel nutrient film technique (NFT) units, illuminated with LED-light. A balanced standard nutrient solution (NS) was used, and the electrical conductivity (EC) and pH were adjusted regularly to constant average values of 1.16 mS cm^‑1 and 6.2 with standard deviations of ±0.12 and ±0.5, respectively. The volume of NS in each unit was kept at 20 L by adding refill solution to replace nutrient uptake and transpiration. Lettuce growth during the first six weeks in the NFT-system was normal and stable. After six weeks, a decrease in concentrations of N, P, and K was observed, with a corresponding decline in yield of lettuce. After ten weeks, lettuce weight at harvest was reduced by 56% in average compared to the control, and the concentrations of N, P and K in the NS were reduced by 54.5, 90.5 and 96.6%, respectively. Contrarily, more slowly absorbed nutrients like Ca, S, Zn, Cu, and B experienced increases by factors of 2.2, 2.9, 6.6, 4.9 and 2.5, respectively. The depletion and accumulation of nutrients in the NS were reflected in corresponding deficiency and excess levels of nutrients in leaf tissue compared to norm-values of healthy lettuce. The study showed that after six weeks, corresponding to a yield of 1 kg lettuce per 10 L tank volume of NS, the reduced growth implied that the recirculated NS should have been discharged and replaced, or a “tailor-made” refill solution should have been used to avoid depletion of some nutrients. Based on the foliar analysis and calculations of actual nutrient absorption rates, the composition of such a refill NS was suggested.