Effect of electrical conductivity on the productivity and nutrient uptake of Lactuca sativa L. grown using nutrient film technique (NFT)

Nutritional recommendations for hydroponic crops in nutrient film techniques (NFT) are given based on electrical conductivity (EC) levels of the fertilizer solution. Growers rely on the recommended EC levels of 1.8 mS cm^‑1 for lettuce production, despite how growing conditions vary in the greenhouses throughout the year. Disorders like tipburn are a common occurrence in hydroponic lettuce due to this overreliance on EC. Variation of yield with different EC levels were reported in hydroponic systems, however, reliable data are not available for the interactions between EC, nutrient uptake patterns and yield in NFT systems under greenhouse conditions in northern USA. Therefore, two experiments were conducted in fall 2016 and spring 2017, with lettuce cultivars grown under four EC levels, 0.8, 1.2, 1.8 and 2.4 mS cm^‑1. The temperature set points for fall and spring trials were 65 to 70°F/18.3 to 21.1°C and 70 to 75°F/21.1 to 32.8°C, respectively. During the fall trial, maximum yield was observed at EC of 1.8 mS cm^‑1 with 225±9 g per head in ‘Green Butter’ lettuce and 213±13 g per head in ‘Red Butter’ lettuce. Nitrogen and potassium uptake by the leaves increased with increased EC, however, there was no influence on yield above 1.8 mS cm^‑1 EC, since both fresh and dry weight were reduced with EC greater than 1.8 mS cm^‑1 in both cultivars. Phosphorus, Ca, Mg, S, Mo and Mn content of leaves increased from EC of 1.2 to 1.8, however, remained similar between 1.8 and 2.4 mS cm^‑1. Photosynthetic rate increased with increasing EC and leveled off after 1.8 mS cm^‑1. Tipburn severity is greatest at 1.8 mS cm^‑1 EC. During spring trial there were no yield differences between the 1.8 and 2.4 mS cm^‑1 EC. Increasing EC beyond 1.8 mS cm^‑1 did not contribute to yield gains, however, tipburn symptoms were greatest at EC at 1.8 mS cm^‑1.