Yield reduction resulting from high temperatures and tipburn are common issues during the summer for hydroponically grown lettuce using the nutrient–film technique (NFT). We investigated the yield and degree of tipburn of lettuce ‘Red Butter’, ‘Green Butter’, and ‘Red Oakleaf’ of the Salanova® series under different-solution electrical conductivity (EC) and pH levels. We also quantified the effect of foliar spray application of calcium chloride (CaCl2) on the yield and degree of tipburn using the lettuce cultivar Green Butter. For the EC experiment, the plants were grown at four EC levels (1.4, 1.6, 1.8, or 2.0 mS·cm–1) and a constant pH of 5.8. For the pH experiment, the plants were grown at and four pH levels (5.8, 6.0, 6.2, or 6.4) and a constant EC of 1.8 mS·cm–1. For the foliar spray experiment, CaCl2 was applied 1 week after transplanting into NFT channels at three different concentrations: 0, 200, 400, or 800 mg·L calcium (Ca). During the EC trial, the maximum yields were observed at or more than 1.8 mS·cm–1 for ‘Green Butter’ (263 ± 14 g/head) and ‘Red Butter’ (202 ± 8 g), and more than 1.6 mS·cm–1 for ‘Red Oakleaf’ (183 ± 6 g). The yield of ‘Green Butter’ was 75 g less at 1.4 mS·cm–1 compared with 1.8 mS·cm–1. Tipburn symptoms were less at 1.4 mS·cm–1 for ‘Green Butter’ whereas other cultivars were not highly susceptible. In pH trials, the maximum yield for all cultivars was found at pH 6.0 and 6.2. There were no differences in tipburn symptoms among all pH levels. The foliar spray treatment, twice a week at 400 or 800 mg·L–1 Ca, provided improved tipburn control, as the tipburn symptoms were minimal and the impact on yield was minor compared with reducing EC. This series of experiments found evidence in proper EC and pH management for optimum yield and tipburn control in NFT lettuce grown in summer conditions.
Opportunities and constraints in hydroponic crop production systems: A review