Airflow, fertilizer solution recipes, and calcium concentrations influence lettuce and spinach growth in an indoor vertical farm

Indoor vertical farming is a potential solution to address the high food production demands generated by the increased global population and urbanization. However, the maximum potential of indoor farms can only be achieved when optimal cropping systems are used. There is a lack of information concerning vertical farms in the Southeast U.S. since this technology is new and in constant development. The objective of our study was to evaluate the effects of airflow rates and nutrient concentrations for lettuce 'Rex' (Lactuca sativa) and spinach 'Lizard' (Spinacia oleracea) production in a deep water culture system inside an indoor vertical farm. We tested four airflow rates (0.4, 0.7, 1.0, and 1.3 m s−1), two fertilizer solution recipes with different nutrient concentrations (Mattson and Resh), and three levels of calcium (Ca) concentration (regular with 90 and 200 mg L−1 Ca for Mattson and Resh, low with 40–50 mg L−1 Ca lower than regular, and high with 40–50 mg L−1 Ca higher than regular). Results showed that a high airflow rate (1.3 m s−1) significantly increased plant water use and solution electrical conductivity (P < 0.01), negatively affected lettuce P, K, and Mg concentrations (P < 0.05) while having no significant effects on spinach growth performance and mineral concentrations. Lettuce and spinach plants grown using the Resh recipe had higher leaf area, yield, shoot, and root biomass than the Mattson recipe. High Ca concentrations in the solution further accelerated these improvements, with additional benefits on increasing tissue Ca concentrations (P < 0.01). No tipburn symptoms were found, indicating no Ca deficiency induced by low Ca availability, and reduced plant transpiration was presented using these airflow rates and solution Ca concentrations. Overall, slow airflow rates (0.4–0.7 m s−1) and a Resh recipe with high Ca concentration can be used for lettuce and spinach production in vertical farms to increase yield.