Nutrient Use in Vertical Farming: Optimal Electrical Conductivity of Nutrient Solution for Growth of Lettuce and Basil in Hydroponic Cultivation

Abstract

During the past few decades, vertical farming has attracted a lot of interest as an alternative food production method. Vertical farms use engineered growth environments and hydroponic cultivation techniques for growing plants indoors. One of the important factors in vertical farming for the cultivation of different plants is the amount of nutrients, which can be measured as electrical conductivity (EC). Studying the optimal EC is important for avoiding nutrient loss and deficiency in vertical farms. In this study, we investigated the effect of five EC levels (2, 1.2, 0.9, 0.7, and 0.5 dS m−1) of Hoagland nutrient solution on the growth and development of basil cultivar ‘Emily’ and lettuce cultivar ‘Batavia-Caipira’. During the study, the environmental parameters were kept fixed using an automatic dosing machine. The experiment was done in automatic vertical farms using the hydroponic ebb–flow cultivation technique with a temperature of 20 ± 1 °C, relative humidity of 50–60%, CO2 concentration of 450 ppm, pH = 6, the PPFD (photosynthetic photon flux density) of 215 ± 5.5 μmol m−2 s−1, and the photoperiod of 16:8 h (day/night). Each treatment was replicated four times. We studied the effects on several growth parameters (including the dry and fresh weight of leaves and roots, number of leaves, and leaf area) as well as the chlorophyll and nitrogen concentration of the leaves. According to the results, the basil and lettuce growth parameters among the five treatments have been significantly higher in the treatment with EC of 1.2 and 0.9 dS m−1. These EC values are lower than the recommended EC value given as the optimum in the previous studies. However, the concentration of chlorophyll and nitrogen show different trends and were higher in full strength of nutrient solution with EC = 2 dS m−1.

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“…Moreover, in our study, we did not observe any significant change in fresh yield in basil grown in HS (1 dS m −1 ) and FS (2 dS m −1 ) nutrient solutions. Our result is in line with the observations of Hosseini et al (2021) , who reported reductions in fresh yield in basil and lettuce grown on nutrient solutions with lower EC of 0.9 dS m −1 and corroborated the studies of Walters and Currey (2018) on “Sweet,” “Lemon,” and “Holy” basil, which did not observe yield increase with EC between 1 and 4 dS m −1 . This shows that excess nutrients in the solution provide no benefit in terms of basil yield and negatively affect resource efficiency, economic viability, and environmental sustainability of hydroponic systems.…”

“…It should be noted that “Eleonora” showed better adaptability to the selected cropping system, producing more leaves per plant, and thus providing a higher fresh yield than “Italiano Classico,” which, in contrast, had a higher dry matter percentage ( Table 1 ). In hydroponic systems, yield is primarily determined by the formulation of the NS, and to this end, numerous studies have focused on seeking optimal mineral levels to achieve ad hoc crop-specific “recipes” ( Hosseini et al, 2021 ). For example, some studies have shown reduced yields in spinach ( Spinacia oleracea L.; Cocetta et al, 2014 ) and lettuce ( Hosseini et al, 2021 ) when grown in nutrient solutions with suboptimal mineral concentrations.…”

“…In hydroponic systems, yield is primarily determined by the formulation of the NS, and to this end, numerous studies have focused on seeking optimal mineral levels to achieve ad hoc crop-specific “recipes” ( Hosseini et al, 2021 ). For example, some studies have shown reduced yields in spinach ( Spinacia oleracea L.; Cocetta et al, 2014 ) and lettuce ( Hosseini et al, 2021 ) when grown in nutrient solutions with suboptimal mineral concentrations. Moreover, in our study, we did not observe any significant change in fresh yield in basil grown in HS (1 dS m −1 ) and FS (2 dS m −1 ) nutrient solutions.…”

“…In addition, the potential to control and manipulate the composition of the nutrient solution (NS) would positively change secondary metabolism by enhancing the phytochemical properties of grown horticultural products ( Scuderi et al, 2011 ; Maggini et al, 2014 ; El-Nakhel et al, 2019 ; Prinsi et al, 2020 ; Aktsoglou et al, 2021 ). The unclear effects of dilute NS (nutrient stresses) on leafy vegetable yield parameters ( Fallovo et al, 2009 ; El-Nakhel et al, 2019 ; Ciriello et al, 2020 ; Hosseini et al, 2021 ) have prompted growers to use concentrated NS that exceed crop needs ( Yang and Kim, 2020 ). Consistent with the guidelines of the European Commission, the need to reduce chemical input while improving yield and quality in intensive production environments has prompted the agricultural sector to become increasingly interested in biostimulants ( Carillo et al, 2019 ; Kerchev et al, 2020 ; Teklić et al, 2021 ).…”

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“…When the actual EC values fell below the target EC, the dosing system pumped stock solution into the reservoir until the target value was reached again. This is a well-established system for nutrient dosing used in many different hydroponic applications [16,51]. However, when used in re-circulating systems, the disadvantages become obvious.…”