A very practical definition of “soilless culture” is the growth of nonaquatic plants with roots in a substrate without mineral soil, where plant nutrient needs are supplied with a nutrient solution. Soilless culture has various classification systems and methods, such as hydroponics, aeroponics, gravel culture, and rockwool culture (Maxwell, 1986). NFT was developed by Allen Cooper and his colleagues during the 1960s at the Glasshouse Crops Research Institute in Littlehampton, UK, and they defined it as a practice of growing plants in a shallow film of nutrient solution flowing near bare roots in a water-tight channel (Resh, 1995).
Selection of a suitable fertilizer is one of the main challenges when growing with NFT. The fertilizer used in NFT should have balanced amounts of essential elements and should not form any precipitate during its use. The plant should grow as a normal plant without facing any type of nutrient deficiency (Jones, 1982). As the nutrient solution in the NFT system recirculates, macro- and micronutrients accumulate in the nutrient solution, and some of the nutrients are depleted quickly. This necessitates frequent renewal of nutrient solution, which can lead to environmental pollution resulting from the release of mineral elements if the wastewater is not treated properly (Giuffrida et al., 2002). One of the reasons for suitable fertilizer selection in hydroponics is, in field conditions to some extent, the plant can influence nutrient availability by releasing root exudates or exploring some new soil regions by growing their roots, whereas in hydroponics, all nutrients should be provided in adequate amounts unnaturally and may vary from crop to crop (Page and Feller, 2013).
There are various hydroponic fertilizers available, but the selection varies according to crop and the system used. In most studies, self-made nutrient recipes were used for growing leafy vegetables in hydroponics, including Cooper’s, Imai’s, Massantini’s, and Hoagland’s solution (Karimaei et al., 2001; Shah and Shah, 2009). This method of preparing nutrient recipes is also known as the made-from-scratch method (Mattson and Peters, 2014). Mattson and Peters (2014) stated it is hard for small hydroponic growers to manage concentrations of nutrients while preparing their own hydroponic recipes, which has resulted in an interest in commercially prepared water-soluble fertilizers. This method is also known as the one- or two-bag approach and is suitable for hydroponic production (Mattson, and Peters, 2014; Shah, and Shah, 2009). In hydroponics, plant roots are exposed directly to the nutrient solution, whereas in soil, the conditions are different. Nutrients are not directly available to plants, and plants can influence nutrient availability (Page and Feller, 2013). Thus, if the nutrients are supplied in excess in nutrient solution, they may accumulate in the plant structure and, when consumed, may pose health risks (Cavarianni et al., 2008). In particular, if a food product high in nitrate is ingested, it is transformed into nitrite. Subsequently, nitrite, in combination with amines, may form carcinogenic compounds (Boink and Speijers, 1999).
Another challenge for NFT production of leafy greens is the selection of suitable cultivars. There are various studies that focus on the selection of cultivars for leafy greens in field environments, but the growth conditions in the field are incomparable with growth conditions of closed-environment culture using hydroponics. Thus, the cultivar ranking from the data derived from field experiments cannot be applied directly to cultivar selection for hydroponics (Molders et al., 2012). Vital and Teixeira (2002) evaluated different lettuce cultivars (Cinderella, Monica, Elizabeth, and Princess) and hydroponic fertilizers for shoot weight. They reported different performance of cultivars among various nutrient solutions, with cultivars Monica and Princess performing the best. Walters and Currey (2015) reported differences among basil cultivars and types, but not among hydroponic systems. Therefore, there is a need to evaluate different cultivars of leafy greens in hydroponics.
According to Ali et al. (2009), color is an influential trait for leafy greens because it affects preference and acceptability, and is also an indicator of antioxidant properties of leafy vegetables. Vittum (1963) concluded greenness of leafy vegetables is also an index for carotene content. Ferrante et al. (2004) also suggested that the color of leafy vegetables can be considered an indicator of antioxidant properties. Colonna et al. (2016) used chlorophyll meter readings [soil–plant analyses development (SPAD); Spectrum Technologies, Plainfield, IL] as an indicator of greenness for 10 different green leafy vegetables. They concluded that chlorophyll meter readings were affected by the light intensity at time of harvest, and recommended harvesting leafy vegetables during low light intensity. The objectives of our research were to evaluate the effect of two different commercial hydroponic fertilizers on growth and quality parameters of different cultivars of leafy greens (lettuce, basil, and swiss chard) in an NFT system.
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