The vertical structure was developed using hardboard. The hardboard was first divided into four equidistance (1 m) blocks followed by further division of blocks into eight plots. Each of the plots was mounted with a growth device with the help of electric borer, iron threads, and ladder. The containers were built by removing a rectangular portion (0.70 × 0.15 m²) from the upper surface of 4-inch diameter PVC pipes, followed by covering both ends with pipe-end caps. Each container weighted around 0.75 kg. The containers were hung on the hardboard wall in such a manner that the base of the container at lowest alleviation was placed at a height of 0.5 m from the ground level (Figure 1A).

Four types of organic growing medium were applied, namely, 40% soil + 40% vermicompost + 20% cocodust (P1), 50% soil + 50% vermicompost (P2), and 20% soil + 40% vermicompost + 40% spent mushroom compost (P3) and 100% soil (P0, control). The chemical properties including organic carbon, nitrogen, C:N, P, K, and S of the different growing media are displayed in Table 1. Two lettuce cultivars, such as Green Wave (V1) and New Red Fire (V2) were used in this study. The study was carried out in a randomized complete block design (RCBD) with four replications (Figure 1B).

This study was conducted on the wall of the Horticultural farm building at the Sher-e-Bangla Agricultural University, Dhaka, Bangladesh from May to July 2019. Seeds of two lettuce cultivars those were Lactuca sativa cv. Green Wave (Lot No. 72 WN, Takii & Co. Ltd, Kyoto Japan) and Lactuca sativa cv. New Red Fire (Lot No. 96 WL, Takii & Co. Ltd, Kyoto Japan) put for germination in pots. Ten days old healthy and uniform sized lettuce seedlings were transplanted in a single row with 15 cm spacing in 1 m long growth devices, containing the organic substrates. The average temperature was 32.5–34.5°C (Table 2) and the relative humidity was 65.3–81.6% during the study. Transplanting as done in the afternoon and shade was provided by hanging a dense net over the wall until establishment of seedlings. All necessary intercultural operations, such as gap filling, weeding, and irrigation, were carried out accordingly. Total four hand weeding was conducted. Irrigation was provided as per evaporative demand of the plant (Once in every 2 days) with tap water. To avoid runoff of the irrigation water and leaching of nutrients into the lower devices, water was sprinkled on each device in two split doses (125 ml per container). A thin layer of foam as also provided at the bottom layer of each goring devices for the same reason. As part of the plant protection measures against pests (birds, insects, and thieves) the wall as covered with a coarsely woven net. There was no incidence of insect attack during the experiment. There was an infestation of Alternaria spp. in the lower leaf of plant, which was removed immediately and taken away. No additional plant nutrient was supplemented to the various organic substrates during this experiment. Lettuce was harvested at 40 days after transplanting and data were recorded.

Our study shows that, the composition of the growing medium has significant effect on the lettuce growth characteristics (Table 3). As expected, when no additional fertilizer (P0) was supplemented, the lettuce yield was low, irrespective of the cultivar (Figure 2). The plant height, leaf length, leaf width, leaf area, fresh and dry weight of the V1 and V2 lettuce was significantly increased in P1 with few exception among the treatments It is observed that V1 plant height was nearly double in P1 (34.88 cm) compared to P3 18.63 cm) and V2 has higher plant height in P0 (30.40 cm) compared to P3 (16.95 cm). At the same way, leaf area of V1 increased in P1 (397 cm) and lowest in P3 (263 cm) whereas, for V2, the highest leaf area was in P1 (286 cm) and lower in P3 (198 cm). Likewise, fresh and dry weight of the lettuce cultivars were increased in P1 substrate.

It is well-known that, physical, chemical and microbiological parameters associated with growing media affect the agronomic performance of the grown crops (Santamaria et al., 2000; Boroujerdnia and Ansari, 2007; Islam et al., 2012; Tsiakaras et al., 2014; Jahan, 2017). Growing media or substrates include all materials that can be used to grow plants in a variety of production systems, such as greenhouse cultivation, containerized ornamental plant production, and urban agriculture (Cao et al., 2014). Vermicompost, cocodust, and others organic/inorganic components has been traditionally used as growing substrate (Méndez et al., 2011). However, a mixer of these organic and inorganic substances would be an excellent combination to improve the substrate physical and chemical properties. It is reported that a proper combination of different substrate enhanced the plant agronomic characteristics due to the optimal C/N ratios (Dumroese et al., 2011). The growing media constituents in the present study were chosen arbitrarily and not optimized with respect to the needs of a lettuce crop. Even though, the highest nutrient content was found when soil was amended with vermicompost and spent mushroom compost, about two times higher in N, P, and K (P3) than in the vermicompost-coir amended variant (P1), the higher growth attributes of both lettuce cultivars in substrate P1 might be due to the optimum bioavailable percentage of N, P, and K compared to P3. These findings prove that nutritional compositions in a growing medium are not only important factor for crop growth but should be rational percentage. The beneficial impact of coir on water holding capacity and porosity on plant biomass has previously been reported (Lokesha et al., 1988; Evans and Stamps, 1996).

In the framework of this proof of concept, we show that it is possible to grown leafy vegetables vertically on non-utilized building walls and that choice of growing medium composition and cultivar affects the agronomic performance. Our results also showed that the plant cultivar and growing medium is selective therefore, the cultivar and the growing medium properties must be adjusted to the specific plant variety/cultivar.

Sensory quality, such as crispness, sweetness, bitterness, and appearance of lettuce leaf influence its consumers' acceptance. The red-leafed cultivar V2 displayed the top and bottom score among the treatments in terms of crispiness, when grown in P1 and P0, respectively (Figure 3). The V2's sweetness was perceived as highly and slightly acceptable by all panel members. With the respect of sweetness, produce from V2P1 obtained highest acceptable score whereas V1P1 got highest unacceptable score. The highest acceptable appearance score was obtained by leaves from V2P1 treatment. However, the sensory quality taste results showed that crispiness, sweetness, and appearance of the red-leafed cultivar grown in the soil-vermicompost-coir mix was perceived higher than in all other treatments. The sensory quality including crispness and bitterness of leafy vegetables is greatly affected by the availability of the mineral in the growing medium (Gül et al., 2005). Other researcher also demonstrated that growing media has a significant control over the sensory attributes of leafy vegetables due to the varied physical and chemical properties of the substrate (Fontana et al., 2018). This emphasizes the need for adjustment of chemical and physical properties of the growing media to the crop's demand when aspiring sustainable urban food production.

It is reported that an enhanced consumption of fresh food is one of the reasons for increased number of foodborne outbreaks (Matthews, 2014). Usually, the outbreaks related to the fresh products are associated with pathogens, such as Salmonella, Listeria monocytogenes or pathogenic Escherichia coli (Heaton and Jones, 2008). Manures are widely used in organic farming, therefore, potentially, microbial contamination of organically grown plants may be higher than in conventional cultivation (Maffei et al., 2013). Consumers frequently consider that organic vegetables are better because they have less pesticide residues, but they ignore the microbiological safety issues resulting from organic production (Denis et al., 2016).

In the current study, none of the growing media showed a consistent picture when retention of the selected microbes was compared on the two cultivars (Table 4). Lettuce grown in the same growing medium, significantly lower CFU of the selected microbial groups were detected on the V2 cultivar as compared to the V1 cultivar, when grown in the P0, but considerably higher counts were found on the red-leafed than the green-leafed one, when the cultivars were grown in P2. Previously, it is noted that, plant growing in organic media is more prone to microbial infestation (Center for Science in the Public Interest, 2015). To produce safe ready-to-eat food, to assess risks and suggest hurdles, these factors must be clarified for future studies and uses. Despite the absence of Salmonella spp. in the present study, the high prevalence Enterobacteriaceae-genera is a concern. In food safety contexts, E. coli is used as an indicator organism, thus acting as a proxy for less abundant fecal contaminants (Yu et al., 2018). But the high prevalence of Klebsiella spp. in some of the treatments is disturbing from a food safety perspective. If such high numbers are an artifact or not, needs to be tested in future studies, considering standard food safety techniques and verification of presumptive colonies. To ensure that the urban produced ready-to-eat leafy greens are suitable for human consumption is a fundamental priority.

In the economic analysis, the cost benefit ratio was calculated in the Bangladesh context and the unit was expressed as Bangladeshi currency (BDT). After calculation, it is clearly shown that V1P1 has highest net economic return (4,619 BDT) having benefit cost ration (BCR) 1.37, whereas least economic return was attained at V1P3 combination having BCR 1.04 (Figure 4).

In this experiment, the vertical garden was installed for 10 years and installation cost was 8,960 BDT. Since this amount will not be included in input cost after the first season; the garden will be economically more productive for further cultivation, at least by 8,960 BDT.

Cost reduction, effective production, and better quality produce increase the incentives of food productions in urban areas. However, high installation costs, labor cost in urban area are problems and need to be analyzed (Krusemana and Bade, 1998). Establishment costs for urban farm, their economic feasibility, and food–energy nexus are significant issues for food production system (Kan et al., 2018; Schlor et al., 2018).

In this research, it is seeking the application of techniques that assist the rural producers in obtaining greater monetary returns, encouraging them to continue in the agricultural activity in unutilized area. The ultimate net economic return is depending on the yield of the production. Higher yield of plant food is directly related to the economic thus ensure the production sustainability. Materials that are easily available and accessible to the local people, such as organic manure can be used as growing media (Vendruscolo et al., 2019).