Substrate Properties and Fertilizer Rates on Yield Responses of Lettuce in a Vertical Growth System

The increased demand for food and the challenge for space for agriculture production in urban centers have made the vertical growth system an interesting trend. Agriculture is no longer only the horizontal, traditional, and soil grown method. Urban agriculture has created ways for inner city growers to be able to farm in a restricted space. Vertical farming is the practice of growing crops, especially leafy vegetables such as lettuce, in vertically stacked layers, as this results in significantly higher plant population per unit area. Two research trials were conducted in the fall of 2018 and the spring of 2019 to determine the effect of substrate properties and fertilizers on lettuce yield to optimize the urban production of lettuce. Three substrates (P3 + CF7 = Perlite 30%: Coco fiber 70%, PB7 + C3 = Pine Bark 70%: Compost 30%, and PB9 + C1 = Pine Bark 90%: Compost 10%) along with full and half rates of a fertilizer blend (VertiGro Organics fermented molasses, Ohrstrom’s Maxicrop liquid seaweed, and organic mineral blend) were evaluated on fresh and dried weight yield of lettuce. Substrate physical properties (air space, water holding capacity, total porosity, and bulk density) and substrate volumetric water content were also determined. ‘Nevada’ and ‘Optima’ lettuce varieties showed similar yield responses (fresh and dried weight) to substrate and fertilizer rates in both trials. In the fall 2018 trial, the highest fresh weight was observed in substrate PB7 + C3 with half fertilizer rate, with a mean plant weight of 41.13 g and 49.75 g for ‘Optima’ and ‘Nevada’, respectively. The least mean fresh weight was observed in half fertilizer rate of substrate P3 + CF7. For the spring 2019 trial, PB7 + C3 in half fertilizer strength gave the highest fresh weight for ‘Optima’ and ‘Nevada’ (45.64 g and 41.13 g, respectively). These values were statistically comparable to all other treatments except for substrate P3 + CF7 in full and half fertilizer which gave the least mean fresh weight. Volumetric water content in substrates P3 + CF7 recorded the highest average, while PB7 + C3 gave the least. Higher water holding capacity, total porosity, and lower airspace were observed in substrate P3 + CF7. Higher airspace was observed in PB9 + C1 but was comparable to PB7 + C3. PB7 + C3 gave the highest bulk density in both trials.