Improving the Fertigation of Soilless Urban Vertical Agriculture Through the Combination of Struvite and Rhizobia Inoculation in Phaseolus vulgaris

Abstract

Soilless crop production is a viable way to promote vertical agriculture in urban areas, but it relies extensively on the use of mineral fertilizer. Thus, the benefits of fresher, local food and avoiding the transportation and packaging associated with food import could be counteracted by an increase in nutrient-rich wastewater, which could contribute to freshwater and marine eutrophication. The present study aimed to explore the use of mineral fertilizer substitutes in soilless agriculture. Phaseolus vulgaris (common bean) was fertilized with a combination of slow-releasing fertilizer struvite (a source of N, P, and Mg), which is a byproduct of wastewater treatment plants, and inoculation with Rhizobium (a N2-fixing soil bacteria). The experiment included three bean-production lines: (A) 2 g/plant of struvite and rhizobial inoculation; (B) 5 g/plant of struvite and rhizobial inoculation, both irrigated with a Mg-, P-, and N-free nutrient solution; and (C) a control treatment that consisted of irrigation with a full nutrient solution and no inoculation. Plant growth, development, yields, and nutrient contents were determined at 35, 62, and 84 days after transplanting as well as biological N2 fixation, which was determined using the 15N natural abundance method. Treatments A and B resulted in lower total yields per plant than the control C treatment (e.g., 59.35 ± 26.4 g plant–1 for A, 74.2 ± 23.0 g plant–1 for B, and 147.71 ± 45.3 g plant–1 for C). For A and B, the nodulation and N2 fixation capacities appeared to increase with the amount of initially available struvite, but, over time, deficient levels of Mg were reached as well as nearly deficient levels of P, which could explain the lower yields. Nevertheless, we conclude that the combination of struvite and N2-fixing bacteria covered the N needs of plants throughout the growth cycle. However, further studies are needed to determine the optimal struvite quantities for vertical agriculture systems that can meet the P and Mg requirements throughout the lifetime of the plants.

Plant Beneficial Bacteria and Their Potential Applications in Vertical Farming Systems

“…In a study with common bean, plants in the treatment groups inoculated with Rhizobia spp. and irrigated with a N-free nutrient solution sustained no signs of N-deficiency throughout the growth cycle [ 58 ]. Another study with hydroponic bean found that a rhizobial inoculation led to the successful nodulation and sustenance of normal N levels in tissue, but only when the inorganic N supply was restricted [ 59 ].…”

“…Another study with hydroponic bean found that a rhizobial inoculation led to the successful nodulation and sustenance of normal N levels in tissue, but only when the inorganic N supply was restricted [ 59 ]. In both of these studies, complications with cation uptake (resulting from the absence of the NO 3 − anion from the nutrient solution) led to smaller plants in general, but the N demand was satisfied [ 58 , 59 ]. For N-fixing bacteria to gain a more widespread commercial use in soilless systems, electrochemical imbalances resulting from reduced-N nutrient solutions should be resolved, and applications to non-leguminous plants—specifically, common hydroponic plants such as lettuce, tomato, and other leafy greens—should be further researched and developed.…”

Hydroponic Common-Bean Performance under Reduced N-Supply Level and Rhizobia Application

“…Despite this potential, only a small number of research studies have been conducted aiming to optimise BNF and minimise mineral-N application to the common bean grown in soilless systems [ 27 ]. A range of mineral-N application levels, in combination with rhizobia inoculation in the hydroponically cultivated common bean was evaluated by Kontopoulou et al [ 28 ] and Arcas-Pilz et al [ 29 ]. However, given the severe N starvation applied in both studies, BNF capacity was too low to meet the plant requirements, and so yield and profitability were compromised.…”

A simplified non-greenhouse hydroponic system for small-scale soilless urban vegetable farming

“…There is already a move from traditional farming systems to protected farming systems mainly soilless farming [4] . Fresh farming systems to produce vegetables specifically while reducing land use are being exploited [5] . Some studies have already shown soilless culture to be an alternative cropping system to traditional farming in terms of reducing the problems related to conventional farming such as; low profitability [6] .…”

“…The purpose of this study was to develop a simplified low-cost vertical hydroponic unit to increase vegetable production within urban centers in developing states that still have low adoption of soilless farming technologies and yet battling with food insecurity challenges. Soilless farming is a sustainable method for supporting vertical agriculture in urban areas [5] . Vertical farming in itself has the capacity to produce food in a climate-resilient way while using low land as compared to traditional soil culture systems.…”