Resource Efficiency of Swiss Chard Crop in Vertical Hydroponic Towers Under Greenhouse Conditions ^†

Abstract

Resource efficiency is essential in today’s approach to horticulture. The global problems of water scarcity, soil pollution, biodiversity loss, and rapid growth of the global population require increased food production with fewer resources. Resource efficiency is an indicator that allows defining how much biomass an agri-food system can produce per unit of the resource used. Closed hydroponic systems, such as vertical hydroponic towers (VHTs), exhibit high resource efficiency. In these systems, the water use efficiency (WUE) and the nutrient use efficiency (NUE) can be calculated in terms of the water loss through transpiration and the ion concentration in the nutrient solution. The research aimed to determine the WUE and NUE for chard crops in VHT under greenhouse conditions and to evaluate its feasibility as an urban and peri-urban system for leafy vegetable production. Trials were carried out with chard in the fall 2024 in a tunnel-type greenhouse at the facilities of the Autonomous University of San Luis Potosi. The VHTs were built with a 20 L square lower deposit on which a cylindrical pipeline of 11.5 cm in diameter and 1.6 m in height was vertically placed. Each pipe had 45 growing containers distributed on 15 levels of three containers spaced vertically 9 cm and a density of 25 plants·m⁻². The experimental design was completely randomized with three treatments (75, 100, and 125% of Steiner’s nutrient solution) and three replications. The transpiration (Tr) of the crop (recording weight loss in the deposit) and the shoot fresh weight (SFW) of the plants were measured daily using a scale. An ANOVA and Tukey’s test for mean differentiation were performed with p < 0.05. Significant differences were found between treatments for SFW, WUE and NUE obtaining the best results at 75% of Steiner’s nutrient solution. Results show that WUE increased between 3 and 6 times, and NUE between 3 and 12 times compared to chard grown in soil. These results were equal and even higher than horizontal hydroponic systems or vertical farms. Vertical hydroponic closed towers installed in greenhouses are an optimal horticultural production system with high resources use efficiency. The implementation of VHT is feasible in areas where there is water scarcity or have a high population density.

1. Introduction

Food insecurity is a reality suffered by almost a third of the world’s inhabitants (approximately 2300 million people), among other things due to climate change and the scarcity of resources that affect food production [1,2]. Each year the population increases by 70 million inhabitants who require approximately 9.8 million tons of food. To achieve this production, a cultivation area of 1.5 million hectares is needed, of which 331,000 hectares would be irrigated. The necessary resources would range from twelve million m³ of water (drip irrigation) to 772 million m³ (surface irrigation); and around 36,400 tons of inorganic fertilizers would be consumed [3,4,5]. This perspective makes it necessary to transition towards more efficient agrifood systems in the use of resources such as soil, water, and fertilizers, while maintaining high yields.

In this sense, concepts such as water use efficiency and nutrient use efficiency are becoming increasingly important within horticultural food production. Determining the production capacity of an agri-food system per unit of resource consumed (whether soil, water, fertilizer, energy, etc.) and comparing it with other systems will improve the efficiency of the systems until the technology and available resources allow it. For instance, a tomato grown in Spain under effectively managed greenhouse conditions can increase 13 times the WUE compared to an open field system grown with poor water management. The same crop can increase water efficiency by up to 174 times in the Netherlands using closed hydroponic growing systems, in high-tech greenhouses with advanced climate control and CO₂ fertilization [6]. Reusing drain water and recovering transpired water stands out as effective techniques to improve water efficiency between 25% and 70%, respectively [7].

Introducing closed VHT in greenhouse conditions is an innovative approach to the production of green vegetables that, using one-fifth of arable land and consuming 85% less water increases yields by about 20 times. It can achieve WUE 133 times greater than open-field systems while consuming less energy than plant factories [8,9]. Overall, the water use efficiency of this system ranges 53 g FW·L⁻¹ H₂O for wheat and lettuce until around 245 g·L⁻¹ for tomato and strawberries, a lot higher than 3 g·L⁻¹ obtained for outdoor soil crops [10]. Closed VHT systems in greenhouses are an agri-food system that can be implemented in urban and peri-urban areas to ensure food security from the point of view of environmental sustainability. However, it is necessary to further investigation not only to boost resource efficiency used by these systems but also to determine their adaptability to produce a wide range of crops. The objective of the research was to determine the efficiency in the use of water and nutrients for cutting chards, grown in a closed VHT under greenhouse conditions.

2. Materials and Methods

The experiment is part of preliminary studies aimed at providing guidance for decision-making on the validation of the methodology and its macroscale application. To this end, Assessed Swiss chard (Beta vulgaris L. cv. “Ford Hook Giant”) for 11 days between 9–20 December 2024 (32–43 days after transplant, DAT) were assessed on a tunnel-type greenhouse at the Faculty of Agronomy and Veterinary of the Autonomous University of San Luis Potosi. Vertical system consists of vertical hydroponic towers (VHTs) 1.6 m height, which has a cylindrical pipeline 11.5 cm diameter vertically mounted over a square deposit of 20 L capacity at bottom. Each VHT has forty-five plants capacity, distributed in 15 levels, setting a vertical distance of 9 cm. VHTs were arrayed 1.2 m between them and 1.5 m among halls, resulting in a density of 25 plants·m⁻². A completely randomized design was followed in which three concentrations (75, 100 and 125%) of Steiner’s nutrient solution (mM): 12 NO₃⁻, 1 H₂PO₄⁻, 3.5 SO₄⁻², 7 K⁺, 4.5 Ca²⁺ and 2 Mg²⁺, and complemented with a micronutrients recipe (µM): 30 Fe, 25 B, 0.5 Cu, 5 Zn, 10 Mn and 0.5 Mo were evaluated in triplicate. Fertigation was programmed with an analogic timer with four 15-min pulses every 4 h during the day and one 15-min pulse at midnight.

The water use efficiency per plant (WUE, g.L⁻¹ H₂O) was computed as shoot fresh weight (SFW, g) divided by crop transpiration (Tr, L H₂O) as shown below:

WUE = SFW·Tr^−1,

(1)

Tr = (1000 × 45⁻¹)·(DW_t+1 – DW_t)·ρ^−1,

(2)

SFW = 0.81·(1000 × 45⁻¹)·(CPW_t – CPW_i),

(3)

where (DW_t+1 – DW_t, kg) is the difference between deposit weight at time (t + 1) and (t), and (ρ, kg·m⁻³) is the density of the nutrient solution. The (CPW_t – CPW_i, kg) is the difference between the cylindrical pipeline weight (with plants) at time (t) and the cylindrical pipeline weight (without plants) at initial time (i). To estimate the SFW in Equation (3), a shoot/root index of 4.26 was used, equivalent to an 0.81 SFW·TFW^-1 ratio (Supplementary Materials Table S1)

The nutrient use efficiency per plant (NUE_ni, g·g⁻¹ of consumed ion) was calculated as the quotient of SFW divided by consumed nutrients per plant (CNP, g) for three nutritive ions ([K⁺]: potassium UE, KUE; [Ca²⁺: calcium UE, CaUE; [NO₃⁻]: nitrate UE, NitUE), and one no-nutritive ion ([Na⁺]: sodium UE, NaUE) in (NUE_n-ni, g·mg⁻¹ of consumed ion), whose expressions see following:

NUE_ni = SFW·CNP⁻¹; NUE_n-ni = 1000⁻¹·SFW·CNP^−1,

(4)

CNP = 45⁻¹·MW·(IC_f·DW_f·ρ_f⁻¹ – IC_i·DW_i·ρ_i⁻¹),

(5)

where (MW, mg·mmol⁻¹) is the molecular weight of ion, and (IC_f·DW_f·ρ_f⁻¹ – IC_i·DW_i·ρ_i⁻¹, mM·m³) is the difference between products of ion concentration (IC, mM or mmol·L⁻¹) multiplied by deposit weight (DW, kg) and density of the nutrient solution (ρ, kg·m⁻³) at final (f) and initial (i) time.

Measurements of weight were determined each morning via a 1 g accuracy weight scale (Figure 1), and density weighing a 500 mL graduated cylinder with a 0.001 g accuracy weight scale. While the IC of each ion was measured in ppm via the LAQUAtwin 4M Kit by HORIBA^®, (HORIBA Advanced Techno, Co., Ltd.: Kyoto, Japan) at 32 and 43 DAT (initial and final time), taking 50 mL samples of the nutrient solution in the deposit of each VHT.

Analyzing data was performed via IBM SPSS Statistics^® v.26 software, applying an ANOVA and a post hoc Tukey test (p-value < 0.05) for differences between means. Data was checked for normal distribution using Shapiro-Wilk’s test and homogeneity of variance by Levene’s test to perform an ANOVA parametric.

3. Results

The Steiner’s nutrient solution concentration (SSC) did not significantly affect both transpiration and cumulative transpiration (Figure 2a,b). Swiss chard crop transpired an average of 15.4 mL·plant⁻¹·day⁻¹, ranged between 8.0 and 28.8 mL·plant⁻¹·day⁻¹ among 33–43 DAT for winter season. Overall, each chard plant loses 167.8 mL of water by transpiration process for 11 days, with a compound daily growth rate of 30%. If the allowable depletion is set at 25% of the tank of each VHT, it should be refilled with at least 4.8 L of water each week. With a maximum allowable depletion of 50%, water consumption should be adjusted to 9.6 L every two weeks. Water loss through transpiration presents a temporary accumulation that resembles positive linear growth.

Reducing SSC by 25% (one-quarter less fertilizers) increased the FW of the shoot part by 17% over control and treatment with 125% SSC (Figure 2c). Within the experiment period, plants fertigated with 75% of SSC, increasing their FW from 7.3 to 13.6 g, 6.3 g over 11 days; 0.57 g·day⁻¹ or a compound daily growth rate of 6%. The extrapolated results show that VHT gains an average of 180.4 g of shoot FW for cutting on a weekly basis. Figure 2d shows that WUE decreased as SFW (Figure 2c) and CTr (Figure 2b) grew. This is because the growth ratio of CTr was about 5 times higher than SFW, characteristic of an early stage of crop growth. This decrease in WUE varies from around 70 g·L⁻¹ to values close to 40 g·L⁻¹ between 33 and 43 DAT. The compound daily decline rate was -8% for the first 8 days, but it seems to stabilize over the last three days until it reaches almost zero. Plants at 75% SSC had a WUE 18% higher than control and 125% SSC.

Figure 3 shows that 75% of SSC treatment was visibly the most yielding and efficient. By reducing 25%, SSC increased: chard shoot FW by 22% (Figure 3a; 13.6 g·plant⁻¹), WUE by 24% respect 125% SSC (Figure 3c; 40.0 vs 32.3 g·L⁻¹). Potassium Use Efficiency (KUE) increased by 69% (Figure 3e), Calcium Use Efficiency (CaUE) by 84% (Figure 3f), and NO₃⁻ Use Efficiency (NitUE) by 58% (Figure 3g) compared to control. In general, reducing a quarter of fertilizers, each chard plant presented an average efficiency of 129.6 g·g⁻¹ KUE, 362.6 g·g⁻¹ CaUE and 45.5 g·g⁻¹ NitUE. Conversely, EC increases with the increasing SSC (Figure 3d). During the test time, SSC had no effect on either CTr (Figure 3b; 167.8 mL) or Sodium Use efficiency (NaUE) (Figure 3h; 3.6 g·mg⁻¹).

4. Discussion

Closed VHT systems inside greenhouses have a high capacity to transform resources (water and fertilizers) into biomass, which gives them a high productive potential with higher WUE and NUE values. Swiss chard plants grown in closed VHT inside greenhouses, fertigated at 75% SSC produce 3 to 6 times more biomass L⁻¹ of water than some green vegetables (lettuce, basil, arugula and chicory) grown in open field (average WUE: 6.5–12.0 g·L⁻¹) and up to 4 times more than these crops on traditional greenhouse (average WUE: 10.0–32.5 g·L⁻¹) [11]. Regarding fruit vegetables, VHT produces at least 2 times more than tomatoes grown in unheated ventilated greenhouses in Spain, but between 12 and 40% less than in high-tech closed greenhouses in the Netherlands [6]. Our results show that chard plants have WUEs comparable to artificial lighting plant factories (PFAL), exceeding by 4% (38.5 g·L⁻¹), the basil WUE, but 36% lower than lettuce WUE (62.5 g·L⁻¹) [11]. Other researchers have found WUE values in PFAL like ours. Sweet basil and lettuce produce between 44.5 and 56.3 g L⁻¹ in indoor growth with controlled light [12,13]. Righini et al. [10] present WUE values for different crops in PFAL and show that highly WUE-efficient crops (such as potato, soybean, cucumber, tomatoes, or strawberries) are four to six times more productive, while moderately WUE-efficient crops (such as lettuce and wheat) had WUE values only 30% higher than our Swiss chard plants with 75% SSC grown in VHT.

Swiss chard plants grown in closed VHT inside greenhouses, fertigated at 75% SSC also produced more biomass per unit of nutrient consumed than some crops in the soil. Potassium Use Efficiency increased by 3.8 times over that of maize, NitUE was 12 times higher than that of sugar beet and CaUE increased by 16% over oil palm [14,15,16]. However, contrasting our results with the production in PFAL, KUE decreased 5.3 times, CaUE 6.9 times and NitUE 7%, compared to lettuce [13]. Swiss chard also has KUE and CaUE values that were 3.2- and 3.8-times lower bat conversely, they were 66% more efficient in NitUE than those of basil and sweet basil [12].

Growing chard in closed VHTs in low-tech greenhouses can be a strategy that easily increases WUE and NUE compared to production in the open field and in traditional greenhouses. However, it is necessary to study other aspects such as climate control and radiation deficits to reach values close to those obtained in high-tech production systems or PFAL without requiring their energy resources. In addition, it is necessary to carry out an in-depth economic feasibility analysis to establish concrete strategies for its implementation.

5. Conclusions

Closed VHT in unheated greenhouses with natural ventilation offers high potential to produce environmentally sustainable leafy vegetables. They efficiently convert resources into food, which is very interesting for their implementation in densely populated urban areas or with water scarcity. These systems can produce three times more biomass than open field cultivation, more than twice as much as the traditional greenhouse, and comparable to some industrial plant crops, consuming the same amount of water. They also generate one-third more biomass per unit of nutrient consumed, particularly for potassium, calcium and nitrates, compared to open-field systems. However, it is necessary to evaluate construction costs, operational management issues and their scalability potential before proposing them as an urban agriculture alternative.

Reducing the concentration of dissolved nutrients in fertigation is a simple strategy to improve water and nutrient use efficiency. However, this approach is limited by the crop requirements. Therefore, adopting new devices, equipment, and decision-making tools to regulate the climate and light conditions, key factors in transpiration and nutrient uptake, is essential to achieve efficiency levels comparable to vegetables grown indoors in plant factories.

Future research should focus on both determining the efficiency of macro and micronutrient use and the accumulation of non-nutritive ions, avoiding imbalances caused by their different absorption rates through real-time monitoring using low-cost ion-selective electrodes to perform correct fertigation, maximizing the yield of different crops and minimizing the use of resources.