Plant growing containers: (A) Dutch bucket, (B) 25-cm regular plastic container (RPC) used for growing lettuce using different liquid organic fertilizer in Expt. 1. The 10-inch RPC was only used in Expt. 2.
Visual comparison of lettuce grown under different containers, fertilizers, and harvesting time: (i) 14 and (ii) 28 d after transplanting. Pictures shown include (A, B, C) DB with earthworm casting, sugarcane molasses, and fish emulsion, respectively, (D, E, F) RPCs with earthworm casting, sugarcane molasses, and fish emulsion, respectively.
Root growth under different containers, fertilizers, and harvesting time: (i) 14 and (ii) 28 d after transplanting. Pictures shown include (A) Dutch bucket (DB) with earthworm casting, (B) DB with sugarcane molasses, (C) DB with fish emulsion, (D) regular plastic container (RPC) with earthworm casting, (E) RPC with sugarcane molasses, and (F) RPC with fish emulsion.
Visual comparison of lettuce grown under different liquid organic and synthetic fertilizers after 30 d of transplanting in regular plastic containers: (A) earthworm casting (F1), (B) sugarcane molasses (F2), (C) synthetic fertilizer (F4), and (D) lettuce roots under F1, F2, and F4 fertilizers.
Growth comparison of dwarf tomato grown under different liquid organic and synthetic fertilizers after 60 d of transplanting in regular plastic containers: (A) earthworm casting (F1), (B) sugarcane molasses (F2), (C) synthetic fertilizer (F4), and dwarf tomato roots under F1, F2, and F4 fertilizers.
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Organic production is becoming increasingly popular among producers in controlled environment agriculture. However, selecting a suitable fertilizer for organic production can be complicated, as commercially available organic fertilizers have widely different nutrient compositions. The goal of this study was to evaluate the effectiveness of several liquid organic fertilizers and compare their performance with a synthetic fertilizer for growing lettuce and dwarf tomato in containerized production systems under a controlled environment. Two consecutive experiments were conducted. In Expt. 1, three commercial liquid organic fertilizers [earthworm castings (F1), sugarcane molasses (F2), and fish emulsion (F3)] were evaluated under two different containerized systems [Dutch bucket (DB) and regular plastic container (RPC)]. The best-performing fertilizers (F1, F2) were then compared with synthetic fertilizer (F4) in Expt. 2. In Expt. 1, lettuce was harvested 14 and 28 days after transplanting to assess shoot growth. In Expt. 2, dwarf tomato was also considered along with the lettuce, which were harvested 60 and 30 days after transplanting, respectively. Besides evaluating the regular growth parameters in both experiments, lettuce leaf tissue and leachate analyses were performed in Expt. 2. In Expt. 1, the F1 fertilizer outperformed F2 and F3, resulting in a 28% and 32% higher fresh weight in the DB system, and a 57% and 41% higher fresh weight in the RPC system, respectively. In addition, F1 led to improvements in the RPC system compared with the DB system, with increases of 28% in fresh weight, 20% in dry weight, 48% in leaf area, 26% in shoot width, 126% in root fresh weight, and 47% in root length. In Expt. 2, results showed that F1 performed similar to or better than F4 for growing lettuce and dwarf tomatoes in container hydroponic systems. Leaf tissue and leachate analyses also showed similar results. The findings of this study indicate that synthetic fertilizer could be replaced by some liquid organic fertilizers, and a single organic fertilizer could be used instead of several for organic leafy green production. Fruit crops such as tomato may require more than one organic fertilizer to provide the correct ratio of all nutrients.
Controlled environment agriculture (CEA) facilities, such as greenhouses and indoor farms, are becoming popular for increasing local food production within a smaller footprint and with less resource consumption (Cowan et al. 2022; Dsouza et al. 2023; Vatistas et al. 2022). CEA allows for control over plant growing factors, such as temperature, humidity, CO2, and light levels, which would be left to chance in traditional agriculture (Chowdhury et al. 2021a, 2023). Other advantages of CEA facilities include uniform growth rates, high-quality yields, lower rates of disease and pest infestation, and year-round crop production (Gruda et al. 2019). In addition, hydroponic cultivation, a method of growing crops without soil, under CEA facilities has boosted production rates and is becoming popular among growers. Recently, the US Department of Agriculture–National Organic Program (USDA-NOP) has permitted the certification of hydroponically grown crops as organic (NALC 2022). Consequently, organic farming methods, such as the use of organic substrates, fertilizers, pesticides, and biological control, have become more popular in the hydroponic and CEA industries (Ahmed et al. 2021; Rogers 2017).
Organic products are becoming increasingly popular among consumers due to concerns about the potential health risks associated with the excessive use of synthetic fertilizer and pesticides in crop production (Sabry 2015; Tagkas et al. 2024; Zhou et al. 2025). In addition, growers are motivated by economic benefits and ecological awareness (Ahmed et al. 2021; Kacira et al. 2017; Murakami et al. 2021). The demand for organic produce is rising globally. US sales of organic food products increased by $25.1 billion from 2010 to 2021 (ERS 2023). Since organic food became available in stores, sales of fresh vegetables and fruits have consistently surpassed other types of organic produce. In 2021, the annual sale of organic lettuce (Lactuca sativa L.) reached about $276 million (NASS 2021), highlighting its significant role in the American diet and nutrition.
Typically, CEA growers use water-soluble synthetic fertilizers and inorganic media (such as rockwool or foam) for lettuce and other crop production. Synthetic fertilizers and inorganic media have numerous problems with environmental sustainability (Chowdhury et al. 2021b; Kumar and Cho 2014; Smith 2016). In contrast, organic fertilizers have less impact on the environment, as they use waste plant or animal-based materials that are often byproducts from agricultural industries and release nutrient ions slowly through microbial activities (Niemiera and Wright 1986; Ruiz and Salas Sanjuan 2022; Wang et al. 2023). Organic fertilizer can be substrate incorporated or applied via irrigation in a water-soluble form. Currently, organic growers primarily use substrate-incorporated fertilizer for organic crop production (Alneyadi et al. 2024). However, there are several limitations to using solid-state or substrate-incorporated organic fertilizers in soilless cultivation systems. In our previous study, three organic fertilizers were applied to lettuce alongside a conventional fertility source standardized to 100 mg·L−1 N (nitrogen) to assess the efficacy of each fertilizer type. A fulvic acid-based liquid organic fertilizer (Espartan) outperformed other substrate-incorporated compost-based fertility sources, namely food scrap, yard trimming, home compost, and dairy manure compost (Floom et al. 2024). Use of water-soluble fertilizers may offer better control over nutrient management for organic crops.
Commercially available liquid fertilizers are derived from natural plant extracts, fish emulsion, seaweed, molasses, yucca extract, humic acids, earthworm castings, kelp, fermented sugar beet molasses, wheat barley, corn, animal bone, and blood. In our previous study, we identified that earthworm casting and sugarcane molasses–based liquid organic fertilizers contain relatively similar amounts of macro- and micronutrient ions as commercial synthetic fertilizers (Chowdhury et al. 2024a). Due to the convenience of applying water-soluble fertilizer using existing irrigation equipment, CEA growers prefer liquid organic fertilizers over substrate-incorporated fertilizers. Organic fertilizers contain nutrients in complex forms that must be broken down by microbes into basic components before plants can absorb them. The dissimilation processes and nutrient ion availability can vary based on the inoculated microbes, root zone environment, type of hydroponic system (i.e., open or recirculating), and hydroponic methods (i.e., liquid culture or substrate-based systems) (Niemiera and Wright 1986). Our previous studies indicate that liquid organic fertilizers perform better in substrate-based hydroponic systems compared with liquid culture systems, such as nutrient film technique or deep water culture (Chowdhury et al. 2024b).
Water-soluble organic fertilizers may provide better control over nutrient supply to the crop compared with substrate-incorporated fertilizers; however, they vary widely in their nutritional composition. Therefore, selecting a suitable fertilizer can be complicated for CEA growers. Comparing plant growth performance between liquid organic and traditional synthetic fertilizers is necessary to ensure market-compatible production and competitive pricing. In addition, cultivation containers (i.e., DB and RPCs) may have a significant impact on plant growth as water and nutrient availability vary due to their physical structure. Therefore, this study aimed to evaluate the effectiveness of three liquid organic fertilizers for growing lettuce in two different containerized hydroponic systems and compare the production performance between the selected liquid organic and synthetic fertilizers under controlled environment conditions.
The study was conducted in a Venlo-type polycarbonate-covered greenhouse at the Ohio State University, Wooster, OH, USA (40.78°N, 81.93°W). The greenhouse was equipped with a thermal shade screen and a supplemental lighting system. The ambient temperature was maintained using an evaporative fan-pad cooling system. The ambient environmental parameters and light conditions were automatically monitored and controlled by a climate control system (Wadsworth Control Systems, Arvada, CO, USA). In Expts. 1 and 2, the temperature, humidity, and photosynthetically active radiation (PAR) level were 23.2 ± 3.4 °C, 63.1% ± 12.7%, and 187.0 ± 44.1 W·m−2 and 19.0 ± 0.8 °C, 48.9% ± 4.8%, and 174.9 ± 50.8 W·m−2, respectively. The total photoperiod was 16 h (6:00 AM to 10:00 PM), and supplemental lights were turned on and off when PAR levels were ≤250 W·m−2 and ≥350 W·m−2, respectively.
Two separate experiments were conducted. Commercial liquid organic fertilizers were evaluated in Expt. 1, which ran from 15 May to 26 Jun 2023. The best-performing fertilizer(s) from Expt. 1 were compared with a synthetic fertilizer in Expt. 2, conducted from 12 Oct to 12 Dec 2023. A commercial lettuce cultivar, Green Butter (Johnny’s Selected Seeds, Winslow, ME, USA), was used in both experiments. Two different containers, the DB and RPC, were considered in Expt. 1 to assess suitability for production (Fig. 1). The Dutch buckets can retain nutrient solution at the container bottom due to its elevated drain line, which maintains a zone of saturation at the container bottom. Moreover, the Styrofoam lid on the DB provides protection from light and debris, reduces algae growth, and decreases evaporation from the surface. Both the DB and RPC containers were filled with commercial growing media (Pro-Mix-BX; Premier Tech, Rivière-du-Loup, QC, Canada), and all containers were saturated before transplanting seedlings. The nutrient solution was provided through a drip irrigation system. Submersible pumps (Hydrofarm AAPW400, Petaluma, CA, USA) were set to deliver the nutrient solution. As the containers (growing media) were fully saturated before transplanting, the pumps ran for only 30 s every hour for the first 2 weeks and 1 min per hour for the last 2 weeks from 6:00 AM to 10:00 PM, resulting in 400 mL and 800 mL of nutrient solution delivery per container per day in the first and last 2 weeks, respectively, in both experiments.
Citation: HortScience 60, 5; 10.21273/HORTSCI18466-25
Fertilizers certified by the Organic Materials Review Institute were sorted previously based on their nutrient profile (Chowdhury et al. 2024a). Among the considered liquid organic fertilizers, earthworm casting (F1) (Grow Big®; FoxFarm, Arcata, CA, USA), sugarcane molasses (F2) (Pre-Empt; Hort Americas, Fort Worth, TX, USA), and fish emulsion (F3) (Aqua Power; SaferGro Laboratories, Ventura, CA, USA) were selected for Expt. 1. Earthworm casting contains vermicast and essential micronutrients, with an N–P–K ratio of 3–2–6. Sugarcane molasses is derived from fermented sugarcanes and has an N–P–K ratio of 5–3–2. Fish emulsion (N–P–K: 5–1–1) is derived from marine fish species. A total of 60 containers (30 DBs and 30 RPCs) were used and 10 DBs and 10 RPCs irrigated with each fertilizer. All the containers were arranged in a completely randomized design with a total of six treatments with 10 replications. Each fertilizer was mixed in 1 L of water with the amount needed to attain 150 mg·L−1 nitrogen in the solution, and the respective electrical conductivity (EC) was recorded. The achieved EC levels for the F1, F2, F3, and F4 fertilizers were 1.42, 0.90, 1.25, and 1.30 mS·cm−1, respectively. The EC was then used as a target for further mixing of the nutrient solutions. EC and pH monitoring occurred using EC and pH meters (COM-100, PH-200; HM Digital Inc., Redondo Beach, CA, USA) after refilling the tanks every 2 d.
Based on the results of Expt. 1, two liquid organic fertilizers, earthworm casting (F1) and sugarcane molasses (F2), were considered for Expt. 2, and a commercial water-soluble fertilizer (F4) (Jack’s Professional 20–10–20; J.R. Peters, Inc., Allentown, PA, USA) was selected for comparison of lettuce growth. This synthetic fertilizer was derived from ammonium nitrate, potassium phosphate, potassium nitrate, magnesium sulfate, boric acid, iron EDTA, manganese EDTA, zinc EDTA, copper EDTA, and ammonium molybdate. A 100 ppm N solution contains macronutrients N, P, K, Ca, and Mg: 100, 21.8, 83, 0, and 0.8 ppm and micronutrients including, B, Cu, Fe, Mn, Mo, and Zn: 0.034, 0.018, 0.250, 0.125, 0.005, and 0.013 ppm, respectively. In this experiment, a dwarf tomato variety (Micro Tom; Victory Seed Company, Irving, TX, USA) was also considered alongside the Green Butter lettuce. A total of 60 RPCs (30 for lettuce and 30 for dwarf tomatoes) were randomly placed for a total of three treatments with 10 replications.
Lettuce seeds were sown in 105-cell plug trays filled with a commercial seedling mix (Pro-Mix BX Mycorrhizae Growing Mix; Premier Tech, Rivière-du-Loup, QC, Canada) and irrigated with a nutrient solution at an EC of 1.1 mS·cm−1 and pH of 5.8 twice a day. The nutrient solution was prepared using a water-soluble fertilizer (Hydro-Gro Leafy Greens; 4.3% N–9.3% P–35% K: Crop King, Lodi, OH, USA) at 100 mL·L−1 and calcium nitrate (CropKing) at 78 mL·L−1. The ambient environmental conditions were the same as mentioned previously. Two weeks after seeding, healthy lettuce seedlings were transplanted into the DBs and RPCs. In Expt. 1, lettuce was harvested twice, at 14 and 28 d after transplanting (DAT). In Expt. 2, lettuce and tomato were harvested at 30 and 60 DAT, respectively. Before transplanting, growth characteristics of the lettuce seedlings, such as leaf length, number of leaves, relative foliar chlorophyll (SPAD), leaf area, and fresh weight, were measured, as summarized in Table 1.
The lettuce physiological parameters were measured in the same manner in both experiments. Shoot parameters, such as plant width, number of leaves, leaf area, relative foliar chlorophyll (SPAD), fresh weight, and dry weight, were measured after harvest. Roots from three plants were washed from each treatment, and root growth was measured by taking root length, fresh weight, and dry weight after the shoot harvest. Images of the shoots and washed roots were taken for comparison. Root images were used to determine root length and area using ImageJ software (Version 1.54f; National Institutes of Health, Bethesda, MD, USA) in Expt. 1. Dwarf tomato plants were grown until the early fruiting stage. The measured shoot and root parameters for dwarf tomatoes were plant height, number of leaves, stem diameter, SPAD, number of flowers, number of flower clusters, number of fruits, fruit fresh and dry weight, plant fresh and dry weight, root length, and root fresh and dry weight. All fruits, from small immature green to fully ripe red, were counted for number of fruits. Relative foliar chlorophyll values were measured using a commercial chlorophyll meter (SPAD-502; Minolta Corporation, Ltd., Osaka, Japan). Three SPAD readings were taken randomly from fully grown lettuce and tomato leaves before destructive harvesting for other data collection. The values were then averaged and recorded. Fresh weights were measured using an electric scale (ML3001T; Mettler Toledo, Columbus, OH, USA), and then the samples were placed in a drying oven (Heratherm OGS400; Thermo Fisher Scientific, Waltham, MA, USA) at 68 °C until a constant dry weight was reached before being weighed again.
Leaf tissue nutrient measurement was conducted in Expt. 2. Fully expanded leaves from the middle of each plant were collected and oven-dried for leaf nutrient analysis. All dried leaf samples were ground with a sample mill (Cyclotec™ 1093; FOSS Analytical, Hillerød, Denmark) and stored in plastic vials for tissue nutrient analysis. The plant leaf nutrient analysis was conducted at Ohio State University’s Service, Testing, and Research laboratory (Wooster, OH, USA). The total concentrations of essential plant elements (P, K, Ca, Mg, S, Al, B, Cu, Fe, Mn, Mo, Na, and Zn) were determined by microwave digestion with HNO3, followed by inductively coupled plasma emission spectrometry, according to Jones et al. (1991). Total nitrogen content in plant tissue samples was determined using the Dumas method, according to the Association of Official Analytical Chemists (AOAC 1990).
The total nitrogen and nutrient ions (P, K, Ca, Mg, S, B, Cu, Fe, Mn, Mo, and Zn) of leachates collected from tomato in Expt. 2 were also analyzed. Leachate was collected by placing a container underneath the RPCs 1 d before the final harvest, and samples were analyzed at the USDA laboratory in Wooster, OH, USA with an ion chromatography system (IC 600, Thermo Fisher Scientific).
The significance of differences between mean values was determined by analysis of variance using Minitab 21.4.2.0 software (Minitab, State College, PA, USA). Expt. 1 had a factorial arrangement of treatments with fertilizer and container type as main factors. Means were separated using Fisher’s protected least significant difference test where α = 0.05. The graphs were prepared using MS Excel (version 2023; Microsoft Corporation, Redmond, WA, USA).
The effects of fertilizers and containers on lettuce growth in Expt. 1 varied with harvesting time. At the first harvest, lettuce shoot parameters varied significantly due to different liquid organic fertilizers, except for plant width and number of leaves. However, only plant width, number of leaves, and leaf area varied significantly for containers. The interaction between the container and fertilizer was significant only for biomass, not for other physical growth parameters, such as plant width, number of leaves, leaf area, and SPAD. In contrast, the effects of liquid organic fertilizers and containers were significant at the second harvest, except for plant width and SPAD, respectively. Similarly, the interaction between the container and fertilizer was significant, except for leaf area and shoot fresh weight, as shown in Table 2.
In most cases, lettuce grown in RPCs using earthworm castings (F1) showed better performance. In the RPC system, F1 produced an average fresh weight 57% higher than F2 and 41% higher than F3. In the DB system, the fresh weight of F1 was 27% and 31% higher than F2 and F3, respectively. Similarly, the average dry weight under F1 was 50% higher than F2 and 35% higher than F3 in the RPC system. In the DB system, it was 23% and 14% higher than F2 and F3, respectively. F1 also resulted in a 17% higher average leaf number than F2 and 30% higher than F3. F3 had the lowest average fresh weight in the first harvest and the lowest average leaf number in the second harvest. This may be due to clogging of pumps and irrigation lines with F3, caused by the accumulation of large amounts of biofilm. When using F1 in the RPC system, fresh weight was 28% higher, dry weight 20% higher, leaf area 48% higher, and plant width 26% higher compared with the DB system. Fresh weight and leaf area were also 19% and 20% higher in the RPC than in the DB system with F3. Figure 2 shows the shoots of lettuce grown under different fertilizers and container systems.
Citation: HortScience 60, 5; 10.21273/HORTSCI18466-25
In this study, tip-burn symptoms were also observed in all fertilizer-treated lettuce, which is often caused by a lack of calcium in developing leaves. Tip burn has been previously reported for Green Butter lettuce and has been used in tip burn control studies (Samarakoon et al. 2020). This condition can result from several factors, including rapid growth, water stress, low evapotranspiration, high soil fertility, high temperatures, high soluble salts, and pH (Collier and Tibbitts 1984; Frantz et al. 2004; Samarakoon et al. 2020; Westerdahl and Ploeg 2016). Expt. 1 was conducted in the summer, so the high temperature due to strong sun light and high humidity due to the continuous operation of the evaporative cooling system might have contributed to the tip-burn occurrence.
After the final harvest (28 DAT), it was observed that root parameters were not significantly affected by fertilizers and containers, except for root dry weight by fertilizer. However, the interaction between fertilizer and container type was significant for root fresh weight and root length (Table 3). Figure 3 shows the roots of lettuce grown under different fertilizers and container systems.
Citation: HortScience 60, 5; 10.21273/HORTSCI18466-25
There was no significant difference between the organic and synthetic fertilizers for lettuce shoot and root parameters, except for plant width and leaf area (Table 4). Plant width with F1 fertilizer was 4% greater compared with the F4 fertilizer, whereas the leaf area showed a 15% increase. The visual appearance of shoot and root conditions of lettuce grown under different fertilizers is shown in Fig. 4.
Citation: HortScience 60, 5; 10.21273/HORTSCI18466-25
Several tomato growth parameters, such as stem diameter, number of leaves, number of flower clusters, fruit fresh weight (FW), dry weight (DW), shoot FW, DW, and root FW, varied significantly depending on the fertilizer used. In contrast, plant height, SPAD, number of flowers per cluster, number of fruits per plant, root length, and root DW were not significantly affected by the fertilizers (Table 5). Specifically, there were no significant growth differences between the F1 and F4 fertilizers, except for the number of leaves and number of flower clusters. Plants treated with F4 fertilizer showed 40% and 71% more leaves and flower clusters, respectively, compared with those treated with F1 fertilizer. In contrast, there was a significant growth difference between the F2 and F4 fertilizer-treated plants, except for a few parameters such as SPAD, number of flowers per cluster, and root parameters. Overall, plants treated with F2 fertilizer showed the lowest performance among the fertilizers. Figure 5 shows the shoot and root conditions of dwarf tomatoes grown under different fertilizers.
Citation: HortScience 60, 5; 10.21273/HORTSCI18466-25
The accumulation of some macronutrients (i.e., N, K, Ca) and micronutrients (i.e., B, Fe, Mn, Cu) varied significantly depending on the fertilizer used for lettuce. However, other nutrient ions (i.e., P, Mg, S, and Zn) were not significantly affected by the fertilizers (Table 6). When comparing F4 and F1 fertilizers, no significant difference was observed for N, Ca, B, Mn, and Zn ion concentrations; however, K, Fe, and Cu ion concentrations varied significantly, with differences ranging from 9% to 90%. Similarly, no significant differences were found in K, Ca, Fe, and Zn ion concentrations between leaves treated with F4 and F2 fertilizers. F4-treated leaves had 12% higher N and 35% and 28% lower Mn and Cu ion concentrations, respectively, compared with F2-treated leaves. Regarding nutrient accumulation in dwarf tomato leaves, most of the nutrient ions (N, Ca, Mg, B, Fe, Mn, Zn, and Cu) were not significantly affected by the fertilizers, except for P, K, and S (Table 6). Between F4- and F1-treated tomato leaves, P and S ion concentrations varied significantly, with differences of 26% and 14%, respectively. Similarly, significant differences were observed only for K (25%) ion concentrations when comparing F4 and F2 fertilizers. On average, lettuce plants treated with organic fertilizers performed less effectively compared with those treated with synthetic fertilizer. However, both organic fertilizers (F1 and F2) led to nutrient accumulation in tomato leaves comparable to that achieved with synthetic fertilizer (F4). In addition, the nutrient levels in lettuce and dwarf tomato leaves were within or slightly exceeded the sufficiency range outlined by Pickens et al. (2022) and UM (2022).
According to the dwarf tomato leachate analysis, most of the nutrient ions varied significantly depending on the fertilizer used, except for TN, P, K, and Mo ions (Table 7). In general, the F1 fertilizer leached more ions (i.e., Mg, S, B, Fe, Zn, and Cu) compared with the other fertilizers (F2 and F4). In contrast, F4-treated containers leached the fewest nutrient ions. Specifically, F1-treated containers leached 170%, 76%, 254%, 198%, 390%, 297%, and 1163% more Mg, S, B, Fe, Mn, Zn, and Cu ions, respectively, compared with F4-treated containers. Similarly, F2-treated containers leached 26%, 27%, 157%, 398%, and 483% more Ca, S, Fe, Mn, and Cu ions, respectively, compared with F4-treated containers.
Liquid organic fertilizers perform better in substrate-based hydroponic systems compared with liquid culture systems (Chowdhury et al. 2024b); however, plant growth performance is greatly influenced by the nutrient composition of liquid organic fertilizers. The growing system is another factor that affects plant growth and yield through water and nutrient retention (Yang et al. 2024). According to the results of Expt. 1 (Table 2), lettuce grew similarly in the first few weeks after transplanting, and the effects of fertilizers and containers became prominent ≈4 weeks after transplanting. Among the fertilizers, earthworm casting (F1) and sugarcane molasses (F2) performed better than fish emulsion (F3). However, Shaik et al. (2022) found that fish fertilizer performs better than other plant-based organic fertilizers, even though the N–P–K ratio of both fish-based fertilizers was 5–1–1. This could be because of clogging issues due to biofilms we observed with fish-based fertilizer. Variation in nutrient composition and interaction with the substrate might be a reason for this yield difference.
When comparing solid organic fertilizers, such as soybean and bone meal, livestock manure, food scrap, yard trimming, crop residues (Barker et al. 2017; Zandvakili et al. 2019), and liquid organic fertilizers, such as fulvic acid, fish emulsion, sugarcane molasse, and oilseed extract (Floom et al. 2024; Shaik et al. 2022) with synthetic or conventional fertilizers, previous studies have shown that organic fertilizers often result in lower yields and growth parameters. Synthetic fertilizers used for growing different varieties of lettuce in a containerized hydroponic system were found to increase yield by 67% over organic fertilizers (Zandvakili et al. 2019). Other studies have shown that synthetic fertilizers yield 12% to 38% more than organic ones (Shaik et al. 2022). Barker et al. (2017) reported 64% lower yields of cabbage grown with organic fertilizer compared with chemical fertilizer. Lower yields with organic fertilizers have been attributed to lower and inconsistent nutrient availability; however, as evident in the current experiment optimized fertility management with organics could decrease the yield gap. In Expt. 2, no significant differences were observed between the earthworm casting (F1) and synthetic (F4) fertilizers for most of the growth parameters of lettuce and dwarf tomato (Figures 4 and 5). The possible reason behind this similar growth is the screening of liquid organic fertilizers based on nutrient composition. In our previous study, the survey and selection process of proper liquid organic fertilizers for optimizing fertilizer dosing was demonstrated (Chowdhury et al. 2024a).
Lettuce grown in RPCs showed similar or better results compared with the DB system in Expt. 1, despite the hypothesis that the DB system would promote higher yields due to its ability to retain water and nutrients at the bottom. During the washing of lettuce roots, it was observed that roots in the DB system, particularly under the F3 fertilizer treatment (Fig. 3), tended to grow outward rather than downward. In contrast, roots in the RPC system typically reached the bottom of the container. This difference may be attributed to roots in the DB system avoiding saturation and potentially anoxic conditions at the bottom, potentially contributing to the lower yield. In addition, the drainage holes in the RPCs may have improved root zone aeration, which could have influenced the mineralization rate of microorganisms and ultimately supported better lettuce growth in the RPC system.
The tissue analysis results indicate that fertilizer significantly influenced mineral accumulation in lettuce. The slow mineralization of organic fertilizers may explain this outcome. Zandvakili et al. (2019) also noted that fluctuations in organic fertilizer mineralization affect nutrient accumulation in lettuce tissue, whereas nutrient ions from synthetic fertilizers are immediately available to plants. Similarly, Song et al. (2020) highlighted the interaction between nutrient solution concentration and light intensity in determining the nutritional quality, mineral content, and antioxidant levels in lettuce. In contrast, most nutrients accumulated in dwarf tomato leaves were not significantly influenced by the organic or synthetic fertilizers used in this study. Because dwarf tomato plants were grown for a longer period, it is plausible that microbiomes mineralized sufficient nutrients over time. In addition, in our preliminary study on the screening and selection of liquid organic fertilizers (Chowdhury et al. 2024a), we observed that fertilizers based on earthworm castings and sugarcane molasses contained nutrient ion concentrations comparable to those in synthetic fertilizers. The results of the current study also demonstrated that liquid organic fertilizers can perform similarly to synthetic fertilizers. Moreover, nutrient accumulation in lettuce and dwarf tomato leaves was within or slightly above the sufficiency range reported by Pickens et al. (2022) and Byers (2022), as shown in Table 6. Therefore, selecting an appropriate liquid organic fertilizer is essential for successful organic crop production.
A similar phenomenon was observed in the leachate nutrient analysis. In organic cultivation, several studies have demonstrated how microbial activities accelerate mineralization and influence nutrient enrichment in the root zone (Lang and Elliott 1997; Niemiera and Wright 1986; Ruiz and Salas Sanjuan 2022; Wang et al. 2023). In Expt. 2, most leached nutrient ions under the control treatment (F4) were either statistically similar to F1 or F2, indicating that a comparable amount of nutrient ions was mineralized from the liquid organic fertilizers as provided in the control treatment. However, the concentrations of S, Fe, Mn, and Cu ions were significantly lower in the F4-treated containers, which might be due to the slower mineralization of these ions compared with macronutrients like nitrogen and phosphorus from the F1 and F2 fertilizers (Dhaliwal et al. 2019).
Generally, the cost of organic production under controlled environments greatly influences its adoption. Growers use many fertility sources on a single crop, but the findings of the current study indicate that a single fertilizer is enough for growing leafy greens. This will also increase the ease to use and cut down on the cost of production. Growers can even use familiar irrigation equipment for fertigation. In this study, the same volume of nutrient solution was provided to the plants of each treatment, but the volume of stock solutions required in the nutrient blending tanks to reach 150 mg·L−1 N were different. The dosing rates of the F1 (Grow Big® from FoxFarm), F2 (Pre-Empt from Hort Americas), F3 (Aqua Power from SaferGro Laboratories), and F4 (Jack’s Professional from J.R. Peters, Inc.) fertilizers were 2.5 mL/L, 5.9 mL/L, 3 mL/L, and 0.75 g/L. Therefore, the costs per liter of nutrient solution preparation were $0.083/L, $0.032/L, $0.089/L, and $0.006/L, respectively, clearly indicating that the use of synthetic fertilizers is significantly cheaper than any liquid organic fertilizers. However, because the production rate and nutrient quality of organically grown lettuce could be similar to that of synthetically grown lettuce, the higher market price of organic produce can compensate for the higher production costs. Fertilizer recommendations for high-wire crops (i.e., tomato, cucumber, or pepper) need further studies with the use of indeterminate cultivars and additional fertilizer supplementation.
The current study investigated the effectiveness of different liquid organic fertilizers (Expt. 1) and compared their performance with synthetic fertilizers for growing lettuce and dwarf tomatoes (Expt. 2) in containerized production systems under greenhouse conditions. Findings from Expt. 1 showed that lettuce grew better in RPCs with the earthworm casting and sugarcane molasses–based fertilizers compared with the fish emulsion–based liquid organic fertilizer and the DB system. In Expt. 2, it was observed that earthworm casting and sugarcane molasses–based liquid organic fertilizers could perform similar to synthetic fertilizers for growing lettuce in containerized production systems; however, further research is required to achieve similar growth and development with tomato. The findings of the current study indicate that with the selection of a proper liquid organic fertilizer, organic production in CEA could be achieved with no yield reduction. Moreover, a single organic fertilizer could be used instead of several for organic leafy green production, whereas high-wire crops, such as tomato, need further investigation.
Plant growing containers: (A) Dutch bucket, (B) 25-cm regular plastic container (RPC) used for growing lettuce using different liquid organic fertilizer in Expt. 1. The 10-inch RPC was only used in Expt. 2.
Visual comparison of lettuce grown under different containers, fertilizers, and harvesting time: (i) 14 and (ii) 28 d after transplanting. Pictures shown include (A, B, C) DB with earthworm casting, sugarcane molasses, and fish emulsion, respectively, (D, E, F) RPCs with earthworm casting, sugarcane molasses, and fish emulsion, respectively.
Root growth under different containers, fertilizers, and harvesting time: (i) 14 and (ii) 28 d after transplanting. Pictures shown include (A) Dutch bucket (DB) with earthworm casting, (B) DB with sugarcane molasses, (C) DB with fish emulsion, (D) regular plastic container (RPC) with earthworm casting, (E) RPC with sugarcane molasses, and (F) RPC with fish emulsion.
Visual comparison of lettuce grown under different liquid organic and synthetic fertilizers after 30 d of transplanting in regular plastic containers: (A) earthworm casting (F1), (B) sugarcane molasses (F2), (C) synthetic fertilizer (F4), and (D) lettuce roots under F1, F2, and F4 fertilizers.
Growth comparison of dwarf tomato grown under different liquid organic and synthetic fertilizers after 60 d of transplanting in regular plastic containers: (A) earthworm casting (F1), (B) sugarcane molasses (F2), (C) synthetic fertilizer (F4), and dwarf tomato roots under F1, F2, and F4 fertilizers.
Contributor Notes
We would like to thank Hunter Myers and Sarah Lanphear for help with system preparation, crop management, and data collection, and Lesley Taylor and Leslie Morris for leaf tissue and nutrient solution preparation and analysis.
This work was supported by US Department of Agriculture–Agricultural Research Service (grant number GR60071885) and OSU Research Internship Program.
Current address for M.C.: School of Engineering and Technology, Kentucky State University, Frankfort, KY 40601, USA.
Plant growing containers: (A) Dutch bucket, (B) 25-cm regular plastic container (RPC) used for growing lettuce using different liquid organic fertilizer in Expt. 1. The 10-inch RPC was only used in Expt. 2.
Visual comparison of lettuce grown under different containers, fertilizers, and harvesting time: (i) 14 and (ii) 28 d after transplanting. Pictures shown include (A, B, C) DB with earthworm casting, sugarcane molasses, and fish emulsion, respectively, (D, E, F) RPCs with earthworm casting, sugarcane molasses, and fish emulsion, respectively.
Root growth under different containers, fertilizers, and harvesting time: (i) 14 and (ii) 28 d after transplanting. Pictures shown include (A) Dutch bucket (DB) with earthworm casting, (B) DB with sugarcane molasses, (C) DB with fish emulsion, (D) regular plastic container (RPC) with earthworm casting, (E) RPC with sugarcane molasses, and (F) RPC with fish emulsion.
Visual comparison of lettuce grown under different liquid organic and synthetic fertilizers after 30 d of transplanting in regular plastic containers: (A) earthworm casting (F1), (B) sugarcane molasses (F2), (C) synthetic fertilizer (F4), and (D) lettuce roots under F1, F2, and F4 fertilizers.
Growth comparison of dwarf tomato grown under different liquid organic and synthetic fertilizers after 60 d of transplanting in regular plastic containers: (A) earthworm casting (F1), (B) sugarcane molasses (F2), (C) synthetic fertilizer (F4), and dwarf tomato roots under F1, F2, and F4 fertilizers.
