Abstract
Rockwool and peatmoss are commonly used substrates in the greenhouse industry due to their quality, stable pH, exceptional water retention properties and air porosity that is important for plant root development. Although rockwool is commonly used in deep water culture (DWC) hydroponic systems as the base support, there is a lack of studies comparing different types of substrates that could be used in DWC systems, especially considering the increasing market value and awareness of sustainable production in controlled environment agriculture. We identified 13 commercial substrate mixes with different compositions and conducted a series of studies in a DWC system in a greenhouse for three seasons to evaluate their effects on arugula ‘Slow Bolt’ (Eruca sativa L.) and lettuce ‘Summer Crisp’ (Lactuca sativa L.) growth, yield, and quality. The substrates tested significantly influenced the growth, yield, and quality of both arugula and lettuce. The average leaf fresh weight per plant could range from 44 to 190 g for arugula and 89 to 265 g for lettuce. The peat-based products outperformed the coir and other inorganic substrates (phenolic foam, rockwool). The substrate with 75% peat + 25% fine coir produced the greatest plant height, width, and biomass for arugula and lettuce over three growing seasons. Examining arugula and lettuce growth, the fall season produced plants with higher water and nutrient use efficiency, while plants grown during the winter had lower resource use efficiency. Further research is needed to engineer hydroponic substrates suitable for various seasons of leafy green production that results in comparable yield and quality.
Hydroponic food production in controlled environments has existed for decades, but the recent advances in technology and awareness of food safety in urban areas have dramatically promoted the hydroponic industry from $64 million in 1988 to $800 million in 2014 (Walters et al. 2020). Even though the United States expanded its hydroponics industry, reaching 1046 ha of greenhouse vegetable production in 2017, there is an opportunity to further expand, considering the amount of food imported from Canada (2852 ha in 2009) and Mexico (3676 ha in 2015) (Resh 2022). The choice of substrates for hydroponics production is primarily based on their properties to simultaneously provide sufficient water and oxygen levels to the roots. Commonly used substrates can be divided into two groups, inorganic and organic. Inorganic substrates include unmodified natural minerals (sand, gravel, volcanic tuff, pumice), synthetic materials (polyurethane and polystyrene foam, phenolic foam, hydrogel, and felt), and processed minerals (rockwool, perlite, vermiculite, expanded clay, zeolite). Organic substrates include natural organic matter (pine sawdust, pine bark, wood chips, peatmoss, coconut coir, and rice hulls) (Papadopoulos et al. 2008).
Since the introduction and utilization of rockwool in hydroponic production, it has become a widely used inorganic substrate due to its high aeration and water-holding capacity. However, the high energy demand of production, short-term use, and costly and environmentally unfriendly disposal of used rockwool, due to its slow decomposition, has become an increasing problem (Hardgrave and Harriman 1995; Nichols and Savidov 2009). Peat moss is another commonly used substrate, offering an option to rockwool in commercial greenhouse vegetable production. However, harvesting peat is associated with lowering the water table and land subsidence. In addition, mining peat from bogs releases stored CO2, a greenhouse gas that contributes to global warming (Poppe et al. 2021). Worldwide CO2 emissions from drained peatlands are estimated at 2.0 gigatons of CO2 annually (Günther et al. 2020). Having more options for selecting different substrates to use in the hydroponic systems has become a topic of interest due to economic and environmental concerns.
Previous research compared several organic substrates to grow cucumber (Cucumis sativus L.) in a run-to-waste hydroponic system as a comparison with rockwool, including coir, shredded pine wood, peat, bark, high-density wheat straw, and wood-waste, and found that plants grown in these organic substrates had similar yield compared with the ones produced in rockwool (Hardgrave and Harriman 1995). Coir, a by-product obtained from coconut husk with a potential annual production of 15 million metric tons, has been evaluated and used as an alternative to rockwool and peat due to its excellent aeration and water-holding capacity. In a Dutch bucket system, cucumber plants grown using coir showed similar canopy growth, photosynthesis and gas exchange, water use efficiency, nutrient concentrations, fruit yield, and dry biomass compared with peat or perlite substrates (Yang et al. 2023). Seeking coir as an alternative or a portion substitute for peat could provide economic and environmental benefits.
Unlike other hydroponic systems, deep water culture (DWC) typically uses fewer substrates that act as a physical support, and there are fewer substrate options to use in the system other than rockwool cubes. Typically, in the DWC systems of leafy greens, substrate is important to establish a healthy plant during the seedling stage and then upon transplanting to anchor the plant into DWC where the roots are suspended into the nutrient solution. The objective of this research was to evaluate and compare commercially available substrates that use different organic or inorganic materials for hydroponic leafy green production in DWC systems for three consecutive growing seasons. Seasonal plant growth variations, nutrient use efficiencies, and solution changes were also assessed to better understand the environmental effects on hydroponic DWC production.
Materials and Methods
Study location and environmental conditions.
This study was conducted in a greenhouse at the University of Georgia, Athens, GA, USA (33.93°N, 83.36°W) for three consecutive growing seasons to determine the environmental effects on plant production. Environmental parameters inside the greenhouse were measured to determine air temperature and relative humidity (HMP60; Vaisala, Vantaa, Finland), CO2 concentration (GMP252; Vaisala), and photosynthetic photon flux density (PPFD) (SQ-610 ePAR; Apogee Instruments, Logan, UT, USA). Environmental sensors were connected to a datalogger (CR1000X; Campbell Scientific, Logan, UT, USA), and daily light integral (DLI) and vapor pressure deficit (VPD) were calculated automatically. Average temperature, relative humidity, VPD, CO2 concentration, and DLI for the first season (summer, Jun 2022–Jul 2022) were 26.3 °C, 83.8%, 0.62 kPa, 397.9 mg·L−1, 28.14 mol·m−2·d−1, respectively. For the second season (fall, Sep 2022–Nov 2022) were 21.4 °C, 53.1%, 1.27 kPa, 417.4 mg·L−1, 19.35 mol·m−2·d−1, respectively. For the third season (winter, Nov 2022–Dec 2022) were 21.0 °C, 49.8%, 1.30 kPa, 432.7 mg·L−1, 9.34 mol·m−2·d−1, respectively.
Growth conditions, plant material, and treatments.
Two leafy greens, arugula ‘Slow Bolt’ (Eruca sativa L., Mountain Valley Seed, Salt Lake City, UT, USA) and lettuce ‘Summer Crisp’ (Lactuca sativa L., Johnny’s Selected Seeds, Winslow, ME, USA), were sown in 13 substrates (Table 1) and placed inside a walk-in growth chamber (25 °C, 800 ppm CO2) with an automated ebb-and-flow irrigation system, where the root zone was watered twice daily with a fertilizer solution at the rate of 50 mg/L N (Jack’s professional LX 15–5–15 Ca-Mg; JR PETERS, Allentown, PA, USA). Photoperiod inside the walk-in chamber was 16 h/d using light-emitting diode lights (RAY series with Physiospec indoor spectrum; Fluence Bioengineering, Austin, TX, USA) with PPFD of 200 µmol·s−1·m−2. Seeds were covered in the dark for the first 3 d to facilitate germination. After 2 weeks, we used an imaging procedure to quantify the germination rates as indicators for the initial growth differences between each substrate (Supplemental Fig. 1), and only minor differences were detected. The most similar seedlings with similar initial growth were transferred to a greenhouse and placed inside a 4.45-cm top diameter × 3.18-cm bottom diameter × 4.76-cm deep net cup (Orimerc Garden, Seattle, WA, USA) and then transplanted in a DWC hydroponics system. The DWC system used 243.8 × 121.9 × 20.3-cm (L × W × H) trays (OD black tray; Botanicare, Vancouver, WA, USA) covered with 243.8 × 121.9 × 2.5-cm polystyrene foam insulation boards (GreenGuard XPS; Kingspan Insulation, Atlanta, GA, USA) with 12 × 6 holes (3.8-cm diameter) for a total of 72 plants per tray. The plants located at the edge of the tray were used as a border, and each experimental unit had three plants per substrate tested. Each tray was aerated by four 5.08-cm air stones (Aquaneat, Madison, WI, USA) connected by 0.79 × 0.48-cm (outside × i.d.) clear extruded acrylic tubes (Dernord; Tangxia, Dongguan, China) to 3.75 L/s at 0.048 MPa aeration pump with a 1.27-cm outlet (EcoAir 7; EcoPlus, Vancouver, WA, USA). The system was filled with a fertilizer solution prepared using calcium nitrate, potassium sulfate, mono ammonium phosphate, potassium nitrate, magnesium nitrate, magnesium sulfate, boric acid, copper sulfate, manganese sulfate, ammonium molybdate, chelated iron, and zinc sulfate, resulting in a fertilizer solution described in Table 2. The solution’s initial pH and electrical conductivity (EC) were ∼6.5 and 1.5 dS/m, respectively. We used municipal water as our water source, whose quality parameters are shown in Table 2. During the entire growth period (4–5 weeks), we did not control the solution pH and EC, and the water was not refilled due to the large size of the DWC tray and more than half of the solution was not consumed, except for a water refill in the end of the summer season for lettuce before measuring the solution pH and EC.
List of commercial substrate products used in this study and their physical properties, including substrate type, shape, dimensions (fully expanded), bulk density, and porosities.
Detailed nutrient concentrations of fertilizer solution in deep water culture system and water used to make nutrient solution in this study.
Arugula plants were grown in the DWC system for 3, 4, and 4 weeks after transplanting (WAT), respectively, during summer, fall, and winter trial seasons, and lettuce plants were grown for 4, 5, and 5 WAT during the three trial seasons.
Substrate physical properties.
The substrate’s physical properties largely determine the growth and quality of plants. We measured the bulk density, air porosity, water porosity, and total porosity of the 13 substrates (Table 1) following the method described by Huang and Fisher (2013). We filled and soaked all the substrates into known-volume plug trays until saturation, measured the weight immediately (W1), drained the substrates for 15 min, and measured the weight again (W2). Substrates were put into the dryer for 3 d at 100 °C until constant weight, and the tray and substrate were weighed again (W3). Substrates were emptied from the trays and weighed (W0). Parameters were determined by air porosity (%) = 100 × (W1 − W2)/V, where V was the volume of the trays; water porosity (%) = 100 × (W2 − W3)/V; total porosity (%) = air porosity + water porosity; bulk density (g/mL) = (W3 − W0)/V. The measurements were repeated three times (n = 3).
Measurements.
Before we started the measurement, we used an imaging system to automatically calculate seed germination rate without statistical analysis; we added this information in the supplemental file (Supplemental Fig. 1). During the plant growth periods, weekly measurements were taken for plant height, width (average of two side widths), leaf chlorophyll content (CCM-200plus portable meter; Opti-Sciences, Hudson, NH, USA), leaf anthocyanin content (ACM-200plus portable meter; Opti-Sciences), solution pH and EC (HI5522-01 bench meter; Hanna instruments, Smithfield, RI, USA), and dissolved oxygen (DO, HI98193 portable meter; Hanna instruments).
At the end of the growth period, plants were harvested, shoot fresh weight was determined by weighing the canopy biomass, leaf area was measured using an area meter (LI-3100; LI-COR, Lincoln, NE, USA), and soluble solids content (SSC) was determined using a digital refractometer (HI 96801; Hanna instruments). Plant dry weight was determined after oven drying at 80 °C for 3 d. After drying, leaf samples were collected and shipped to Waters Agricultural Laboratories (Camilla, GA, USA) for tissue mineral concentration analysis. Leaf nitrogen (N) was determined by high temperature combustion process (Nelson and Sommers 1973). Leaf phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca) concentrations were determined by inductively coupled plasma atomic emission spectrophotometer (ICP-AES) after wet acid digestion using nitric acid and hydrogen peroxide (Twyman 2005). After plants were removed from the system, total water use was recorded. The solution samples were sent to Waters Agricultural Laboratories for water mineral concentration analysis for N, P, K, and Ca, determined directly by ICP-AES analysis. Plant water and nutrient use efficiencies were calculated at tray level as follows: plant water (dry weight/total solution used), N (dry weight/total N consumed from solution), P (dry weight/total P consumed from solution), K (dry weight/total K consumed from solution), and Ca (dry weight/total Ca consumed from solution). Nutrients consumed (N, P, K, Ca) were estimated by subtracting the initial nutrient content from the remaining nutrients in the solution determined after the end of the experiment.
Experimental design and statistical analysis.
For each crop, the study was arranged on a randomized complete block design (RCBD) with 13 substrates and three growing seasons (summer, fall, and winter) considered as treatment factors, three DWC trays were used as three blocks for each crop (total six DWC trays), and the substrate treatments were randomly assigned to one DWC tray as a replication, with three plants per replication (measured individually). Analysis of variance (ANOVA) regarding the effects of substrate and season as well as their interaction effects on plant growth, leaf quality, nutrient use efficiency, solution pH, EC, and DO were analyzed using R (R Core Team 2022) with package “agricolae,” and the mean comparison was conducted using Tukey’s honestly significant difference test at 5% probability (α = 5%) when treatment effects differ according to ANOVA test.
Results
Effects of substrates and seasons on arugula and lettuce growth performance.
Arugula plants grown using JiHPC overall had the greatest height and width regardless of season, and RiC had the least growth in height and width (Fig. 1). The ElC and ElOC substrates resulted in lower arugula height and width, especially during summer and fall, whereas arugula grown on other substrates (other than JiHPC and RiC) showed no statistically significant differences (Supplemental Table 1). The effects of substrates on arugula chlorophyll and anthocyanin contents were more challenging to interpret during the summer. ElOC had more positive effects of increasing leaf chlorophyll and anthocyanin content during summer, whereas during fall and winter, substrates had similar effects, except plants grown using RiC tended to have lower leaf chlorophyll and anthocyanin contents in fall (Supplemental Table 1). The JiHPC substrate also resulted in the highest arugula leaf area, shoot fresh and dry weight, whereas plants on JiPP had the highest SSC and JiP had the highest K concentrations (Table 3). Seasonal variations also existed: plants grown in fall had the highest leaf area, shoot fresh and dry weight, and K and Ca concentrations, whereas plants grown in the winter had the least leaf area, biomass, and SSC but the highest N and P concentrations (Table 3). Significant interaction effects were found between season and substrate treatments on arugula leaf area, biomass, and SSC (Table 3). During summer, arugula on RiC had the lowest canopy and biomass but the highest SSC, which had similar results with ElC, JiPP, and ElOC, whereas plants grown using JiHPC substrate had the highest biomass but lowest SSC (Table 3). During fall and winter, the high SSC caused by the RiC substrate was no longer seen. JiPP substrate had better results in increasing arugula SSC (Supplemental Table 1, Supplemental Fig. 2).
Plant height, width, leaf chlorophyll content (expressed as an index, CCI), and anthocyanin content (expressed as an index, ACI) of arugula (n = 3) cultivated for three seasons, summer (A-1 to A-4), fall (B-1 to B-4), and winter (C-1 to C-4) as affected by selected commercial substrates: Gr36: Grodan Rockwool AO 36/40 Plug; JiHPC: Jiffy 75% peat and 25% coir Preforma HP; RiC: Riococo coir PCM Coco. Arugula plants were grown in the deep water culture system for 3, 4, and 4 weeks after transplanting (WAT) in summer, fall, and winter.
Citation: HortScience 59, 3; 10.21273/HORTSCI17606-23
Plant growth performance and mineral concentration of arugula cultivated for three seasons, summer, fall, and winter. The 13 commercial substrates tested are OaS: Oasis phenolic foam single seed dibble; OaM: Oasis phenolic foam multiseed dibble; Gr25: Grodan Rockwool AO 25/40 Plug; Gr36: Grodan Rockwool AO 36/40 Plug; JiP: Jiffy 98% Peat Pellet; JiH: Jiffy 98% Peat Horticulture Pellet; JiHPC: Jiffy 75% peat and 25% coir Preforma HP; JiPP: Jiffy 98% Peat Horticulture Peat Pellet; ElC: Ellepot 100% Coir Universal 6–9 paper; ElPC: Ellepot Peat & Coir Mix Universal 6–9 paper; ElOC: Ellepot 100% Coir Organic 2.0 paper; ElOPC: Ellepot Peat & Coir Mix Organic 2.0 paper; RiC: Riococo coir PCM Coco.
There were fewer differences between substrates on lettuce growth. Plants grown using JiP, JiH, JiHPC, and JiPP substrates had greater height, width, leaf chlorophyll, and anthocyanin content than other substrates (Supplemental Table 2). JiHPC also had the most pronounced effects on improving lettuce leaf area, shoot fresh and dry weights, and RiC resulted in the lowest leaf area and shoot fresh weight of lettuce (Fig. 2), whereas plants on OaS and OaM substrates had the least shoot dry biomass. No differences were found in foliar nutrients among different substrates (Table 4). Unlike arugula, lettuce grown in summer had the highest leaf area, fresh weight, and SSC, whereas plants grown in winter had the lowest canopy, biomass, and SSC but the highest N, P, K, and Ca concentrations (Table 4). Interactions between seasons and substrates were reduced in lettuce growth, mainly reflected in canopy and biomass. During summer, lettuce growth using JiHPC, ElOPC, ElPC, JiP, and JiPP substrates had the top performance. Plants grown using RiC showed higher shoot dry weight than other substrates, whereas during the fall and winter seasons, ElOPC dropped from the beneficial performer groups. The positive effect of RiC on shoot dry biomass accumulation was not presented during the fall seasons (Table 4, Supplemental Fig. 3). Overall, from both arugula and lettuce results, we found the substrate type with mixed use of peat and coir had better hydroponic leafy greens production in DWC system compared with other inorganic or 100% coir substrates.
Plant height, width, leaf chlorophyll content (expressed as an index, CCI), and anthocyanin content (expressed as an index, ACI) of lettuce (n = 3) cultivated for three seasons, summer (A-1 to A-4), fall (B-1 to B-4), and winter (C-1 to C-4) as affected by selected commercial substrates: Gr36: Grodan Rockwool AO 36/40 Plug; JiHPC: Jiffy 75% peat and 25% coir Preforma HP; RiC: Riococo coir PCM Coco. Lettuce plants were grown in the deep water culture system for 4, 5, and 5 weeks after transplanting (WAT) in summer, fall, and winter.
Citation: HortScience 59, 3; 10.21273/HORTSCI17606-23
Plant growth performance and mineral concentration of lettuce cultivated for three seasons, summer, fall, and winter. The 13 commercial substrates tested are OaS: Oasis phenolic foam single seed dibble; OaM: Oasis phenolic foam multiseed dibble; Gr25: Grodan Rockwool AO 25/40 Plug; Gr36: Grodan Rockwool AO 36/40 Plug; JiP: Jiffy 98% Peat Pellet; JiH: Jiffy 98% Peat Horticulture Pellet; JiHPC: Jiffy 75% peat and 25% coir Preforma HP; JiPP: Jiffy 98% Peat Horticulture Peat Pellet; ElC: Ellepot 100% Coir Universal 6–9 paper; ElPC: Ellepot Peat & Coir Mix Universal 6–9 paper; ElOC: Ellepot 100% Coir Organic 2.0 paper; ElOPC: Ellepot Peat & Coir Mix Organic 2.0 paper; RiC: Riococo coir PCM Coco.
Effects of seasons on the changes of solution pH, EC, DO, plant water, and nutrient use efficiencies.
Different seasons significantly affected plant growth and development and consequently pH, EC, and resource use efficiencies as the trials were conducted under varying environmental conditions (DLI levels varied from 9 to 28 mol·m−2·d−1). For arugula growth, winter had slightly higher EC and lower pH at the end of the growth stage compared with fall and summer, and solution DO was elevated in winter, followed by fall and then summer (Fig. 3A). Overall, plants grown during summer and fall had higher water, N, and P use efficiencies than winter, and the highest Ca use efficiency was found in summer (Fig. 3B).
Effects of three seasons on arugula solution pH, electrical conductivity (EC), and dissolved oxygen (DO) changes (n = 3) during the plant growth period (A-1 to A-3), and the end-season arugula plant water, nitrogen (N), phosphorous (P), potassium (K), and calcium (Ca) use efficiencies (B-1 to B-5).
Citation: HortScience 59, 3; 10.21273/HORTSCI17606-23
The lettuce fertilizer solution had more dynamic changes than the arugula. Except for the end growth stage, solutions in summer had the lowest pH and DO but the highest EC. During the summer season, we accidentally added 75 L of pure water before the last pH and EC measurements, which caused the EC to have a steep decrease and pH had a steep increase. Winter again had the highest solution DO. The solution in winter tended to have higher pH, EC, and DO during the end of the growth stage than in fall (Fig. 4A). Unlike arugula, lettuce plants grown during fall had the highest water, N, P, K, and Ca use efficiencies than other seasons, whereas summer had the worst P, K, and Ca use efficiencies (Fig. 4B).
Effects of three seasons on lettuce solution pH, electrical conductivity (EC), dissolved oxygen (DO) changes (n = 3) during the plant growth period (A-1 to A-3), and the end-season arugula plant water, nitrogen (N), phosphorous (P), potassium (K), and calcium (Ca) use efficiencies (B-1 to B-5). During the summer season, we accidentally added 75 L pure water before the last pH and EC measurements, which caused the EC to have a steep decrease and pH had a steep increase.
Citation: HortScience 59, 3; 10.21273/HORTSCI17606-23
Discussions
Overall, our results showed that plants had higher performance under the peat-based media with moderate bulk density compared with other materials (rockwool, phenolic foam) or substrates with high bulk density. Due to the high water-holding capacity, low oxygen concentrations in rockwool could unfavorably affect plant root respiration and lead to hypoxia, especially in a non-flood and drain water systems such as DWC (Bhattarai et al. 2008). This could explain why rockwool products did not perform as well as others. Compared with other products, Ellepot’s peat and coir-based substrates had higher bulk density and lower air porosity. Studies have shown that increased bulk density is associated with decreased seedling root development (length, area, and dry mass) in nursery plants (Kormanek et al. 2015), and decreased air porosity was a key influencer to reducing diffusive oxygen in soil (Grable and Siemer 1968). One hundred percent use of coir is not ideal for hydroponics production, as it could contain high levels of salts (Velazquez-Gonzalez et al. 2022). However, we did not measure the initial EC level of the coconut coir–based substrate, and the nutrient solution in the DWC system could greatly dilute the salt level (if they might have) from coir-based substrates, the bad performance (lower growth and yield) from coir-based substrates could be explained by their physical properties. Inorganic substrates, such as phenolic foam, decreased lettuce yield compared with vermiculite and coir in an NFT system (Jordan et al. 2018), and our results showed that phenolic foam–based substrates were not suitable for producing arugula and lettuce in DWC, even when the products are recommended for this system. From our results, lower performance from these inorganic substrates all had lower bulk density, but their air and water porosity are very different. Substrates with low bulk density normally require frequent irrigation in the greenhouse to avoid oxygen deficiency but may not provide sufficient physical support for plant growth (Patil et al. 2020). Therefore, this could be another explanation for the lower performance from the phenolic foam and rockwool substrates (all with < 0.1 g⋅cm−3 bulk density) tested in our study.
Although several attempts have been made to find competitive products for rockwool and peat using perlite, vermiculite, coir, bark, and sand (Vinci and Rapa 2019), our study revealed that despite testing various materials, none proved to be a superior alternative to peat, which continues to outperform all other substrates in hydroponic production. This explains the challenge in convincing growers to transition away from peat use.
Several nontraditional materials were tested that could be potentially used to replace peat in potted production, which could provide some suggestions in selecting substrate materials used in hydroponic DWC systems. Dannehl et al. (2015) found that sheep wool and hemp fiber could produce similar or higher leaf areas, flower numbers, yield, and nutrient concentration in ‘Pannovy’ tomato (Solanum lycopersicum L.) production. Komorowska et al. (2023) further evaluated sheep wool to produce cucumber and learned the carbon footprint could be reduced by 30% compared with conventional substrates while maintaining similar crop production. Yu et al. (2019) found that hardwood and sugarcane bagasse biochar could produce similar tomato and basil seedling growth and quality compared with the commercial peatmoss-based substrate. Other types of organic substrates, such as compost, wood fiber, and coir dust, also showed the potential to substitute for peat (Fascella 2015), especially alternative organic substrates that have adequate physical characteristics such as high C:N ratios, nutrient concentrations (e.g., K), optimal bulk density, porosity, wettability, physical and biological stability, high availability, and potential disease suppressive properties (Atzori et al. 2021).
In addition, using mixed substrates from different raw materials, such as peat, coir, perlite, and biochar, could provide multiple benefits to reduce the disadvantages of using one specific material, providing economic and environmentally friendly solutions to address the recycling issues from by-products. In our study, using a mix of peat and coir (75%:25%) had higher beneficial effects for plant growth than 100% peat or 100% coir. There were beneficial effects from this substrate during early development, shown in the first week after transplanting; however, later, the plant growth rate is similar among substrates, indicating the early beneficial effects had more influence on the final harvest. The advantages of mixed use of substrate materials were also shown in other hydroponics studies: compared with 100% peat, a mix of 60% perlite and 40% peatmoss produced higher strawberry (Fragaria ×ananassa) fruit numbers and soluble solids (Jafarnia et al. 2010); a mix of 33% coir and 67% perlite produced higher strawberry yield and shoot dry biomass (Ebrahimi et al. 2012), whereas the mixed growing substrates containing 75% coir and 25% perlite led to higher root development (Roosta and Afsharipoor 2012). Compared with the 100% use of perlite or biochar, substrate mixed with 50% rice husk biochar and 50% perlite produced doubled yield of leafy greens, including cabbage (Brassica oleracea L.), dill (Anethum graveolens L.), mallow (Malva sylvestris L.), red lettuce, and tatsoi (Brassica rapa L.), and similar leaf nutrient contents were also achieved (Awad et al. 2017). Although most substrate studies were conducted under potted production, their results could suggest selecting the optimal mixed substrates used in the hydroponic DWC systems.
One study has shown that water and air porosities of the substrate could significantly affect plant growth by affecting plant water and nutrient uptake, and substrates with higher total porosity and a larger ratio of air porosity tend to have more benefits for plant growth (Ghazvini et al. 2007). Ideal physical properties for a slab substrate are 75% to 85% total porosity, 10% to 30% air porosity, and <0.4 g⋅cm−3 bulk density (Dannehl et al. 2015). However, in our study using the DWC system, porosity seemed to have less effect on plant growth after seedling stage as the substrates were partially submerged into the water system with a consistently full water capacity. In addition to the fixed volume in pellet or plug substrates when fully saturated, the determined effects could also be related to the growth and the packing materials (e.g., paper wood fibers for Ellepot products, fine netting for Jiffy products, paper cellulose for Riococo product) for the substrate, which needs further investigation to test the packing materials’ properties regarding their degradation rate, existing microbes, hardness. The different effects also could be caused by the different product formats, JiHPC (mixture of peat and coir) product comes with a pre-filled plug tray, while the other substrates that use peat or coir come as pellets.
Dickson and Fisher (2019) showed that arugula had a lower cation:anion uptake ratio than lettuce when supplied with both ammonia and nitrate in hydroponics solutions at the end of growth, which could increase solution pH with increased uptake of NO3−. However, our study found that the solution used for arugula had a relatively unchanged pH, and the lettuce solution decreased pH except during summer. We did not adjust the solution pH, EC, or refilled any solution during the growing season except for the lettuce in summer, where we accidentally added pure water before we measured the solution pH and EC. Therefore, the changes of solution pH and EC were mainly caused by plant nutrient and water uptake. Overall, solution EC was maintained in an ideal range for arugula growth based on Yang et al. (2021). However, in arugula solution, DO had the trend to decrease during summer and fall, especially DO levels below 4 mg/L (in our case) are concerning for stresses such as root respiration and Pythium incidence, which could significantly reduce plant growth and marketable yield (Hendrickson et al. 2022). Even though there was no visible issue with Pythium in our studies, additional air or oxygen supplies should be considered during these seasons. An ideal climate condition for plant growth is required for arugula and lettuce to achieve the highest resource use efficiencies. Alternatively, controlling the temperature inside the greenhouse and using supplemental lighting to optimize DLI when weather conditions are not ideal is required. Our study also found that arugula had higher yields and efficiently used water and fertilizers during the summertime compared with early and winter, which indicated that arugula needed higher temperatures (26.3 °C in summer vs. ∼14.4 °C in fall) and DLI (28.14 mol·m−2·d−1 in summer vs. ∼15.00 mol·m−2·d−1 in fall) to optimize its production. In contrast, lettuce needed lower temperatures during the fall season compared with summer and a relatively higher DLI to achieve the highest production compared with fall (19.35 mol·m−2·d−1) and winter (9.34 mol·m−2·d−1). These temperature responses were relatively consistent with the studies by Tarr (2022). It is worthy to note that lettuce plants grown during summer had reduced plant width, which might be because lettuce plants had morphological changes to avoid too much direct sunlight and reduce water loss during the hot weather in summer (could reach up to 35 °C).
As technology advances, options for substrates have increased. Recently, researchers found that biopolymers (keratin and cellulose) extracted from biowaste (hair, poultry feathers, wood shavings, vegetable trimmings) could be used as a sustainable substrate to grow seedlings of arabidopsis (Arabidopsis thaliana L.), bok choy (Brassica rapa L.), and arugula (Zhao et al. 2022). Testing more mixed or single-use substrates in different hydroponics systems to understand their roles in supporting plant growth is further needed to provide more information for using other substrates. In addition, evaluating cheap, easily accessed, and locally available substrates could provide more economic benefits for hydroponics production.
Conclusions
In this study, 13 commercial substrates were tested and compared in the DWC system. Compared with inorganic materials (phenolic foam OaS, OaM, and rockwool Gr25, Gr36) and coir-based substrates (ElC, ElOC, RiC), substrates made from peat (JiP, JiH, JiHPC, JiPP, ElPC, ElOPC) had higher benefits for plant growth in marketable yield and biomass accumulation, whereas the highest plant performances were achieved using the 75% peat and 25% coir mixture substrate (JiHPC), which could be due to the improved physical properties in bulk density and porosity. In addition, arugula was found to have a higher production and resource use efficiency during summer when temperature and light intensity were higher, and lettuce favored the cooler season to achieve its maximum production potential. Different from potted production, with the increasing trend of using DWC systems in controlled environments (greenhouse, vertical farms), challenges will remain with selecting the miniaturized and compact substrates, and the choice of materials will also become diverse.
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