Substrate Comparison for Tomato Propagation under Different Fertigation Protocols

Abstract

Greenhouse tomato production faces multiple challenges, including the excessive use of nonrenewable substrates that are difficult to dispose of after use. Currently, most growers propagate tomatoes in rockwool, but there is an increasing demand for sustainable media. The objective of this research was to evaluate sustainable and organic alternatives for greenhouse propagation of tomato seedlings intended for high-wire production. Different organic and inorganic substrates were evaluated in three experiments, using a nutrient solution composed of a complete water-soluble fertilizer. Germination and growth parameters, including height, stem diameter, number of leaves, leaf area, foliar chlorophyll levels (SPAD), and shoot fresh and dry weight, were measured. In the first experiment, which employed overhead irrigation, rockwool, coir, wood fiber–coir mix, medium-grade pine bark, pine bark < 0.64 cm, and pine bark < 0.32 cm were evaluated. Tomato germination was faster and achieved higher percentages with pine bark < 0.64 cm compared to other substrates. However, growth performance was similar or better in coir than in rockwool four weeks after transplantation. For the second experiment with sub-irrigation only, rockwool, coir, wood fiber–coir mix, pine bark < 0.32 cm bark, and peat were evaluated at different container heights. Peat resulted in greater growth across all parameters, followed by wood fiber–coir mix in all container heights, while pine bark had the least growth across all measured parameters. In the third experiment with overhead irrigation, rockwool, wood fiber–coir mix, pine bark < 0.32 cm, and a commercial peat-based mixture were evaluated under different fertilizer rates (electrical conductivity of 1.1 and 2.2 mS·cm⁻¹). Wood fiber–coir mix, peat-based mix, and rockwool were the substrates with the highest values for all evaluated parameters. While all the organic substrates showed potential for use in tomato propagation, pine bark < 0.32 cm bark and wood fiber–coir mix provided the best media for germination. Peat and wood fiber–coir mix showed the best media for subsequent seedling growth and demonstrated potential to be used as substitutes for rockwool.

1. Introduction

Tomato (Solanum lycopersicum L.) belongs to the family Solanaceae and is rich in beneficial compounds including vitamins, carotenoids, and phenols that support health and prevent several chronic diseases and dysfunctions [1,2,3]. In addition to their consumption as fresh produce, people also utilize tomatoes in various processed items, like soups, juices, and sauces [4,5]. The United States produced over 12.5 million tons of tomatoes in 2021 [6]. In addition to open-field cultivation, high-wire crop production under controlled environment agriculture (CEA) facilities, specifically high-tech greenhouses, is increasing rapidly as it yields 6.4 times higher per unit area with 0.74 times water footprints compared to open-field production [7]. This production rate largely depends on the quality of the tomato seedlings.

The germination and seedling establishment stages are critical in the life cycle of a plant, playing a critical role in shaping successful reproduction [8,9]. Several environmental factors, including temperature, humidity, light conditions, growing media, irrigation, and nutrient concentrations, affect germination and establishment. The selection of a proper growing media or substrate is crucial as it serves as a reservoir of moisture and nutrients around the plant’s roots, and the pore space of the substrates also provides oxygen, which is essential for seed germination and establishment [10]. Substrates can be classified into two groups: synthetic and organic. Examples of synthetic substrates include rockwool, foam, perlite, tuff, sand, and polystyrene, while coco coir, wood fiber, peat, and pine bark are the commonly used organic soilless substrates [11]. Synthetic substrates offer advantages such as large water retention and air-holding capacity, availability in multiple sizes and shapes, durability, and resistance to breaking down. However, they are not biodegradable, can cause skin and eye irritants, and can have high pH. They also contain very little carbon for microbial populations [12]. On the other hand, organic substrates are derived from biological sources, making them more readily biodegradable and environmentally sustainable. They also provide a suitable environment with sufficient water-holding capacity, aeration, and nutrient content. Additionally, they create a conducive environment for microbial populations. However, many of these substrates are hydrophobic. If they dry out, it takes a long time for them to become wet again [13]. Rockwool cubes are considered the standard propagation substrate in the greenhouse industry. Peat-based mixtures, such as Promix BX, are also commonly used as propagation media for a wide range of crops. They also contain perlite and vermiculite to provide a balance of moisture and nutrient retention. Coir is a widely utilized substrate that exhibits excellent water-holding capacity and consistent water and salinity profiles and is a popular alternative to synthetic substrates in high-wire crop production [13]. Wood-based substrates, including pine bark and wood fiber, generally provide ample airspace and high saturated hydraulic conductivities [14]. These substrates are considered more sustainable than peat and more readily available and cost-effective than peat and coir for U.S. growers. However, there is limited research exploring the performance of these organic substrates for seed propagation and transplant production for high-wire crops in CEA.

Irrigation is another crucial factor that affects seed germination and establishment. The right amount of irrigation, along with fertilizer, at the right time is essential [15]. Irrigation for seed propagation can be classified into two groups: overhead irrigation and sub-irrigation. Examples of overhead irrigation include impulse sprinklers and spray booms, while sub-irrigation methods include ebb–flow, trough, and flooded floor systems. Irrigation systems for the propagation of high-wire crops may include these systems or modified versions of these standard systems to suit the resources available. In overhead irrigation, the foliar canopy sometimes limits the quantity and uniformity of water reaching the growing media, while sub-irrigation is considered more efficient as water is delivered from beneath containers, and the growing media becomes saturated due to the capillary action of water [16]. Sub-irrigation also reduces the waste of water and fertilizer inherent to conventional overhead watering systems used in greenhouses [17]. The ideal irrigation rate varies by substrate, crop, and system. With the diversity of irrigation systems for propagation used in CEA and recent introductions of new substrates, there is a significant knowledge gap regarding irrigation practices for high-wire crop propagation.

In our previous studies, we found that organic substrates, such as peat, pine bark, coir, wood fiber, and their various combinations, perform similar or better than synthetic substrates (such as perlite) for high-wire production of tomatoes and cucumbers [18,19]. In those studies, seedlings were propagated in rockwool cubes to maintain uniform seedling quality at transplantation. For growers who are adapting new organic substrates for production, having organic substrate options for propagating a quality seedling is essential. Limited research has been conducted to assess the potential of organic substrates other than coir specifically for high-wire crop propagation. Furthermore, the impact of fertigation practices during propagation (seed germination and seedling establishment) is a crucial aspect, which is often overlooked. With a focus on sustainable propagation, this study aims to compare different substrates for tomato propagation under various fertigation protocols.

2. Materials and Methods

2.1. Experimental Site, Substrate, and Growing Condition

The experiment was conducted in a Venlo-type glass greenhouse located at the Ohio State University in Wooster, OH, USA (40.781° N, 81.928° W). Throughout the experiment, the average daily temperature, relative humidity, and photosynthetically active radiation (PAR) in the greenhouse were 24.4 °C, 59.8%, and 188.2 mmol·m⁻²·s⁻¹, respectively. The model crop used was tomato (Solanum lycopersicum ‘Favorita F1’; Johnny’s Selected Seeds, Winslow, ME, USA). Tomato seeds were sown at a depth of 0.63 cm in substrates that had been previously saturated. Once the seeds were sown, they were covered with the same substrate, except for rockwool, which was covered with coarse vermiculite (A-3, Palmetto Vermiculite, Woodruff, SC, USA). The evaluated substrates were rockwool (Grodan, Roermond, The Netherlands), coir (Jiffy Products of America Inc., Lorain, OH, USA), wood fiber and coir (WF–coir) mix (60:40) (Profile Products LLC, Buffalo Grove, IL, USA), medium-grade pine bark, pine bark < 0.64 cm, and pine bark < 0.32 cm (Pacific Organics, Henderson, NC, USA). The substrates were prepared by individually wetting or saturating them 24 h before sowing the seeds using the following procedures. Rockwool was soaked in city water, coir was overhead and sub-irrigated, wood fiber was soaked in hot water, and pine bark substrates were overhead irrigated. A commercial water-soluble fertilizer (Hygro-Gro Vine and Ca(NO₃)₂; Crop King, Lodi, OH, USA) was used, and the pH and electrical conductivity (EC) of the mixed solution were 5.8 and 1.2 mS·cm⁻¹, respectively, unless otherwise noted. Electrical conductivity (EC) and pH of the nutrient solution were measured and adjusted daily using an EC meter (COM-100 EC/TDS/Temperature meter, HM Digital Inc., Carson, CA, USA) and a pH meter (PH-200 pH/temp meter, HM Digital Inc., Carson, CA, USA) with stock nutrient solutions and 5% sulfuric acid (Fisher Scientific, Waltham, MA, USA) throughout the experiments. The nutrient composition of the Hygro-Gro Vine was total nitrogen 4.40%, phosphoric acid 13.00%, potash 34.00%, magnesium 3.70%, boron 275 mg·kg⁻¹, copper 95 mg·kg⁻¹, iron 1000 mg·kg⁻¹, manganese 800 mg·kg⁻¹, molybdenum 40 mg·kg⁻¹, and zinc 270 mg·kg⁻¹.

2.2. Experimental Overview

Three separate experiments were conducted to determine the optimal substrate for tomato propagation, considering different irrigation protocols. The first experiment evaluated tomato seed germination and seedling/transplant development under synthetic and organic substrates with overhead irrigation. The second experiment determined how container height (water capillarity effect) affects seed germination and seedling development. The third experiment assessed the effects of nutrient concentration and substrate type on seedling propagation.

2.2.1. Experiment 1: Substrate Selection for Propagation

Six different substrates were considered in this experiment: rockwool (Grodan, Roermond, The Netherlands), coir (Jiffy Products of America Inc., Lorain, OH, USA), wood fiber and coir mix (WF–coir mix) (Profile Products LLC, Buffalo Grove, IL, USA), medium-grade pine bark, pine bark < 0.64 cm, and pine bark < 0.32 cm (Pacific Organics, Henderson, NC, USA). The physical properties and particle size distribution were analyzed following the methodologies described by Altland et al. [20] and [21], respectively. The substrate selection experiment was carried out in two stages, germination and transplant stages, each lasting two weeks. In the germination stage, a total of 30 seed trays (6 substrates × 5 replications) were arranged in a completely randomized design on a test bench. Each seed tray contained 10 small propagation cells and was placed on sub-irrigation trays to collect and hold leachates. During the first two weeks, cell plug trays were overhead irrigated daily using a pump sprayer (7.57 L Multi-use Sprayer, The Fountainhead Group Inc., New York Mills, NY, USA) equipped with nozzles to apply a fine mist applied to the substrate surface. Irrigation was carried out using the nutrient solution until saturation and any excess leaching accumulated on the sub-irrigation tray to mimic irrigation practices used for propagation of hydroponic crops. For rockwool, a 1.25 cm layer of water was left in the sub-irrigation tray to simulate industry practices. During the first three days, trays were irrigated with 40 mL of nutrient solution twice per day, and then 100 mL once per day thereafter.

The transplant stage commenced two weeks after the seeds were sown. Five seedlings from each substrate treatment were transplanted into larger cubes or containers (staging up) comprising the same growing media represented in the germination stage. Thus, seedlings were transplanted from the cells to either rockwool and coir cubes (10 × 10 × 7.6 cm³) or 0.6 L containers filled with loose substrates (i.e., pine bark and wood fiber). The cubes or containers were placed on sub-irrigation trays and the five replications from each treatment were arranged in a completely randomized design on the greenhouse bench. Containers and cubes were watered with 50 mL of the nutrient solution, while 500 mL was poured into rockwool trays to reach a depth of 1.25 cm. Leachate from other substrates accumulated on the seed tray during overhead watering, but the sub-irrigation volume was not adjusted, unlike rockwool.

2.2.2. Experiment 2: Effect of Container Height and Substrate Type on Seedlings under Sub-Irrigation

The experiment was designed to include four substrates selected based on the seedling growth performance from experiment 1 (WF–coir mix, coir, peat, and pine bark < 0.32 cm bark) and four container heights (3.81 cm, 5.08 cm, 6.35 cm, and 8.89 cm). A 7.62 cm diameter PVC pipe was cut to prepare the required containers. The bottom of each PVC container was sealed with a 12-mesh plastic screen and a zip tie. A 6.35 cm rockwool cube was considered as a control treatment. A total of 68 containers (4 substrates × 4 heights × 4 replications = 64 + 4 for rockwool) were filled with substrates and placed in trays. The filled PVC containers were distributed in trays measuring 30 × 40 cm, with each tray containing eight randomly distributed containers. Subsequently, the trays were arranged randomly on the greenhouse bench so that the replicates were arranged in a completely randomized design. Trays were filled with up to 1000 mL of nutrient solution (sub-irrigation) daily. Seeds were sown directly in the containers and grown for three weeks.

The volumetric water content of each container was assessed after seedlings were sampled. Initially, clean aluminum pans were individually weighed. Substrate from each PVC container was then removed to assess root condition. Subsequently, the wet substrate was placed in a clean aluminum pan and weighed again. The PVC container, net, and zip tie were carefully brushed to collect all substrate particles. The weight of the wet substrate was calculated by subtracting the second weight (aluminum pan + wet substrate) from the first weight (empty pan). The substrates were then dried in an oven at 68 °C for three days. The dry weight of each substrate was measured, and the amount of water was determined by calculating the ratio of wet to dry substrate. Finally, the volumetric water content was calculated by dividing the weight of the water by the container volume.

2.2.3. Experiment 3: Effect of Nutrient Solution Concentration and Substrate Type

This experiment focused on the interaction between substrates and nutrient concentration. Four different substrates (peat-based soilless propagation mix, pine bark < 0.32 cm, WF–coir mix, and rockwool) and two nutrient concentrations (1.1 and 2.2 mS·cm⁻¹) were considered. A total of 40 seedlings (8 treatments with 5 replicates each) were grown. Overhead irrigation started at 50 mL per pot but was increased (up to 80 mL) with plant growth. Like experiment 2, seeds were directly planted in the containers, and after three weeks of growth, seedlings were sampled for data collection.

2.3. Sampling and Data Collection

In experiment 1, seedling sampling and data collection were performed twice, representing two growth stages. At the end of the first stage (two weeks after seeding), half of the subsamples (five) were harvested destructively for data collection, and the other half were transplanted for the second stage. As a result, the second stage (four weeks after seeding) had five plants per substrate per replication for sampling. In contrast, sampling and data collection for experiments 2 and 3 were performed three weeks after seeding. Germination data were collected every day for the first two weeks in all experiments. A seed was considered germinated when cotyledons were present. After each stage of the experiment, plant height and stem diameter were measured, and number of leaves was counted. Relative foliar chlorophyll content was measured in one expanded leaf using a chlorophyll meter (SPAD-502 Chlorophyll meter, Konica Minolta Sensing Inc., Ramsey, NJ, USA). Leaf area was recorded at the end with a portable leaf area meter (CI-202 Portable Laser Leaf Area Meter, CID Bio-Science Inc., Camas, WA, USA). Shoot fresh weight was also recorded by severing the shoots immediately above the soil line, and the shoots were dried in an oven at 68 °C until a homogeneous weight and shoot dry weight were obtained.

For the analysis of substrate physical properties, substrates were packed into 347 cm³ aluminum cores following the methods described by [22]. Rockwool was cut cross-sectionally to fit the shape of a core. There were three replications for each substrate. The cores were weighed, oven-dried for four days at 60 °C, and weighed again to determine the container capacity. Total porosity (TP) was calculated as the sum of container capacity (CC) and air space (AS) [23]. All physical properties (TP, AS, CC) were calculated as the algebraic mean of the cores. Bulk density was determined using oven-dried substrate in 347 cm³ cores. Particle size distribution (PSD) of the different substrates was determined using approximately 250 cm³ of oven-dried (60 °C) substrate passed through sieves of various sizes (19.0, 12.5, 6.30, 4.0, 2.8, 2.0, 1.4, 1.0, 0.71, 0.50, 0.35, 0.25, 0.18, and 0.11 mm). Particles were classified into three classes, such as coarse (>2.0 mm), medium (0.5–2.0 mm), and fine (<0.5 mm). Particles ≤ 0.11 mm were collected in a pan. The sieves and the pan were shaken for 3 min using an RX-29/30 Ro-Tap^® test sieve shaker (278 oscillations.min⁻¹, 150 taps.min⁻¹) (W.S. Tyler, Mentor, OH, USA). Three replicate samples for each substrate were analyzed.

2.4. Statistical Analysis

All presented growth data represent the averages of independent measurements. The significance of differences was determined using an analysis of variance (ANOVA) with statistical software (Minitab 21.4.2.0, State College, PA, USA). Means were separated using Fisher’s protected least significance difference test with a confidence level of 95% and two-sided confidence intervals. Some basic statistical analysis was performed, and graphs were prepared using MS Excel (ver. 2023, Microsoft Corporation, Redmond, WA, USA).

3. Results

3.1. Evaluation of Substrates for Propagation

The percentage of air space, water holding capacity, total porosity, and bulk density varied significantly among the considered substrates (

Table 1. Physical properties of six substrates used for tomato (Solanum lycopersicum cv. Favorita F1) propagation, including rockwool, coir, wood fiber–coir mix (WF–coir), medium-grade pine bark, pine bark < 0.64 cm, and pine bark < 0.32 cm.

Treatments	Air Space (%) X	Water Holding Capacity (%) X	Total Porosity (%) X	Bulk Density (g.cm−3) X	pH X	EC (mS·cm−1) X
Rockwool	1.90 c	87.70 a	89.60 a	0.10 c	7.73 a	0.74 b
Coir	18.80 ab	62.00 cd	80.80 b	0.10 d	5.19 d	1.39 a
WF–coir mix	3.30 c	77.30 b	80.50 b	0.10 c	6.59 b	0.29 c
Medium pine bark	20.70 a	61.10 d	81.80 b	0.20 b	5.63 c	0.05 d
Pine bark < 0.64	16.30 b	63.50 cd	79.80 b	0.20 b	5.74 c	0.06 d
Pine bark < 0.32	5.60 c	66.60 c	72.20 c	0.20 a	5.61 c	0.08 d
Ideal range	20–30 Y	60–100 Y	>85 Y	<0.40 Y	5.4–6.4 Z	0.8–1.2 Z

^X Means in columns with different letters are significantly different according to Fisher’s protected least significance difference test (p ≤ 0.05), where N = 3. ^Y According to Abad et al. [24]; ^Z according to Lindberg and Owen [25].

). Medium-grade pine bark, coir, and pine bark < 0.64 had higher (16.30% to 20.70%) air space, while rockwool, and WF–coir mix, and pine bark < 0.32 had lower (1.90% to 5.60%) air space. Rockwool had the highest (87.70%) water-holding capacity, while medium-grade pine bark had the lowest (61.10%). Among the organic substrates, WF–coir mix had the maximum water-holding capacity (77.3%), while it varied between 62.0% and 66.6% for others. Rockwool and pine bark < 0.32 had the highest and lowest percentages of total porosity, respectively, while no significant difference was observed for the others. Although there were significant differences in bulk density among the substrates, they fell within a relatively narrow range from 0.1 to 0.2 g·cm⁻³.

The particle size also varied significantly among the substrates (

Table 2. Particle size distribution of five substrates used for tomato (Favorita F1) propagation, including coir, wood fiber–coir mix (WF–coir mix), medium-grade pine bark, pine bark < 0.64 cm, and pine bark < 0.32 cm.

Class	Particle Size (mm)	Coir	WF–Coir Mix	Medium Bark	Bark < 0.64	Bark < 0.32	LSD
Coarse (>2 mm)	>12.5	0.00	0.05	0.28	0.00	0.00	0.07
	12.5–6.3	0.00 c	0.85 b	7.50 a	0.00 c	0.03 c	1.68
	6.3–4.0	0.00 d	0.64 c	7.12 a	4.48 b	0.07 d	2.46
	4.0–2.8	0.00 d	0.80 c	5.29 b	7.76 a	0.16 cd	2.80
	2.8–2.0	0.29 e	1.20 d	3.85 c	4.25 b	6.05 a	3.13
	∑	0.29	3.54	24.04	16.49	6.31
Medium (0.5–2.0 mm)	2.0–1.4	2.10 e	2.43 d	4.01 c	5.68 b	7.58 a	4.36
	1.4–1.0	3.93 c	3.00 e	3.41 d	4.40 b	7.48 a	4.44
	1.0–0.7	3.78 b	2.73 c	3.05 c	3.74 b	7.49 a	4.16
	0.7–0.5	2.61 d	3.30 c	3.57 c	4.22 b	8.96 a	4.53
	∑	12.41	11.46	14.04	18.04	31.51
Fine (<0.5 mm)	0.5–0.35	0.92 d	2.16 c	2.36 c	2.66 b	6.00 a	2.82
	0.35–0.25	0.38 d	1.43 c	1.66 b	1.76 b	4.12 a	1.87
	0.25–0.18	0.18 c	0.72 b	0.90 b	0.83 b	1.88 a	0.90
	0.18–0.106	0.12 c	0.65 b	0.72 b	0.65 b	1.33 a	0.69
	<0.106	0.04 d	0.36 c	0.51 b	0.35 c	0.76 a	0.40
	∑	1.64	5.32	6.15	6.25	14.09

Means in columns with different letters are significantly different according to Fisher’s protected least significance difference test (p ≤ 0.05), where N = 3.

). Based on the cumulative weight of the retained particles on each sieve, medium-grade pine bark had the maximum percentage of coarse particles, followed by pine bark < 0.64, pine bark < 0.32, and WF–coir mix, while coir had the lowest percentage of coarse particles. The percentage of coarse particles is directly proportional to air space, increasing as the percentage of coarse particles increases. Pine bark < 0.32 had the highest percentage of medium particles, followed by pine bark < 0.64, medium-grade pine bark, and coir, while WF–coir had the lowest percentage. The percentage of fine particles in all substrates was generally low relative to medium particles. Pine bark < 0.32 had the highest percentage of fine particles. Usually, substrates with fine particles have a larger surface area than those with larger particles, allowing them to hold more water.

The germination percentage was similar among all substrates except for coir (Figure 1). Coir was the lowest (68%) due to oversaturation with water. Two weeks after seeding, a higher germination percentage was observed on those pots filled with pine bark < 0.64 (100%), followed by pine bark < 0.32 (96%), medium-grade pine bark (96%), WF–coir mix (90%), and rockwool (84%).

The highest plant height (3.89 cm), leaf area (9.72 cm²), and shoot FW and DW (0.32 and 0.03 g) were observed under the rockwool (

Table 3. Effect of different substrate treatments, including rockwool, coir, wood fiber–coir mix (WF–coir mix), medium-grade pine bark, pine bark < 0.64 cm, and pine bark < 0.32 cm on the growth and development of tomato seedlings (Favorita F1) two weeks after sowing.

Substrate	Plant Height (cm)	Number of Leaves	Leaf Area (cm2)	Relative Foliar Chlorophyll	Shoot FW (g)	Shoot DW (g)
Rockwool	3.89 a	3.80 ab	9.72 a	37.28 b	0.32 a	0.03 a
Coir	2.07 c	2.66 c	4.52 c	39.56 a	0.15 c	0.01 c
WF–coir mix	2.86 b	3.56 b	5.32 bc	35.28 cd	0.18 bc	0.01 c
Medium bark	2.79 c	3.88 ab	5.05 c	37.44 b	0.20 bc	0.02 bc
Pine bark < 0.64	3.25 b	3.88 ab	5.84 bc	34.40 d	0.21 bc	0.02 bc
Pine bark < 0.32	3.17 b	4.24 a	7.14 b	36.15 bc	0.24 b	0.02 b

Means in columns with different letters are significantly different according to Fisher’s protected least significance difference test (p ≤ 0.05), where N = 5.

). The highest number of leaves (4.24) and SPAD value (39.56) were observed in plants grown with pine bark < 0.32 and coir, respectively (

Table 4. Effect of different substrate treatments, including rockwool, coir, wood fiber–coir mix (WF–coir mix), medium-grade pine bark, pine bark < 0.64 cm, and pine bark < 0.32 cm on the growth and development of tomato seedlings (Favorita F1) four weeks after sowing.

Substrate	Stem Dia. (mm)	Plant Height (cm)	Number of Leaves	Leaf Area (cm2)	Relative Foliar Chlorophyll	Shoot FW (g)	Shoot DW (g)
Rockwool	4.92 b	30.02 a	11.09 a	211.24 b	44.56 b	13.67 a	1.47 a
Coir	5.17 a	24.50 bc	10.82 a	258.07 a	50.65 a	13.00 a	1.26 b
WF–coir mix	5.09 a	25.99 b	9.96 b	168.97 c	41.84 c	10.35 b	1.05 c
Medium bark	4.56 c	19.74 d	9.48 c	136.02 d	39.29 d	7.02 d	0.74 d
Pine bark < 0.64	4.76 b	24.04 bc	10.16 b	168.97 c	40.30 cd	9.42 bc	1.03 c
Pine bark < 0.32	4.81 b	23.42 c	10.08 b	166.50 c	38.81 d	8.97 c	1.02 c

Means in columns with different letters are significantly different according to Fisher’s protected least significance difference test (p ≤ 0.05), where N = 5.

). On the contrary, the lowest plant height (2.07 cm), number of leaves (2.66), leaf area (4.52 cm²), and shoot FW and DW (0.15 and 0.01 g) were observed in plants grown with the coir. Among the bark treatments, medium-grade pine bark and pine bark < 0.64 showed lower performance.

After four weeks from sowing or two weeks after transplanting, rockwool and coir showed better performance, with the highest plant height (30.02 cm) and shoot dry weight (1.47 g) observed in plants grown with rockwool. Coir exhibited the highest leaf area (258.07 cm²) and SPAD value (50.65) and combinedly exhibited the highest number of leaves and shoot FW. Similar to the first evaluation, bark treatments displayed lower performance, and except for the SPAD value, the lowest growth performance was recorded in plants grown with medium-grade pine bark. Overall, seedlings transplanted in large rockwool cubes showed better performance in both evaluations. Coir demonstrated either better or similar performance compared to rockwool during the second evaluation.

3.2. Evaluation of Container Height and Substrate Types

The interaction between container height and substrate significantly affected the germination percentage of tomato seeds (Figure 2). Better germination performance was observed under the WF–coir mix for all container heights, followed by peat, coir, and pine bark < 0.32 cm. Although seeds under the WF–coir mix and peat started germinating after 4 to 5 days of seeding, the 3.8 cm peat container showed lower performance (75%) due to oversaturation. Similarly, the 8.9 cm coir and pine bark < 0.32 cm containers exhibited lower (75%) performance due to water stress. Moreover, seeds under the pine bark < 0.32 cm started germinating after 7 to 8 days of seeding. These germination differences were directly related to the water content of each container, which is shown in Figure 3.

The water content significantly varied based on the substrate, and gradually decreased with increasing container height (Figure 3). The highest volumetric water content was observed in the 6.4 cm rockwool cube (93.52%), and among the organic substrates, it was 92.50% in the 3.8 cm peat container. The lowest volumetric water content (59.99%) was found in the 8.9 cm pine bark < 0.32 container. A low water content difference was observed among the coir containers. The pine bark < 0.32 containers had the lowest water content among the substrates due to high porosity and low water-holding capacity. This water content directly affected the establishment and growth rate of seedlings after transplantation, as summarized in

Table 5. Effect of container height (i.e., 3.8 cm, 5.1 cm, 6.4 cm, and 8.9 cm) and substrates (i.e., coir, pine bark < 0.32 cm, peat, WF–coir mix, and rockwool as the control) on the growth and establishment of tomato seedlings (Favorita F1) three weeks after sowing.

Substrate	Container Height (cm)	Plant Height (cm)	Stem Dia. (mm)	Relative Foliar Chlorophyll	Number of Leaves	Leaf Area (cm2)	Shoot FW (g)	Shoot DW (g)
Coir	3.8	5.25 bcd	2.59 cde	39.98 abc	4.50 abc	28.95 efg	1.31 efg	0.12 efg
Pine bark < 0.32		2.13 ef	1.09 fgh	32.83 bcd	2.00 de	4.34 g	0.15 g	0.02 gh
Peat		7.67 abcd	3.23 cde	46.90 abc	4.67 bcd	65.76 de	2.83 cde	0.26
WF–coir mix		7.13 ab	3.23 abc	42.40 ab	5.25 a	56.53 cde	2.53 bcde	0.22 bcde
Coir	5.1	5.50 cde	2.35 efg	35.30 cd	4.33 cd	26.28 fg	0.98 fg	0.09 fgh
Pine bark < 0.32		1.88 ef	0.96 fgh	38.75 d	1.50 ef	2.18 g	0.10 g	0.01 gh
Peat		7.88 ab	3.75 ab	44.30 ab	5.25 a	89.38 ab	3.78 ab	0.33 ab
WF–coir mix		6.50 abc	3.19 abc	42.60 ab	5.00 ab	53.56 de	2.39 cde	0.21 cde
Coir	6.4	3.50 def	1.90 def	35.60 abc	3.50 bcd	16.81 fg	0.74 fg	0.06 fgh
Pine bark < 0.32		1.00 ef	0.86 h	0.00 e	0.25 f	0.44 g	0.02 g	0.002 gh
Peat		8.13 a	3.83 a	45.30 ab	5.50 a	103.91 a	4.22 a	0.37 a
WF–coir mix		6.63 abc	3.25 abc	43.55 ab	5.00 ab	66.17 bcd	2.77 bcd	0.23 bcde
Rockwool		7.25 ab	2.82 bcd	45.03 ab	5.50 a	61.96 bcd	2.53 bcde	0.24 bcd
Coir	8.9	2.38 ef	1.78 efg	35.88 abc	2.75 de	10.80 g	0.43 g	0.04 fgh
Pine bark < 0.32		1.50 f	0.84 gh	0.00 e	0.25 f	0.27 g	0.01 g	0.002 h
Peat		7.50 ab	3.69 ab	46.75 a	5.25 a	84.01 abc	3.41 abc	0.30 abc
WF–coir mix		5.50 abcd	2.61 cde	40.33 ab	4.75 abc	40.23 def	1.79 def	0.15 def

Means in columns with different letters are significantly different according to Fisher’s protected least significance difference test (p ≤ 0.05), where N = 4.

.

Among the 3.8 cm containers, peat and WF–coir mix showed better performance, and coir performed well for some parameters too. With increasing container height, the performance of the coir and WF–coir mix dropped. Finally, in the 8.9 cm container, peat showed the highest performance, except for the number of leaves parameter. The significantly lowest growth rate was observed under the pine bark < 0.32 treatments for all container heights. Among the 6.4 cm rockwool cube, peat, and WF–coir mix containers, peat either performed better or similarly to the rockwool. The differences in plant height, leaf area, and shoot FW and DW between the peat and pine bark < 0.32 treatments were 72–87%, 93–99%, 94–99%, and 93–99%, respectively. These parameters varied from 30 to 68%, 55 to 87%, 53 to 87%, and 54 to 87% for coir treatments and from 7 to 26%, 14 to 52%, 10 to 47%, and 16 to 50% for WF–coir treatments, respectively.

3.3. Evaluation of Nutrient Solution Concentration and Substrate Type

The germination percentage of tomato seeds under the rockwool, peat-based mix, WF–coir mix, and pine bark < 0.32 bark substrates was 80%, 80%, 66.7%, and 93.3%, respectively, for the low EC (1.1 mS·cm⁻¹) condition, while it was 80%, 80%, 93.3%, and 80% under the high EC level (2.2 mS·cm⁻¹), as shown in Figure 4. This indicates that the WF–coir mix and pine bark < 0.32 bark worked well in high and low EC conditions, respectively. Overall, the germination rate was higher under the 2.2 mS·cm⁻¹ EC level.

An increasing growth trend was noted when the EC level increased to 2.2 mS·cm⁻¹ (

Table 6. Effect of nutrient concentrations (EC 1.1 and 2.2 mS·cm⁻¹) on the growth of tomato seedlings (Favorita F1) three weeks after sowing under different substrates, including rockwool, peat-based mix, WF–coir mix, and pine bark < 0.32 cm.

Substrate	EC Level (mS·cm−1)	Plant Height (cm)	Stem Dia. (mm)	Relative Foliar Chlorophyll	Number of Leaves	Leaf Area (cm2)	Shoot FW (g)	Shoot DW (g)
Rockwool	1.1	9.60 bc	3.81 c	44.74 a	6.00 b	119.61 cd	4.64 cd	0.47 b
WF–coir mix		11.90 ab	4.21 bc	43.86 a	6.00 b	121.93 bcd	5.27 bc	0.49 b
Peat-based mix		10.00 bc	4.22 bc	42.92 ab	5.80 bc	112.31 cde	4.83 cd	0.40 bc
Pine bark < 0.32		5.80 d	2.76 d	42.66 ab	4.60 d	43.66 e	1.62 e	0.14 d
Rockwool	2.2	12.40 ab	4.36 bc	44.82 a	6.60 ab	194.68 ab	7.84 ab	0.61 ab
WF–coir mix		14.70 a	5.03 a	44.40 a	7.20 a	238.46 a	10.62 a	0.78 a
Peat-based mix		11.30 ab	4.55 ab	44.58 a	6.80 ab	183.11 abc	7.35 bc	0.56 ab
Pine bark < 0.32		7.10 cd	3.00 d	40.80 b	4.80 cd	65.44 de	2.15 de	0.18 cd

Means in columns with different letters are significantly different according to Fisher’s protected least significance difference test (p ≤ 0.05), where N = 5.

). The seedling growth difference between 1.1 and 2.2 mS·cm⁻¹ EC conditions varied from 9% to 40%, 16% to 50%, 7% to 38%, and 4% to 33% for the rockwool, WF–coir mix, peat-based mix, and pine bark < 0.32 bark, respectively, for the considered shoot parameters. According to the combined mean separation analysis, the WF–coir mix and peat-based mix performed similarly to the rockwool for plant height, number of leaves, leaf area, and shoot FW and DW. They also performed higher than the rockwool for stem diameter under the 2.2 EC mS·cm⁻¹ EC level. The lowest seedling growth was observed in the pine bark < 0.32 bark treatment under both EC levels. However, the relative foliar chlorophyll (SPAD) was the same for all the treatments.

4. Discussion

Typically, seed germination and emergence largely depend on the water content of growing media, and water content relies on the physical properties of the growing media. A significant influence of substrates, fertigation practices, container heights, and nutrient concentration on tomato propagation was observed in this study.

The analyzed physical properties, such as air space, water holding capacity, total porosity, and bulk density, of the substrates used in the first experiment varied in a range of 1.9% to 20.70%, 61.10% to 87.70%, 72.20% to 89.60%, and 0.1 to 0.2 g·cm⁻³, while the ideal range of these properties for the potted substrate is 20% to 30%, 60% to 100%, >85%, and <0.4 g·cm⁻³, respectively [24]. Except for the percentage of air space of medium-grade pine bark, the others are outside the ideal range, especially rockwool (1.9%), WF–coir mix (3.30%), and pine bark < 0.32 (5.60%). Although the water holding capacity and bulk density properties of the selected substrates were within the ideal range, total porosity was outside the ideal range, except for rockwool. However, the results of the physical property analysis were also comparable with previous studies by Dubský and Šrámek [26], Yang et al. [19], and Altland et al. [20]. Usually, substrates with smaller particles have a larger surface area, allowing them to hold more water [27,28]. In experiment 1, a 100% germination rate was observed under the pine bark < 0.64 substrate (20.70% air space and 61.10% water holding capacity) (Figure 1), indicating the importance of a balanced combination of air and water. Coir (18.80% air space and 62.00% water holding capacity) also would show similar performance, but due to over-saturation caused during the implementation of equal conditions between treatments, the lowest germination rate was observed in coir. Verdonck et al. [29] emphasize the use of mixtures of different substrates (fine and coarse grade) to achieve good physical properties. Although the germination rate was higher in the pine bark treatments, seedling growth was not satisfactory, especially under the medium-grade pine bark. One of the reasons is its low water holding capacity. Altland et al. [20] also explained the inverse relationship between air space and water holding capacity in a wide range of pine bark substrates. As expected, seedling growth under the rockwool substrate was significantly higher at both sampling periods (Table 3 and Table 4). Although seedling growth under the coir substrate was not high at the first sampling, it was either similar or better than the rockwool for some parameters (i.e., stem diameter, leaf area, SPAD) at the second sampling (Figure 5a). Our data suggest that the ideal range of physical properties of substrates can vary more widely than the ranges suggested by Abad et al. [24], as the germination rate and seedling growth were higher in some substrates where some physical properties (i.e., air space or total porosity) were below the standard ranges. Even under saturated moisture levels, seedlings performed well under rockwool. Additionally, the interaction between the substrate and irrigation needs further investigation, as hydroponic propagation methods are different from regular overhead irrigation methods used in CEA.

Sub-irrigation methods, such as ebb and flow, trough, and flooded floor, are generally considered more efficient compared to overhead irrigation because they enable more consistent and thorough water distribution. This method involves delivering water from beneath the containers, allowing it to move upward through capillary action and saturate the growing medium more evenly [16,17,30]. However, capillary force depends on various factors, including the surface tension of water, viscosity, temperature, pore size, gravitational pressure, and the height to which capillary action will move the water [13,31,32]. Based on the analysis of volumetric water content (Figure 4), it is observed that water content gradually decreases with increasing container height. However, germination data indicate that container height does not significantly affect the water distribution of the peat and WF–coir treatments. According to the literature [33,34,35,36,37,38], the particle size of these substrates is strongly correlated with water and gas transfer parameters. On the contrary, water content in the bark containers was the lowest due to its large pore size, resulting in the lowest growth performance. Seedling growth revealed specific water distribution patterns among the substrates, particularly in relation to container height. There was no significant growth difference among the seedlings grown in the 3.8 cm peat, WF–coir mix, and coir containers. However, a significant growth difference emerged with increasing container height for the coir and WF–coir mix. Ultimately, seedling growth under the 8.9 cm peat container was the highest and significantly different from the other three substrates. Based on the findings of this experiment, peat was observed as a suitable substrate for tomato propagation when sub-irrigation is applied (Figure 5b).

According to the results of experiment 3, nutrient concentration (EC levels) accelerated the overall germination rate; however, it may vary with substrate types and EC levels. Generally, soilless substrates do not contribute to nutrient supply by themselves. As a result, intensive fertigation is required to support plant growth, especially where the substrate is incapable of holding (bonding) nutrient ions due to low cation exchange capacity [39,40,41]. However, many soilless potting mixes contain grains of slow-release fertilizer to provide nutrients to growing seedlings. In this study, no significant growth difference was observed among the rockwool, WF–coir mix, and peat-based mix substrates for both EC treatments, but seedling growth was better under the high (2.2 dS·cm⁻¹) EC level (Figure 5c). This indicates that the water along with nutrient ion transportation and retention properties of these substrates are better than the pine bark < 0.32 bark. Additionally, it can be suggested that WF–coir and peat-based mix can be used industrially as alternatives to rockwool for sustainable seed propagation. In our previous study, Yang et al. [19] also proved the potentiality of organic substrates to be an alternative to synthetic substrates for high-wire crop production.

5. Conclusions

The current study was performed to select alternate substrates for tomato propagation considering different irrigation protocols used in hydroponic crop production. The performance of the considered substrates (i.e., rockwool, coir, WF–coir mix, peat, peat-based mix, and different pine barks) varies with the irrigation methods. Initially, coir performed equally with rockwool under overhead irrigation coupled with sub-irrigation trays; however, the peat and WF–coir mix showed better performance compared to other organic substrates under the sub-irrigated condition without being affected by different container heights. The peat-based mix and WF–coir mix also worked similarly to rockwool under overhead irrigation, and growth increased proportionally with increasing nutrient concentration. Under both irrigation protocols, rockwool and WF–coir mix treatments generated better seedlings. Overall, WF–coir mix performed similarly to rockwool and can be introduced as a propagation substrate along with peat and coir, whereas bark, including the finest grade, did not perform well as a propagation substrate. Further investigation into the different ratios of WF–coir mix, as well as featured substrates, particle sizes, and aging, along with fertigation methods, volume, frequency, and crop, could be considered to optimize the propagation performance considering economic and environmental sustainability.