Evaluation of a Three-Level Cascade Soilless System Under Saline Greenhouse Conditions

Abstract

A three-level cascade hydroponic system was designed to enhance resource efficiency by reusing drainage solutions across sequential crops: tomato (primary-donor crop), herbs (mint, peppermint; secondary receivers from primary), and halophytes (lemon balm, sea fennel; tertiary receivers from secondary). The aim was to address salinity, a common challenge in hydroponics limiting plant growth and resource use. Two fertigation strategies were applied to secondary and tertiary crops to simulate salinity, with electrical conductivity (EC) increasing weekly by 1 dS m⁻¹ to reach 9 dS m⁻¹ for secondary and 11 dS m⁻¹ for tertiary crops. Control (S1) used fresh nutrient solution (FS), while the recycling treatment (S2) used tomato drainage with added NaCl. For tertiary crops, the control (S3) received a salinity-enriched FS, and the recycling treatment (S4) reused 70% of secondary crop drainage combined with 30% of its own, plus NaCl to reach target EC. Under moderate salinity (9 dS m⁻¹), mint produced 2.5 kg m⁻², whereas lemon balm dropped 16.7%, showing sensitivity; peppermint was more tolerant. Sea fennel showed resilience under high salinity (11 dS m⁻¹), with high chlorophyll (97.2) and improved ion uptake. The system reduced nutrient and fertilizer use by 86–88%, highlighting potential for sustainable nutrient recycling and efficient crop production.

1. Introduction

Climate change and population growth drive rising food demand, placing pressure on agriculture, while salinity in hydroponics poses additional challenges to sustainable and efficient crop production. This issue reflects a broader and urgent threat to global food security, namely the increasing salinization of arable land and freshwater resources. Globally, soil and water salinization already affect over 20% of irrigated land and poses a severe threat to food security [1,2]. According to projections by the Food and Agriculture Organization (FAO), this issue could reduce the yield of major crops by up to 50% by 2050, making research into salt-tolerant farming systems a global priority [1,2]. This challenge is particularly critical in arid and semi-arid regions, such as the Mediterranean coastal areas, where limited rainfall and high evaporation rates intensify salt accumulation in both soil and irrigation water [3].

Hydroponics offers an efficient alternative to traditional agriculture by optimizing resource use and environmental control; however, it faces its own salinity-related challenges. In closed or semi-closed systems, the continuous recirculation of nutrient solution (NS) leads to the accumulation of salts and other ions, compromising nutrient availability, causing osmotic stress in plants, and ultimately requiring the discharge of saline drainage [4,5]. This practice is both financially wasteful and environmentally damaging, contributing to nutrient pollution and eutrophication of water bodies [6,7,8]. Recirculation also leads to salinity-induced effects on NS parameters, including electrical conductivity (EC) and pH [9,10]. While EC serves as a key indicator of solution concentration [3,11], long-term recirculation can lead to nutrient imbalances [12], and suboptimal pH conditions, both of which can compromise nutrient availability and plant growth [13]. These salinity-related changes demonstrate the need for optimized nutrient management strategies to maintain plant health and maximize hydroponic system performance.

To mitigate these issues and enhance resource-use efficiency, cascade hydroponic systems have emerged as a promising strategy. These systems align with the principles of the circular economy by reusing the drainage solution from one crop to irrigate subsequent crops with higher salt tolerance [14,15]. This strategy is made possible by the unique physiological adaptations of certain species, known as halophytes, which not only endure high salinity but can, in some cases, exhibit enhanced growth and yield under moderate saline stress [16,17]. These plants employ sophisticated mechanisms to manage salinity, including ion compartmentalization within vacuoles to maintain a favorable cytoplasmic K⁺/Na⁺ ratio, synthesis of compatible osmolytes (e.g., proline and sugars) for osmotic adjustment, and the ability to maintain photosynthetic activity despite external osmotic stress [18,19]. Therefore, the strategic selection of these salt-tolerant species is fundamental to the successful implementation of cascade systems, allowing for the productive reuse of increasingly saline drainage water. Recent studies have confirmed the potential of cascade systems to significantly reduce water and fertilizer consumption compared to monoculture setups [20,21,22]. Reuse of low-quality leachates in successive stages can contribute to more sustainable agricultural models [23,24,25,26,27,28,29].

Despite these advances, a significant knowledge gap remains concerning the full potential of cascade systems. While early implementations in open-field agriculture date back nearly thirty years, and more recent controlled-environment experiments have confirmed their resource-use efficiency and environmental benefits in secondary cascade systems (two crop stages) [30,31,32,33]. However, the application of a tertiary cascade system—sequentially reusing nutrient solutions across three or more crop stages—has received limited attention [34]. This is primarily due to the increased complexity in managing the nutrient solution across multiple cultivation stages. As the number of stages increases, it becomes more difficult to maintain balanced nutrient levels, an optimal pH, and acceptable salinity. Furthermore, these systems demand more complex infrastructure and higher initial investment. The cumulative effects of salinity and nutrient imbalances can also negatively impact plant growth if not managed correctly. For these reasons, earlier research focused on simpler secondary systems, where these challenges were more manageable [35,36,37].

In this sense, a three-level cascade experiment was conducted, using primary crops (tomato), secondary crops (mint and peppermint), and tertiary crops (lemon balm and sea fennel). To simulate salinity, two distinct fertigation strategies were implemented for both the secondary and tertiary crops. This study is the first to evaluate the feasibility of such a system, focusing on specific crop pairings that include a primary food crop (tomato) and secondary/tertiary crops with varying salt tolerance, including halophytes (mint, peppermint, lemon balm, and sea fennel). Specially, the research aims to:

• Quantify water and nutrient flow at each of the three-level to assess overall resource-use efficiency under increasing salinity conditions.
• Examine the physiological and agronomic responses of each crop to rising salinity levels, including halophytes with high salt-absorbing capacity.
• Provide data to optimize crop combinations and management strategies for commercial-scale cascade systems, particularly in regions facing water scarcity and high-salinity irrigation sources.

This research will contribute valuable insights into the feasibility and sustainability of a tertiary cascade system, paving the way for more resilient and resource-efficient food production in a changing climate.

2. Materials and Methods

2.1. Greenhouse Facilities

The experiment was performed in different compartments of a gothic-arch greenhouse located at the experimental farm of the University of Thessaly (Velestino: Latitude 39°23′, longitude 22°45′, altitude 85 m) on the continental area of Eastern Greece. The greenhouse was oriented north–south, and the total ground area of each compartment was 240 m². Four out of the six compartments of the greenhouse were used for the experiments of this work.

All the compartments were covered by a polyethylene film in the roof, while the side walls were covered by polycarbonate sheets. The transmittance coefficient of the cover material was 0.75. The greenhouse was equipped with a shading/thermal screen with light transmission of 50%. Additionally, the greenhouse was equipped with heating, ventilation and pad and fan evaporative cooling system, all connected to a climate controller (SERCOM; Regeltechniek B.V. Automation SL, Lisse, The Netherlands). Fertigation management was performed by means of a hydroponic head (Argos Electronics; Athens, Greece). The complete fertigation system, including the cascade reuse components and monitoring units, is illustrated in Figure 1. The figure illustrates the innovative closed-loop fertigation system designed to maximize water and nutrient efficiency across three sequential crop stages.

The automated fertigation system designed for the cascade cropping setup inside the greenhouse begins at the Hydroponic Head and Mixing Tank, where tap water is combined with fertilizers, acid, and optionally recycled leachate before being distributed to five irrigation tanks (IR-tanks 1–5). The primary crop (tomato) receives fresh nutrient solution via IR-tank 1, and the drainage solution (DS) from these plants is collected in DR-tank 1. This drainage is then reused to irrigate the secondary crops (mint and peppermint) through IR-tanks 2 and 3, while the leachate from these secondary crops is collected in DR-tanks 2 and 3 and subsequently supplied to the tertiary crops (lemon balm and sea fennel) via IR-tanks 4 and 5, following two distinct treatments (S2 and S4). A portion of the drainage from all stages is returned to the recycling unit and mixed back into the main nutrient solution. The recycling loop includes essential treatment components—a UV unit for sterilization and a NaCl addition unit for precise salinity (EC) adjustment—to ensure solution quality for reuse. Sensors for pH, EC, and other parameters are placed throughout the tanks and greenhouse compartments to continuously monitor and control nutrient solution and climate conditions, allowing for multiple and distinct reuse treatments, while maintaining optimal growing conditions across all crop stages.

2.2. Experimental Set up and Crop Management

The experiment was carried out from March 2020 to July 2020. Within this period, the experimental setup included three sequential crop levels:

• Primary (donor) Crop: Tomato plants (Solanum lycopersicum L. cultivar Growdena, an F1 hybrid specifically selected for its high yield potential and indeterminate growth habit suitable for hydroponic cultivation in Mediterranean climates), were grown as the primary crop in two greenhouse compartments, totaling 400 m² (200 m² each). In each of the compartments, six channels measuring 20 m in length and 0.3 m in width that carried 19 rock wool slabs (Grodan Delta, NL 100 × 15 × 7.5 cm, 0.18 g cm⁻³, 90% water retention capacity, Roermond, The Netherlands) were installed. Tomato plants were transplanted on 10 March 2020, at a density of 4 plants m⁻² (864 plants total, with an initial height of 20 cm). The cultivation period lasted until 10 July 2020, for a total of 122 days.
• Secondary (receiver) Crops: Mint (Mentha piperita L. cultivar ‘Mitcham’, known for its robust growth and essential oil production) and peppermint (Mentha spicata L. cultivar ‘Crispa’, chosen for its distinct flavor profile and adaptability) were cultivated in a separate 200 m² compartment. The plants were transplanted into rock wool slabs at a density of 4 plants m⁻² (216 plants each) on 4 March 2020, and 16 April 2020, respectively, with an initial height of 9 cm. The cultivation cycle for mint lasted until 9 July 2020 (128 days), and for peppermint, until 10 July 2020 (129 days).
• Tertiary (receiver) Crops: Sea-fennel (Crithmum maritimum L.) a native Mediterranean halophyte highly tolerant to saline conditions, and Lemon-balm plants (Melissa officinalis L. cultivar ‘Lemonella’, a popular herb but more sensitive to salt stress), were chosen as tertiary crops and obtained from a local nursery. Plants were transplanted on 5 March 2020, and 27 March 2020, respectively, in rock wool slabs at a density of 4 plants m⁻². Initial plant height was approximately 9 cm. The cultivation cycle for the herbs and sea fennel lasted until 7 July and 8 July 2020 (103 and 104 days), respectively.

Agronomic details for all crops are as follows:

• Pruning and Thinning (Tomatoes): Tomato plants were pruned to a single main stem, and lateral shoots (suckers) were routinely thinned (removed) weekly to ensure consistent fruit development, improve air circulation, and direct photosynthetic energy towards fruit production [38,39].
• Staking (Tomatoes): Staking was performed weekly using plastic clips and strings to support the growing tomato vines and prevent lodging [40].
• Harvesting: All crops were subjected to continuous harvesting. Marketable yield for tomatoes was collected twice a week based on fruit ripeness (red color) [41]. Herbs (mint, peppermint, lemon balm) and sea fennel were harvested selectively as needed, by cutting the top 10–15 cm of the shoots, when plants reached an adequate biomass and visual maturity, ensuring continuous production and optimal plant health.
• Pollination (Tomatoes): Natural pollination for tomatoes was facilitated by commercially purchased bumblebee hives introduced into the greenhouse. No artificial pollination was performed for any crop [42].
• Phytosanitary Management: Phytosanitary management involved regular scouting for pests and diseases. Integrated Pest Management (IPM) principles were strictly followed [43,44]. Biological control agents (e.g., predatory mites for spider mites, parasitic wasps for whiteflies) were applied as a primary strategy, supplemented by targeted organic-approved treatments when necessary to minimize chemical intervention and maintain system integrity [45].
• Saline Stress Management: Saline stress was induced by adding NaCl to the nutrient solution. The electrical conductivity (EC) was monitored daily using a calibrated portable sensor (Combo pH-EC-TDS-Temp, 98,130 Hanna Instruments, Woonsocket, RI, USA). The substrate’s electrical conductivity was monitored daily by analyzing the collected drainage solution.

The experimental design for the secondary and tertiary crops was a completely randomized block design with three replications per treatment, with each replication consisting of 36 plants.

The irrigation dose for the primary crop was set to cover at least the 30% of the leaching fraction. The average daily dose was 1.49 L plant⁻¹, varying from 0.2 L plant⁻¹ to 2.4 L plant⁻¹ during the cultivation period. The pH of each NS preparation was set at 5.8. Correction was performed through the addition of NO₃ solution.

To evaluate the cascade system’s efficiency, different fertigation strategies under saline conditions were applied to the secondary and tertiary crops.

Secondary Crop Treatments:

o

S1 (Control): Irrigation with a fresh solution (FS) where salinity was gradually increased with fertilizers to an EC of 9 dS m⁻¹.

o

S2: Irrigation with 100% drainage solution (DS) from the primary crop, supplemented with NaCl to reach the same target EC of 9 dS m⁻¹.

Tertiary Crop Treatments:

o

S3 (Control): Irrigation with FS where salinity was gradually increased with fertilizers to an EC of 11 dS m⁻¹.

o

S4: Irrigation with NS mixed from S2 and S4 treatment DS, with additional NaCl to reach the target EC of 11 dS m⁻¹.

In all saline treatments, the EC was increased by 1 dS m⁻¹ per week. The pH was adjusted to 5.8 using H₂SO₄ 1N solution. The daily irrigation doses for the secondary and tertiary crops were 0.67 L plant⁻¹ and 0.55 L plant⁻¹, and 0.58 L plant⁻¹ and 0.55 L plant⁻¹, respectively.

Table 1. Mean values of pH, EC, and nutrient composition of the target nutrient solutions (NS) used for irrigating the primary, secondary, and tertiary crops at each growth stage, adapted to Mediterranean climatic conditions.

Parameter	Unit	Primary Crop	Primary Crop	Primary Crop	Secondary Crop	Tertiary Crop
		Vegetative Stage	Flowering Stage	Fruiting Stage	Vegetative Stage	Vegetative Stage
pH		5.8	5.8	5.8	5.8	5.8
EC	(dS m−1)	2.2	2.6	2.0	2.4	2.8
NO3	(mmol L−1)	17.4	15.6	11.9	12.5	11.4
P	(mmol L−1)	0.8	1.0	0.8	0.3	0.4
K	(mmol L−1)	6.0	6.0	5.4	2.4	5.4
Ca	(mmol L−1)	4.9	5.6	4.3	4.3	3.2
Na	(mmolL−1)	1.2	2.9	1.3	9.3	11.5
Mg	(mmol L−1)	2.5	2.9	2.5	2.1	2.3
NH4	(mmol L−1)	1.2	1.2	1.2	1.0	0.2
Fe	(μmol L−1)	18.1	24.7	16.5	10.4	29.1
Zn	(μmol L−1)	8.7	5.7	4	2.4	3.5
Mn	(μmol L−1)	13.0	10.6	10.2	2.1	3.2
Cu	(μmol L−1)	0.7	1	0.7	1.3	1.1
B	(μmol L−1)	35.0	30.0	30.0	20.0	50.0
Mo	(μmol L−1)	0.5	0.5	0.5	1	0.1

presents the synthesis of the target NS, as well as the corresponding pH and EC values, tailored to the growth stages of the primary, secondary, and tertiary crops under Mediterranean climatic conditions. Saline treatments started simultaneously across all the treatments, 71 and 28 (12-May) of mint and peppermint, 48 and 70 DAT (13-May) of lemon-balm and sea-fennel, respectively. The treatment cycle lasted for 59 days.

The tap water used in the system had low salinity, with EC of 0.8 dS m⁻¹ and a pH of 7.1. Its composition included 0.200 mmol L⁻¹ NO₃⁻, 0.001 mmol L⁻¹ NH₄⁺, 0.006 mmol L⁻¹ PO₄²⁻, 0.094 mmol L⁻¹ SO₄²⁻, 0.375 μmol L⁻¹ Fe²⁺, 0.884 mmol L⁻¹ Ca²⁺, 1.629 mmol L⁻¹ Mg²⁺, 0.032 mmol L⁻¹ K⁺, and 1.959 mmol L⁻¹ Na⁺. Due to this low baseline salinity, stress conditions could not be induced naturally. To establish the intended saline environment, NaCl was added to the irrigation solution (IS). A concentration of 5 mmol L⁻¹ NaCl was introduced across all treatments starting 9 days before the formal onset of salinity application (defined as 0 days after salinity treatment start, DAS), in order to accelerate the buildup of the intended saline environment. The EC values reported in Table 1 for secondary and tertiary crops correspond to the nutrient solution before this pre-treatment adjustment. Substrate EC was continuously monitored twice weekly using a pour-through method to ensure the root zone salinity matched the applied irrigation solution targets and to detect any unintended accumulation.

Although only a single cultivation cycle was conducted, the experimental design allowed detailed monitoring of water and nutrient flows, salinity accumulation, and crop responses across all three stages, providing robust insights into the feasibility and resource-use efficiency of the cascade system.

2.3. Measurements

2.3.1. Climate and Nutrient Solution Quality Data

Environmental variables—air temperature (Ta, °C), relative humidity (RH, %) and solar radiation (SR, W m⁻²) based on solar pyranometer (SERCOM, Automation SL, Lisse, The Netherlands), were measured with calibrated sensors placed 1.8 m above ground level. Sensors were calibrated prior to deployment following the manufacturer’s instructions to ensure measurement accuracy [46]. To provide a comprehensive view of the growing conditions, these variables are illustrated in Figure A1, which presents their daily trends throughout the experimental period.

EC and pH of the irrigation solution (IS) and drainage solution (DS) were measured daily using a portable sensor (Combo pH-EC-TDS-Temp, 98,130 Hanna Instruments, Woonsocket, RI, USA). The sensor was calibrated weekly with standard buffer solutions (pH 4.01, 7.01, 10.01 for pH and 1.413 and 12.88 dS m⁻¹ for EC), and recalibration was performed immediately if readings drifted. The daily volumes of IS and DS in each crop according to the treatment expressed in L m⁻² were recorded automatically in a database.

Samples of the IS and DS were collected weekly for nutrient analysis. Anions (NO₃-N, PO₄-P) were analyzed using ion chromatography (model DX-500, Dionex, Sunnyvale, CA, USA). Cations (K, Ca, Mg, and Na) were analyzed by inductively coupled plasma-atomic emission spectrometry (model iCAP7400, Thermo Scientific, Waltham, MA, USA) [47], and micronutrients (Fe, Zn, Mn, and Cu) were analyzed using ICP–OES (SPECTRO Genesis spectrometer, Analytical Instruments GmbH, Kleve, Germany) [48]. The quality of these analyses was verified with standard procedures. For all analyses, a series of known concentration standards was used to generate a linear calibration curve. Sample concentrations were then determined by comparing their measured values to this curve. Nutrient concentrations (mg L⁻¹) were calculated using a linear calibration Equation (1):

$$\mathit{Concentration} (\mathrm{mg} {\mathrm{L}}^{-1})=(3.2\times \mathit{Absorbance})+0.5$$

(1)

Analytical methods were subject to rigorous quality control procedures to ensure data reliability. We established the Limit of Detection (LOD) and Limit of Quantification (LOQ) for each analysis, with values consistent with those reported in the literature and detailed in

Table A1. Analytical Quality Assurance and Control for Nutrient Analysis. This table details the Limit of Detection (LOD), Limit of Quantification (LOQ), and Reproducibility (as Relative Standard Deviation, RSD%) for each analytical method. Values are based on standard validation protocols and are consistent with those in the cited literature. The ‘~’ symbol indicates an approximate value.

Analyte	Method	LOD (µmol L−1)	LOQ (µmol L−1)	Reproducibility (RSD, %)
Anions
NO3−	Ion Chromatography	~0.26 1	~0.80 1	<3% 2
PO42−	Ion Chromatography	~0.01–0.03 3	~0.03–0.09 3	<5% 3
Cations
K+, Ca2+, Mg2+	ICP-AES	~0.1–2.0 4	~0.3–6.0 4	<5% 5
Na+	ICP-AES	~0.5–3.0 4	~1.5–9.0 4	<5% 5
Micronutrients
Fe, Zn, Mn, Cu	ICP-OES	~0.01–0.1 6	~0.03–0.3 6	<5% 7
Total N	Potentiometric (ISO 16634–1:2008)	~0.07 8	~0.21 8	<2% 9

¹ Based on data for nitrate analysis in agricultural solutions by Ion Chromatography [68]. ² Based on data for ion chromatography reproducibility [69]. ³ Based on typical values for phosphate analysis in agricultural solutions via ion chromatography [70,71]. ⁴ Based on typical values for ICP-AES analysis of major cations in plant material [72]. ⁵ Based on standard reproducibility for ICP-AES analysis [68,69]. ⁶ Based on typical LOD values for trace elements by ICP-OES [72]. ⁷ Based on standard reproducibility for ICP-OES analysis of trace elements [73]. ⁸ Based on typical LOD and LOQ values for Total N in plant material by combustion/potentiometric methods [74]. ⁹ Based on standard reproducibility for Total N analysis by potentiometric methods [75].

. Precision and reproducibility were verified through triplicate analyses of randomly selected samples, consistently yielding a relative standard deviation (RSD) below 5%.

2.3.2. Plant Growth and Nutrient Data

The photosynthesis rate (P_N; µmol CO₂ m⁻² s⁻¹), stomatal conductance (g_s; mol H₂O m⁻² s⁻¹), and leaf transpiration (E; mmol H₂O m⁻² s⁻¹), were recorded weekly on young, fully developed leaves (n = 15 per crop and treatment). These measurements were performed using a fluo-porometer, a handheld photosynthesis measurement system, model LCpro+ 1.0 ADC (Bioscientific Ltd., Hoddesdon, Hertfordshire, UK), equipped with a 6.25-cm² cuvette. Before recording each measurement, the instrument was allowed to run for several minutes on the leaf until the P_N and g_s values became stable. The measurements were conducted with the cuvette settings at CO₂ concentration of 350 ppm, a photosynthetic photon flux density (PPFD) of 1000 µmol m⁻² s⁻¹, 25 °C leaf temperature, and 65% ambient RH. The PPFD was uniformly set to 1000 µmol m⁻² s⁻¹ across all species to establish a non-limiting light condition within the shared greenhouse compartment. This ensured that observed differences in crop performance were primarily attributable to the applied fertigation treatments (recycling and induced salinity), allowing for a valid comparison of species-specific responses. Measurements were taken weekly on sun-exposed leaves during midday.

Leaf greenness was estimated as a proxy for relative chlorophyll content using a SPAD 502 Chlorophyll Meter (Konica-Minolta Co. Ltd., Tokyo, Japan) to record the SPAD index of leaves. Measurements were made weekly on the upper surface of the canopy (n = 30 per treatment) during midday. For each plant, eight measurements were taken on a fully developed leaf to obtain an average SPAD value.

To evaluate growth and nutrient uptake, two destructive samplings (n = 18 per treatment) were conducted. The first occurred between 15–23 days after transplanting (DAT), depending on species: mint (15 DAT), peppermint (17 DAT), lemon balm (22 DAT), and sea fennel (23 DAT). The second sampling was performed toward the end of cultivation (56–59 DAT): mint (58 DAT), peppermint (59 DAT), lemon balm (57 DAT), and sea fennel (56 DAT). Fresh matter (FM, g plant⁻¹) was measured immediately after harvest, and samples were then dried at 70 °C for 72 h to determine dry matter (DM, g plant⁻¹). FM and DM were recorded for whole plants and separately for leaves and stems.

Dried samples were ground to powder in order to perform mineral analyses of the main macro-micronutrients (N, P, K, Ca, Na, Mg, Fe, Zn, Mn and Cu). The total N extraction was performed by potentiometer using an internal method based on ISO 16634–1:2008 [49,50]. The other elements, including Sodium (Na), were determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, SPECTRO Analytical Instruments GmbH, 180 Kleve, Germany) [51] using an internal method based on EN-13805:2002 [52]. The content of each element concentration of macronutrients (N, P, K, Ca, Na, Mg) and micronutrients (Fe, Zn, Mn, Cu) were expressed in % and ppm of leaf dry weight, respectively, using Equation (1). In total, 36 plants per destructive process were used (n = 9 treatment and crop) to determine the uptake nutrient concentrations of each secondary crop.

Since, the primary objective was to evaluate the effects of the recycled drainage solution, the analyses was focused on the secondary and tertiary crops, which directly received the varying salinity and nutrient treatments.

2.4. Calculations

The total IS and DS during the cultivation period is the sum of the daily IS and DS dosages, respectively. The total volume uptaken (Vup) by the plants the whole cultivation period expressed in L m⁻² consists of the total amount applied minus the amount drained from the plants. Drainage percentage rate expressed in % was calculated as a ratio of volumes of drained NS to applied NS, according to the following formula (Equation (2)):

$$\mathit{Drainage} \mathit{percentage}={\displaystyle \frac{\mathit{NS} \mathit{drained}}{\mathit{Ns} \mathit{applied}}}\times 100$$

(2)

The irrigation doses were dynamically adjusted based on real-time drainage monitoring. If drainage exceeded 30%, the next irrigation was reduced; if it fell below 30%, it was increased. Total biomass (kg plant⁻¹ m⁻²) was determined at harvest by collecting and weighing all aboveground plant parts (stems and leaves) from a representative sample of four plants within a one-square-meter area per plot. The biomass per plant was calculated by dividing the total biomass of the sample by the number of plants. At the end of the growing season, marketable yield (kg m⁻²) was calculated by summing the FM of all harvests collected throughout the crop cycle. Fresh weight was measured using a calibrated electronic scale for all plants within a one-square-meter plot (4 plants m⁻²). FM/DM ratio was calculated as the ratio between the FM and DM and expressed as a percent value per plant to define the plant’s water content.

The water use efficiency (WUE) was assessed by calculating instantaneous (P_N/E) and intrinsic (P_N/g_s) ratios, measured using a portable photosynthesis system, gas exchange analyzer. Instantaneous water use efficiency (P_N/E, expressed in µmol (CO₂) mmol⁻¹ H₂O) was determined as net photosynthesis (P_N, µmol CO₂ m⁻² s⁻¹) divided by transpiration rate (E, mmol H₂O m⁻² s⁻¹), representing carbon fixed per unit of water transpired. Intrinsic water use efficiency (P_N/g_s, expressed in µmol (CO₂) mol⁻¹ H₂O) was calculated as net photosynthesis (P_N) divided by stomatal conductance (g_s, mol H₂O m⁻² s⁻¹), indicating carbon fixed per unit of stomatal opening and reflecting the plant’s strategy for maximizing carbon gain while minimizing water loss, as described by Hatfield & Dold [53].

The water productivity (WP) was determined as the ratio between the FM and the volume of water applied in each crop during the whole cultivation period and expressed as kg FM m⁻³ H₂O (Equation (3)). Fertilizer productivity (FP) was determined as the ratio between total the FM and the quantity of fertilizers applied in each crop during the whole cultivation period and expressed as kg FM kg⁻¹ fertilizer (Equation (4)).

$$\mathrm{WP}={\displaystyle \frac{\mathit{Marketable} \mathit{Fresh} \mathit{Matter} (\mathrm{kg} \mathrm{FM})}{\mathit{Total} \mathit{Water} \mathit{Applied} \left({\mathrm{m}}^3\right)}}$$

(3)

$$\mathrm{FP}={\displaystyle \frac{\mathit{Marketable} \mathit{Fresh} \mathit{Matter} (\mathrm{kg} \mathrm{FM})}{\mathit{Total} \mathit{Fertilizer} \mathit{Applied} (\mathrm{kg} \mathrm{fertilizer})}}$$

(4)

WP and FP were assessed at both the individual crop and system levels, allowing for evaluation of input-use efficiency across different cropping configurations. At the individual crop level, water productivity was denoted as WPP, WPS, and WPT for the primary, secondary, and tertiary crops, respectively. Correspondingly, fertilizer productivity was expressed as FPP, FPS, and FPT, representing the FM produced per unit of fertilizer applied to each respective crop. At the system level, combined water productivity (SWP) was determined by dividing the total FM of all crops in a given system (e.g., primary + secondary) by the total water applied to those crops. These were denoted as SWP-PS, SWP-PT, and SWP-PST. System-level fertilizer productivity (SFP) was similarly calculated by dividing the total FM of the system by the total fertilizer input, and denoted as SFP-PS, SFP-PT, and SFP-PST.

The hydroponic cascade system enabled substantial fertilizer savings for the secondary and tertiary crops. Fertilizer savings were estimated by comparing the fertilizer requirements under a monoculture hydroponic setup with the actual input used in the cascade system. For the secondary crop, the estimated monoculture fertilizer requirement was 0.27 kg m⁻² (based on 53.65 kg over 200 m²), while no additional fertilizer was applied in the cascade system, resulting in a saving of 0.27 kg m⁻². Similarly, for the tertiary crop, the corresponding estimated monoculture requirement was 0.19 kg m⁻² (38.01 kg over 200 m²), with actual usage in the cascade system being zero, leading to a full saving of 0.19 kg m⁻².

2.5. Statistical Analysis

Data analysis was performed with SPSS 26.0 (Statistical Product and Service Solutions, IBM Corp, Armonk, NY, USA). The assumptions of ANOVA, including normality and homogeneity of variances, were verified before analysis. One-way ANOVA at a confidence level of 95% (p ≤ 0.05) was used to test for significant differences of different variances crossed with the four salinity treatments. LSD (Least Significant Difference) multiple comparisons test to identify differences among the groups. The mean values and standard deviations (±SD) of the measured parameters are reported; p < 0.05 was set as the threshold for significant differences. While the lack of statistical validation is a constraint, the observed trends in nutrient concentrations across different growth stages and irrigation conditions offer valuable insights.

3. Results

During the experimental period, the average daily Ta outside the greenhouse was 18.7 ± 4.0 °C (mean ± standard deviation, SD), with a daily RH of 64.3 ± 3.1%. Additionally, the average global SR reached 221.4 ± 0.7 W m⁻². Inside the greenhouse, the average daily Ta in the tomato cultivation area was 18.8 ± 5.8 °C, with a daily RH of 75.4 ± 5.6%. The average daily SR within the greenhouse was 164.5 ± 0.5 W m⁻². For the secondary crop, the daily average Ta was 19.8 ± 6.3 °C, the RH was 66.9 ± 2.8%, and the SR measured 162.8 ± 0.5 W m⁻². In the tertiary crop, the Ta and RH were maintained under stable conditions, similar to the secondary crop, at 19.9 ± 7.4 °C and 65.0 ± 4.0%, respectively. The average daily SR in the tertiary crop area was 160.3 ± 0.5 W m⁻².

3.1. Quality Assessment of Cascade Solution

To assess the quality of the NS, both pH and EC variations were monitored throughout cultivation. In all the crops, pH of irrigated NS remained stable at 5.8, while the pH of DS ranged according to the cultivation. In primary crop, pH ranged from 6.2 to 7.6, averaging 6.9 ± 0.5. In secondary crop, under S1 conditions, mint and peppermint experienced notable pH instability, with sharp decreases at 24 and 51 DAS. These fluctuations were likely linked to nutrient uptake dynamics. Meanwhile, S2 provided a more stable pH environment, consistently averaging near 8. This elevated pH in the S2 DS, significantly above the target NS pH of 5.8, suggests reduced availability and potential precipitation of certain micronutrients particularly iron (Fe), which is less soluble at higher pH. This deviation likely influenced the subsequent accumulation patterns observed in the drainage solutions. In contrast, sea fennel (7.25 ± 0.17) and lemon balm (7.61 ± 0.26) showed minor interspecies differences, yet sea fennel exhibited a slight (~5%) pH reduction under the same salinity treatment compared to lemon balm.

For tomatoes, EC in NS ranged from 1.7 to 3.2 dS m⁻¹ (avg. 2.1), with higher values during the vegetative stage (Table 1). Correspondingly, the EC of the DS for tomatoes ranged between 2.5 and 5.3 dS m⁻¹ (avg. 4.1). Figure 2a illustrates the gradual increase in EC of the IS for the secondary crops, attributed to the weekly additions of saline water in pre-treatment and follow onset of salinity treatment. Correspondingly, Figure 2b presents the EC values of the DS, which increased by 92% relative to the IS. Although mint exhibited slightly higher EC levels than peppermint during mid-cultivation, no statistically significant differences were observed at final harvest (p > 0.05), with all treatments converging at approximately 17 dS m⁻¹. In tertiary crops, the composition of the secondary crop DS influenced the EC of the new irrigation solution. Figure 3a presents the increase in IS EC until the target threshold was reached, while Figure 3b shows the EC trends in DS over time. Notably, in the S4 treatment for sea fennel, the drainage EC exceeded the sensor’s maximum limit, resulting in some data loss, but the rising trend highlights the species’ high salt tolerance and provides valuable qualitative insight. This accumulation indicates that factors such as plant uptake, evapotranspiration, or substrate interactions contributed to salt buildup, especially in this halophytic species.

3.2. Crop Water Consumption

Table 2. Water Balance Analysis. Total nutrient solution (NS) applied in, drained and uptake (L m⁻²) by each crop species according to the treatment. The NS is divided based on the quantity of water and drainage solution introduced.

Crop Species	Volume of NS Applied		Volume of NS Drained	Water Uptake	Percetage Drained (%)
	(L m−2)		(L m−2)
Tomato	409		160.5	248.5	39.2
	Volume of NS applied (L m−2)		Volume of NS drained (L m−2)	Water uptake (L m−2)
Mint	Water	DS
S1	92	0	29	63	31.5
S2	0	86	29	57	33.7
Peppermint
S1	75	0	32	43	42.7
S2	0	70	32	38	45.7
Lemon balm
S3	59	0	18	41	30.5
S4	0	54	18	36	33.3
Sea fennel
S3	48	0	19	29	39.6
S4	0	40	15	25	37.5

details the volumes of water and drainage mixed to prepare the irrigation solutions, alongside the NS uptake and drainage by each species under different treatments. In the primary tomato crop, 409 L m⁻² of NS was applied, with a water uptake of 160 L m⁻² and a resulting drainage of 39%. For mint, treatments S1 and S2 received 92 L m⁻² and 86 L m⁻² of NS, respectively, with a drainage volume of 29 L m⁻². Similar trends in applied NS, water uptake, and drainage were observed in the tertiary crops, as presented in Table 2. According to the table, the irrigation needs of both the secondary and tertiary crops were fully (100%) covered by reused drainage solution (DS). However, the total DS generated (352.5 L m⁻²) was not entirely reused; only 250 L m⁻² was applied for irrigation, resulting in a 71% reuse rate. The 94 m³ of irrigation supplied to S2 and S4 originated from the DS of the secondary (65%) and tertiary (35%) crops, indicating a surplus of drainage and a non-closed-loop system. Moreover, the higher salinity sensitivity of lemon balm, used as a tertiary crop, underscores the importance of accounting for species-specific salt tolerance when aiming to maximize water reuse and minimize drainage. Further research could explore reasons for the unrecovered drainage and strategies for enhanced water use efficiency.

3.3. Nutrient Concentration

The concentrations of elements in the IS, leaf tissue, and DS are interconnected in hydroponic systems. Understanding these relationships is crucial for maintaining optimal nutrient levels and preventing nutrient deficiencies or excesses. Macronutrient and micronutrient levels by crop and treatment are shown in

Table 3. Mean values of macronutrients (mmol L⁻¹) and micronutrients (μmol L⁻¹) measured in irrigation solution (ΙS) according the specie and treatment, in six sampling dates before and after salinity period.

IRRIGATION	NO3	P	K	Ca	Na	Mg	Fe	Zn	Mn	Cu
SOLUTION (IS)	mmol L−1	mmol L−1	mmol L−1	mmol L−1	mmol L−1	mmol L−1	μmol L−1	μmol L−1	μmol L−1	μmol L−1
6DBS
Mint_Peppermint_S1	12.46	0.34	2.36	4.32	9.27	2.10	10.44	2.37	2.10	1.29
Mint_Peppermint_S2	15.36	0.46	4.30	4.62	4.83	3.09	24.48	6.26	3.36	3.72
Lemon balm_Sea fennel_S3	11.37	0.42	5.44	3.20	11.48	2.25	29.09	3.46	3.21	1.13
Lemon balm_Sea fennel_S4	16.68	0.58	6.09	4.98	4.43	3.42	23.91	6.17	3.69	2.73
8DAS
Tomato NS	16.83	1.13	6.85	5.53	1.49	2.75	24.46	6.05	12.80	0.70
Mint_Peppermint_S1	12.64	0.54	3.23	6.05	14.03	2.18	10.23	2.24	2.30	0.70
Mint_Peppermint_S2	19.30	0.79	7.44	6.24	8.22	3.99	32.34	6.71	7.20	2.26
Lemon balm_Sea fennel_S3	11.23	0.53	6.57	3.68	14.50	2.63	32.43	2.51	5.31	1.10
Lemon balm_Sea fennel_S4	21.87	0.58	7.50	7.21	11.57	4.59	35.18	5.23	6.30	2.45
29DAS
Tomato NS	11.90	0.77	5.36	4.33	1.35	2.48	16.50	3.98	10.23	0.66
Mint_Peppermint_S1	21.35	0.18	1.65	9.42	27.81	2.42	15.61	1.45	2.19	0.75
Mint_Peppermint_S2	20.79	0.43	4.31	7.19	26.70	4.41	29.46	5.56	6.94	2.59
Lemon balm_Sea fennel_S3	13.18	1.07	10.24	4.79	38.22	2.38	143.20	4.03	2.89	1.52
Lemon balm_Sea fennel_S4	23.54	0.33	5.24	9.43	32.57	6.05	41.68	3.93	3.84	3.05
37DAS
Tomato NS	13.80	0.80	5.41	4.59	1.66	2.39	16.81	5.40	9.91	0.74
Mint_Peppermint_S1	20.81	0.56	3.05	9.16	37.14	2.11	10.04	3.39	1.98	0.86
Mint_Peppermint_S2	16.42	0.34	2.85	6.88	34.62	4.87	26.29	5.02	4.60	1.46
Lemon balm_Sea fennel_S3	12.70	0.96	7.74	4.31	40.34	2.26	149.38	2.64	2.61	1.22
Lemon balm_Sea fennel_S4	17.72	0.32	2.98	7.14	51.43	4.83	24.39	3.65	5.19	1.10
43DAS
Tomato NS	14.13	0.72	5.29	4.97	1.28	2.38	28.79	5.24	11.71	0.74
Mint_Peppermint_S1	41.34	3.86	19.38	17.96	12.00	3.50	26.50	5.16	1.56	4.73
Mint_Peppermint_S2	21.21	1.19	7.38	8.46	27.89	4.91	45.98	6.09	2.37	2.25
Lemon balm_Sea fennel_S3	16.65	7.56	40.23	7.13	16.98	4.89	153.45	11.83	3.45	10.34
Lemon balm_Sea fennel_S4	26.46	0.60	5.89	12.87	55.96	7.74	49.27	4.48	2.43	3.40
51DAS
Tomato NS	14.79	0.77	6.42	4.30	1.97	2.52	37.32	6.32	12.52	0.95
Mint_Peppermint_S1	41.99	4.12	23.41	18.13	15.58	3.95	30.13	12.64	0.84	3.92
Mint_Peppermint_S2	25.39	1.36	8.91	9.54	44.65	4.73	54.59	7.08	2.42	2.66
Lemon balm_Sea fennel_S3	23.67	7.71	50.41	8.09	14.63	5.37	252.86	13.43	3.68	7.90
Lemon balm_Sea fennel_S4	31.94	0.54	6.97	14.15	58.26	8.75	81.54	3.77	3.56	3.86

for the IS and in

Table 4. Mean values of macronutrients (mmol L⁻¹) and micronutrients (μmol L⁻¹) measured in drainage solution (DS) according the specie and treatment, in six sampling dates before and after salinity period.

DRAINAGE	NO3	P	K	Ca	Na	Mg	Fe	Zn	Mn	Cu
SOLUTION (DS)	mmol L−1	mmol L−1	mmol L−1	mmol L−1	mmol L−1	mmol L−1	μmol L−1	μmol L−1	μmol L−1	μmol L−1
6DBS
Tomato NS	20.20	0.40	5.51	5.91	2.62	3.51	23.64	8.61	4.41	2.89
Mint S1	15.10	0.09	2.06	8.13	18.40	4.24	17.05	0.75	0.28	5.22
Mint S2	19.80	0.12	5.60	7.08	6.81	4.47	33.58	0.77	0.21	5.45
Peppermint S1	12.20	0.10	1.97	5.25	12.12	2.71	10.34	2.49	0.33	3.08
Peppermint S2	14.98	0.25	4.58	5.39	4.99	3.38	27.11	2.27	0.21	4.99
Lemon Balm S3	15.97	0.18	7.07	4.82	14.36	3.36	28.58	2.08	0.21	2.49
Lemon Balm S4	24.14	0.44	10.18	6.88	10.12	4.69	24.54	3.64	1.78	3.37
Sea fennel S3	21.43	0.21	8.93	5.52	19.05	4.70	35.08	3.99	0.77	3.63
Sea fennel S4	32.12	0.24	12.05	8.24	12.79	6.55	33.90	3.20	1.57	4.08
8DAS
Tomato NS	28.91	0.59	7.97	8.78	4.26	6.15	45.86	5.94	6.98	4.34
Mint S1	26.61	0.03	0.29	21.96	51.57	8.29	34.78	1.78	0.31	9.86
Mint S2	55.05	0.05	5.41	25.48	34.03	18.89	152.42	3.80	0.21	14.69
Peppermint S1	16.70	0.02	0.33	13.94	34.82	4.35	21.58	2.29	0.46	5.52
Peppermint S2	35.99	0.06	5.58	17.08	24.75	10.86	88.32	3.74	0.21	8.69
Lemon Balm S3	23.42	0.06	11.89	11.03	70.23	7.97	132.93	8.26	0.38	6.43
Lemon Balm S4	34.68	0.14	7.71	16.23	32.83	10.21	64.20	2.64	1.14	7.31
Sea fennel S3	21.43	0.21	8.93	5.52	19.05	4.70	35.08	3.99	0.77	3.63
Sea fennel S4	31.15	0.25	8.35	9.89	19.60	7.20	39.52	2.65	1.42	4.35
29DAS
Tomato NS	19.03	0.68	5.81	6.48	3.62	4.21	26.54	5.50	11.51	0.90
Mint S1	33.23	0.05	2.72	16.68	52.38	4.22	16.60	4.00	0.21	4.01
Mint S2	34.14	0.12	7.05	13.07	41.19	7.70	41.76	2.67	0.21	4.20
Peppermint S1	28.93	0.04	2.73	13.03	42.94	3.34	14.77	4.97	0.21	2.41
Peppermint S2	31.98	0.09	6.72	11.10	40.52	6.86	38.23	1.75	0.21	3.46
Lemon Balm S3	16.95	0.04	8.72	7.20	74.54	3.56	134.26	4.78	0.21	1.11
Lemon Balm S4	42.97	0.04	8.11	18.75	52.09	11.80	85.71	2.10	0.21	6.28
Sea fennel S3	37.94	0.11	8.72	14.12	89.75	10.54	157.19	6.95	0.21	5.12
Sea fennel S4	55.47	0.12	9.40	20.61	85.71	15.33	157.12	9.44	0.21	7.65
37DAS
Tomato NS	15.12	0.24	2.23	5.90	3.76	4.30	19.75	2.34	<0.01	2.42
Mint S1	52.81	0.09	4.30	31.13	100.57	10.44	53.76	3.91	0.56	7.39
Mint S2	46.18	0.05	4.42	26.34	101.52	12.50	65.66	3.18	0.21	7.87
Peppermint S1	45.92	0.14	5.08	25.37	74.64	5.53	28.74	6.62	6.42	5.17
Peppermint S2	36.33	0.03	6.20	18.91	88.77	12.61	72.44	2.34	0.21	6.12
Lemon Balm S3	23.48	0.59	19.20	9.23	101.14	5.63	270.00	5.64	0.70	3.36
Lemon Balm S4	40.03	0.03	7.66	18.99	102.29	11.18	72.29	2.82	1.43	5.71
Sea fennel S3	28.71	0.62	17.14	10.74	119.39	7.20	293.93	13.91	5.09	3.77
Sea fennel S4	74.46	0.05	9.27	32.92	150.87	23.35	168.64	5.92	4.23	11.23
43DAS
Tomato NS	16.94	0.32	3.60	6.80	5.07	4.32	29.29	3.48	2.08	1.42
Mint S1	104.42	0.17	2.58	52.91	72.35	7.17	47.09	6.71	4.19	9.14
Mint S2	38.44	0.03	2.27	21.83	95.30	13.78	76.39	2.79	0.21	4.83
Peppermint S1	83.61	0.19	3.98	40.70	49.49	5.08	38.32	13.14	12.21	5.66
Peppermint S2	37.20	0.02	4.10	19.18	89.78	11.88	73.82	2.11	0.21	4.72
Lemon Balm S3	26.27	0.53	16.63	11.06	75.54	4.89	338.69	5.69	0.41	3.26
Lemon Balm S4	39.94	0.07	5.23	19.25	84.23	10.29	70.92	3.18	1.76	4.41
Sea fennel S3	29.83	1.22	18.29	10.93	79.91	6.20	351.13	9.85	4.97	4.28
Sea fennel S4	55.83	0.09	6.47	23.48	107.81	14.90	96.54	4.82	3.49	6.36
51DAS
Tomato NS	11.04	0.47	4.26	3.65	4.29	2.83	38.30	4.16	4.50	1.52
Mint S1	133.51	9.34	60.64	45.53	48.51	15.64	88.26	19.86	7.35	24.28
Mint S2	47.62	0.27	7.41	22.13	121.74	13.06	109.83	2.87	0.21	4.75
Peppermint S1	97.47	8.51	52.93	36.26	37.29	11.93	68.71	17.79	6.47	14.72
Peppermint S2	50.46	0.37	11.34	20.56	115.93	11.73	109.21	1.82	0.21	4.70
Lemon Balm S3	48.14	11.44	99.32	15.07	49.28	14.34	514.46	13.61	2.12	22.18
Lemon Balm S4	56.99	0.25	7.82	24.64	113.74	15.44	117.52	2.92	1.32	6.38
Sea fennel S3	39.24	11.75	71.15	13.68	40.17	10.45	370.77	19.54	11.18	15.31
Sea fennel S4	57.58	0.46	9.00	22.22	99.78	14.66	113.36	4.97	4.23	6.56

for the DS. Statistical analysis was not performed in nutrient balance analysis (Table 3 and Table 4) due to the limitation of a single sample per treatment; prevents formal statistical analysis.

According to Table 3, the initial irrigation solutions (6 days before salinity treatment start, DBS) displayed moderate macronutrient concentrations across all treatments. In the tomato IS, nitrate (NO₃⁻) was 17.42 mmol L⁻¹, potassium (K) at 5.98 mmol L⁻¹, and sodium (Na) was relatively low at 1.22 mmol L⁻¹. In contrast, S1 (mint and peppermint) presented notably lower nitrate (12.46 mmol L⁻¹), potassium (2.36 mmol L⁻¹), and higher sodium (9.27 mmol L⁻¹), while iron (Fe) was 10.44 μmol L⁻¹. As salinity increased (S3 and S4), sodium levels in the IS rose further—up to 11.48 mmol L⁻¹ in S3—indicating a clear gradient of salinity input. Iron and other micronutrients like zinc and manganese also showed slightly elevated values in the higher salinity treatments, consistent with the expected composition of more saline irrigation sources. At the final stage, 51 DAS, nitrate in S1 reached 41.99 mmol L⁻¹, potassium 23.41 mmol L⁻¹, and calcium 18.13 mmol L⁻¹, showing strong nutrient enrichment in the irrigation solution. In S3, iron rose drastically to 252.86 μmol L⁻¹—the highest recorded value—while potassium peaked at 50.41 mmol L⁻¹. S4 maintained high sodium (58.26 mmol L⁻¹) and iron (81.54 μmol L⁻¹) levels, with magnesium also increasing to 8.75 mmol L⁻¹, highlighting the growing ionic burden in the system.

While EC (Figure 2 and Figure 3) provides an overall measure of total dissolved salts, a more detailed analysis of the nutrient solution’s composition reveals the specific dynamics of ion uptake and accumulation. In the primary crop, a significant reduction in essential macronutrients like nitrate (NO₃⁻) and potassium (K⁺) was observed in the drainage solution, a clear sign of active uptake by the tomato plants. In contrast, the concentrations of non-essential ions, particularly sodium (Na⁺) showed a steady increase in the drainage due to both their presence in the irrigation water and minimal plant uptake (Table 3). This differential uptake, where plants selectively remove nutrients while leaving behind non-essential ions, was the primary driver of the rising EC observed across the cascade. A mass balance analysis further confirmed that while essential nutrients were actively partitioned into plant biomass, the proportion of non-metabolized ions steadily rose, particularly in the later stages. This explains why the EC of the tertiary drainage (Figure 3b), especially in the halophytic treatments like sea fennel, reached very high levels (e.g., 20 dS m⁻¹), as these species were able to tolerate and accumulate salts without a corresponding decrease in EC from nutrient uptake.

According to Table 4, at 6 DBS, mint (S1) had 15.10 mmol L⁻¹ NO₃⁻, 0.09 mmol L⁻¹ P, 2.06 mmol L⁻¹ K⁺, 8.13 mmol L⁻¹ Ca²⁺, 18.40 mmol L⁻¹ Na⁺, 4.24 mmol L⁻¹ Mg²⁺, and 17.05 μmol L⁻¹ Fe²⁺. Peppermint (S1) showed lower NO₃⁻ (12.20 mmol L⁻¹) and K⁺ (1.97 mmol L⁻¹) but similar Ca²⁺ and Mg²⁺. Lemon balm and sea fennel under higher salinity (S3–S4) displayed elevated K⁺ and Na⁺, with lemon balm S4 reaching 10.18 and 6.88 mmol L⁻¹ respectively, and sea fennel S4 reaching 12.05 and 8.24 mmol L⁻¹. Fe levels ranged between 24–35 μmol L⁻¹. By 51 DAS, nutrient concentrations rose markedly across all treatments, particularly under high salinity. In mint (S1), NO₃⁻ increased to 133.51 mmol L⁻¹, P to 9.34 mmol L⁻¹, K⁺ to 60.64 mmol L⁻¹, and Ca²⁺ to 45.53 mmol L⁻¹. Similar trends were seen in peppermint. In lemon balm (S3), Fe spiked to 514.46 μmol L⁻¹ and K⁺ to 99.32 mmol L⁻¹, indicating nutrient accumulation likely due to reduced uptake efficiency. Sea fennel (S4) maintained ~100 mmol L⁻¹ Na⁺, confirming sustained ion buildup. Micronutrients like Mg²⁺, Zn, and Mn showed variable trends, suggesting different mobility or regulation under salinity. Iron peaks may reflect transient availability linked to rhizosphere pH shifts or redox changes under stress.

The nutrient composition of the IS and DS appears to align with the leaf nutrient content reported in Table A1 (Appendix A), particularly in treatments with elevated salinity.

3.4. Effect of Salinity on Plant Physiology

3.4.1. Photosynthetic Performance

Figure 4a–c, presents photosynthetic rate (P_N, μmol CO₂ m⁻² s⁻¹), stomatal conductance (gs, mol m⁻² s⁻¹) and CCI. The mean P_N values for mint, peppermint, lemon balm, and sea fennel were 12.0, 12.3, 11.3, and 10.0 µmol CO₂ m⁻² s⁻¹, respectively, with no significant treatment effect (p > 0.05) (Figure 4a). Stomatal conductance (g_s) showed significant differences (p ≤ 0.05) among treatments, with mean values of 0.20, 0.13, 0.11, and 0.08 mol m⁻² s⁻¹ for peppermint, mint, lemon balm, and sea fennel, respectively (Figure 4b). Mint plants were more affected, with four out of seven measurement dates showing significant differences. Under heavy salinity, g_s responses slowed toward the end of the experiment, with lemon balm and sea fennel recording similar values of 0.09 mol m⁻² s⁻¹.

CCI varied among species, with mint and peppermint showing similar values (~43.1 and 45.3), lemon balm lower (34.9), and sea fennel notably higher (78.4). Peppermint was unaffected by nutrient solution (NS) composition, while mint, lemon balm, and sea fennel showed significant differences on three of eight dates (Figure 4c). The highest CCI (97.2) was recorded in sea fennel under S4 on May 16, matching the P_N trend. No significant differences (p > 0.05) were found between control and NaCl treatments in other parameters except CCI.

3.4.2. Fresh and Dry Matter

Table 5. Morphological parameters in g plant⁻¹ ($\overline{x}$ ± SD, n = 108). Columns with same letter are not significantly different between means according to Least Significant Difference test (LSD) at the confidence level p ≤ 0.05. FM: fresh matter; DM: dry matter, FM/DM ratio.

Mint_1st Harvest (15DAS)	FM g Plant−1	DM g Plant−1	FM/DM
S1	441.78 a ± 98.05	51.94 a ± 10.64	8.53 a ± 0.90
S2	543.67 b ± 96.56	71.67 b ± 12.12	7.65 b ± 1.00
2nd harvest (58DAS)
S1	736.89 a ± 161.05	117.67 a ± 25.75	6.37 a ± 1.10
S2	596.76 b ± 82.51	120.33 a ± 12.48	4.96 b ± 0.31
Peppermint_1st harvest (17DAS)
S1	426.39 a ± 92.05	89.83 a ± 18.08	4.74 a ± 0.19
S2	388.05 a ± 65.58	80.28 a ± 14.70	4.86 a ± 0.46
2nd harvest (59DAS)
S1	441.17 a ± 133.32	102.61 a ± 27.59	4.27 a ± 0.25
S2	392.05 a ± 81.89	92.89 a ± 18.93	4.23 a ± 0.32
Lemon balm_1st harvest (22DAS)
S3	237.17 a ± 51.81	50.39 a ± 12.97	4.79 a ± 0.54
S4	264.28 a ± 69.51	49.89 b ± 14.65	5.34 b ± 0.26
2nd harvest (57DAS)
S3	387.55 a ± 87.90	87.89 a ± 19.02	4.43 a ± 0.30
S4	284.05 b ± 56.80	70.61 b ± 16.74	4.06 b ± 0.31
Sea fennel_1st harvest (23DAS)
S3	339.50 a ± 72.49	41.05 a ± 9.52	8.32 a ± 0.41
S4	328.28 a ± 56.44	38.00 a ± 7.47	8.82 a ± 1.40
2nd harvest (56DAS)
S3	888.00 a ± 231.83	133.39 a ± 31.98	6.65 a ± 0.44
S4	792.78 a ± 179.26	128.11 a ± 30.43	6.21 b ± 0.42

provides a detailed overview of FM, DM and FM/DM for the studied species across different treatments and harvest periods. Irrigating with salinized water (NaCl) showed no significant effects (p ≥ 0.05) on the morphological parameters of sea fennel, except for the FM/DM ratio, where significant differences were observed between S3 and S4, with lower values at higher EC. For lemon balm, salt stress caused a decrease in all parameters with visible salt injury symptoms such as leaf burning, curling, and drying. Specifically, the total biomass of lemon balm in S4 was 1.0 kg m⁻², representing approximately a 16.7% reduction compared to its control S3 at 1.2 kg m⁻² (

Table 6. Total fresh biomass of aerial part per m² ($\overline{x}$, n = 108) (columns with same letter are not significantly different between means according to Least Significant Difference test (LSD) at the confidence level p ≤ 0.05).

	S1 (kg)	S2 (kg)	S1 (kg m−2)	S2 (kg m−2)
Mint 1st harvest (15 DAS)	51.3	55.9	1.0 a	1.1 b
Mint 2nd harvest (58DAS)	75.0	62.0	1.5 a	1.2 b
Mint: Total biomass crop−1 treatment−1	126.2	117.9	2.5	2.4
Peppermint 1st harvest (17DAS)	37.5	41.3	0.8 a	0.8 a
Peppermint 2nd harvest (59DAS)	48.5	47.8	1.0 a	1.0 a
Peppermint: Total biomass crop−1 treatment−1	86.0	89.0	1.7	1.8
	S3 (kg)	S4 (kg)	S3 (kg m−2)	S4 (kg m−2)
Lemon balm 1st harvest (22DAS)	22.1	21.8	0.4 a	0.4 a
Lemon balm 2nd harvest (57DAS)	37.3	27.3	0.7 a	0.5 b
Lemon balm: Total biomass crop−1 treatment−1	59.4	49.1	1.2	1.0
Sea fennel 1st harvest (23DAS)	36.7	35.5	0.7 a	0.7 a
Sea fennel 2nd harvest (56DAS)	62.5	59.4	1.2 a	1.2 a
Sea fennel: Total biomass crop−1 treatment−1	99.1	94.9	2.0	1.9

). In peppermint, salt stress significantly reduced leaf FM and DM at EC levels above 9 dS m⁻¹, with an 18.6% reduction in FM and 15% in DM under high salinity. Mint growth also responded negatively to salinity, with the EC of the nutrient solution significantly affecting all measured traits (FM, DM, and FM/DM). In the first cut, mint productivity increased in S2 (EC 6 dS m⁻¹) compared to S1, but in the second cut, reductions of about 20% were observed across treatments (S1, S2). The interactive effect of salinity was significant (p ≤ 0.05) in both cuts. Among secondary crops, peppermint was less affected by salinity stress compared to mint, which can be explained by species-specific stress thresholds and the different planting intervals of 43 days between the two cultivars (mint, peppermint).

3.4.3. Plant Biomass

Table 6 summarizes biomass production across treatments and harvests. Herbs showed the highest yields, with S2 slightly outperforming S1 (p < 0.05). Salinization with NaCl had no significant impact on peppermint and sea fennel (p ≥ 0.05), while lemon balm was negatively affected, showing a 16.7% biomass reduction in the second harvest. Mint reached a maximum of 2.5 kg m⁻² (S1), whereas lemon balm dropped to 1.0 kg m⁻² under saline conditions (S2), indicating low tolerance at 11 dS m⁻¹.

3.5. Water Conservation

Analysis of intrinsic (P_N/g_s) (Figure 5a) and instantaneous (P_N/E) water use efficiency (Figure 5b) revealed maximum values of 198.6 µmol (CO₂) mol⁻¹ (H₂O) for P_N/g_s in the S4 (salinity) treatment of sea fennel and 4.35 µmol (CO₂) mmol⁻¹ (H₂O) for P_N/E in the S2 treatment of peppermint. However, average measurements across all sampling times showed no statistically significant differences (p ≤ 0.05) between treatments for each crop.

The values of P_N/g_s and P_N/E were influenced by the application of fertilizers or salt in the S2 and S4 treatments for all crops. Significant differences (p ≤ 0.05) were observed throughout the experimental period, with increasing values until May 18th (early vegetative stage). Intrinsic water use efficiency (P_N/g_s) was higher in the S2 and S4 (NaCl-treated) plants compared to the S1 and S3 (control) plants. Subsequently, P_N/g_s decreased to a minimum at the maturity stage (last cut). For instantaneous water use efficiency (P_N/E), the average value decreased, and differences between treatments were observed. Contrastingly, in sea fennel, P_N/E increased by 20% (4.17 µmol (CO₂) mmol⁻¹ (H₂O)) on May 18th but decreased to 3.09 µmol (CO₂) mmol⁻¹ (H₂O) at the stem elongation stage (last cut).

3.6. Water and Fertilize Productivity

The results, in Figure 6a, indicated that system production under cascade system used water more efficiently as compared to the monoculture crop. To produce 1 ton of tomatoes and 35.9 kg of hydroponic fresh herbs and halophytes required 16.4 m³ of water for 4—month cultivation. WP in cascade systems (SWP-PS, SWP-PT, SWP-PST, was not significant compared to monoculture growing system of tomato (WPP) as shown in Figure 6a. But the same trend was not obtained in no-recycling treatments. The lowest WP was calculated (5.7 kg m⁻³) in tertiary crops, as opened system without recirculation. Totally, SWP increased more with decreasing the irrigation regime than CWP, as opened system. The cascade crop production of five cultivars was estimated at 65.9 kg fruit m⁻³, whereas water productivity in secondary and tertiary crops, determined according to Equations (3), was 12.7 and 5.7 kg fruit m⁻³, respectively. This represents a substantial 86% improvement in water use efficiency.

Likewise, results for fertilizer productivity (FP), determined according to Equations (4), and are illustrated in Figure 6b. Hydroponically, producing 1 ton of tomatoes and 35.9 kg of hydroponic fresh herbs and halophytes required 41.6 kg of fertilizers over a 4-month cultivation period. Fertilizer productivity significantly increased (p < 0.05) in the closed system compared to the open system. The cascade crop production of five cultivars was estimated at 34.8 kg kg⁻¹, while fertilizer productivity in secondary and tertiary crops was 4.0 and 4.2 kg kg⁻¹, respectively, demonstrating an 88% improvement in fertilizer use efficiency. Both water productivity (WP) and FP were calculated until the final harvest of the secondary, tertiary, and primary crops.

The implementation of fertilizer recirculation in the hydroponic cascade system resulted in a significant saving of fertilizers. A total of 91.75 kg of fertilizers was saved across the secondary and tertiary crops. Specifically, the secondary crop demonstrated a saving of 53.65 kg of fertilizers, while the tertiary crop contributed a saving of 38.1 kg. This substantial reduction in fertilizer usage highlights the effectiveness of the recirculation system in minimizing nutrient waste and improving overall nutrient use efficiency within the multi-crop hydroponic setup. The complete elimination of fertilizer input for both the secondary and tertiary crops in the cascade system, compared to the estimated usage in a monoculture scenario, underscores the potential of this approach for resource conservation in hydroponic agriculture.

4. Discussion

Our experimental evaluation of the tertiary cascade hydroponic system provides critical insights into the feasibility of nutrient and water recycling for various horticultural crops, particularly under induced salinity stress. The observed environmental conditions within the greenhouse compartments remained largely consistent, indicating that variations in crop performance and nutrient dynamics were primarily attributable to the applied fertigation strategies and species-specific responses to salinity and recycled solutions.

4.1. Evaluation of the System Based on Crops Physiology

The differential performance of the cultivated species under salinity highlights the importance of strategic crop selection in multi-tier cascade systems. Mint demonstrated notable resilience, maintaining high productivity (total biomass 2.5 kg m⁻² in S1 and 2.4 kg m⁻² in S2 as per Table 6) and relatively stable physiological parameters even under moderate salinity (S2, EC 6 dS m⁻¹). This suggests its suitability for intermediate tiers where nutrient solution quality may fluctuate due to upstream crop uptake. In contrast, lemon balm proved to be the most sensitive species to saline conditions. Its total biomass in S4 (1.0 kg m⁻²) experienced approximately a 16.7% reduction compared to its own control (S3 at 1.2 kg m⁻²), with visible symptoms of salt injury such as leaf burning and curling. This comparison focuses on lemon balm’s performance relative to its own appropriate control, acknowledging that species like mint operate under distinct physiological thresholds and growth dynamics. The strong tolerance of sea fennel (total biomass 1.9 kg m⁻² in S4 vs. 2.0 kg m⁻² in S3), a known halophyte, was confirmed, showcasing its potential as a robust tertiary crop capable of utilizing highly saline drainage solutions with minimal adverse effects on growth and chlorophyll content (average CCI 78.4, peaking at 97.2). This stratification of species based on salinity tolerance is crucial for the efficient management of increasingly saline recycled solutions.

This study investigated the physiological responses of mint, peppermint, lemon balm, and sea fennel to varying salinity levels within a cascade system. No significant differences (p > 0.05) in photosynthetic rate (P_N) were observed between control and NaCl treatments, with P_N ranging from 10.0 to 12.3 µmol CO₂ m⁻² s⁻¹. This stability suggests that carbon fixation was not greatly affected within the tested salinity range, in line with studies showing that plants can maintain photosynthesis under moderate saline stress through osmotic adjustment [54]. The consistent P_N across all crops also indicates that the cascade system did not negatively affect photosynthetic capacity, supporting findings in two-stage cascade systems. However, the lack of change in P_N does not rule out other potential salinity effects, like reduced growth or altered metabolism. In line with photosynthetic responses, transpiration in peppermint likely decreased under salt stress, reflecting stomatal closure and reduced root water uptake, mechanisms commonly observed under osmotic stress [55]. While the scope of this study did not include the direct biochemical analysis of osmolytes (e.g., proline or soluble sugars), the observed stability in P_N is consistent with an effective physiological strategy for osmotic adjustment.

In contrast to P_N, g_s exhibited significant differences (p ≤ 0.05) among the treatments, highlighting species-specific responses. Mint plants were particularly sensitive, with significant differences observed on four out of seven measurement dates, indicating a dynamic response to salinity. The decrease in g_s under salinity is a common plant response to minimize water loss [56]. The trend of lemon balm and sea fennel converging to similar low g_s values (0.09 mol m⁻² s⁻¹) under heavy salinity indicates a strong stomatal closure mechanism to conserve water, which is crucial under saline conditions. This highlights species-specific adaptations to salinity stress, with mint showing a more volatile response and sea fennel demonstrating a more conservative strategy. The consistent g_s levels further suggest effective water management and minimal plant stress, crucial for maintaining water balance under increased salinity [54], while increases in leaf temperature may reflect impaired transpirational cooling [57].

CCI varied across species, with mint and peppermint fluctuating, lemon balm decreasing, and sea fennel maintaining significantly higher values, likely as a compensatory stress response [58]. Peppermint was less affected by salinity, while other species showed significant shifts on three of eight dates. The highest CCI was observed under S4 towards the middle of the cultivation period, coinciding with peak P_N. Variability in mint and peppermint suggests adaptive responses. Sea fennel biomass matched hydroponic data, while lemon balm produced 15–20% less than in monoculture, confirming its sensitivity to salinity. These findings underline the importance of species selection and salinity control in cascade systems.

Salinity affected FM, DM, and biomass production, indirectly reflecting nutrient dynamics. Reductions in FM and DM for lemon balm and mint, along with the lower biomass of lemon balm, suggest impaired nutrient uptake under saline conditions. In contrast, mint and peppermint maintained higher yields, indicating better nutrient acquisition. Interestingly, moderate salinity (S2) slightly improved herb yields compared to the control (S1), suggesting that the nutrient balance under mild stress supported growth. However, the sharp biomass decline in lemon balm (1.0 vs. 2.5 kg m⁻² in mint), along with visible salt damage, points to nutrient imbalances due to salinity. Salt stress is known to disrupt nutrient uptake and cause ion toxicity [59]. The reduced FM/DM ratio in sea fennel at higher EC supports this, indicating shifts in water-nutrient relations. Peppermint’s relative stability under stress suggests more efficient nutrient handling. The sensitivity of lemon balm, especially at 11 dS m⁻¹, underscores the challenge of maintaining balanced nutrition under salinity. These findings highlight the importance of species selection and adaptive nutrient and irrigation management in saline systems.

4.2. Evaluation of the System Based on Nutrients Concentration

The detailed analysis of nutrient solution composition revealed complex dynamics, particularly concerning micronutrient accumulation. While the primary crop showed efficient uptake of macronutrients like nitrate and potassium, leading to their reduction in the drainage solution, non-essential ions, especially sodium, steadily increased. This ion imbalance was further exacerbated in the secondary and tertiary stages. A notable observation from our data was the exceptionally high concentrations of certain micronutrients, particularly iron (Fe), in the drainage solutions (Table 4) on specific dates. For instance, lemon balm in S3 drainage reached 514.46 μmol L⁻¹ Fe at 51 DAS, and sea fennel in S3 drainage reached 351.13 μmol L⁻¹ Fe at 43 DAS. These values are significantly higher than typical optimal nutrient solution concentrations for iron (which are usually in the range of 10–30 μmol L⁻¹).

Several factors could contribute to these elevated micronutrient levels. Firstly, while plants actively absorb macronutrients, the uptake of micronutrients can be less proportional to their availability, especially under stress conditions. The observed high EC levels in the drainage, particularly for halophytic species like sea fennel which can tolerate and even accumulate salts, suggest significant water uptake through transpiration without a proportional uptake of all dissolved ions, leading to their concentration in the remaining solution.

Secondly, the pH of the drainage solution (ranging from 6.2 to 8) could play a role. Although iron is typically less soluble at higher pH, it can form complexes that remain in solution, or its bioavailability might be reduced for plant uptake, leading to its accumulation in the drainage. Additionally, the continuous recycling within the closed system, even with periodic replenishment, can lead to a gradual buildup of ions that are either minimally utilized by the plants or are present as impurities in the initial water source and fertilizers.

Such high concentrations of micronutrients, while not immediately toxic to all plants in this study due to their complex interactions, raise concerns for long-term system stability and the potential for phytotoxicity in subsequent cultivation cycles or for less tolerant species. This necessitates a more detailed investigation into the speciation and potential phytotoxicity of these accumulated micronutrients in recirculating systems. Future research should focus on advanced filtration or selective removal techniques for specific micronutrients to maintain optimal balance and prevent detrimental accumulation, thereby ensuring the long-term sustainability and productivity of multi-cascade hydroponic systems.

In this study, all species grew satisfactorily in the multi-loop cascade system; indicating strong resilience despite the increased salinity of the drainage solution. To provide physiological support for this species resilience, differential nutrient absorption was quantified and found to vary significantly among species, suggesting diverse capacities for resource utilization and adaptability. For instance, sea fennel, a halophyte, showed higher absorption of sodium (Na⁺) and potassium (K⁺) compared to lemon balm, a non-halophyte. This is consistent with studies showing that halophytes have evolutionarily adapted to regulate ion uptake under high salinity conditions [60].

In terms of nutrient uptake, sea fennel—a halophyte—exhibited higher sodium (Na⁺) and potassium (K⁺) uptake compared to lemon balm, a non-halophyte. This aligns with previous studies showing that halophytes have evolved mechanisms to regulate ion absorption under saline conditions [16].

The observed species-specific differences in ion uptake rates provide strong correlative evidence for the effectiveness of these ion regulation patterns. A deeper exploration of these ion regulation patterns reveals that the observed species-specific responses are driven by sophisticated ion regulation. Halophytes like sea fennel employ a two-pronged strategy to manage high salt concentrations: ion exclusion at the root level and ion compartmentalization within their cells [59]. The accumulation of toxic ions such as Na⁺ is actively controlled, with excess sodium being sequestered into the large central vacuole, preventing it from reaching the cytoplasm where it would disrupt metabolic processes [60]. In contrast, glycophytes like mint are less efficient at this vacuolar sequestration. Additionally, plants must maintain a high cytoplasmic K⁺/Na⁺ ratio, as Na⁺ can mimic K⁺ and interfere with essential enzymatic functions. The ability of a species to tightly regulate these ion channels and transporters, such as Na⁺/H⁺ antiporters, is a critical determinant of its salinity tolerance and explains why sea fennel performed better under high EC conditions compared to the more sensitive mint [61,62].

4.3. Evaluation of the System Based on Water and Fertilizer Use Efficiency

In terms of water and fertilizer efficiency, the cascade system demonstrated promising results. The term, ertilizer Productivity (FP) has been standardized throughout the manuscript for clarity and consistency (referencing the ‘Calculations’ section). The reuse of drainage solution significantly reduced the overall water consumption, with irrigation needs for secondary and tertiary crops being met entirely by recycled water (100% DS). While the system achieved a high reuse rate, zero drainage leaching was not possible during this work. As, to our knowledge, this was the first time a three levels cascade system is reported, it was not possible to estimate in advance the optimal combination of crops and areas that would give the perfect combination for zero drainage. However, this manuscript provides data to aid future works on optimising the combination of crops and primary to secondary and tertiary crop area rations. It has to be noted that zero drainage leaching is clearly related to water consumption in the different crop levels and that crop leaf area and transpiration will have to be considered for the design of the system and the estimation of the cultivation areas of each crop. The improved water use efficiency (WUE) for mint and peppermint under moderate salinity and the enhanced Fertilizer Productivity (FP) for peppermint underscore the system’s potential for resource conservation. These efficiencies are crucial for sustainable agriculture, particularly in regions facing water scarcity and concerns over nutrient runoff.

Water Productivity and Nutrient Use Efficiency, these are the key metrics for the final evaluation of cascade system. Water reuse reduces overall water consumption compared to traditional hydroponics with constant solution replacement. Comparing the water productivity of cascade system in experimental trial, the system offer the potential for significant water savings compared to traditional hydroponic methods as monoculture, around 65%. The reduction in water volume in the three-loop cascade system was significantly higher (86%) than the results reported in other cascade cropping systems (around 18%) [34,35], that were based on a dual-crop cascade system.

Concerning the primary crop, water productivity during the four months (10 March until 10 July) was calculated at 63.7 kg fruit⁻¹ m⁻³. This value, although slightly lower, aligns more closely with the water productivity observed in our study compared to Rodríguez et al. [63]. Specifically, their studies reported water use efficiency values of 46.03 kg fruit⁻¹ m⁻³ and 34.42 kg fruit⁻¹ m⁻³, respectively, in open systems, highlighting the importance of optimizing water management.

The main advantage of cascade systems is regarded nutrient use efficiency, calculating the total quantity of fertilizers during the experimental trial (March–July). By reusing the nutrient solution, cascade systems significantly reduce the amount of fertilizer required compared to open hydroponic systems. This translates to lower operational costs for growers. Minimized fertilizer waste and reduced need for constant nutrient production contribute to a more sustainable agricultural practice. Nutrient recycling can provide essential nutrients for plants in the lower tiers, reducing the need for constant nutrient addition.

Comparing the fertilizer productivity of cascade system, the fertilizer use efficiency in three-loop cascade system was reduced by 80–90%, taking into account the fertilizer productivity of cascade system and fertilizer in secondary and tertiary crop, as monocultures. Regarding the primary crop, fertilizer productivity over the four-month period (10 March–10 July) was calculated at 43.4 kg kg⁻¹, aligning with the findings of Rodríguez et al. [63], who reported a nutrient use efficiency of 45.44 kg kg⁻¹ in an open system.

Overall, the system’s performance in terms of water and fertilizer use efficiency was influenced by salinity, leading to species-specific adaptations and growth variations. This approach would optimize resource utilization and minimize the negative impacts of salinity, thereby enhancing overall water and fertilizer use efficiency.

4.4. Policy and Market Implications

The findings of this study have significant implications for both agricultural policy and market dynamics, especially in the context of global food security and climate change. The demonstrated efficiency of the three-loop cascade system in conserving water (86%) and fertilizer (80–90%) directly aligns with key policy objectives like the European Green Deal and the UN’s Sustainable Development Goals (SDGs) [64,65]. By providing a practical model for minimizing resource use and waste, this system can inform policy-making aimed at promoting circular agriculture and reducing the environmental footprint of food production. Furthermore, the study addresses a growing market demand for sustainably produced, locally sourced food. Consumers are increasingly willing to pay a premium for products grown with minimal environmental impact [66]. The ability of this system to successfully grow high-value, aromatic crops like mint and peppermint in water-scarce coastal regions, while reducing resource dependency, creates a compelling case for commercial adoption and contributes to a more resilient and sustainable food supply chain [67].

5. Conclusions

This study significantly contributes to the growing body of knowledge on sustainable agricultural practices by experimentally evaluating the combined effects of salinity and nutrient recycling within a novel tertiary cascade hydroponic system. By assessing crop physiology and resource use efficiency in various herb and halophyte species, our findings offer valuable insights into optimizing soilless cultivation under challenging conditions.

Our research successfully demonstrated the differential adaptability of selected crops within the cascade system, which directly addresses our aim to examine the physiological and agronomic responses of each crop to rising salinity levels. Peppermint exhibited remarkable resilience, maintaining high productivity under moderate salinity (S2, EC 6 dS m⁻¹), biomass production (2.5 kg m⁻²) and stable chlorophyll content. Conversely, lemon balm proved to be the most sensitive species, with biomass dropping to 1.0 kg m⁻² under S4—representing approximately a 16.7% reduction compared to its control conditions (S3 at 1.2 kg m⁻²). This highlights the importance of species selection in multi-stage systems. As anticipated, sea fennel demonstrated high inherent tolerance to salinity, maintaining both robust biomass and high chlorophyll content index (average 78.4, peaking at 97.2 under S4), thereby confirming its strong potential for integration into saline hydroponic environments.

Beyond individual crop performance, the cascade system notably enhanced resource efficiency, which addresses the aim of the current study to quantify water and nutrient flow at each of the three levels to assess overall resource-use efficiency under increasing salinity conditions. Water use efficiency (WUE) improved for certain species under moderate salinity, with peppermint and mint showing commendable values (2.4 g FM L⁻¹ and up to 3.1 g FM L⁻¹, respectively, under S2). This indicates an effective conversion of water into biomass, a critical advantage in water-scarce regions. Furthermore, (FP) improved, particularly in the secondary crop peppermint (2.2 g FM L⁻¹ N compared to 1.5 in monoculture controls), suggesting superior nutrient recovery and reduced reliance on external inputs. These improvements underscore the system’s potential to minimize nutrient discharge and align with circular economy principles by maximizing resource utility.

In conclusion, this pioneering evaluation of a tertiary cascade system provides a robust framework for developing more resilient and resource-efficient hydroponic food production. Our findings strongly support the strategic pairing of crops with varying salinity tolerances to optimize nutrient recycling and maintain productivity, a resilience that is substantiated by the observed differences in their ion regulation strategies. This work provides key data for optimizing crop combinations and management strategies, thereby addressing our final aim. To fully characterize the underlying adaptive mechanisms, future research should build upon these findings by exploring the long-term nutrient dynamics and cumulative effects of extended recycling in multi-cycle studies. While the present study focused on key physiological metrics, a more comprehensive understanding of plant tolerance requires direct biochemical analyses. Therefore, subsequent studies should incorporate methods such as quantifying osmolytes like proline or stress-related enzyme activities to fully validate and explain the physiological tolerance observed in these species. Recognizing the importance of commercial implementation, further investigation into the economic viability and scalability of such systems, along with the development of predictive models for nutrient management in tertiary cascades, will be essential for their successful adoption.