Experimental Evaluation of Oxygen and Dissolved Solids Levels in Hydroponic Crops Using Organic Nutrients as a Function of the Number of Daily Recirculations

Abstract

By 2030, the world’s population is projected to reach 8.5 billion, posing significant challenges for food production. Traditional agriculture, which requires large amounts of water, soil, and energy, can contribute to the depletion of natural resources and environmental degradation. In this context, organic hydroponic systems emerge as a sustainable alternative, allowing for more efficient, controlled, and resilient production in the face of climate change. In this research, the physical development of romaine lettuce and the physicochemical parameters of the crop water are evaluated as a function of the number of daily recirculations. The crop variables are measured with the help of an intelligent control system, which allows the real-time monitoring of the process variables. The methodological approach is mixed: quantitative, for the recording of physicochemical variables, and qualitative, for the physical analysis of the crop throughout the process, With the experiments conducted it was found that the treatment with four daily recirculations promoted the most significant physiological growth of the plants. Despite having a pH of approximately five and dissolved oxygen of 6 mg/L, this treatment maintained adequate levels of TDS (2050 ppm) and hardness (1000 ppm), favoring the development of the crop. The treatments with less recirculation presented lower growth values. These results suggest that increased recirculation can optimize yields in floating-root hydroponic systems, addressing global food challenges from an environmentally responsible perspective.

1. Introduction

Greenhouses intended for vegetable production for human consumption often present management challenges. They generally require large areas of land and the use of topsoil, fertilizers, and pesticides, which not only increase production costs but also degrade the soil, reducing its fertility over time [1]. With this background, the need to continuously develop alternative agricultural methods has become important, especially in response to the multiple problems faced by conventional agriculture [2]. In this regard, the use of hydroponic systems for plant growth is becoming increasingly popular. The products generated in this type of cultivation are of higher nutritional quality [3,4].

Plant production systems without agricultural land use are of two types: open systems, where water is replenished periodically, and closed systems, where water containing nutrients is recirculated [5,6]. Many experiments have been conducted to evaluate the effectiveness of these crops; for example, hydroponic crops can reduce the harvest time of lettuce from 70 days to 30 days [7], and enable sage to bloom 8 days earlier [8], reduce water consumption [9], reduce the use of fertilizers, and increase yields [10].

Among the foods around the world in which this method of cultivation is adopted, cucumber and strawberries stand out as having better physical characteristics [11,12] with this type of cultivation. In Asian countries, experiments have been conducted with medicinal plants to reduce their production times [13].

Soilless cultivation systems are also used to reduce environmental impacts by treating wastewater and utilizing desalinated water for cultivation, thereby reducing freshwater consumption. In arid and urban areas, hydroponics is presented as a sustainable alternative, as it reduces the use of agrochemicals. Additionally, it is possible to practice hydroponics in educational environments to promote the use of sustainable technology in food production [14,15,16,17,18,19,20,21,22,23,24,25,26,27].

However, there are difficulties in several areas, such as obtaining more efficient organic nutrient solutions, due to limited research on maintaining the optimal parameters of aqueous crop solutions (including electrical conductivity, pH, and dissolved oxygen levels) and developing better methods of biological-based pest control [18,28,29,30]. However, the use of organic nutrients offers several environmental advantages: they are biodegradable and can reduce the accumulation of salts in the recirculated solution, thereby minimizing the risks of phytotoxicity, which consists of toxic damage to plants caused by natural or synthetic chemicals, in the root system [31].

There is a debate in the scientific and regulatory community. Among the most common organic nutrients are fish emulsions, compost extracts, bat guano, bone meal, blood meal, feather meal, and aquaculture sludge; all of them are 100% organic and free of synthetic mineral salts. They release macro and micronutrients gradually, promoting the activity of beneficial microorganisms for the plant. These approaches encourage sustainability, utilize local organic by-products for lettuce nutrition, and reduce the dependence on mineral fertilizers [8,32].

In the scientific and regulatory community, there is a debate about whether hydroponics can be considered organic. On the one hand, in the United States, the USDA-NOP allows hydroponic systems to be certified as organic if they use only approved inputs—compost extracts, fish emulsions, or algae extracts—while, in the European Union and other countries, cultivation in living soil is required, prohibiting such labeling [15,33]. Faced with such disparities, this work adopts a strict definition of “organic hydroponics”: as a closed system that exclusively uses nutrients of organic origin—without synthetic mineral salts—entirely dispensing with chemical pesticides and, guaranteeing compliance with the most demanding standards of organic production.

Despite the advances demonstrated by hydroponics, few studies have evaluated the impacts of the daily frequency of recirculating the nutrient solution (number of pumping cycles per day). This information is essential in controlling the yield and quality of lettuce [34]. It is also possible to compare energy efficiency in lettuce cultivation, but without varying the recirculation frequency within each system [35]. Alternatively, it is possible to evaluate how the flow-recirculation frequency in a floating system affects leaf and root growth using an ANOVA analysis with the mean ± SD to validate differences, as described in [36].

Investigating the effects on plant growth of increasing or decreasing the number of daily nutrients recirculations becomes essential. By using closed hydroponic systems, energy is saved compared to maintaining recirculation throughout the day, and water consumption is reduced. Therefore, it is necessary to develop an electronic system that performs recirculation intelligently and controls the nutrients supplied to the plants to improve the efficiency of hydroponic crops. Part of this research is dedicated to the development of such a control system.

Selecting appropriate components, which are efficient and economical to control the recirculation process of the crop, is a difficult task. When reviewing previous research, work has been found regarding programmable logic controls (PLC) and embedded systems such as Arduino cards, applied in the process of recirculating nutrients from soilless production systems. However, their costs and levels of management may be limitations [37,38,39,40].

In this sense, the use of modern microcontrollers is the most viable option to control the recirculation and nutrients needed by plants, since they offer greater storage capacities, Wi-Fi connection, low energy consumption, and compatibility with sensors, actuators and communication systems; they are ideal for monitoring and automated decision making and are economical compared to other controllers [41].

As can be seen from the background of this research, conducting experiments on a nutrient-based cultivation system using organic nutrients is vital for environmental conservation. Hydroponic cultivation has many advantages: it reduces environmental impact, improves food production in terms of quality and quantity, reduces production times, minimizes water and energy consumption, and can also be used in urban areas and be controlled by people with a reduced capacity.

In this work, tests were conducted with hydroponic crops fed organic nutrients. The test was applied to romaine lettuce, and the control system for recirculation and the automatic measurement of the physicochemical characteristics of the water state where the nutrients are done is developed, as well as the manual measurement of the physical characteristics of the plants such as stem thickness, length of leaves and roots with a frequency of measurements every 5 days.

In the experimental processes, the effect on plant growth was evaluated by varying the number of daily waters recirculation’s in the crop. During the process, temperature, humidity, pH, number of dissolved solids (TDS), hardness, and water oxygenation levels (ppm) were measured, variables that are monitored by a network system of physical objects connected to the Internet (IoT). It was confirmed that higher recirculation (4 recirculation’s) favors vegetative and root development, which translates into higher yield and quality of the commercial product, as well as better crop water conservation.

Since it is essential to correctly dose plant nutrition, the organic nutrient formula proposed by Mr. Angel Llerena was used during the experimentation. This formula, when tested in hydroponic cultivation, proved to be highly effective.

2. Materials and Methods

2.1. Hydroponic System Design

To hold the water used in the crop, three wooden boxes were assembled with dimensions of 90 cm long, 90 cm wide, and 25 cm high. In these boxes, black plastic is placed on the internal part to retain water for the crop, as shown in Figure 1. Water is poured up to a level of 10 cm, which gives a volume of 81 L, which is calculated with Formula (1), to this amount of water is placed the nutritive solution indicated by Angel Llerena [3], who recommends pouring 5 mL of solution A for each liter of water and 2 mL of solution B for each liter of water. The values of the nutritive substance to be placed are obtained with Equations (2) and (3).

$$V=a\times b\times h=\left(0.9\right)\left(0.9\right)\left(0.1\right)=0.081 {\mathrm{m}}^3=81 \mathrm{L}$$

(1)

In the hydroponic boxes, 1/2-inch pipes were installed for water recirculation (hydroponic tubes C1, C2, and C3). Additionally, a vent was placed at the bottom to facilitate the cleaning of each container. A 100-watt water pump (B1) and sensors were connected to the hydroponic tubs to recirculate the culture water and measure process variables, including the pH, oxygen, dissolved solids, and water hardness. These configurations are illustrated in Figure 1 and Figure 2.

2.2. Formulation and Dosage of Organic Nutrients

The nutrient solution used for plant development was provided by Angel Llerena [3], and was composed of two compounds: Concentrated Solutions A and B, in the latter, micronutrients are added. The elements of each of these concentrated solutions and their micronutrients are presented in

Table 1. Concentrated Solution A For 5 Liters of Water.

Chemical Compound	Quantity
Potassium Nitrate 13.5% N, 45% K2O	550 g
Ammonium Nitrate 33% N	350 g
Triple Superphosphate 45% P2O5, 20% CaO	180 g

,

Table 2. Concentrated Solution B For 2 Liters of Water.

Chemical Compound	Quantity
Magnesium Sulfate 16% MgO	220 g
Iron Chelate 6% Fe 33% N	17 g
Micronutrient Solution	400 mL

and

Table 3. Concentrated Micronutrient Solution For 1 Liter of Water.

Chemical Compound	Quantity
Magnesium Sulfate	5.0 g
Boric Acid	3.0 g
Zinc Sulfate	1.7 g
Copper Sulfate	1.0 g
Ammonium Molybdate	0.2 g
Sulfominors I	0.5 g

. The proportion of each of these concentrates that is dosed to the plant depends on the amount of water used in hydroponic cultivation.

To the 81 L of water in the hydroponic tubs the nutritive solution indicated by Eng. Angel Llerena [3] was placed; he recommends adding 5 mL of solution A to each liter of water and 2 mL of solution B to each liter of water. The values of the nutritive substances to be applied ware obtained with Equations (2) and (3), respectively:

$$Va=\left(V\right)\left(SolutionA\times lt\right)=\left(81\mathrm{L}\right)\left({\displaystyle \frac{5mL}{l}}\right)=405 \mathrm{m}\mathrm{L}$$

(2)

$$Vb=\left(V\right)\left(SolutionB\times lt\right)=\left(81\mathrm{L}\right)\left({\displaystyle \frac{2mL}{l}}\right)=162 \mathrm{m}\mathrm{L}$$

(3)

2.3. Automatic Recirculation System Design

For the implementation of the control system, a microcontroller (PIC) was selected. This offers greater storage capacity, Wi-Fi connection, low power consumption, and compatibility with sensors, actuators and communication systems; it presents an excellent response for monitoring and automated decision making, and In addition, its cost is low compared to PLCs and embedded systems [41].

To monitor the sensor data, three analog inputs of the PIC were used, configured as follows: the humidity sensor was connected to channel A1, the pH sensor to channel A0, and the temperature sensor to channel A3. Additionally, a digital pin was used for One Wire communication, and two digital outputs were utilized to control the relays that activated the pump connected to channel D2 and the solenoid valve connected to channel D1. Figure 3 shows the pin layout.

The clock circuit utilized a serial communication protocol (I2C), which employed the SCL and SDA pins, connected to terminals B1 and B0 of the microcontroller. To send the data of the monitored variables, an XBee was used, which was connected to the RX and TX pins of the controller through RS-232 communication. To display the orchard data, a 16 × 2 LCD was used, which was connected to port B (from B2 to B7) of the microcontroller. Figure 4 and Figure 5 show the system connections of each pin of the integrated circuit.

2.4. Sensors

To obtain the temperature values, the DS18B20 sensor was used (DALLAS SEMICONDUCTORS EE. UU.:1-888-476-5130INT:+1-984-234-5366), a high-fidelity digital thermometer with accuracy of 9 to 12 bits in degrees Celsius. Its operating temperature ranges from −50 °C to 125 °C, and it utilizes One Wire communication, which enables the transmission and reception of data over a single cable [42].

Humidity values were obtained using the DHT11 sensor (ALLDATASHEET EE. UU.:1-888-476-5130INT:+1-984-234-5366), which provides a digital data output and is powered by a voltage between 3.5 volts and 5.5 volts. It can measure relative humidity between 20% and 95% with accuracy of 5%. [43].

As a sensor that allows the measurement of the pH in an analog way, the SEN0161 model was selected, which has a measuring range of 0 to 14 pH with an accuracy of ±0.2% pH [44], (APLICATION-DATASHEET EE. UU) Figure 6 shows these sensors.

To maintain a constant time count and prevent the system from deviating from its configuration, a DS1307 real-time clock was included, which was responsible for reporting the exact time to the PIC controller [45].

2.5. Recirculation Logic

The system that automatically controlled the process is the PIC18F4550 [46] serial microcontroller, which defines the exact time and date. The integrated circuit is placed in an infinite loop, where it reads the sensors and converts the values to be displayed on the liquid crystal display (LCD). The orchard variables are sent to the user interface for the operator to view.

To determine if it is time to recirculate the water, the microcontroller checks the time against the clock. If it is the programmed time, the microcontroller proceeds to send a power signal to the electro valve and the pump, which are placed in standby mode when the recirculation time has been fulfilled.

2.6. IoT System

The XBee modules (Figure 7) perform wireless communication and allow interconnection between the hydroponic garden and the computer that monitors crop variables. These modules are based on the IEEE 802.15.4 standard [47].

2.7. Monitoring Interface

The interface to observe the variables of the crop was developed using the graphical programming environment LabVIEW [48]. The screen consists of three graduated bars reflecting the magnitudes of the three monitored variables temperature, pH and humidity. It has a stop button and a selector for the reading or not of the variables. It also features a space to select the communication port, complete with icons to configure the data reading of the port. Figure 8 shows this communication interface.

To perform all the programming, the NI-VISA software V8, NI-488.2 [49], and the LabVIEW Datalogging and Supervisory Control (DSC) Module [50] were installed, which allowed for co-communication between the computer and the module in addition to storing data on the computer.

Table 4. History of the temperature and humidity variables.

Time	pH	Temperature (°C)	Humidity (%)
10:15:18.603	4	37	50
10:15:23.104	4	37	50
10:15:27.604	4	37	50
10:15:32.103	4	37	50
10:15:36.607	4	37	50
10:15:41.106	5	37	49
10:15:44.107	5	37	50

shows how the data of the variables are stored. With these data, the averages of the values were obtained, and an alarm was set in the program to be activated when any of these data points were not within the expected range.

3. Implementation

3.1. Recirculation System Coupling

In the experiment, lettuce seedlings were placed in a floating raft with holes, which allowed the roots to remain submerged in the nutrient solution throughout the growing cycle. In the first experiment, C1 and C2, water was oxygenated throughout the process using a fish tank pump (B1). In C3, it was oxygenated with the control system developed in this research (a 100-watt pump [B2]). The B2 pump was installed on the walls of all tanks, in addition to a solenoid valve that opened and closed the recirculation. Sensors were adapted to monitor water parameters in real time. Twenty plants were planted per box, strategically distributed in the floating pond area.

The system is powered by a DC power supply, where the +5 V channel is used to power the PIC and the +3V channel is used to power the transmission equipment (XBee). Figure 9 illustrates the connections between all elements of the recirculation system.

3.2. Electronic System Functional Tests

To verify the operation of its components, the system underwent several tests. The sensors were operated to confirm that the values of the variables that measured could be read. Then, we proceeded with the clock test (Figure 10), observing that the times established in the program to activate the elements that facilitated water recirculation in the crop were met. Once the operation of the automated system was verified, it was applied to the lettuce crop according to the experiment, one or two.

3.3. Physicochemical Monitoring in Hydroponic Cultivation

The experimental process of hydroponic cultivation was developed in the province of Guayas, Guayaquil, Ecuador, which has an average annual temperature of 24.1% C with minimal seasonal variations, the average relative humidity is 77% with values that fluctuate between 72% in December and 81% in February, the hydroponic cultivation vats were placed in an open field. Two experiments were conducted:

In the first experiment, the recirculation system attached to boxes C1 and C2 was blocked, and a recirculation system with pump B1 was used throughout the day (Traditional Recirculation System). These boxes are referred to as Control Box 1 and Control Box 2, respectively. In the hydroponic C3 boxes, the recirculation system was maintained, coupled with pump B2. These hydroponic boxes were initially programmed to operate with daily recirculation for the first 15 days after transplanting. After this period, the programming was modified to operate with four daily recirculation’s until harvest time.

In the second experiment, the boxes used in the first experiment were washed, and pump B1 was removed from C1 and C2. Subsequently, all three tanks (C1, C2, and C3) were operated with the B2 pumping system in place. For this experiment, C1, C2, and C3 were programmed to operate with 1, 2, and 3 recirculation’s per day, respectively. The number of recirculation’s was kept constant throughout the romaine lettuce life cycle until harvest.

To obtain the value of the physicochemical variables in the hydroponic boxes, electronic sensors and a microcontroller were used, for which it was necessary to develop a control program: The variables that were taken with these sensors were: PH, Dissolved Solids (TDS), Dissolved Oxygen (O), Water Hardness

In parallel, physiological variables were also measured manually such as: stem and head thickness, leaf and root length were measured with a flexible ruler considering the most developed leaf of each plant, then the average was obtained to record as day data in the corresponding box (C1, C2, C3); the number of leaves was counted visually, to get the value to be placed in the tables, the average was obtained in each vat.

All measurements were carried out every five days, from the beginning to 40 days of cultivation, in a morning schedule to minimize environmental variations. Data were systematically recorded on digitized record sheets, classified by day and treatment.

Once the data were obtained, two MATLAB (V2024B) programs were created: one that included all the physicochemical variables of the crops and another to represent the physical variables of the crop. The first program generated four graphs: pH, dissolved solids, oxygen, and water hardness. The second program generated five graphs, showing the leaf length, stem thickness, head size, number of leaves, and root length of romaine lettuce. In each case, the consolidated data for each measured variable from the two experiments are presented.

3.4. Lettuce Planting

To determine the result of automating the process, we proceeded to grow romaine lettuce, which takes approximately 50 days to mature. First, the seeds were placed in a container with organic matter, as shown in Figure 11. After 7 days, the plants had begun to sprout, so they were transplanted into hydroponic boxes.

To support the plants in the aqueous solution, the floating root technique was employed, which involved placing them in Plumafon sheets to float above the water level in the hydroponic box. A total of 20 plants were placed in each box during transplanting, as shown in Figure 12.

4. Results

4.1. Standardization of Recirculations

To facilitate water flow and minimize losses, the boxes were designed with the pump located underneath. Activation of the solenoid valve was performed automatically at the programmed times.

The humidity and temperature sensors were in an elevated area of the orchard and showed accurate readings for each of their variables. It was also possible to compare the data obtained with the provided limit parameters. The interface displays an alarm that is activated in the event of variable decompensation, as illustrated in Figure 8.

After one month and one week, the lettuce had completed its growth (Figure 13), so the plants harvested.

4.2. Lettuce Physical Characteristics

This research project evaluated the growth of romaine lettuce subjected to a floating root hydroponic culture in four levels of recirculation with Llerena nutrient solution (1, 2, 3, and 4 daily recirculation’s) as well as two control groups (control box 1 and control box 2). The latter were subjected to sustained recirculation throughout the day. The parameters measured were the stem thickness, leaf length, head thickness, and number of leaves at 40 days from planting, obtaining the following results:

4.2.1. Stem Thickness

Figure 14a illustrates the consolidation of all tests, indicating that this variable exhibited a progressive increase across all evaluated conditions. Plants subjected to four daily recirculation reached the highest average growth rate at day 40 (1.80 cm), significantly exceeding the other conditions especially the controls, which presented lower values (1.10 cm and 1.00 cm). This pattern suggests that greater recirculation promotes better nutrient transport, favoring the structural development of the stem.

4.2.2. Leaf Length

The leaf length (Figure 14b) showed an upward trend in all treatments. The treatment with four recirculation’s produced leaves up to 16.0 cm in length at day 40, followed by the treatments with three and two recirculation’s (14.4 cm and 13.0 cm, respectively). In contrast, the controls only reached an average maximum length of 8.0 cm. This suggests a positive relationship between recirculation frequency and leaf development.

4.2.3. Head Thickness

Regarding the head thickness (Figure 14c), plants with four recirculation’s had the highest value (3.6 cm), while the lowest values were observed in the controls (2.5 cm and 2.6 cm). Treatments with intermediate recirculation’s (one to three) showed gradual increases in this parameter, supporting the hypothesis that adequate recirculation promotes the compaction and development of the lettuce head.

4.2.4. Number of Leaves

The number of leaves per plant (Figure 14d) increased significantly with increased recirculation. The treatment with four recirculation’s reached up to 16 leaves per plant, while the controls barely exceeded seven leaves. This behavior underscores the importance of maintaining constant movement and oxygenation of the nutrient solution to stimulate meristematic activation and leaf growth.

4.2.5. Root Length

As shown in Figure 14e, root development was more significant when greater water recirculation was applied in hydroponic cultivation, especially in the cases of four and three daily recirculations, where the average values obtained were 9 cm and 8 cm, respectively. The boxes used as controls (Control 1 and Control 2) with more static solutions showed smaller root systems (Control 1: 4.2 cm, Control 2: 3.8 cm). Longer root lengths generate greater nutrient absorption, indicating that the oxygenation and mobility of nutrient solutions are key determinants.

4.3. Physicochemical Variables

During the 40-day evaluation period, significant differences were observed in the physicochemical properties of the water, depending on the recirculation levels in the two experiments.

4.3.1. pH Characteristics

The systems with three and four recirculations maintained a pH between 5.6 and 6.2, which is the optimal range for nutrient absorption. In contrast, in the control boxes, the pH dropped to values close to 4.4, indicating the progressive acidification of the medium (Figure 15a).

4.3.2. Total Dissolved Solids (TDS):

In the recirculation systems, the TDS levels increased moderately, reaching between 1400 and 1600 ppm. In contrast, in the control boxes, values above 2000 ppm were recorded on day 40, suggesting the accumulation of salt and unabsorbed nutrients (Figure 15b).

4.3.3. Dissolved Oxygen (DO):

The systems with three and four recirculations maintained DO levels above 6.0 mg/L, while in the control boxes, DO progressively decreased to 5.8 mg/L, compromising root oxygenation (Figure 15c).

4.3.4. Water Hardness:

Hardness was higher in the control boxes (up to 1000 ppm), indicating the accumulation of calcium and magnesium without solution replacement. In recirculation systems, the values remained between 600 and 800 ppm, exhibiting greater stability over time (Figure 15d).

These results indicate that water recirculation enhances the chemical stability of the hydroponic system, whereas its absence leads to the progressive deterioration of water quality.

5. Discussion

The physiological data obtained in this research show that the number of nutrient recirculations in the hydroponic system significantly influences the growth of romaine lettuce. In the graphs of the variables, it can be observed that for a greater number of recirculations, the development of the stem, leaves, and roots improves Figure 14. This is consistent with previous studies indicating that the oxygenation, movement and constant renewal of solutions promote nutrient availability and prevents the accumulation of toxic residues in the roots [51,52].

Figure 16a shows the roots of the control tubs, which could reach 4 cm in length, while Figure 16b shows the roots of the plants that grew in box C3 with four recirculations, which were equipped with an electronic control to move the water from the crop. The length of their roots reached 8 cm on average, which allowed the plants to reach their growth levels more quickly, as they absorbed greater amounts of nutrients.

The hydroponic tub with four daily recirculations showed the highest values in all the physiological variables analyzed, surpassing those obtained in the control tubs. This shows that the greater the number of recirculations, the better the homeostasis of the root environment, and it promotes greater leaf development and allows the more efficient formation of the bud.

In contrast the experiment with the control tubs where recirculation was less intense, the physiological values of the lettuce showed smaller growth in all parameters, which was justified by the fact that there was a smaller amount of dissolved oxygen, greater accumulation of metabolic waste and less access to nutrients. These findings indicate the importance of implementing active flow management techniques in plant production systems without the use of floating root agricultural soil.

The results obtained for the physicochemical variables show that the systems with the greater recirculation of nutrient substances (three and four recirculations) maintained better water quality in the culture compared to those obtained with the control tubs. The pH was more stable, avoiding acidic conditions which affect the absorption of nutrients such as calcium and magnesium, this is consistent with the results found in [53,54].

The increase in TDS was lower in the tubs that were recirculated, which reduced the risk of osmotic stress associated with salt accumulation [55]. Likewise, oxygen levels remained higher which allowed root respiration and the efficient absorption of nutrients [34].

The build-up of hardness was greater the control tub treatments, which may have led to ionic imbalances affecting the availability of essential nutrients [34,55].

These findings confirm that adequate recirculation allows the maintenance of optimal physicochemical parameters, which is important for hydroponic crop yields.

Given that in this study only the evolution of the physiological development of lettuce and the nutrient status were evaluated through the measurement of the pH, TDS, dissolved oxygen and water hardness for four levels of daily recirculation, it is necessary to expand the number of recirculations evaluated to identify extreme values and determine under which conditions the ideal point of recirculation that favors plant growth is obtained.

With regard to the control system implemented in this research, the number of daily recirculations had to be manually changed in the program. However, an intelligent control system could have been developed, using microcontrollers (such as Raspberry Pi or Arduino) and AI algorithms to dynamically adjust the number of recirculations based on variations in pH, oxygen and TDS, improving crop efficiency in real time.

In this study, no measurements of the amount of energy used were made performed, so we did not evaluate the energy consumption associated with each level of recirculation and its correspondence with the biometric gain of the crop (sizes of leaves, stems and roots), in order to determine the energy efficiency of the system.

Since the energy consumption of the system was not quantified, future research should analyze the relationship between the energy expenditure per level of recirculation and the physiological yield of the crop, with the aim of establishing energy efficiency indicators.

A progressive and positive pattern was observed in all physiological variables with the increase in the amount of recirculation. The evolution of the data taken in the two experimental tests indicates that from one to four recirculations using the B2 pump the lettuces exhibits a consistent increase in growth, which indicates that the flow of solutions contributes to the best root and leaf development.

The frequency of recirculation of nutrient solutions has a significant impact on the physicochemical parameters of the culture medium pH, TDS, Dissolved Oxygen and Water Hardness. In particular, the treatments with three and four recirculations showed greater chemical stability and optimal conditions for plant growth.

6. Conclusions

When analyzing the graphs of the tests carried out on the hydroponic system of romaine lettuce grown with four daily recirculations, the highest values were presented in all physiological variables at day 40: stem thickness (1.8 cm), leaf length (16 cm), head thickness (3.6 cm), number of leaves (16) and root size (9 cm). This indicates better efficiency in the absorption of nutrients and the oxygenation of the solution.

Compared to the results for the control hydroponic tubs whose recirculation was more limited, significantly less growth was observed, with a stem thickness of 1.1 cm (control 1) and 1 cm (Control 2) and a limited root size (4.2 cm and 3.8 cm respectively). This demonstrates the importance of recirculation to maintain a homogeneous and oxygenated nutrient solution.

Root size is a key indicator of physiological success. Lettuce that was subjected to the greater recirculation of nutrients developed longer and more vigorous roots, which probably favored the greater absorption of water and nutrients, boosting the overall growth of the plant.

The pH remained more stable in systems with four recirculations which facilitated the absorption of essential nutrients. This is consistent with research that underscores the importance of automatic pH control to optimize mineral absorption in hydroponic lettuce [53,56].

The control electronics used in hydroponic crops are necessary, as they allow the number of daily recirculations to be calibrated, which a number that can be established by the user of the IoT system so as not to reduce production. In this task, the remote monitoring of the crops is necessary, since it can provide information about the variables involved in the system thanks to the IoT system developed in this research work.

This project presents a solution that facilitates and optimizes the care of hydroponic gardens by automating the recirculation of nutrients and the monitoring of variables, which frees up time for personnel, since they do not have to monitor the oxygenation of the water or empty the drawer when the water is no longer useful.

Thanks to the interface developed, time is saved in the collection of variables that influence plant growth, since it is only necessary to approach the garden in the event of alerts for example, if the interface indicates sudden changes in weather or a lack of nutrient solution.

With the help provided by the automated garden, personnel can work more easily and efficiently, as they place more plants per square meter.

These findings support the use of technologies and hydroponics by properly calibrating the number of daily recirculations as a viable solution to address global food challenges from an environmentally responsible perspective with the use of organic nutrients.

The results of this research offer the possibility of obtaining a mathematical model of the physiological growth of lettuce as a function of the recirculation flow, incorporating variables such as pH, TDS and dissolved oxygen.