Current perspective on nutrient solution management strategies to improve the nutrient and water use efficiency in hydroponic systems

Abstract Hydroponics, a soilless cultivation technique using nutrient solutions under controlled conditions, is used for growing vegetables, high-value crops, and flowers. It produces significantly higher yields compared to conventional agriculture despite its higher energy consumption. The success of a hydroponic system relies on the composition of the nutrient solution, which contains all the essential mineral elements necessary for optimal plant growth and high yield. This review delves into the discussion of enhancing nutrient solution management strategies across different hydroponic systems. The aim of this review is to discuss various techniques for monitoring nutrient solutions in order to improve nutrient use efficiency (NUE) and water use efficiency (WUE). The conventional approach of monitoring the hydroponic nutrient solution using electrical conductivity measurement may not provide precise information about ion concentrations, potentially resulting in poor yields or excessive fertilizer usage. To overcome these limitations, alternative management strategies have been developed to enable more accurate monitoring and efficient management. One such strategy is the nitrogen-based approach, where nitrogen concentration becomes the primary controlled element in the nutrient solution and leads to WUE and NUE development by prolonging nutrient solution recirculation. Furthermore, various methods have been devised to improve nutrient solution strategies. These include using ion-selective electrodes to measure individual ions in the hydroponic nutrient solution, using sensors to monitor substrate moisture content, estimating water requirements, and implementing programmed nutrient addition methods. In addition to introducing different management techniques to optimize hydroponic performance, this review provides a better understanding of hydroponic systems.


Introduction
The global population is rapidly increasing, estimated to reach ∼9.7 billion in 2050 (United Nations 2014).Thus, it is estimated that 70% more food production will be required to feed this growing population (Silva 2018).To meet this increasing populations' food and feed demands, there is an urgent need to use innovative approaches to enhance the availability of fresh food produced across the globe (Pascual et al. 2018).However, water shortage is one of the most important challenges for food production, and increasing food production could negatively affect water resources (Mancosu et al. 2015;Nicola et al. 2020).The agricultural sector is the major consumer of freshwater, with over 70% of annual water withdrawals (FAO 2017), and some traditional open-field soilbased farming increases water usage due to deep leaching, runoff, and evaporation (Bar-Yosef 2008;Putra and Yuliando 2015).On the other hand, climate change will bring drought or uneven precipitation, negatively affecting agricultural ac-tivity and productivity (Barbosa et al. 2015;Abukari and Tok 2016).
Greenhouse cultivation systems increase water and fertilizers productivity compared with open-field soil-based cultivation systems due to better control of environmental conditions and inputs (Rouphael and Colla 2009;Rosa-Rodriguez et al. 2020).However, it is necessary to minimize water and nutrient consumption to decrease the costs and water requirements in the greenhouse systems as well as to minimize adverse environmental impacts (Rouphael et al. 2004).One of the promising approaches to boost vegetable production to enhance food security is growing vegetables in hydroponics: the cultivation of plants in nutrient solution and a soilless growing medium under controlled environmental conditions (Jafarnia et al. 2010;Agrawal et al. 2020).
The nutrient solution supplies the essential elements containing macro-and micronutrients with optimum concentrations for plant growth and metabolism (Sharma et al. 2018).It is essential to keep the nutrient solution in an optimum range of nutrient concentration by adjusting the solution (Wada 2019).However, imbalanced nutrients may cause nutrient deficiencies in crops grown in hydroponics, significantly restricting crop production.Additionally, excessive use of fertilizer can increase the cost of production, plant toxicity, and environmental pollution and decrease crop quality (Patra et al. 2016;Rahman and Zhang 2018).It is important to underline that hydroponically grown plants quickly show the symptoms of nutrient toxicity or deficiency compared to soil-based grown plants (Sathyanarayan et al. 2023).
Due to the reasons aforementioned, the nutrient solution is a substantial factor in hydroponics; thus, proper nutrient solution management is essential to improve nutrient use efficiency (NUE) and water use efficiency (WUE) in hydroponic cultivation systems.This review discusses different nutrient solution management strategies to enhance NUE and WUE in different hydroponics systems.

Hydroponic systems
The hydroponic system is the technique of growing vegetable crops in a nutrient-rich solution or soilless environments such as rockwool, coir, perlite, peat moss, coconut husk, gravel, coarse sand, mineral wool, vermiculite, or sawdust (Asao 2012;Sharma et al. 2018).Hydroponics is derived from the Greek words that mean water work, consisting of "hydro" which means water, and "ponos" means labor.The word hydroponics was coined by Professor William Gericke in the 1930s (Sharma et al. 2018).Similar to hydroponics, the floating gardens of Babylon, Egypt, and Mexico in the Aztecs times indicate that water gardens have been practiced for centuries.Similar to the floating raft hydroponic systems, the floating gardens were practiced as a cultivation method (Pachauri et al. 2014).In 1887, the first nutrient solution for soilless cultivation systems was developed by Sachs and Knop (Hershey 1994;El-Ramady et al. 2014).The hydroponic method is successfully used for fast-growing leafy vegetables and commercial crops, such as lettuce (Lactuca sativa L.) (Holmes et al. 2019), spinach (Spinacia oleracea L.) (Janeczko and Timmons 2019), potato (Solanum tuberosum L.) (Chang et al. 2012), tomato (Solanum lycopersicum L.) (Verdoliva et al. 2021), kale (Brassica alboglabra L.) (Yanti et al. 2020), pepper (Capsicum annuum L.) (Singh et al. 2019), cucumber (Cucumis sativus L.) (Zhang et al. 2023), and strawberry (Fragaria ananassa L.) (Talukder et al. 2019).
Hydroponics has multiple advantages compared to openfield soil-based farming (Table 1).However, hydroponics has a few limitations, including high initial setup cost, energy, vulnerability to power outage due to water and air pump utilization, and knowledge requirements for operation and maintenance (Domingues et al. 2012;Hashida et al. 2014).Moreover, higher energy consumption in hydroponic systems may increase greenhouse gas emissions (GHG), which can be optimized using longer service life materials and renewable energy.High fuel and electricity utilization, machinery, irrigation systems, and transportation increase energy consumption, which may lead to high GHG emissions (Martinez-Mate et al. 2018).
Delivering a nutrient solution to the root zone is the key process in hydroponics systems.The hydroponic systems employ different techniques to supply nutrient solution, such as deep water culture (DWC), ebb and flow system, nutrient film technique (NFT), wick system, and drip system (Fig. 1) (Agrawal et al. 2020).The DWC technique is one of the most common hydroponic methods in which the plant roots are submerged in an aerated nutrient solution (Bodenmiller 2017).The ebb and flow system is a flood and drain hydroponic system in which the roots are periodically flooded with the nutrient solution through a water pump.A water pump supplies nutrient solution to plants, and the roots are allowed to absorb moisture and uptake nutrients; then, the excess nutrient solution drains to a reservoir, and the roots can take oxygen during this draining time (Nicola et al. 2006).
In the NFT, the roots are constantly submerged in a thin flowing nutrient solution (Suhl et al. 2019).One of the simplest types of hydroponic systems is the wick system in which the nutrient solution is delivered to the roots through wicks via capillary action.Indeed, wicks distribute nutrient solutions from the reservoir to the growing media (Ali et al. 2021).In a drip hydroponic system, the nutrient solution is applied directly to the growing media through drippers, and plant roots absorb water and nutrients from the media (Graham et al. 2011).
Another method of growing leafy vegetables in hydroponics is the vertical hydroponic system, which can increase crop production per unit area to meet the demand for food production in an urban area that suffers from a lack of enough space (Al-Chalabi 2015).In this system, the nutrient solution is delivered to the top and drained under gravity to the bottom of the vertical hydroponic system (Singh and Dunn 2017).Researchers have suggested that vertical hydroponics appears to be a promising solution for urban areas to increase land and crop productivity and support local food security targets (Zhang et al. 2018;Martin and Molin 2019).

Classification of hydroponics as a recirculation system
Hydroponics increases water and fertilizer productivity due to better control of environment and water and nutrient management by recirculating the nutrient solution (Rouphael and Colla 2009;Rosa-Rodríguez et al. 2020).Two main hydroponic systems are based on recirculating the nutrient solution, that is, open-and closed-loop hydroponic systems.

Open-and closed-loop hydroponic systems
In open hydroponic systems (Fig. 2a), the nutrient solution is drained after passing through the crop root zone, and the nutrient solution is not recirculated back to the root zone after usage (Maboko et al. 2011).Nevertheless, a proper hydroponic system that can reduce water and nutrient application and negative environmental impacts is essential.A closed-loop hydroponic system (Fig. 2b) is an effective nutrient solution management approach in which leachate is reused and reduces the negative aspects such as the dis- posal of nutrient solution to the environment (Rouphael andColla 2005a, 2005b;Sanjuan-Delmás et al. 2020).The open hydroponic system can be run as a drip (Méndez-Cifuentes et al. 2020;Fayezizadeh et al. 2021), NFT (Santos et al. 2022), or DWC (Shohael et al. 2017) system, and nutrient solution application in a closed-loop system can be carried out in the form of drip (Verdoliva et al. 2021;Rosa-Rodríguez et al. 2020), NFT (Silva et al. 2020), DWC (Silva et al. 2020;Hebbar et al. 2022), or ebb and flow (Rouphael and Colla 2005a;Incrocci et al. 2006) system.Fayezizadeh et al. (2021) compared WUE in tomato production in open-and closed-loop drip hydroponic systems.The authors concluded that closed-loop systems had 54.3% higher water productivity compared to open systems, with an average WUE of 33.7 g of tomato produced per liter of water used in a closed-loop system followed by the open system with 21.84 g L −1 .Furthermore, the closed-loop system reduced fertilizers consumption (2.53 kg) during the entire crop cycle by 96% compared to the open system (4.95 kg).Accordingly, the closed-loop hydroponic system was able to enhance water and fertilizer savings without a significant crop yield reduction compared to the open hydroponic system.These findings are consistent with Méndez-Cifuentes et al. (2020) who compared an open drip hydroponic system and a closed-loop ebb and flow hydroponic system.The closed-loop system produced 9.5% lower biomass, compared to the open system.However, the open system consumed twofold water (41 L) to produce 1 kg of fresh tomatoes than the closed-loop system with 22 L of water consumption.Furthermore, fertilizers consumption was around 59%-75% lower in the closed-loop system.
Rosa-Rodríguez et al. ( 2020) and Katsoulas et al. (2013) observed a similar trend that closed-loop hydroponic systems have a higher WUE and NUE than open hydroponics due to the recirculation of the nutrient solution.Maboko et al. (2011) in a study on the plant growth performance in open drip and closed-loop NFT hydroponic systems concluded that the closed-loop system increased the marketable yield of tomato as well as nutrient solution efficiency compared with the open system.Rouphael and Colla (2005a) compared the WUE of two closed-loop hydroponic systems (drip-irrigation and ebb and flow) while growing zucchini (Cucurbita pepo L.).The water requirement of zucchini was 53% lower in the summer-fall season compared to the spring-summer season due to low air temperature and reduction in the evaporative demand.Likewise, both hydroponic systems had higher WUE in the summer-fall season (around 34 g L −1 ) compared to the springsummer season (around 23 g L −1 ).However, ebb and flow produced higher WUE (reduced water requirement by around 24%) than drip irrigation during the spring-summer season.During the spring-summer season, the ebb and flow system used 40.5 L of the nutrient solution to produce 1 kg of fruits, while the drip irrigation system needed 44.2 L of the same nutrient solution.A significantly lower WUE was observed in drip irrigation, that is, 22.6 g L −1 in the spring-summer season, than in the ebb and flow system by 24.7 g L −1 .

Advantages and disadvantages
The advantages of closed-loop hydroponic systems over open systems include reduced water use, nutrient usage, and environmental pollution (Rouphael and Colla 2005a;Valenzano et al. 2008;Rodríguez-Jurado et al. 2020;Fayezizadeh et al. 2021).However, the recycling of nutrient solution in closed-loop hydroponic systems can cause excess ions accumulation, such as sodium and chloride, in the sub- strates and the root zone, resulting in higher salinity (Ehret et al. 2005;Rouphael and Colla 2005a;Rouphael et al. 2006).High consumption of nutrients by crops from highly concentrated nutrient solutions in a closed-loop system may cause negative effects such as nutrient toxicity and reduction in yield and crop quality (Pardossi et al. 2002).
Nutrient imbalance often occurs in closed-loop hydroponic systems (Ko et al. 2013a).Nutrient concentration can rise in the nutrient solution over time due to water loss by evap-otranspiration, which increases the electrical conductivity (EC) of both the nutrient solution and the growing media (Ahn and Son 2011;Eridani et al. 2017).Therefore, the regular monitoring of ions concentrations or EC in the nutrient solution in a closed-loop system is extremely important (Ko et al. 2013a).The ion imbalance commonly occurs in a longterm crop growth cycle, which leads to a crop yield reduction due to deficiency or toxicity of nutrients (Nakano et al. 2010); therefore, the recycled nutrient solution should be refreshed or changed periodically (Savvas et al. 2005;Incrocci et al. 2006).Moreover, high EC can cause extreme osmotic conditions and plants stomatal conductance reduction (Rodríguez-Ortega et al. 2019;Nemeskéri et al. 2019;Fayezizadeh et al. 2021).
On the other hand, open hydroponics are easy to manage compared to closed-loop hydroponics, as the used nutrient solution is drained; then, a new nutrient solution is prepared and supplied.However, the discharge of nutrient solution, especially containing nitrate nitrogen (NO 3 − −N) and phosphorous, contaminates water resources, which may cause eutrophication in the surface water bodies as well as some diseases such as blue baby syndrome because of high NO 3 − −N concentration in drinking water (Putra and Yuliando 2015;Zamora-Izquierdo et al. 2019;Kwon et al. 2021).Moreover, open hydroponics indicates low water and nutrients productivity and economic returns due to the high cost of production with increased hydroponics inputs (Kitta et al. 2015).
The closed-loop system is more economically efficient than open hydroponics in terms of water and fertilizer savings, up to 90% and 85% in closed-loop versus 85% and 68% in open systems (Castillo et al. 2014;AlShrouf 2017).However, an open hydroponic system prevents salt accumulation in the root zone and growing media (Schröder and Lieth 2002).The studies described above have been selected to reveal that closedloop hydroponic systems have higher WUE and NUE than open hydroponic systems due to the recycling of nutrient solution in closed-loop systems.Future research is required to introduce a nutrient solution management technique to resolve ion imbalance in the closed-loop systems, which may lead to a reduction in ion toxicity or deficiency for crops.

Nutrient solution management in hydroponic systems
Optimal nutrient solution management can lead to a high water and nutrient efficient system.A better management of nutrient solution in hydroponic systems requires optimum pH, EC, or ions concentration (Rijck and Schrevens 1995).The pH of a nutrient solution is one of the most important factors affecting nutrient availability, uptake, and solubility.The op-timum pH range for plants is between 5.5 and 6.5 in which the plants have readily available nutrients (Domingues et al. 2012;Majid et al. 2021;Gillespie et al. 2021).For example, high pH increases the precipitation of calcium and magnesium and reduces the solubility of iron and phosphate in the nutrient solution, which forms the ions as the unavailable nutrients for roots and inhibits the absorption of micronutrients such as iron, copper, zinc, and manganese (Singh et al. 2019;Gillespie et al. 2020;Velazquez-Gonzalez et al. 2022).On the other hand, low pH decreases the absorption of macronutrients, including nitrogen, phosphorus, potassium, calcium, and magnesium (Velazquez-Gonzalez et al. 2022).Although pH stabilization is important in the nutrient solution, the pH fluctuation frequently occurs in hydroponics due to low buffering capacity of the substrates in hydroponics compared to soil.Moreover, roots release anion and cation, such as HCO 3 − and H + , to absorb nutrients, which leads to unbalanced anion and cation exchange and pH fluctuation in the substrate (Singh et al. 2019).Therefore, an optimum pH range should be maintained for proper plant growth.Adopting the optimal nutrient solution management strategy to reduce water and nutrient consumption and the cost of production to increase crop growth is essential (Rouphael et al. 2016;Gumisiriza et al. 2022).

EC-based management strategy
Since the EC value represents the nutrient concentration of the solution, in most previous studies and general practice, the monitoring of the nutrient solution is based on the measurement of EC few times daily (Wortman 2015;Majid et al. 2021).Nutrient concentration alteration occurs over time due to plant nutrient uptake, crop growth, and evaporation.When the EC value drops from a specific threshold or exceeds the optimum range of 1.5-2.5 dS m −1 , the nutrient solution with a corrected concentration should be recirculated (Rouphael et al. 2016;Majid et al. 2021;Kannan et al. 2022).Plant nutrient uptake decreases the EC depending on the crop growth stage, while evaporation may increase the EC and salt concentration in the coco coir bags or any other substrates (Majid et al. 2021).However, plants uptake more water than mineral nutrients which may, in general, cause an increase in the nutrient concentration and, subsequently, increase the EC or salt concentration in the nutrient solution over time (Eridani et al. 2017;Lee et al. 2017;Son et al. 2020;Fayezizadeh et al. 2021).
EC monitoring is the most commonly used approach as the nutrient solution management strategy because its measurement is fast, simple, low-cost, and can be used directly in situ.However, the EC value indicates only the total amount of dissolved ions in the nutrient solution without indicating the individual ions' concentrations (macronutrients or micronutrients) in the solution (Massa et al. 2008;Neto et al. 2014).Regardless of the importance of the balanced nutrient feed, plants require a high rate of specific macronutrients such as nitrogen to produce more leaves in leafy vegetables or potassium and calcium to produce high-quality fruits in crops such as tomatoes.Thus, the nutrient solution adjustment requires applying the correct amount of required ions at different growth stages based on the crop requirements (Lee et al. 2017).Additionally, plants' uptake of each ion is different, which may cause a nutritional imbalance in the solution, leading to some plant physiological disorders (Pardossi et al. 2002;Lee et al. 2017).For example, tomatoes require high amounts of calcium, potassium, and nitrogen for increasing plant growth, productivity, and fruit quality (Lee et al. 2017;Tavallali et al. 2018).Lee et al. (2017) investigated EC-based nutrient supplementation in a closed-loop hydroponic system for tomato cultivation.The mineral composition or the concentrations of the ions were analyzed to compare the micro-and macronutrients concentration variation with the EC variation.Different techniques were applied to determine the concentrations of the ions, including ion chromatography, ion-specific electrodes, colorimetric, and inductively coupled plasma optical emission spectroscopy.The results showed that EC variation followed the NO 3 − , SO 4 2− , Mg 2+ , Ca 2+ , and K + concentration in the nutrient solution over the crop growth due to slow ions uptake in the initial stage and rapid uptake during the flowering stage.However, the concentration of PO 4 3− -P, Na + , Cl − , and the other micronutrients did not follow the EC variation.Therefore, a specific ion-based nutrient solution management is necessary compared to the EC-based nutrient control to improve crop growth and yield while minimizing the production cost.

Nitrogen-based management strategy
An effective alternative to manage the nutrient solution in hydroponics is the measurement of macronutrients in the solution, which will help to understand useful information on the main ions in the nutrient solution.The essential nutrients such as NO 3 − −N, phosphorous, and potassium can be controlled using ion-selective electrodes, which efficiently control macronutrient supply.This nutrient solution management technique may decrease water and nutrient consumption by prolonging the recirculation of the solution in closed-loop hydroponic systems (Pardossi et al. 2006;Massa et al. 2010).
Among fertilizers, nitrogen is an essential macronutrient required for leaves, crop growth, quality, taste, and enhanced grain yield, as well as biosynthesis of cellular components such as proteins, enzymes, hormones, and amino acids (Goins et al. 2004;Barker and Pilbeam 2007;Maathuis 2009;Geary et al. 2015).Since nitrogen is the most essential nutrient for crop productivity (Robin 1998), the appropriate management of the nitrogen in the nutrient solution could decrease nitrogen wastage in hydroponic systems (Sonneveld and Voogt 2009;Massa et al. 2010).The NO 3 − −N analysis of the nutrient solution can be conducted in the laboratory or in situ using a reflectometer (Massa et al. 2010) or quick test kits (Maggini et al. 2010).− -based strategy, due to higher nutrient solution flushing events.Additionally, the nitrate, phosphate, and potassium losses were 23, 4.5, and 3.5 times higher, respectively, in the EC-based strategy than the NO 3 − -based strategy.The number of flowers and flower dry weight was significantly higher in the NO 3 − -based strategy.However, both nutrient solution management strategies did not significantly affect the other plant growth parameters, such as stem number, number of leaves, total dry weight, and total leaf area in amaryllis production.

Other nutrient solution management strategies
Neto et al. ( 2014) developed a fertigation automatic control system (FACS) for tomato cultivation in a hydroponic system.The FACS method estimates transpiration using the Penman-Monteith model and maintains the EC values of the drained nutrient solution under the specific limits (3 ± 0.8 dS m −1 ).The plant transpiration was estimated by measuring the atmospheric variables of the cultivation system, including air temperature, air humidity, and solar radiation.The results showed that the FACS strategy improved WUE and NUE due to adjusting fertigation frequency and reducing environmental issues related to the discharge of the nutrient solution.
Another alternative method to manage the nutrient solution is pre-arranged nutrient addition in which some specific elements are added to the nutrient solution in a particular period.Pardossi et al. (2002) compared the conventional ECbased nutrient solution management with the programmed nutrient addition to produce melon (Cucumis melo L.) in an NFT system.In the EC-based strategy, the EC value was maintained at 2.5 dS m −1 , and when NO 3 − concentration dropped below 0.085-0.09g L −1 , the nutrient solution was replaced.In the pre-arranged nutrient solution strategy, nitrogen, phosphorus, and potassium were weekly added to the solution at a recommended rate for melon without EC or ion concentration measurement.The results did not show a significant difference in fruit yield or quality, but pre-arranged nutrient addition decreased water and nutrient consumption by 40%-60% compared to the EC-based management method (Pardossi et al. 2002).Rouphael and Colla (2009) studied the effects of drip irrigation and ebb and flow hydroponic systems on the zucchini squash (C.pepo L.) growth with half-strength (1 dS m −1 ) and full-strength (2 dS m −1 ) nutrient solution.Zucchini yield was significantly affected by different nutrient solution application methods and nutrient solution concentration.The yield was higher in the drip irrigation system compared to the ebb and flow system.The ebb and flow system resulted in higher EC in the growing media due to capillary action compared to the drip irrigation system, which led to the accumulation of mineral elements in the growing media and resulted in the unavailability of nutrients for plants.The marketable zucchini yield decreased in the half-strength solution compared with a full-strength nutrient solution by around 58% and 42% in ebb and flow and drip irrigation systems, respectively.Furthermore, the full-strength nutrient solution resulted in the highest nitrogen, phosphorus, and potassium concentration in leaves in both systems.The lowest plant growth, yield, and mineral concentration in the fruit were observed under the half-strength solution under the ebb and flow system.Similarly, the half-strength solution significantly reduced the fruit yield and mineral concentration in the fruit.Ko et al. (2013b) claimed that the recycled nutrient solution's renewal (adjustment of the reused nutrient solution) could reduce the ion imbalance in the solution while improving WUE and NUE in closed-loop and open hydroponic systems.The experiment compared three different nutrient solution renewal intervals of 4, 8, and 12 weeks to investigate the crop yield and the water and nutrient uptake by paprika (C.annuum L.).The results indicated that the 12-week renewal period produced the lowest fruit yield, and the highest deviation of cation ratios, Ca 2+ , K + , Mg 2+ , and Na + , from the initial nutrient values.Authors found higher accumulation of SO 4 2− , Na + , and Cl − in the nutrient solution under a 12week period compared to 4-and 8-week periods potentially contributing to the lowest fruit yield.There was no significant difference between the open system under 4-and 8-week intervals regarding total fruit yield.Further, the 4-week renewal period resulted in the best ion balance and the highest uptake of K + in the closed-loop hydroponic system.Furthermore, the open hydroponic system observed the highest water and nutrient consumption.
Continuous monitoring of the elements in the nutrient solution using ion-specific nutrient management methods leads to efficient use of nutrients in the solution (Jung et al. 2019).Cho et al. (2018) developed an on-situ ion monitoring system using ion-selective electrodes (ISEs) to measure the concentrations of NO 3 − , K + , and Ca 2+ ions in the hydroponic nutrient solution for growing paprika in a greenhouse.The developed ISEs monitored the drainage solution five times daily based on automatic sampling and electrode rinsing.To validate the developed monitoring system, the manually taken samples were analyzed to determine the ion concentrations in the solution using ion chromatography and inductively coupled plasma spectrophotometry.The results showed that the developed monitoring system estimated the NO 3 − concentration in a strong linear relationship with a slope of 0.99 with the ion chromatography results.However, the developed ISEs system overestimated the concentrations of K + with a slope of 1.17 and underestimated the concentrations of Ca 2+ with a slope of 0.75.Despite the deviations in measuring K + and Ca 2+ concentrations, the linear relationships above 0.97 indicate that ISEs could be feasible for nutrient solution management in hydroponic systems.This response is in close agreement with the results of Kim et al. (2023) who showed that the ISE technology can be considered a potential method to control macronutrients in the nutrient solution.
The effects of two different nutrient solution management strategies were evaluated by Solis-Toapanta et al. ( 2020) on hydroponically grown tomato.In the first treatment, the nutrient solution was replaced biweekly; in the second treatment, the same amount of nutrient was added to the solution biweekly without replacement.The treatment without solution replacement affected the fruit fresh weight by about 18% compared to biweekly replacement due to the high EC of the nutrient solution.In fact, the EC of the solution with replacement remained up to 2 dS m −1 , while the EC value reached around 4.6 dS m −1 in the solution by adding nutrients at constant intervals.However, the treatments did not significantly affect the nutrient uptake by tomatoes.
Based on the drawbacks of the open-and closed-loop hydroponic systems, some researchers conducted studies to develop a drainage-free open hydroponic system in which the irrigation schedule was based on the crop water requirement to avoid nutrient solution leaching (Choi et al. 2013a).In this nutrient solution management approach, substrate moisture was measured using a moisture sensor to maintain the moisture content within the plant water requirement level to avoid leaching (Choi et al. 2013a).Choi et al. (2013a) used three-rod probe frequency domain reflectometry (FDR) sensor to monitor the EC and moisture content of coir substrate in an open drip hydroponic system.The results indicated that an automated irrigation technique at 40% or 50% volumetric moisture content led to no leachate in an open hydroponic system, which increased WUE.In other words, using an FDR sensor to manage nutrient solutions based on water demand increased WUE without water stress.In a study by Choi et al. (2013b), an FDR sensor was evaluated in tomato cultivation under a drip hydroponic system with two different irrigation schedules, 40% or 50% and 60% of volumetric moisture content, with a time-clock schedule for the fixed irrigation intervals.The results indicated that no leachate was observed at 40% or 50% volumetric moisture content compared to 60%, and 70 days after transplanting, no leachate was observed at 60% treatment.Plant growth significantly reduced at 40% or 50% moisture content compared to 60%.Generally, to have an efficient irrigation schedule by FDR, 40% or 50% volumetric moisture content at the beginning of crop season during spring and summer was suggested.
The WUE in an open hydroponic system was investigated using FDR sensors and a conventional timer-set system to manage the irrigation schedule in a large hydroponic farm (Choi et al. 2015).The experiments resulted in higher WUE, around 1.9-fold, for the FDR-managed irrigation schedule than the timer-set managed irrigation schedule from autumn to winter and spring to summer crop cycles.In addition to a reduction in drained nutrient solution in the FDR system, 61% of fertilizer costs were saved compared with a timerbased irrigation schedule.These results are consistent with Choi et al. (2016), who reported a 1.2-fold higher WUE and 41% fertilizer cost-saving with the FDR schedule system than the timer-based schedule in a hydroponically grown strawberry.
Other nutrient solution management techniques studied are partial root-zone drying (PRD) and regulated deficit irrigation (RDI) methods which are based on the crop water requirement.For example, Hooshmand et al. (2019) investigated the effects of PRD and RDI methods on tomato, consisted of five treatments of PRD at 85% and 70% of water requirement of plant, RDI at 85% and 70% of water requirement of plant, along with the control treatment in a drip irrigation/fertigation system.The results showed that the highest WUE was observed in PRD at 85% water requirement, while PRD at 70% water demand showed the lowest WUE, 16.07 and 9.02 g L −1 , respectively.PRD 85% increased crop growth after the fruiting stage and decreased irrigation water volume, leading to the highest WUE showing the best nutrient solution management approach in a hydroponic cultivation system for the tested crop under drip irrigation.Goins et al. (2004) compared three nutrient solution management approaches on potato yield and nitrogen use efficiency in an NFT.The first approach included the EC management, 0.3, 0.6, 1.2 dS m −1 during the first 42 days and 0.3 dS m −1 at the second 42 days, and 1.2 dS m −1 as the con-trol with a constant pH of 5.8.The next strategy was 0.07, 0.23, 0.57 g L −1 NO 3 − −N at the first 42 days and 0.07 g L −1 NO 3 − −N at the second 42 days, and 0.57 g L −1 NO 3 − −N as control with the constant EC of 1.2 dS m −1 and pH of 5.8.The last management strategy included 0.57 g L −1 NO 3 − −N with (control) or without pH management or mixed-N sources with or without pH management.The results indicated that the control treatments in all experiments had the highest plant growth and total plant dry mass.The lower nitrogen reduced plant canopy, root, and tuber dry weight at 0.07 and 0.23 g L −1 NO 3 − −N treatments compared to the other treatments in this experiment, which revealed that nitrogen supply reduction after tuber initiation produced almost the same biomass as the control.High nitrogen supply during the early growth stage is enough for plants to accumulate sufficient internal nitrogen reserve capacity to sustain high yield in the presence of low nitrogen supply, which can lead to high NUE.The findings are consistent with Alva et al. (2002) and Walker et al. (2001), who observed that increasing nitrogen concentration increased potato yield.
Some research employed Internet of Things (IoT) technology to monitor the nutrient solution and automate hydroponic cultivation systems.In this methodology, the EC and pH of the nutrient solution undergo measurement, transmitting these data to a microcontroller.Subsequently, the data are evaluated, facilitating control over the nutrient solution through the operation of a relay switch (Ludwig et al. 2013).Furthermore, to achieve effective control and monitoring of hydroponics, the application of artificial intelligence, specifically machine learning, is essential (Mehra et al. 2018).Incorporating sensors for nutrient solution monitoring and using artificial neural networks result in automatic hydroponic system control.In many studies, several inputs, including pH, EC, water level, temperature, humidity, light intensity, and plant age, were used for giving output decisions and predicting the values of pH and EC in automatically controlling the hydroponics (Pitakphongmetha et al. 2016).Tatas et al. (2022) designed IoT-based control and monitoring of hydroponic systems by employing wireless sensors to monitor the essential parameters and control of the pump.The system monitors the nutrient solution quality, including pH, EC, dissolved oxygen, temperature, air temperature, and humidity to ensure that crops are in an optimum condition.The sensors network transmitted the collected data to the user through a web-based tool to monitor the crop health and system performance.Afterward, a fuzzy inference engine controlled the duration for nutrient solution supply.Another study by Stevens et al. (2023) developed an IoT-based sensor system to monitor nitrogen changes in the nutrient solution for lettuce cultivation.To evaluate the accuracy of the system, samples of nutrient solution were analyzed for nitrogen concentration in a laboratory.The results presented that the IoT system successfully monitored changes in the nitrogen concentration in the DWC hydroponic system.
In their study, Chowdhury et al. (2020) conducted an evaluation of an IoT-based automated vertical NFT hydroponic system to store, monitor, and control the system parameters for remote monitoring.This innovative system was designed for cultivation of a variety of crops, including lettuce, cu-cumber, tomato, strawberry, mint (Mentha piperita L.), coriander (Coriandrum sativum), and pepper.The core of the system was a microcontroller, and various sensors were employed to monitor all the system parameters.Once the data were transmitted to the user, the system adjusted and maintained the growth parameters within a specific range for the crops.The results demonstrated the effectiveness of this automated system in monitoring environmental parameters and regulating nutrient and water supply to facilitate the stable growth of plants.Arora et al. (2021) conducted a study where they employed machine learning to control an automated dosing system.Probes and sensors continuously measured plant-affecting factors including pH, temperature, and EC, transmitting data to the controller every 5 min.If values deviated from optimal thresholds, the system activated the solenoid valve to adjust the nutrient solution.The research concluded that microcontrollers and sensors can enable automatic monitoring and control of EC and pH in hydroponic nutrient solutions.Rau et al. (2017) designed a smart IoT-based sensing and actuation system for rice cultivation by controlling the concentration of magnesium and nitrogen in a hydroponic solution and monitoring the greenhouse's environmental parameters.In another study by Bakhtar et al. (2018), supplying nutrient solution was conducted using a microcontroller kit connected to a wireless sensor network with the internet.The input data considered pH and water level for spinach; then, the real-time value and the required value were compared.If the values did not match, the required amount of nutrient value was sent to the user to adjust the nutrient solution.
The above studies demonstrate that the common nutrient solution management technique, the EC-based strategy, presents the total amount of ions concentrations in the nutrient solution.In other words, the EC value cannot be considered an appropriate indicator of the vital elements for plants in the nutrient solution.Based on the results of the present literature, it is suggested that an EC-based nutrient solution management strategy can be applied along with another nutrient solution management technique to successfully improve crop growth, WUE, and NUE, thus reducing the cost of production and negative environmental impacts.Moreover, with the advent of IoT technology, many hydroponic monitoring systems have been developed, many of which are accompanied by mobile applications.In terms of incorporating intelligence, specifically machine learning, to analyze the captured data for precise plant growth control in hydroponics, there has been a notable research effort focused on applying artificial neural networks.Figure 3 shows that the different types of hydroponic systems align with different strategies to manage the nutrient solution.

Nutrient use efficiency
Unlike the open-field soil-based cultivation system, the success of a hydroponic system enormously depends on the appropriate nutrient solution directly applied to the system (Djidonou and Leskovar 2019;Kwon et al. 2021).The nutrient solutions for hydroponic systems differ from fertilizers used for soil-based agriculture.Hydroponic systems use high concentrations and more extensive elements since the buffering capacity and micro nutrient content are lower in the hydroponic growing media compared to soil-based cultivation system (Huang 2009;Seaman 2017).Several factors affect NUE in hydroponic systems, including recycling of nutrient solution, types of hydroponic system, and crop varieties (Kwon et al. 2021).
A simple change in the nutrient solution application, such as recycling of nutrient solution, can improve NUE in hydroponic systems (Bar-Yosef 2008;Grewal et al. 2011).In five hydroponic systems, open-and closed-loop beds, openand closed-loop bags, and a DWC system, NUE was evaluated for tomato production by Castillo et al. (2014).The nutrient solution was supplied using drip tape to the bag and bed hydroponics.Recirculating the nutrient solution resulted in 35%-41% higher nutrient savings than the open hydroponic systems.The closed-loop bag had the highest NUE, followed by the closed-loop bed system.Furthermore, it was observed that the closed-loop bag system exhibited higher WUE compared to the closed-loop bed system.This difference can be attributed to the larger exposed surface area in the bed system, which resulted in increased evapotranspiration and subsequent water consumption.Further, DWC and closed-loop bags produced a higher yield than open hydroponics, showing the possibility of fertilizer saving without affecting crop growth, which is aligned with Oztekin et al. (2007).This is consistent with findings by Rosa-Rodríguez et al. (2020), who reported 22.69% higher NUE in a closed-loop drip hydroponic system than in open drip hydroponics for tomato production.
Grewal et al. ( 2011) investigated the effect of recycling drainage water in a closed-loop drip hydroponic system on water and nutrient usage for cucumber production in a greenhouse.Results indicated that the recycling of drainage water reduced 33% of potable water consumption for cucumber production, 49.1 g L −1 of WUE, which was the exact yield as the typical yield for the region.Moreover, the results demonstrated that the plants took up 41% of the total applied nitrogen of the recycling irrigation solution, reducing the cost of fertilizers by reusing the wastewater.The study revealed that the drainage water recycling led to 566 kg ha −1 nitrogen reusage.
As an essential nutrient to crop growth, the nitrogen concentration in the plant is an indication of the applied nitrogen concentration in the growth media or nutrient solution.A higher nitrogen concentration in the solution resulted in higher nitrogen in plants when it is unable to convert all the absorbed nitrogen to dry matter or other structures like cellulose (Walker et al. 2001;Stefanelli et al. 2011).Djidonou and Leskova (2019) investigated the application of different nitrogen concentrations (containing half of the nitrate form and half in the ammonium form), from 100 to 400 mg L −1 , to find the optimum nitrogen concentration to maximize the lettuce yield in a closed-loop NFT hydroponic system.The results revealed that the accumulated nitrogen in lettuce is increased by increasing nitrogen in the nutrient solution.Moreover, fresh weight yield increased as the nitrogen concentration in- creased to 300 mg L −1 .Furthermore, nitrogen use efficiency was reduced by increasing the nitrogen concentration, and 400 mg L −1 nitrogen concentration caused the lowest nitrogen use efficiency, which means that high nitrogen concentrations did not proportionally produce higher yield.Therefore, the optimum range of the solution nitrogen concentration of 100-150 mg L −1 was suggested to maximize lettuce yield.

Water use efficiency
Plants' water consumption may be affected by the cultivation system.Hydroponics facilitates easy water absorption by crops, leading to higher WUE (Rouphael et al. 2004;Tomasi et al. 2015).Studies indicate that the soil-based cultivation system is the least water-efficient system compared to hydroponic systems (Sanyé-Mengual et al. 2015;Barbosa et al. 2015;Verdoliva et al. 2021;Majid et al. 2021).Rouphael et al. (2004) compared the WUE in soil-based and closed-loop hydroponic systems with three growing media, that is, coco-fiber, perlite, and pumice.The results showed higher water consumption in the soil-based system, due to excess leaching and higher application rate of water, compared to the closed-loop hydroponic systems.Closed-loop hydroponic systems with pumice, coco-fiber, and perlite increased WUE by 114%, 76%, and 76%, respectively, compared to the soil-based system.Furthermore, the hydroponic systems produced 33% (coco-fiber), 23% (pumice), and 19% (perlite) higher yield of zucchini squash as well as higher carbohydrate concentration compared to the soil-based system.In a study by Majid et al. (2021)  system on the WUE and NUE and the environmental impact of the drainage water discharge.
It is important to underline that climate factors, such as high air temperature and solar radiation, increase plant water consumption (Rouphael and Colla, 2005b;Williams Ayarna et al. 2020).High temperature leads to stomatal closure, photosynthesis rate depletion, respiratory deficit, and WUE reduction (Rouphael et al. 2008).Hebbar et al. (2022), in an open farm hydroponic system, reported that high vapor pressure losses due to high temperature and low humidity during summer resulted in higher water use and WUE reduction.Table 2 presents some studies on the effects of different conditions on WUE in hydroponic systems.

Summary and future perspectives
This review revealed the need to emphasize the importance of hydroponics to further enhance food security due to challenges faced by the conventional agriculture industry impacted by extreme weather and poor soil conditions.A hydroponic system is essential in improving agricultural productivity as a sustainable and resource efficient system to achieve food security.Innovative approaches to overcome the overuse of water and fertilizers are required to keep up with the increasing food demand while minimizing negative environmental impacts.This review discusses different nutrient solution management strategies to enhance nutrient use efficiency (NUE) and water use efficiency (WUE) in hydroponic systems.From the overview on the recirculation of the nutrient solution, closed-loop hydroponic systems have shown success in increasing NUE and WUE compared to open hydroponics, around 90% and 85% water and nutrients savings, respectively, compared to open hydroponic systems.
On the other hand, innovative techniques are required to manage the nutrient solution to operate the hydroponic system at an optimum level to maximize productivity and minimize the cost of production.The EC-based strategy is the simplest method.However, it cannot follow the nutrient variations in the solution over time.Hence, ion-based strategies have been studied to improve the quality of the nutrient solution, thus increasing the yield.Monitoring the concentration of nutrients could be the most effective contribution to reducing water and fertilizer consumption and achieving the ambition of having an eco-friendly hydroponic system.The nutrient-based strategy can reduce water and nutrient consumption by up to 60% more than the EC-based technique.
Different approaches to managing the nutrient solution have shown some success and have become appropriate alternatives for nutrient solution management due to reducing water and fertilizer consumption.Fertigation scheduling by measuring the atmospheric variables enhanced NUE and WUE due to fertigation frequency adjustment according to plant transpiration estimation.Additionally, deficit irrigation with partial root-zone drying (PRD) in 85% of the plant water requirement increased crop growth and WUE.Likewise, utilizing an frequency domain reflectometry (FDR) sensor to monitor substrate moisture and EC increased WUE without water stress.Besides, FDR can save 41%-61% of the fertilizer costs and lead to 1.2-to 1.9-fold higher WUE compared to a time-based schedule.Although these studies indicate that monitoring sensors increase NUE and WUE, providing the required equipment might be costly.Therefore, a pre-arranged nutrient solution addition can be effortless and affordable if the crop requirements are accessible.
This review has introduced several opportunities to attain a productive and efficient hydroponic system to increase crop yield, WUE, NUE, and environmental pollution control while minimizing the cost of inputs.Although an open hydroponic system has some advantages, the major disadvantages of using the open system are waste of water and fertilizer along with the environmental pollution resulting from used nutrient solution discharge.Studying the possibility of reusing nutrient-rich hydroponic waste to cultivate plants in hydroponics can introduce an environmentally friendly cultivation system.Next to nitrogen, phosphorus and potassium are essential nutrients for plant growth and productivity, and their limitation affects crop yield and quality.Therefore, phosphorus and potassium need to be investigated to control the nutrient solution along with nitrogen to maximize the hydroponic performance.In addition, some nutrient solution strategies discussed in this review are costly, infeasible, or time-consuming.Thus, further studies could introduce the most straightforward nutrient solution management strategies, which make the techniques more appealing and practical to growers.

Fig. 2 .
Fig. 2. Illustration of (a) an open-loop drip hydroponic system and (b) a closed-loop drip hydroponic system.
Massa et al. (2010) compared the effects of three fertigation strategies on the NUE and WUE in a semi-closed hydroponic system for tomato.In strategy A, the recirculating nutrient solution was flushed out whenever the EC value reached 4.5 dS m −1 .In strategy B, NO 3 − −N was measured with a reflectometer every 2-4 days, and the recirculating nutrient solution was discharged whenever NO 3 − −N concentration dropped below 0.07 g L −1 .In strategy C, when the EC value reached 4.5 dS m −1 , water was added to reduce NO 3 − −N concentration below 0.07 g L −1 .The results showed that a shortterm lack of nutrients and by prolonging the recirculation of nutrient solution due to frequent EC and NO 3 − −N concentration measurements reduced the water and fertilizers use and nitrogen losses.Indeed, strategy B led to the highest nitrogen use efficiency, around 344 g g −1 , and strategy C resulted in the best WUE of 22 g L −1 by prolonging the recirculation of the nutrient solution.The NO 3 − -based strategy indicates the nitrogen concentration of the solution; however, the ECbased method exhibits the ions concentration of the solution.The EC-based method does not indicate the amounts of vital elements separately, which may show fertilizers requirement more quickly than the NO 3 − -based method.In other words, by accurately monitoring the nitrogen concentration, this nutrient solution management strategy might extend the solution's recirculating time, which can enhance WUE and NUE.Rouphael et al. (2016) compared two nutrient management strategies, EC based and NO 3 − based, in a hydroponic system to assess the amaryllis (Hippeastrum hybridum) growth and WUE.In the EC-based strategy, when the EC value exceeded 3 dS m −1 , the nutrient solution was discharged, and in the NO 3 − -based strategy, the nutrient solution was recharged when the NO 3 − −N concentration dropped from 1.42 to 1 mol m −3 .The results demonstrated that the NO 3 − -based strategy did not significantly affect plant growth and quality compared with the EC-based approach.The EC-based strategy increased total water use by about 61.5% compared with the NO 3
, the NFT system indicated 64% water saving compared to the soil-based system.Reusing the nutrient solution in closed-loop hydroponics decreases water consumption and raises WUE (Rosa-Rodríguez et al. 2020).Grewal et al. (2011) investigated the effect of recycling drainage water in a closed-loop hydroponic

Table 1 .
Advantages of hydroponic systems compared to the open-field soil-based cultivation system.

Table 2 .
Comparison of water use efficiency (WUE) under different conditions of hydroponics.