Hydroponic Production Systems in Greenhouses

Abstract

Hydroponic production means the growing of vegetables, herbs and ornamental plants and fruits in a nutrient solution (a solution of water and macro- and micronutrients) with or without the use of a substrate that gives the mechanical support to plant. The most important advantages of hydroponics are as follows: continuous cultivation of one crop, better control and supply of plants with water and plant nutrients, reduced occurrence of plant pests and minimized environmental impact and increased water use efficiency. The main hydroponic cultivation technique of fruit vegetables is cultivation on substrates, often called soilless system. Growing substrate (organic, inorganic or synthetic) provides an aseptic environment, good oxygenation and an adequate nutrient solution flow, so the most important substrate properties are biological and chemical inert, porosity and capillarity. Its choice depends on climatic conditions, the type of equipment in the greenhouse and the plant requirements. Hydroponics is also suitable for growing crops with a shorter growing period such as leafy vegetables and herbs. Plants are grown by different growing techniques in a nutrient solution without a substrate (nutrient film technique, floating hydroponics, ebb and flow and aeroponics). These are closed hydroponic systems, which means that drainage nutrient solution is collected, sterilized and reused.

1. Introduction

Human population increasing and market demands require major adjustments in the way food is produced, and turning from previous traditional forms of cultivation to new and sustainable ones. One way to increase the food production sustainability is to grow plants in different hydroponic production systems. Hydroponics represents a climate-smart production method, due to environmental concerns, resource sustainability and efficient use as well as climate changes [1, 2]. This mean, when compared to the open field production, which is often exposed to biotic and abiotic stress factors that hinder production, hydroponics use less resources such as land space, pesticide, and water. As in the most cases hydroponics are placed in the greenhouses, the control of production factors such as temperature, relative humidity, light and carbon dioxide, as well as extension of the growing season is possible. These production systems also make the supply and distribution of nutrients to crops easier and more uniform to enhance crop growth and yield [3].

Hydroponically grown plants in greenhouses are optimally supplied with water and nutrients and have optimal growth and development conditions due to climate control. Production mostly takes place in heated greenhouses, which allows the production and supply of the market throughout or most of the year, depending on the culture grown. Vegetables, herbs and ornamental plants and fruits are grown in a nutrient solution (solution of water and macro- and micronutrients) with or without the use of substrate that gives the mechanical support to plant. Plant nutrients are in optimal relation, and concentration determined by the electrical conductivity (EC-value) and the pH value.

Mentioned above results are with the advantages of hydroponics [2, 4]:

• plant cultivation in locations and areas where there is no soil or the soil is unsuitable for growing,

• continuous cultivation of one crop in the same production area (no crop rotation required),

• better control and supply of plants with water regarding time and amount,

• better control and supply of plants with plant nutrients (during the growing season, the concentration, composition, time and amount of nutrient solution are changed as needed, depending on the plant development phase and on the microclimatic conditions of the greenhouse),

• reduced occurrence of plant pests (diseases, pests, nematodes and weeds) that need greenhouse soil for their development and overwintering,

• minimized environmental impact and increased water use efficiency with closed hydroponic systems.

These advantages result in higher production of biomass in the time and area unit in hydroponic cultivation compared to the soil cultivation, and thus earlier harvesting (faster entry into technological maturity), more harvests in crops that multiple harvested and higher total yields. Besides that, hydroponics represents an appropriate and sustainable growing technology for urban and peri-urban areas, where higher yield could be achieved by using vertical space (vertical farming systems) to meet food demands in densely populated areas [5].

Disadvantages of hydroponic cultivation techniques are high initial investments, that is, higher costs of installing hydroponic systems in relation to conventional soil cultivation. Successful hydroponic production requires a high degree of knowledge and expertise in the field of agronomy and technical skills and knowledge to manage the equipment applied. If diseases and pests occur, the infection spreads rapidly due to optimal conditions for their development in a greenhouse. Due to significantly higher costs, the successful application of hydroponic technology is limited to species of high economic value, in some regions often to a certain part of the year. An additional problem of hydroponic cultivation techniques on substrates is also a disposal and recycling of inorganic and synthetic substrates after use [2, 4].

Hydroponic production systems include both cultivation on different inert substrates or growing media (soilless culture) and water culture with nutrient solution as root environment (without substrate). Regarding drainage solution usage hydroponic systems could be divided to open or closed. In open systems, the drainage solution is discharged, while in closed systems the drainage solution is collected, sterilized and reused [6]. Velasquez-Gonzales et al. [2] stated that choose of hydroponic growing technique depends on the plant species, local climate and budget, among other factors.

Despite some disadvantages mentioned above, hydroponic production is a rapidly growing sector that has seen tremendous growth in recent years. According to various statistics, the global hydroponic system market is projected to reach 16.03 billion USD by 2028 and Europe represents the largest market for this industry, accounting for 41% of its share. The compound annual growth rate of the hydroponics between 2022 and 2028 is estimated at 11.3%. Hydroponic greenhouse vegetable production is growing at a rate of 5–10% annually worldwide, and tomato is the most popular crop in the commercial hydroponics, accounting for over 30% of hydroponic production. When using hydroponic production systems, producers could achieve up to 2–4 times higher yield with approximately 90% less water consumed than traditional soil-based agriculture. At the same time, environmental pollution is decreased by nearly 70% [7].

To reach positive financial results, hydroponic production systems should be placed in a well-equipped, high-tech greenhouses where soilless culture equipment represents only a small fraction of the total investment of about 200 €·m^–2. However, low- or mid-tech greenhouses may sometimes be modernized and used for hydroponics, depending on the economic and technical conditions, such as region, farm characteristics, type of greenhouse, soil problems, water resources, market requirements, establishment costs and, last but not least, restrictions on environment pollution. Low-cost alternatives are suitable for growers with limited capital or in regions with a fluctuating demand. In low-tech hydroponics, the heart of the system is the growing medium or water, while a simple system controls and distributes the nutrient solution or a drip irrigation system can be used [8].

This chapter will discuss about growing technology and substrates used in soilless culture and about water culture growing techniques.

2. Soilless culture

2.1 Growing substrate

There is no material or mixture that would be universal for growing all crops in all growing conditions. Growing substrate properties should match the requirements of the crop and the growing technology. Substrate should provide an aseptic environment, good oxygenation and an adequate nutrient solution flow, so the most important substrate properties are biological and chemical inert, porosity and capillarity. Biologically inert substrate means the absence of pathogens of plant diseases, pests and weed seeds. The chemical inert substrate does not contain any nutrients and does not affect, and they do not change the composition of the applied nutrient solution. In last years, life cycle and substrate sustainability (economic, social and environmental viability) becomes more and more important properties. For sustainable production of vegetables in growing media, priority should be given to locally available and not very expensive or locally manufactured and standardized products. The choice of substrate as a growing medium depends on climatic conditions, the type of equipment in the greenhouse and the requirements of the plants that need to be met [2, 3, 4, 5, 6, 8].

Hydroponic growing substrates are divided into organic (peat, coconut fiber, sawdust, corn, straw), inorganic (rockwool, perlite, sand, expanded clay, pumice, vermiculite, zeolite) and synthetic derived from petroleum (polystyrene, polyurethane and urea-formaldehyde foam). Also, they are divided into fibrous and granular. Fibrous are characterized by a high fiber content of different dimensions giving the substrate high water capacity and low for air. Retained water is easily accessible to the plant, and the volume is significantly reduced and varies from 2 to 7 liters per plant. Granulated substrates (sand, perlite) as opposed to fibrous ones have increased air capacity and reduced water by 10 to 40%. Retained water is more difficult to access to the plant, and the volume of substrate for one plant must be much higher than the fibrous substrates and amounts to between 10 and 40 liters [9].

2.1.1 Rockwool

Rockwool is a natural material obtained by heat treatment of volcanic rocks (bazalt and diabaz), which, with the addition of coke and limestone, are talent and refined to the final product, which, under the influence of high temperatures, acquires a fibrous structure. These fibers are then pressed into blocks or cubes (Figure 1) of light volume weight (80–90 kg·m⁻³) [6]. It absorbs water very well and has good drainage properties. Total porosity is from 95 to 97%. Of these, 75 to 80% are water micropores and 10 to 15% are air macropores (Table 1). One of the most significant features of rockwool is its sterility, that is, the complete absence of pathogenic microorganisms and everything else that could contaminate soilless cultivation. It has mild alkaline reaction (pH value from 7 to 8.5). Because rockwool is an inert pH value can be easily reduced to optimal in hydroponic cultivation (from 6 to 6.5) using a slightly acidic nutrient solution. After use, it can be thermally sterilized and reused for one or 2 years, which reduces environmental pollution. However, after each use, the fiber structure worsens and reduces the proportion of air pores. In areas with colder climates, less density rockwool with vertical threads is most used, while for warmer appetizers, higher-density stone wool with horizontal threads is recommended to allow for better water retention [9, 10].

Figure 1.

Inorganic growing substrates (A: rockwool; B: perlite; C: expanded clay).

Substrate	Volume weight, kg/m^3	Total porosity, vol. %	Water porosity, vol. %	Air porosity, vol. %	pH value
Sand	1400-1600	40–50	20–40	10–20	6.4–7.2
Pumice	570–630	80–90	2–5	75–85	7.0–8.0
Expanded clay	300–700	40–50	5–10	30–40	4.5–9.0
Perlite	90–130	50–75	15–35	30–60	6.5–7.5
Rockwool	55–90	95–97	75–80	10–15	7.0–7.5
Peat	60–400	55–97	52–88	6–42	3.0–7.3
Coconut fiber	65–110	94–96	80–85	10–12	5.0–6.8

Table 1.

Physical and chemical characteristics of some growing substrates [9, 10].

2.1.2 Perlite

Perlite (Figure 1) is an aluminum silicate of volcanic origin containing 75% SiO₂ and 13% Al₂O₃. It is a sterile material, neutral in pH (6.5–7.5) and no decay, with light volume weight (90–130 kg·m⁻³) [6]. Its porosity (50–75%) ensures good breathability important for the growth of the root system (Table 1). Several different perlite granulations are produced (<3 mm, <5 mm, etc.). It can be purchased on the market packed in bags of volume 10 to 15 L on which the plants are planted or in large bags of volume about 100 L when filled into breeding vessels. It can be used alone or in a mixture with other substrates. If it is represented in a higher ratio in the mixture, attention should be paid to the pH value, which should not be lower than 5. It is often mixed with organic materials (peat) that improve its elasticity, permeability and other physical characteristics [9, 10].

The main disadvantage of rockwool and perlite is high energy consumption during production and their high price [6].

2.1.3 Expanded clay

Expanded clay (Figure 1) is obtained by roasting natural clay at 1200°C over 3 hours, giving a porous medium in the form of balls with a diameter of 4 to 20 mm, depending on the purpose. It is an inert substrate without a nourishing, neutral pH reaction. Capillarity on the surface of the ball provides a nutrient solution near the roots of the system. The balls dry easily and do not contain excess water that provides enough oxygen near the roots The lack of expanded clay is a fairly large volume mass, making it difficult to manipulate and very low water porosity (Table 1), which requires frequent and short fertigation [9, 10].

2.1.4 Coconut fiber

Coconut fiber (Figure 2) is increasingly used in hydroponic cultivation, and can be found on the market under different names, most often in the form of pressed blocks (plate). This substrate combines high water capacity of vermiculite and air capacity of perlite. However, it is completely organic in origin obtained by peeling coconuts. By its physical properties (Table 1), it is most similar to rockwool. Coconut fiber has physical stability [6], light weight (65–110 kg·m⁻³), good air content, high total pore space between 94 and 96%, and water holding capacity, subacid-neutral pH (5–6.8). It is rich in hormones and sterilized by pressurized water vapor, which ensures ideal conditions for rooting and protects against the causes of plant diseases. Also, unlike peat, coconut fiber is a completely renewable resource. The lack of coconut fiber as a substrate can be the content of NaCl [6], which affects the ion concentration in the root zone and has a detrimental effect on its development. Pressed blocks require soaking in aqueous solution before use. During soaking, the substrate rehydration and swelling occur up to six times the initial size. It is very often mixed with perlite or vermiculite in equal proportions [9, 10].

Figure 2.

Organic substrates: coconut fibers (a), and peat (b).

2.1.5 Peat

Peat is the most important material of organic origin and is obtained from the remains of Sphagnum moss (Figure 2). It is characterized by good drainage and structure, that is, physical stability, good air and water holding capacity with total pore space ranging 85–97%, low microbial activity, light volume weight (60–200 kg m⁻³), low and easily to adjusted pH, and low nutrient content [6]. According to the degree of decay, the amount of hinges is divided into white, brown and black peat. White peat has great absorption power and high acidity, and pH values between 3.5 and 4.2 (Table 1). It contains very few nutrients so it improves the water regime and air capacity. Black peat contains larger amounts of minerals that are suitable for plant growth. The reaction varies from 6.5 to 7.2 so that it is suitable for growing plants to suit a neutral or weakly alkaline reaction [9, 10].

Disadvantages of peat are that it is finite resource, environmental concerns and contribution to CO₂ release due to peatlands use, increasing cost due to energy crisis. It may be strongly acidic; shrinking may lead to substrate hydro-repellence [6].

2.2 Soilless growing technology

The main form of fruit vegetables production is cultivation on substrates. The substrate is a medium whose role is to strengthen the root system, maintain water in the form of accessible plants, runoff of excess nutrients, and ensure air exchange. Soilless culture of fruit vegetables on substrates is technologically similar to soil cultivation in the greenhouse.

2.2.1 Greenhouse preparation

Before planting, it is necessary to prepare a greenhouse. In the greenhouse, equipment for nutrient solution preparation and a drip irrigation system should be installed. Substrate plates are placed in rows or double-row strips. The substrate is placed on hanging gutters, which serve to runoff an excess nutrient solution. The distance between the rows is 120 to 150 cm. If planted in double-row strips, the distance between the rows in the strip is 70 to 80 cm, and between the strips 100 to 120 cm. If cubes with two prickled plants are planted, two rows of plants are obtained from one row of substrate. After the substrate is placed, the planting sites are cut at the polyethylene foil into which the substrate is packaged. The distance between the plants in the row is 33 to 50 cm. Capillary carriers are inserted vertically into the cut openings so the substrate could be soaked with a nutrient solution before planting.

2.2.2 Sowing and planting

The seed sowing is most often done in rockwool plugs and planting on a selected inert substrate. Another possibility is sowing in rockwool blocks, 2.5-cm brides and 4 cm high. Fifty to sixty blocks are connected by an upper edge so that they form a larger sowing unit. The plugs are placed in polystyrene containers with 240 pots. Sowing is most often done in late November or early December. After sowing in rockwool plugs, seeds are covered with vermiculite, which keeps constant temperature and retains moisture needed for emergence.

Emerged plants are pricked at the phase of developed cotyledon leaves and the first true leaf (Figure 3) into the rockwool cubes. The cube size depends on the culture and the number of plants being prickled into one cube. If one plant is prickled per cube, 7.5-cm edge cubes and 6.5 cm high or 10-cm edge cubes and 7.5 cm high are used, and if two plants are pricked per cube, cubes with 10- or 12-cm edge are used. Since seedlings are produced during a short day, supplemental lighting should be used in order to shorten the growing period.

Figure 3.

Tomato plants in rockwool plugs ready for pricking.

Seedlings are grown in rockwool cubes until planting. During cultivation, they are fertigated with a nutrient solution of reduced concentration every day or every other day. If necessary, after watering with the solution, the leaves are rinsed with tap water to wash out the remains of nutrient salts. When the plants begin to touch each other, the cubes need to be spaced apart to prevent the seedling elongation. The cube is separated once and twice during the cultivation of seedlings (Figure 4). The seedlings are ready for planting when the root grows through the volume of the cube, that is, in late January or in the first half of February. Tomatoes are planted in the developing phase of 7 to 8 leaves and with a visible the first bloom, peppers in the phase of 10 to 12 leaves and a visible branching and the first flower, and cucumbers with 3 to 4 leaves.

Figure 4.

Tomato seedlings in rockwool cubes.

The volume of inert substrate per package is most often between 10 and 20 liters. These bags (plates) are 1 m long, 15 to 20 cm wide and 7.5 to 10 cm high. Granular substrates such as perlite and expanded clay can be filled into pots or bags (Figure 5). The volume of substrate per plant is most often between 2.5 and 5 liters. Due to the small volume of substrate per plant, frequent fertigation is required, and the number and duration of a single ration depend on the substrate capacity for water (nutrient solution), the development stage of plant microclimatic conditions in the greenhouse.

Figure 5.

Cucumber planted in peat bags.

Planting is done by placing the cube with the seedling/s on the openings provided on the substrate plates. When planting, it is necessary to remove the capillary carrier from the plate and insert it into the cube. Due to favorable temperature and humidity conditions, the rooting lasts for 2 to 3 days and plants continue their growth. Because the substrate is soaked with a nutrient solution before planting, only a few rations of fertigation are needed daily after planting. A few days after planting, the substrate plate is cut into two places, about 2 cm from the bottom of the plate, to allow the runoff of the excess nutrient solution.

2.2.3 Plant care measures and harvest

After rooting, plants are wrapped to prevent the stem breaking. It is necessary to maintain the plants by training them up a vertical supporting twine, removing older leaves as the lower fruit clusters are harvested, and by lowering the main plant stem to keep the whole plant within easy reach of workers.

During vegetation, the daily number and duration of the fertigation rations gradually increases. It is needed to control pH and EC values of nutrient solution in root zone, and ant to perform periodic laboratory analysis of nutrient solution composition. At the same time, in a greenhouse it is necessary to maintain the microclimatic conditions as close as possible to the optimal ones. Harvesting of fruit vegetables in soilless culture is performed at technological maturity, and begins 70 to 90 days after planting for tomato (Figure 6) and pepper, and about 30 to 40 days for cucumber. Pepper fruits could be also harvested at physiological maturity. The frequency of harvest depends on the time of harvest and species grown: every 2 to 3 days in cucumbers, every 3 to 5 days in tomatoes and every 10 to 14 days in peppers.

Figure 6.

Tomato plants at the beginning of harvest.

Forty to fifty days before the planned end of the harvest, plants are topped to improve the maturation of formed fruits. A few days before the harvest end, fertigation is stopped.

After the harvest end, plant residues, substrate and parts of the drip irrigation system are moved out from the greenhouse, the greenhouse is cleaned and disinfected and preparations for the next season begin. If the substrate is planned to be reused, it should be stored in a greenhouse to prevent freezing and disrupting the structure.

3. Water culture

Hydroponic techniques for growing plants in a closed system in a nutrient solution without substrate (water culture) are appropriate for growing crops of shorter vegetation, such as leafy vegetables (lettuce, arugula, lamb’s lettuce, spinach, Swiss chard, chicory, endive and cress salad) and herbs (parsley, basil, oregano, marjoram, thyme, sage and dill). As the most commonly used in growing leafy vegetables, nutrient film technique, floating hydroponics, ebb and flow and aeroponics could be pointed out.

3.1 Nutrient film technique (NFT)

The nutrient film technique is based on maintaining a thin layer (up to 1 cm) of aerated nutrient solution that continuously flows over the plants root in shallow channels laid under a slope from 0.3 to 2%, which allows the solution to be circulated with a free fall (Figure 7). As stated by Velasquez-Gonzales et al. [2], nutrient solution flow can be periodic also. The nutrient solution is supplied by the pump from the container to the channel with the plants, and the solution not used by the plants is collected in the storage tank, analyzed and returned to the system. It is precisely the recirculation of the nutrient solution that is the main advantage of this hydroponic technique. Depending on the culture grown, the channel width varies from 10 to 20 cm, while the maximum length is 20 m.

Figure 7.

NFT channel with tomato plants.

The channels can be located on the ground or gutters, and are most often made of polymer materials (polyethylene, polyvinyl chloride). The channels contain openings in which seedlings or pots with plants are placed and their root is continuously supplied with water and nutrients, with an ideal solution flow rate of 3 to 8 L/m² per hour for crops such as chrysanthemums and salad. The disadvantages of this technique are the risk of interrupting the flow of a nutrient solution that very quickly causes root drying, stress and excessive channel warming in the summer due to which young plants may suffer in the initial growth phase. Contrary to growing on substrates, the ion concentration in the root zone does not increase due to continuous solution flow [11].

3.2 Aeroponics

3.2.1 System work out

In aeroponics, the plant root is in the air of dark space, and the nutrient solution is supplied by spraying every 3 to 4 minutes for 15 to 20 seconds in the form of an aerosol, which ensures high humidity (> 95%) in the root zone [11]. The optimum EC and pH values of nutrient solution in aeroponics system lie between 1.5 to 2.5 dS/m and 5.5 to 7.0, sprayed in different intervals, depending on species grown. Nutrient-rich solution is used as a growing medium and provides essential nutrient for sustain plant growth [12]. Velasquez-Gonzales et al. [2] pointed that there is no need for aeration system as oxygen is delivered to root with the sprayed nutrient solution.

In aeroponics, Styrofoam plates with plants are attached to a structure that can be horizontal, or at an angle of 45 to 60 degrees (A-frames). The pump distributes a nutrient solution from the tank to the spray pipe, which is located inside the structure and supply the root of the plant. The nutrient solution is returned to the tank by free fall [11]. The nursery plants might be either raised as seedlings using specially designed lattice pots or cuttings could be placed directly into the system for rapid root formation. Lattice pots allow the root system to develop down into the growth chamber where it is regularly misted with nutrient under controlled conditions [12].

3.2.2 Advantages and disadvantages

The aeroponics provides numerous advantages including a free extension of the root system, direct and sufficient oxygen uptake, and rapid and provision of uniform nutrient spray mist with best root growth environment. Aeroponics uses less water and nutrients because the plant roots are sprayed at intervals using a precise droplet size that could utilize most efficiently by osmosis to nourish the plant [12]. Using A-frames aeroponics (Figure 8) results in good utilization of the greenhouse volume because the number of plants has doubled, but due to the variation of light intensity, uneven plant growth may occur.

Figure 8.

A-frame aeroponics.

High initial investments and the application of complex electronic devices justify the application of this hydroponic technique only to high-income cultures [11]. Lakhiar et al. [12] stated that the main problem in aeroponics is related to water nutrient droplet size. The larger droplets permit the less supply of the oxygen availability in the root zone, while the smaller droplets produce too much root hair without developing a lateral root system for sustainable growth. The main potential challenge and drawback of the system is constant power supply throughout the plant growth. Any prolonged rupture of power energy shuts down the nutrient supply and contributes to permanent plant damage.

3.3 Ebb and flow

3.3.1 System workout

The ebb and flow technique is also called “flood and drain” because of its principle of time intervals between dry and wet periods. Nutrient solution is available periodically by soaking the benches filled with plants in containers (Figure 9), or with pot plants. After a certain time interval that is programed according to plant species and development stage, the nutrient solution is drained from the bench. The system is closed and the solution is recycled [11, 13].

Figure 9.

Stinging nettle in ebb and flow system.

Benches are covered with an impermeable rigid plastic profile that directs all water to the lowest point at one end of the bench where a siphon device (unpowered) drains nutrient water from the bench surface to a gutter below to return the water to the nutrient storage tank. The supply water is pumped from the water and nutrient management storage tank to each bench or group of benches, filling to a depth of 1–2 cm within 5 min and draining within 10 min for a total water cycle per bay of 15 min. Water and nutrient management system includes freshwater filter and disinfection, nutrient dosing device, storage tank with pump, sensors and controls to distribute irrigation water and nutrients. Mechanical filtering devices are required to remove particulates from the drainage water [14].

3.3.2 Advantages and disadvantages

Ebb and flow has many advantages such as root moisture optimization, water saving and fertilizer saving as compared to top sprinkler irrigation. The nutrient solution concentration may be reduced by up to 50% when compared to nutrient solutions for top sprinkle irrigation, with no detrimental effects on plant growth and quality. Subirrigation systems improve the uniformity and quality of bell pepper and tomato if grown with minimal nutrient and drought stress. When used for potted plants grown on concrete floor, some specific advantages of ebb and flow include: elimination of manual watering, flexibility in design of internal transport of potted plants, heating the root zone with low temperature water, and reducing bacterial and fungal diseases because of cultivation surfaces that were easy to clean and disinfect between cultivation cycles [14].

3.4 Floating hydroponics

The floating hydroponics was first applied in the production of tobacco seedlings, and today they are used efficiently in the production of vegetable seedlings and in the cultivation of leafy vegetables and herbs. It is important to emphasize that in the cultivation of seedlings, the solution is not aerated to prevent the root growth of plants outside the container pot [4].

In floating hydroponics plants are grown in a nutrient solution. The basic advantage of this system is that plants provide access to water, and macro- and micronutrients in the form of ions and oxygen over 24 hours, which they can optimally use during all stages of growth. This results in faster growth and earlier harvesting, which provides more production cycles throughout the year and higher yields [11, 13].

This hydroponic system consists of shallow pools filled with a nutrient solution on which Styrofoam plates or containers with plants float. The nutrient solution is raised capillary through the openings of the pot of containers or the slit of the plates to the substrate in them, that is, to the root of the plant. Styrofoam containers can have a different number of pots, and the plates can be of different dimensions, depending on the type of vegetables and the purpose of cultivation, respectively, whether leafy vegetables are grown due to young leaves for cutting or due to rosette or head. Container pots or slots on plates are filled with perlite or some other substrate into which the seeds of vegetables or herbs are sown (Figure 10).

Figure 10.

Seed sown in Styrofoam plates filled with perlite.

3.4.1 Greenhouse preparation and pool construction

The most demanding part of the work in floating hydroponics growing is preparing the terrain for pool construction, and includes precise straightening, with minimal drop along the greenhouse to keep the water level in all parts of the pool uniform. To allow a simpler pool emptying, it is sufficient to ensure a pool drop of 0.1%. If the surface of the terrain is rough, it is recommended to apply the sand in a layer of 2.5 to 5 cm before rolling and final straightening. Due to the good drainage under the pool, the level of terrain subjected for floating hydroponics construction should be raised 10 to 15 cm above the level of the surrounding terrain. The production surface of the pool, that is, its width and length, depends on the dimensions of the greenhouse and floating Styrofoam plates or containers for growing leafy vegetables. It is very important that the surface of the entire pool is completely covered with Styrofoam plates to prevent the development of algae that cannot develop without light, and which pollute the nutrient solution and create unfavorable conditions for growing vegetables. The pool frame height should ensure a nutrient solution depth of 20–25 cm and the floating of Styrofoam plates with plants (Figure 11).

Figure 11.

Floating hydroponics.

Agrotextile is first laid on the aligned soil, followed by PE-film of 0.5 mm thick, with complete frame coverage. At the pool bottom, a pipe system for occasional replenishment and daily circulation of the nutrient solution (to enrich the solution with oxygen) is placed. The nutrient solution is gradually added to the pool depending on its consumption, and the transpiration of the plants, respectively. For the entire production of leafy vegetables, it is also recommended to set up a pipe system to maintain the required nutrient temperature [4].

3.4.2 Growing technology

Leafy vegetables (Figure 12) harvested by cutting in the developed phase of 5 to 6 leaves (baby leaf) sown in Styrofoam plates (96 × 60 × 2.7 cm), with narrow conical slits filled with perlite of coarse granulation (0 to 6 mm). Sown plates are covered with finer perlite, moistened with water and stacked on each other until seed germination, when the plates are laid in pools filled with aerated nutrient solution. Optimal conditions for germination (temperature from 18 to 20°C and relative humidity around 95%) are provided in the germination chamber [4].

Figure 12.

Growing lettuce in floating hydroponics.

If leafy vegetables is grown for harvest of rosettes or heads, seeds are sown into rockwool plugs (cubes) 3 × 3 cm. Cubes with seedlings are placed in lattice pots, in holes (planting sites) distanced 20 × 20 cm in Styrofoam plates [11]. The plates with seedlings are laid in pools filled with a nutrient solution of a certain chemical composition and optimal temperature.

In this hydroponic technique, plants are constantly absorbing a nutrient solution, especially at higher air temperatures when transpiration is more intense, so the level of the solution decreases and it is necessary to ensure a pool supplement. The pH and EC values, the amount of dissolved oxygen and the nutrient solution temperature should be measured daily, and the nutrient solution composition by chemical analysis should be done every 2 weeks. The optimal pH value of the solution is from 5.8 to 6.2, while the EC value in leafy vegetable cultivation should be in the range between 2.5 (lettuce, lamb’s lettuce) and 3.2 dS/m (arugula). The availability of nutrients for plant is affected by the pH value and temperature of the nutrient solution and the amount of dissolved oxygen in the solution. The recommended temperature of the leafy vegetable growing solution should be from 21 to 23°C, while the optimal amount of dissolved oxygen is 4 to 9 mg/L [13, 15]. If the solution temperature is higher, the ability of the solution to retain oxygen decreases and the breathing of the roots is more intense and oxygen consumption is higher. Lack of oxygen in the nutrient solution (below 3 mg/L) results in less root permeability to the water so the plant cannot adopt nutrients in the required amount, and toxin accumulation can occur. Plant growth is slower and plant damage and leaf chlorosis are possible. Lowering the solution temperatures too high will ensure that larger amounts of oxygen are retained and root respiratory is reduced [16].

The length of the vegetation from sowing to harvest depends on the type of leafy vegetables and growing conditions, and the equipment of the protected area (side and roof ventilation, heating and shading equipment and supplemental lighting, nutrient solution heating and cooling system). Lettuce, lamb’s lettuce, endive and chicory are harvested once, while arugula and herbs can be harvested repeatedly. However, the vegetation tip should not be damaged during the first harvest, so the plants could grow again. The annual yield of arugula and lamb’s lettuce in floating hydroponics may be 40 to 50% higher than the yield in the case of soil grown in greenhouse [17, 18].

After a year-round production period, the pools are cleaned from the rest of the nutrient solution, perlite particles and organic matter, than disinfected and prepared for a new year-round cycle with the preparation of a nutrient solution, filling the pool and continuous sowing and harvesting.

4. Water and nutrient solution

4.1 Water quality

Water is the basis of any nutrient solution and therefore, it is necessary to provide sufficient amounts of quality water. High water quality is determined by the low concentration of dissolved substances, especially salts. The higher the water quality, the easier it is for producers to formulate an optimal nutrient solution. If the water quality is lower, more water is needed to dissolve the nutrient salts in open systems, that is, to remove excess salt from closed systems. Low quality can be supplemented by more water [4].

The quality of water should be taken into account at each beginning of the production season in the greenhouse since low-quality water is not usable and is expensive to “process” by filtration and/or reverse osmosis. Quality primarily depends on the available water source (rainwater, surface water-treated waste water and ground water). Rainwater is one of the best sources regarding quality. Before water can be used, it must be analyzed to determine the basic level of all minerals and ions (Ca²⁺, Mg²⁺, SO₄²⁻, HCO₃⁻, Na⁺, Cl⁻,) present and the pH and alkalinity. Without this information, it will be difficult to prepare the optimal nutrient solution [19, 20]. Water quality depends on the concentration of dissolved substances, and the presence of microorganisms such as algae, fungi and bacteria, and certain sediments. The overall analysis should show anions and cations, and special attention should be paid to salinity, alkalinity, and excessive concentrations of sodium, sulfate, and chloride. When using a drip irrigation system, high water quality is required to avoid possible interference by clogging the droppers with iron and manganese [20].

The required amount of water is mainly determined by microclimatic conditions and a leaf surface [4], which also affects the optimal composition of a nutrient solution [6] and EC value [19]. Under conditions of high humidity, low light and low temperature, water consumption can be very low. It is very important to know how to estimate the maximum amount of water used when the irrigation system is constructed and installed. The amount of water that plants consume is caused by the degree of growth of the plant, solar radiation, relative humidity and air movement.

Salinity is the amount of all dissolved salts quantified as water electrical conductivity (EC value), and is expressed in mS/cm or dS/m. An important assumption is that the EC value of the spring water should be below 1 dS/m. In some cases, the use of water with higher EC value is possible for so long while ions, which cause high EC-value, are used as plant nutrients. Even then, the concentration of these ions should not be excessive. Water with EC value 0.75–2.25 dS/m has slight to moderate restriction in use, while with >2.25 it has severe restrictions. The use of salted water in hydroponic cultivation in arid areas results in a slightly lower yield of cultivated crops, but therefore of excellent quality. Harmful effects on plant growth are caused by water and salinity stress. The maximum acceptable level of Na⁺ in the nutrient solution varies between 1 and 8 mmol/l, while the maximum acceptable Cl⁻ level in the root zone is 0.2–0.5 mmol/l higher than the maximum acceptable level for Na⁺ [19, 20, 21].

4.2 Nutrient solution preparation and distribution

In addition to water, nutrient salts or water-soluble complex fertilizers and acids are necessary to prepare a nutrient solution. The advantage of nutrient salts is that they represent high-purity chemical compounds composed of two to three nutrients. Complex water-soluble fertilizers most often contain nitrogen phosphorus, potassium and magnesium with the addition of microelements, which means that when correcting the composition of the nutrient solution, it is not possible to change the concentration of only one nutrient than to change all the concentrations of all nutrients found in the fertilizer [4]. Acid (nitric or phosphoric) needs to be added to the nutrient solution to lower the pH value of water (7.2 to 7.5) to optimal for hydroponic cultivation, which is between 5.5 and 6.8 [8, 17, 19, 21], although values between 5.0–5.5 and 6.5–7.0 may not cause problems in most crops [22, 23]. The EC value measured in fresh nutrient solution ranges from 1.5 to 3 dS/m [1, 8]. Lieth and Oki [24] stated that EC in soilless production may vary between 0 and 5 dS/m. It has been advised to maintain the EC below 3 dS/m to assure rapid plant growth, but this is impossible if the water is high in dissolved salts, and the addition of nutrients will raise EC to higher value than 3 dS/m.

The preparation of fresh nutrient solution is performed using a dosatron, mixer or fertigation unit (Figure 13) depending on the greenhouse. Regardless of the hydroponic cultivation technique, the finished nutrient solution is prepared from 100-fold concentrated solutions in relation to the concentration of the solution that is brought to plants by the system. Therefore, in each hydroponic production there are at least three tanks for concentrated solution [11]. Two tanks are filled with different stock solutions to separate calcium from sulfate and phosphate fertilizers, thereby avoiding precipitation of low-soluble compounds. The third tank contains a solution of nitric or phosphoric acid, which serves to regulate the solution pH value, by neutralization of HCO₃⁻ ion [19]. An equal volume of stock solutions used for fresh solution preparation is necessary in order to avoid nutrient misbalance. The volume of acid used depends on the water pH and the desired pH value of the nutrient solution [4].

Figure 13.

Fertigation unit.

In modern hydroponic growing systems, the nutrient solution parameters (oxygen concentration, temperature, pH and EC) are automatically controlled by a computer system that uses special sensors. The software sets the target values, and the fertigation unit measures water parameters and compares them with target values to add proper volumes of concentrated solutions and acid until the target values are reached. Additionally, probes are immersed in the growing substrate or in nutrient solution to collect data in root zone. The data is transmitted in real time to the cloud, from where it can be read at any time via a mobile app or computer. In this way, a faster response is possible when the parameters of the nutrient solution need to be corrected, which undoubtedly has a positive effect on the success of the cultivation [25].

Lieth and Oki [24] stated that nutrient solution in hydroponic growing systems could be delivered to plants by overhead, surface or subsurface irrigation. However, the dominant way of irrigation is surface, particularly drip irrigation in substrate grown crops. Nutrient solution is delivered by drippers, pinned or laid on the upper side of substrate. One of the most significant problems of drip irrigation is dripper clogging, mechanically or chemically, directly related to the quality of irrigation water and its physical, chemical and microbiological properties. Therefore, a water quality analysis should be performed before installing the drip irrigation system. The filtering site must certainly be an integral part of the drip irrigation system [4].

4.3 Nutrient solution composition

Although there are no significant differences in the nutrient solution composition among crops, crops can vary significantly in the absorption of individual nutrients, especially in certain parts of the vegetation. Nutrient absorption is affected by many abiotic (air and substrate temperature and humidity, light intensity and CO₂ concentration) and biotic (growth and development phase, fruit load and pest presence) factors. However, for more or less standardized growing conditions on substrates, a strong correlation between fresh fruit yield and nutrient absorption has been established [4, 23]. Contrarily, Sonneveld and Voogt [19] and Savvas et al. [26] quote that specialized nutrient solution for each greenhouse crop or even for developmental stage is available. Use of this kind of solution is optimal when nutrient uptake ratios are similar with the relative proportions between the same nutrients in fresh solution. This principle should be strictly followed in closed hydroponic systems to avoid nutrient accumulation and/or depletion. Vox et al. [27] stated that the more concentrated nutrient solutions are used for fast-growing crops, such as vegetables, while for ornamental plants and strawberry lower nutrient concentrations are normally used. Plenty of different nutrient solution formulas have been published and some of them are summarized in Table 2.

	Tomato	Pepper	Cucumber	Lettuce*	Strawberry	Seedlings
Macronutrients, mmol/L
NO3^−	13.75–16.00	15.50–16.00	16.00	16.00–19.00	11.25–12.00	15.00–16.75
H2PO4^−	1.25–1.50	1.25–1.75	1.25–1.50	1.50–2.00	1.00–1.25	1.50–2.50
SO4^2−	3.75–4.40	1.75	1.375–1.50	1.125–2.00	1.50	2.50–3.00
NH4^+	1.20–1.25	0.75–1.25	1.25	1.00–1.25	1.00	1.25–2.00
K^+	8.75–9.50	6.50–7.00	8.00	9.50–11.00	4.80–5.50	6.00–8.00
Ca^2+	4.25–5.40	5.00	4.00	4.50	3.50–3.60	4.00–5.00
Mg^2+	2.00–2.40	1.50	1.375–1.50	1.00	1.35–1.50	3.00–3.50
Micronutrients, μmol/L
Fe^3+	15.00	15.00	15.00	40.00	20.00–30.00	25.00
Mn^2+	10.00	10.00	10.00	5.00–7.00	10.00	10.00–15.00
B^3+	30.00	30.00–35.00	25.00	30.00–40.00	10.00–15.00	35.00
Zn^2+	5.00	5.00	5.00	4.00–7.00	7.00	5.00
Cu^2+	0.75	0.75–1.00	0.75	0.75–1.00	0.75	1.00
Mo^6+	0.50	0.50	0.50	0.50–1.00	0.50	0.50
EC, dS/m	2.30–2.60	2.20	2.20	2.20–3.20	1.60	2.20–2.60
pH	5.5–6.2	5.5–6.2	5.5–6.2	5.8–6.2	5.5–6.2	5.5–6.2

Table 2.

Nutrient solution composition in tank for greenhouse crops according to different authors [9, 19, 21, 22, 23].

^*

Lettuce and leafy vegetables.

4.4 Nutrient solution sterilization and recirculation

According to the use of a nutrient solution, hydroponic systems are divided into the following: open ones where once used nutrient solution is not used again in the system but is drained into evaporation channels or used to fertilize soil-produced crop; and closed ones where drained nutrient solution is passed through a sterilization system, supplemented with a fresh nutrient solution and reused [6]. If the hydroponic system is open, the irrigation system should ensure the amount of nutrient solution or water, which will maintain or reduce the salt concentration. Due to that, the part of supplied nutrient solution should be drained from the substrate. In practice, the drained solution volume varies between 10 and 30%, depending on the quality of the water and/or on the crop sensitivity to salinity [4, 6, 23, 27]. In closed systems, salt accumulation in the root zone is more common, resulting in reduced yields. To avoid this kind of problem, the nutrient concentrations and injection rates of fresh and recycled nutrient solution should be monitored and regulated. Also, irrigation with freshwater, which washes away excess nutrients, could be applied.

Root’s zone in hydroponic systems needs to be pathogen free to efficiently produce good-quality products [2]. Due to hydration, there is a high potential for the rapid spread of root diseases [28], especially in closed hydroponic systems. Closed hydroponic systems reduce or limit the runoff of drained nutrient solution into the environment [3], so in closed systems the drained solution should be filtered and disinfected before it is recycled, to avoid spread of pathogens [29]. There are five main methods of pathogen control in these systems: heat, filtration, chemical, radiation and biological control. Sterilization (heat, oxidizing chemicals and UV-radiation) and membrane filtration methods are generally very effective, but may adversely affect beneficial microorganisms in the recirculated solution (Figure 14). Slow filtration and microbial inoculation methods are less disruptive of the microflora, but effectiveness may vary with the pathogen. Microbial inoculation is perspective in targeted disease suppression, but still just a few products are commercially available [28, 29].

Figure 14.

UV-sterilization unit.

From a sustainability perspective, it is important to recirculate the nutrient solution to minimize water consumption and residuals to dispose into environment. However, it is not always possible to implement systems that balance the consumption of natural resources, energy and financial costs [2]. Besides the environmental benefits, closed hydroponic systems can provide higher economic profits, since they reduce the quantity of water and fertilizers used during production, and they are more efficient in using water and nutrients than open systems, respectively [30]. In their research, De la Rosa-Rodríguez et al. [30] achieved 26.9% (13.5 kg) higher tomato yield per liter of water in closed than in the open system.

5. Conclusions

Hydroponic growing systems include plant growing techniques without soil, on inert substrate (soilless culture), or without substrate (water culture). Inert substrates used are mainly of inorganic or organic origin. The advantage of organic substrate use is their sustainability with no or minimal impact to the environment, so they could be recommended. Water culture techniques represent closed hydroponic systems, which are more efficient in water and fertilizer use compared to open systems (mostly on substrates), and especially compared to soil production. Due to high-quality yield regardless of grown crop, hydroponic systems could be a way to increase the food production sustainability in the future, characterized by population growth, climate changes and the reduction of natural resources.

Future development of hydroponics through research and particularly through application should be focused on vertical farming and plant factories, which will ensure continuous production increase with sustainable use of resources by controlled environment agriculture.

Conflict of interest

The authors declare no conflict of interest.