**3. Hydroponic cultivation of vegetables and fruits**

Hydroponics is an agrotechnology for plant cultivation outside the nutritious solution and in nutrient solution, becoming a promising alternative for the diversification of agribusiness. This system of production provides greater yield per area, lesser incidence of pests and diseases, greater ease of execution of cultivation practices, better programming of production, and shorter cycles, due to better environmental control [1].

Among the different hydroponic systems that do not use substrates, the Nutrient Film Technique (NFT) system is the most widespread in Brazil and worldwide [2]. This technique favors the continuous or intermittent circulation of the nutrient solution in cultivation channels, which may have varying dimension sizes and made by different materials, poly(vinyl)chloride (PVC), polyethylene, polypropylene, asbestos, and masonry being the most widely used [3]. Currently, hydroponic cultivation has great importance in several countries, such as Holland, the United States, France, Spain, Japan, and Israel among others. However, one must consider the cost of implementation and the high level of technology required in this system.

The most planted vegetables in this system are lettuce, arugula, and tomato. Other vegetables are restricted to smaller areas, such as cress, parsley, peppers, strawberries, and melons. In general, hydroponic crops require permanent monitoring, mainly as regards the uninterrupted supply of electricity and the control of the chemical and physical characteristics of the nutrient solution [4].

All essential nutrients must be supplied at levels compatible with the requirements of each species, according to the development stage [5]. In order to minimize experimental errors in the analysis of symptoms induced by excess or deficiency nutrient in nutrient solution, it is recommended to use minimum concentrations [6]. The definition of these minimum concentrations should be studied in view of the genotypic, environmental, and demand differences associated with the different phases of development. In general, there is a tendency to reduce the ionic concentration of the nutrient solution in commercial hydroponic crops, especially in environments whose temperature, luminosity, and relative humidity are high in the hottest seasons of the year [7].

in forms unavailable to plants. If the pH is lower than 5.5, magnesium, calcium, and molyb-

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Using a conductivity meter, we established the ability of the nutrient solution to conduct the electric current. As this capacity changes according to the content of the mineral salts, the value of the electric conductivity allows estimating the total concentration of the nutrients in the solution. The higher the EC, the higher the content of mineral salts in the nutrient solution. Normally, when the electrical conductivity is reduced to a certain level of the initial solution (about 30–50%), it is advisable to replace it. The level at which EC value should be maintained

The pH and EC characteristics of the water used and then of the nutrient solution (water and nutrients diluted in it) should be those indicated for each type of crop. In theory, pH may range from 0 to 14, but in practice, extreme values are incompatible with plant life. Second to the pH values, the quality indexes for the water used in hydroponics can be clas-

In preparing the solutions, fertilizers contain macronutrients that must be weighed in the correct amount, indicated by the chosen formulation, then diluted one by one in the tank with water to approximately two-thirds of its capacity. Posteriorly, added the micronutrients are in the form as concentrated solution, in finally, the solution is added with chelated iron. The main fertilizers used for the preparation of nutrient solutions are found

**Table 4** presents the adapted solutions [3] for use in the preparation of the nutrient solution

denum, in particular, have reduced availability, as shown in **Figure 2**.

varies according to climate and culture.

in the NFT system, for leafy vegetables and fruits.

**Figure 2.** Relationship between pH and element availability.

sified (**Table 1**).

in **Tables 2** and **3**.

It is worth mentioning that the rational use of fertilizers, in addition to reducing costs and guaranteeing production quality, minimizes contamination of the environment and its consequences. These are the eutrophication of surface and groundwater and the accumulation of high levels of nitrate in the groundwater and plants [8]. In the handling of the nutrient solution, factors such as temperature (optimum levels around 24°C), pH (suitable values between 5.5 and 6.5), and electrical conductivity (EC) of the nutrient solution (optimum range between 1.5 and 4.0 dS m−1) should be monitored and controlled periodically [9].

### **4. Nutrient solution management**

One of the basic principles for plant production is the provision of all the nutrients the plant requires [9]. In this environment, when nutrient imbalance occurs, production will be limited. For the adequate development of the cultures, macro-and micronutrients that are essences for the growth and production of the plants are necessary, which are presented in **Figure 1**.

That division, between macro- and micronutrients, takes into account the amount that the plant requires of each nutrient for its cycle, all being equally important in nutritional terms. In this way, it is important to observe that the total amounts absorption are of secondary importance since, in hydroponic cultivation, the concentration of nutrients in the growth medium is maintained constant, which does not occur when cultivated in the nutritious solution.

The optimum pH values for the nutrient solution are between 5.5 and 6.5, being important to keep these values in the solution to favor the availability of nutrients to the plants. If the pH is above 6.5, elements such as phosphorus, manganese, and iron begin to precipitate, remaining


**Figure 1.** Chemical elements used in plant production [3].

in forms unavailable to plants. If the pH is lower than 5.5, magnesium, calcium, and molybdenum, in particular, have reduced availability, as shown in **Figure 2**.

especially in environments whose temperature, luminosity, and relative humidity are high

48 Potassium - Improvement of Quality in Fruits and Vegetables Through Hydroponic Nutrient Management

It is worth mentioning that the rational use of fertilizers, in addition to reducing costs and guaranteeing production quality, minimizes contamination of the environment and its consequences. These are the eutrophication of surface and groundwater and the accumulation of high levels of nitrate in the groundwater and plants [8]. In the handling of the nutrient solution, factors such as temperature (optimum levels around 24°C), pH (suitable values between 5.5 and 6.5), and electrical conductivity (EC) of the nutrient solution (optimum range between

One of the basic principles for plant production is the provision of all the nutrients the plant requires [9]. In this environment, when nutrient imbalance occurs, production will be limited. For the adequate development of the cultures, macro-and micronutrients that are essences for the growth and production of the plants are necessary, which are presented in **Figure 1**.

That division, between macro- and micronutrients, takes into account the amount that the plant requires of each nutrient for its cycle, all being equally important in nutritional terms. In this way, it is important to observe that the total amounts absorption are of secondary importance since, in hydroponic cultivation, the concentration of nutrients in the growth medium is

The optimum pH values for the nutrient solution are between 5.5 and 6.5, being important to keep these values in the solution to favor the availability of nutrients to the plants. If the pH is above 6.5, elements such as phosphorus, manganese, and iron begin to precipitate, remaining

maintained constant, which does not occur when cultivated in the nutritious solution.

1.5 and 4.0 dS m−1) should be monitored and controlled periodically [9].

in the hottest seasons of the year [7].

**4. Nutrient solution management**

**Figure 1.** Chemical elements used in plant production [3].

Using a conductivity meter, we established the ability of the nutrient solution to conduct the electric current. As this capacity changes according to the content of the mineral salts, the value of the electric conductivity allows estimating the total concentration of the nutrients in the solution. The higher the EC, the higher the content of mineral salts in the nutrient solution. Normally, when the electrical conductivity is reduced to a certain level of the initial solution (about 30–50%), it is advisable to replace it. The level at which EC value should be maintained varies according to climate and culture.

The pH and EC characteristics of the water used and then of the nutrient solution (water and nutrients diluted in it) should be those indicated for each type of crop. In theory, pH may range from 0 to 14, but in practice, extreme values are incompatible with plant life. Second to the pH values, the quality indexes for the water used in hydroponics can be classified (**Table 1**).

In preparing the solutions, fertilizers contain macronutrients that must be weighed in the correct amount, indicated by the chosen formulation, then diluted one by one in the tank with water to approximately two-thirds of its capacity. Posteriorly, added the micronutrients are in the form as concentrated solution, in finally, the solution is added with chelated iron. The main fertilizers used for the preparation of nutrient solutions are found in **Tables 2** and **3**.

**Table 4** presents the adapted solutions [3] for use in the preparation of the nutrient solution in the NFT system, for leafy vegetables and fruits.

**Figure 2.** Relationship between pH and element availability.


**Table 1.** Quality indices for water used in hydroponics [3].


**5. Potassium salinity in nutrient solution and its effects on** 

**Table 4.** Values of mineral nutrients in nutrient solutions used for the NFT system.

**Fertilizers Nutrient %**

H3

Na<sup>2</sup> BO2 · 10H2

CuCl2 · 2H2

MnSO4 · H2

FeCl3 · 6H2

(NH<sup>4</sup> ) 6 Mo7 O24 · 4H2

NaMoO<sup>4</sup> · 2H2

CuSO4 · 5H2

MnSO4 · 7H2

ZnSO4 · 7H2

**Culture N–NO3**

**mg L−1**

Na<sup>2</sup> B4 O7 · 5H2

Potassium is an essential nutrient for all living beings, playing a key role in photosynthesis, which is the transformation of light energy into chemical energy (ATP and NADPH). As all vital plant functions depend directly or indirectly on ATP and NADPH, the influence of K on plant

**B Cu Fe Mn Mo Zn**

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BO3 17.0 — — — — —

ZnCl2 — — — — — 48.0

O 11.0 — — — — —

O — 37.0 — — — —

O — — — 43.0 — —

O — — 21.0 — — —

O — — — — 54.0 —

O — — — — 34.0 —

O — 25.0 — — — —

O — — — 32.0 — —

O — — — — — 20.0

O 14.0 — — — — —

**<sup>−</sup> P K Ca Mg S B Cu Fe Mn Mo Zn**

MoO3 — — — — 66.0 —

Lettuce 238 62 426 161 24 32 0.3 0.05 5.0 0.4 0.05 0.3 Tomato 169 62 311 153 43 50 0.2 0.03 4.3 1.1 0.05 0.3 Pepper 152 39 245 110 29 32 0.3 0.03 3.7 0.4 0.05 0.3 Eggplant 179 46 303 127 39 48 0.3 0.05 3.2 0.6 0.05 0.3 Cucumber 174 56 258 153 41 54 0.2 0.03 4.3 1.1 0.05 0.3 Melon 170 39 225 153 24 32 0.2 0.03 2.2 0.6 0.05 0.3 Strawberry 125 46 176 119 24 32 0.2 0.03 2.5 0.4 0.05 0.3

**Table 3.** Main sources of micronutrients used for the preparation of nutrient solutions.

**metabolism**

**Table 2.** Main sources of macronutrients used for the preparation of nutrient solutions.


**Table 3.** Main sources of micronutrients used for the preparation of nutrient solutions.

**Fertilizers Nutrient %**

**Table 1.** Quality indices for water used in hydroponics [3].

NH<sup>4</sup> H2

HCO<sup>−</sup> 3

Na<sup>+</sup>

Cl<sup>−</sup>

SO2 4

NH<sup>4</sup> H2

Ca(H2 PO4 )2 H2

KH2

NH<sup>4</sup>

Ca(NO<sup>3</sup> )2 4H2

Mg(NO<sup>3</sup> )2 6H2

(NH<sup>4</sup> )2

CaSO4 · 2H2

MgSO4 · 7H2

K2

K2 SO4 · **N P K Ca Mg S**

KCl — — 49.8 — — —

**Units Good Acceptable Maximum limit**

50 Potassium - Improvement of Quality in Fruits and Vegetables Through Hydroponic Nutrient Management

EC mS cm <0.75 <0.75–1.50 2.0 pH 6.50 6.80 7.5

mmol L−1 1.60 3.30 7.5

mmol L−1 0.87 1.30 2.61

mmol L−1 1.14 1.71 2.86

 mmol L−1 0.83 1.26 2.08 Ca+2 mmol L−1 6.50 10.00 14.00 Fe μmol L−1 — — 0.08 Mn μmol L−1 — — 0.04 Zn μmol L−1 — — 0.02 B μmol L−1 — — 0.03

O — 24.6 — 15.9

PO4 10.0 21.8 — — — —

PO4 11.0 21.8 — — — —

PO4 — 22.8 28.7 — — —

NO<sup>3</sup> 34.0 — — — — —

KNO<sup>3</sup> 13.0 — 36.5 — — — NaNO<sup>3</sup> 16.0 — — — — —

O 15.0 — — 20.0 — —

O 7.0 — — — 6.0 —

SO4 20.0 — — — — 24.0

SO4 — — 41.5 — — 17.0

O — — — 21.4 — 17.0

O — — — — 9.7 13.0

2MgSO4 — — 18.2 10.8 — 22.0

**Table 2.** Main sources of macronutrients used for the preparation of nutrient solutions.


**Table 4.** Values of mineral nutrients in nutrient solutions used for the NFT system.
