**6. Dissolved oxygen levels in nutrient solution**

Maintaining enough dissolved O2 in a nutrient solution in a hydroponic system is crucial for plant health. Oxygen availability to roots grown in soilless culture can become limiting in case O2 demand exceeds O2 supply, inducing a reduction in root growth rate, ion, and water uptake, eventually reducing plant production [84]. Plants grown in hydroponic systems can quickly deplete the dissolved O2 in the nutrient solution resulting in poor root aeration, especially when greenhouse temperatures are high, **Table 5** shows O2 solubility in pure water at different temperatures. Jong et al. [86] noticed that cucumber growth was significantly affected by root-zone aeration. Roosta et al. [87] found improve eggplant growth with rising O2 levels in the nutrient solution in floating hydroponic cultures and higher O2 levels seemed to alleviate signs of ammonium toxicity among the tested plants. Root respiration also decreases when O2 supply in the root environment falls below a critical O2 concentration [88]. The sensitivity of roots to low O2 concentration depends on its effect on mitochondrial respiration because it supplies most of the energy required for root function. Reduction in O2 levels in the nutrient solution could lead to poor roots, an increase in the incidence of diseases and pests, and a reduction in plant growth. Oxygen around a plant's roots affects the beneficial microorganisms that provide protection from pathogens and improve nutrient uptake. Tomato plant roots would be


#### **Table 5.**

*Solubility of oxygen in pure water at various temperatures.*

much more susceptible to Pythium infection if root zone O2 dropped below 2.8 mg/L [89]. Dissolved O2 concentration, is strongly dependent on solution temperature and flow rate near the root zone, as well as on the growth rate of the crop, and may be influenced by the bacterial community present in the solution. The temperature has a direct relationship to the amount of oxygen consumed by the plant and a reverse relationship with dissolved oxygen from the nutrient solution. The consumption of O2 increases when the temperature of the nutrient solution increases. Consequently, it produces an increase in the relative concentration of CO2 in the root environment if the root aeration is not adequate [90]. For overcoming the limited oxygen exchange between the atmosphere and the nutrient solution in static deep water culture hydroponics, the nutrient solution is aerated by an air bubbler connecting with the pump to provide adequate root oxygenation [6]. Roots of loose-leaf lettuce grown in a floating raft hydroponic system were found to have a better condition with oxygen enrichment done in nutrient solution up to aeration pressure of 0.012 mPa and concentration of 600 ppm, with indicators of increasing length and total root surface area [68]. So, it is important to make sure the nutrient solution is properly aerated to maintain enough oxygen for the plant cells found in the root mass since this is crucial to the function of the plant's cells and the microbial world.

#### **7. Nutrient solution management**

An optimized and well-balanced supply of nutrients is a prerequisite for efficient use of the resources by hydroponically grown vegetables, not only to ensure a high yield but also to guarantee the quality of the edible tissues. In hydroponics, because of the limited nutrient-buffering capacity of the system and the ability to make rapid changes, careful monitoring of the system is necessary. The frequency and volume of the nutrient solution applied depends on the type of substrate, the crop and growth stage, the size of the container, the irrigation systems used, and the prevailing climatic conditions. Depending on the stage of plant development, some elements in the nutrient solution will be depleted more quickly than others and as water evaporates from the nutrient solution, the fertilizer becomes more concentrated and can burn plant roots. In hydroponics, nutrient management is very important and must be done as highly efficient as possible to improve productivity without harming the environment.

#### *Nutrient Solution for Hydroponics DOI: http://dx.doi.org/10.5772/intechopen.101604*

Nutrient management included- application the right fertilizers source (e.g., ammonium or nitrate as nitrogen source), balanced nutrient solution according to plants needs and according to plant growth stages and climatic conditions. The main principle of crop nutrient management is to prevent overapplication of nutrients, which prevents loss due to low yield from toxicities of some nutrients resulting from the unnecessary use of fertilizers. It was reported that the strong difference between the ion ratios presented in the nutrient solution and those absorbed by plants led to the accumulation of certain ions in the nutrient solution, which caused an imbalance of mineral elements in the nutrient solution and created more energy to absorb the suitable ions [6]. Recycling exhausted solutions may also represent an efficient strategy to prevent groundwater and environmental pollution. However, the main problem with the reuse of exhausted nutrient solutions is the shortage of some key macro and micronutrients [91] and their increased salinity [92] causing, in turn, problems for crops [93, 94]. Thus, it is very important to develop management practices/tools that reduce salinity in recycled solutions and/or minimize the physiological impact of salinity on plants. The salinity increase could be contrasted by treating the recycled water with appropriate osmotic systems, including forward and reverse osmosis.

In closed hydroponic systems, accumulation of potentially toxic organic compounds released by the roots of cultivated plants may occur and to overcome this issue, several treatment techniques have been proposed for root exudates degradation or removal. However, for the treatment to be effective, it should be able to remove root exudates without interfering with the inorganic mineral nutrients in the solution. As above-mentioned, the regulation of the solution flow rate in hydroponic production affects plant growth, which in turn affects crop yield and quality. The influence of nutrient solution flow rate on plant growth is related to the plants' physical environment. The flow of nutrient solution not only promotes nutrient ion diffusion but also increases the kinetic energy available to plant roots Therefore, adjusting the flow rate can improve plant yield and a reasonable flow pattern must be carefully selected. Because increasing the flow means increasing electricity consumption, it increases the cost of operation. Therefore, it is important to balance plant yield, nutrient management, and energy utilization. According to Baiyin et al. [95], determining the ideal flow rate for hydroponic production may help to increase yield. However, such a determination requires a specific analysis of each crop and growing environment. The hydroponic nutrient solution is the sole source of nutrients to the plant; therefore, it is imperative that a balanced solution, containing all the right plant nutrients, is applied.
