**2. Soil–plant–water relations**

Plants terminate new developments in the above ground parts by minimizing the use of water and carbohydrates in the stem and help the root develop more when they cannot get enough water from the soil, and if the situation lasts or the water in the soil is insufficient, plant activities stop completely. Nevertheless, there must be sufficient air in the soil for satisfactory root development. In the conditions where there is more water in the soil, the amount of air decreases as the spaces between the soil particles are filled with water. Therefore, it is very important to balance the amount of water and air in the root zone of the plant in order to provide the best plant growth. Thus, increasing plant production depends on knowing the relations between soil, plant, and water.

#### **2.1 Soil properties**

Knowing the physical and chemical properties of the soil is fundamental to successful irrigated agriculture. Values such as field capacity and wilting point of a soil are affected by parameters such as soil texture and structure, and organic matter content [5]. Soil moisture availability varies with the amount of water in the soil and the type of the soil. Knowing soil texture and other specific characteristics

*Principles of Irrigation Management for Vegetables DOI: http://dx.doi.org/10.5772/intechopen.101066*

is essential for planning and using an irrigation system. Since sandy soils do not have much specific surface area, they cannot hold much water. Therefore, the field capacity value of a sandy soil can be as low as 10%. Clay particles stacked on top of each other in the form of plates have a large surface area, so the field capacity of clay soils can be over 40%. As a result, clay soil does not have to be irrigated as frequently as sandy soil. Thus, for scheduling purposes it is essential to know the texture of the soil [6]. In a soil at field capacity, a mineral soil with a high organic matter content has more water holding capacity than a mineral soil with a lower organic matter content [7].

#### **2.2 Water quality**

Irrigated agriculture depends on a sufficient supply of high-quality water [8]. Agricultural water quality is also determined based on the effect of water on plant quality, yield and soil properties [9]. Properties that determine water quality for transplant irrigation are: alkalinity, electrical conductivity (EC), sodium absorption ratio (SAR), and elemental toxicities. The direct effects of irrigation water on plant growth arise either due to the creation of high osmotic conditions from plant sap or the presence of phytotoxic compounds in the water [10].

Under the effect of high osmotic pressure, the water usage of plants decrease, which is lethal for the plants. Therefore, the total salinity content of irrigation water is extremely important [11]. Plants suffer more damage from salts in the early stages of their development compared to their ripening periods. This situation leads to either a decrease in yield or no yield at all. Negative effects of dissolved salts in water could be cessation of vegetative growth in the plant. It also appears in the form of reduced plant and seed development. In a study with broccoli plant [12], both irrigation water salinities and irrigation water amounts were effective on plant yield, while only salt levels were effective on dry matter values. There has been a significant decrease in the yield from 6 dS/m, and the increase in the amount of irrigation water has decreased the yield [12]. Not all vegetables respond to salinity in a similar way; some vegetables can give acceptable yields compared to other plants at much higher soil salinity.

pH has chemical and biological importance in water as an increase or a decrease in pH affects the toxicity of some compounds [13]. The quality of irrigation water in the root medium has a direct impact on the pH of the growing media and the nutrient availability. When the proportional amount of Na ion is high, the physical properties of the soil change negatively. Excess Na in irrigation water causes soil dispersion and structural dispersal. For this reason, the Na + content of water should be calculated using % Na SAR values.

The effects of irrigation water quality on soil and plants vary depending on the physical and chemical properties of the soil, the salt resistance of the plant grown, the climate of the region, the irrigation method applied, the irrigation interval and the amount of irrigation water [14].

Due to the depletion and insufficiency of existing water resources, there is a need to seek alternative water resources. In many countries, treated wastewater is considered as an alternative irrigation water. However, heavy metals, salts and harmful chemicals can be lethal especially in raw vegetables when not managed carefully in irrigation. To determine whether the treated wastewater is suitable for irrigation, the total concentration and electrical conductivity of the dissolved substances in the water, the sodium ion concentration, the ratio of sodium ion concentration to other cations, the concentration of boron, heavy metals, other potentially toxic substances, the total concentration of Ca++ and Mg++ ions under some conditions, total solids, organic matter load, and amount of floating matter such as oil-grease and pathogenic organisms should be examined. On the other hand, wastewater irrigation for vegetables has serious concerns since most of the vegetables are consumed raw. Moreover, in most countries using wastewater as an irrigation water for the raw eatable crops is forbidden by law.

#### **2.3 Important soil moisture levels for irrigation**

In irrigation applications, it is necessary to know the amount of moisture held in the soil at certain tensions. These reference soil moisture amounts are called soil moisture constants. Major soil moisture constants in terms of irrigation are saturation point, field capacity, and wilting point. During and immediately after irrigation, all the pore space in the soil is filled with water and the soil becomes saturated. As a result of water molecules filled the pores in the soil, there is a little air in the soil, and for most crops if the soil stays saturated the crop will be damaged due to this lack of air for the roots to breathe. If there are no drainage problems, the water in the soil will drain away under gravity following irrigation, leaving space for air in the soil's pore space [6]. In many types soil, after a rain or irrigation, the water immediately starts draining deeper into the soil. After 1 or 2 days, the water content in the soil will reach, Zaten after 1-2 day diyerek zaman vermissin. To larger areas underneath the surface, a nearly constant value for a particular depth in question. This somewhat arbitrary value of water content, expressed as a percentage, is called the field capacity [15]. Wilting point also called as the permanent wilting point, can be defined as the amount of water per unit weight or per unit bulk volume in the soil, expressed in percentage, that is held so tightly by the soil matrix that roots cannot absorb this water and a plant will wilt. Therefore, in practice, plants only benefit from moisture between field capacity and wilting point. The interval between the field capacity and the wilting point is called the usable/available water holding capacity. As it is known, the most important factor limiting plant growth in arid and semi-arid climates is the lack of available water in the root zone [16]. For this reason, irrigated agriculture is an inevitable necessity in arid and semiarid areas.

The objective of irrigation is to allow the soil moisture to reduce to a safe limit and then to irrigate the soil to bring it back to field capacity. The interval between irrigation will thus depend on the available moisture in the soil and the rate at which the soil water is abstracted by the crop [6].
