**3. Measuring soil moisture**

Accurately measuring soil moisture and evaluating the moisture change in the soil in the vegetable root zone are very crucial in irrigation applications. The monitoring of soil moisture is a standard way to determine when vegetable crops need to be irrigated. An effective irrigation in arid and semi-arid regions is achieved by monitoring the soil moisture and determining the soil moisture content correctly [17].

There are several methods due to the development of science and technology through the years. Gravimetric method, tensiometers, granular matrix sensors or the gypsum blocks, time domain reflectometry (TDR), frequency domain reflectometry (FDR), drill & drop soil moisture probe and neutron probe are commonly used techniques or practices.

Gravimetric method is a basic method in soil moisture measurement. It can be also used to compare different methods with one another as a standard calibration method [18]. In this approach, the moisture content of the soil is determined by

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

drying soil samples in an oven at 105°C to a constant weight and finding the amount of water lost. The moisture content is calculated by ratio as weight of water to the weight of dry soil. Generally, samples of 50 to 100 g of soil are enough in most field tests due to the large samples requiring longer drying times [18].

In other methods, different devices ranging from inexpensive simple moisture meters to much more expensive probe systems are used to measure soil moisture. These devices provide real-time monitoring of soil moisture. In the first group, water potential in soil are measured with tensiometers and the granular matrix sensors such as gypsum block and the watermark sensors. In the second group, soil moisture content considering the time or frequency of electronic pulse traveling between or returning to electrodes and capacitance sensors are measured with water content sensors. Resistance-type moisture sensors work by measuring the resistance between electrodes inserted into the soil [17].

Tensiometer is a device that measures moisture tension inside soil. It is widely used in the practice because it is inexpensive, needs no power supply, and provides direct and continuous readings [19]. They are inserted into the soil to different depths considering the effective root zone of the plants [20]. It has water-filled generally transparent looking tube with a porous ceramic at the bottom and a vacuum gauge at the top. The readings as a suction i.e. negative pressure or a potential are in kpa (kilopascals) or centibars. Tensiometer readings are not affected by the soil temperature or the osmotic potential of the water solution in the soil and work at low water retention tensions (0–85 kPa) which represent a small part of the entire range of available moisture [21].

Granular matrix sensors or the gypsum blocks with electrodes embedded in block of porous material are used to measure a resistance that reflects. The electrodes are connected to cables that extend to the soil surface for neyi read ediyor. Resistance? by a portable resistance meter providing small voltage. If water is present in the soil, the gypsum block gets wet, thus the resistance between electrodes decreases, while on the contrary the resistance increases as soil dries. Increased resistance shows an elevated tension in the soil. Therefore, resistance readings from devices are converted to actual water contents using a calibration curve later as the granular matrix sensors indirectly measure soil water tension using electrical resistance [22]. Being an inexpensive device can be seen as an advantage but it is stated that mistakes in measurements of moisture of wet soils occur frequently [19, 23].

Capacitance sensors, TDR and FDR are the techniques that consider dielectric property of the soil to measure moisture content [24]. These electromagnetic soil moisture sensors have improved throughout the last few decades considering size, cost and precision [22]. A capacitance sensor consists of two electrodes, which provide to be immersed in the soil and measure the dielectric constant that increases. Although they are inexpensive and user friendly, the common restriction for most of the capacitance sensors is that they provide measurements considering a very small soil volume, thus they do not reflect the situation in the soil away from the sensor. Therefore, they are more suitable for small volume container-grown vegetables in greenhouses. In addition, accuracy of capacitance sensors can be affected by many soil properties such as clay and organic matter contents, salinity level, bulk density, and temperature [22].

Soil moisture content is better estimated by determining the dielectric constant based on time domain reflection principle, frequency reflection principle and standing wave principle with the advanced devices. In this context, several types of soil moisture sensors have been developed as TDR and FDR [17, 25]. TDR device works according to the principle of determining the electrical conductivity (dielectric constant) value of a material based on the propagation speed of electromagnetic waves. Although it provides rapid and repeatable measurements with no health risks, unlike neutron probe technique, it is a complex and an expensive measurement equipment. It has also some disadvantages such as reflection loss in saline soils or wet soil increases conductivity [21]. Soil moisture sensors may also require calibration, and thus calibration equations to convert readings to volumetric water content are considered [22].

In the FDR with capacitance probes, by given voltage from two electrodes, soil moisture is determined under the assumption that dielectric constant of water is much higher than soil. Capacitance is measured from variation in frequency of a reflected radio wave or resonance. When the electrodes are given voltage, which induces the frequency oscillations with an oscillator to propagate an electromagnetic signal, at a certain point resonance occurs and soil moisture content is determined through this point. The accuracy and repeatability of the FDR are high, and the FDR probes give faster response time compared to TDR probes. Moreover, FDR is relatively inexpensive and has no health risks as well. However, calibration for the results needs to be done for each soil used. To obtain correct measurements, probes need to have good contact with the soil without air gaps and the moisture measurements in saline soils are generally not reliable [19, 21].

Soil moisture profile probes or drill & drop probes provide continuous soil moisture measurements from different depths over the entire length of the probe. Salinity and temperature sensors can also be found in the probe in addition to the moisture sensors. In this practice, the time taken for an electromagnetic wave to travel along a given length of a transmission line in the soil is measured. Soil dielectric properties are changed with moisture content in the soil. This leads to different electromagnetic wave travels rates in wet and dry soils. Thus, the soil moisture content can be estimated with this approach. This device might be a good option for fast, easy but short-term measurement for monitoring of the vegetable cultivated soils.

In neutron probe method which detects soil moisture using radioactive element, fast neutrons are continuously emitted from the neutron source to the soil environment during measurement. The probe device has a source and a detector. When fast neutrons collide with hydrogen atoms, they lose energy and decelerate. With increasing soil moisture content, the density of slow neutron clouds increases. The neutron meter determines the moisture content in the soil by determining the functional relationship between the density of the slow neutron cloud and the water molecules. Although it allows fast, reliable and repeated measurements at any soil depths, the neutron instrument is expensive, and has radiation hazard risk to health, thus it cannot be widely used [17]. Neutron probes and drill & drop probes can be effectively used in soils cultivated with deep root vegetables. A major restriction of these devices might be their expense for small farms.

#### **4. Water requirements of the vegetables**

Vegetable crops require more and frequent irrigation than other plants as they contain 60–90% water, and thus irrigation in arid and semi-arid regions plays a vital role in vegetable growth. The availability of sufficient water in soil is essential for good crop formation, growth, yield and quality in vegetable production. The application of frequent but low volumes of water for vegetable crop production has been proven to result in more-yield compared to few application [26].

Crops can experience water stress in two different ways, which are the water shortage (drought) and excess water (flooding, saturation) [27]. The excess water causes waterlogging in soils, and the symptoms are similar to the water deficit impeding the oxygen supply and respiration of roots and water uptake. Drought

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

stress occurs when atmospheric conditions cause permanent water loss through evaporation or transpiration. Under stress conditions the stomatal closure occurs with a reduction of net photosynthesis, and these responses depend on the severity and duration of stress and crop growth stage [28]. As a practical approach in controlling water stress level, the leaf photosynthetic activity can be monitored since measuring stomatal conductance or resistance of plant leaves indicate the severity of water stress. In drought stress; plant development is regressed, woody structure occurs, bloom early and growth of the leaf area, stem height and chlorophyll content reduce [29]. In addition, water plays a considerable role in the nutrition consumption of plants by dissolving nutrients in the soil. The encounter of a dry soil layer during the growing period of the vegetables will prevent the enlargement and development of the roots. However, there are many ways to manage drought stress such as mulching, use of plant growth regulators, anti-transpirants, use of water absorbent polymers (e.g. hydrogel), grafting technique, use of resistant varieties, irrigation method selection (e.g. drip irrigation), water harvesting and protected cultivation [27].

Growing areas of tomatoes have increased intensively, green peas moderately, beans and sweet corn slowly between 1997 to 2017 [28]. Corn, soybeans, beans and peas are the crops that are moderately water stress sensitive while tomatoes are within the extremely drought sensitive group. Although most crops are less sensitive to water shortage during the early stages of vegetative growth, changes of many physiological traits causing the disturbance of fertility and reduction of yield appear during the generative stage [28]. Therefore, irrigation scheduling and irrigation water requirements are determined by the water stress tolerance and water use potential of the plant varieties. Water use potential of vegetable crops depends on crop type, field soil properties, irrigation system type, climatic conditions, and crop growth stage.

## **5. Irrigation water amount**

Irrigation is likely to increase the size and weight of an individual fruit and to prevent defects. On the other hand, too much moisture reduces soluble solids in muskmelons (cantaloupes) and capsaicin (what makes the peppers hot) in hot peppers when it occurs during fruit development. In order to determine the amount of water needed for irrigation of plants, it is necessary to know the amount of water they consume, the percentage of this amount met by precipitation (effective precipitation) and the irrigation efficiency, which includes losses in transmission and application of the irrigation. Effective precipitation is ignored for the vegetables grown under greenhouse conditions [4]. Total irrigation water requirement for a crop in the field conditions can be calculated using below equation:

Total irrigation water requirement = Crop ( evapotranspiration – Effective precipitation)/ Transmission and application efficiency. (1)

Part of water delivered from resource is not fully stored in the crop root zone as it is lost through evaporation, runoff and deep percolation within the irrigated area. Therefore, an application efficiency value used for calculating total irrigation water required is the fraction of the available water stored in effective root depth to water conveyed to the field. Irrigation water requirement can be reduced by drip irrigation method as drip irrigation method applies water directly to crop root area (only some parts of the soil root zone watered) which saves a considerable amount of irrigation water [30]. In this case, the total irrigation requirement value should be corrected with a wetting percentage or plant cover percentage value which is lower than 1.
