MEASUREMENT OF WATER MATRIIAL POTENTIAL IN THE SOIL THROUGH INSTRUMENTED TENSIOMETERS WITH PRESSURE SENSORS AND PUNCTURE DEVICE

Objective: The search for a practical and accessible solution for measuring water tension in the soil using tensiometers is fundamental for irrigation management, efficient use of water and increased profitability of irrigated crops. The objective of this study was to propose a technique based on the instrumentation of a tensiometer with a pressure sensor and a device for reading the sensor's response. Theoretical Frame: This study used the following main quotes as a theoretical basis: Freescale and NPX Semiconductors manufacturers of pressure sensors, Azevedo & Silva for the soil water tension correction methodology and Van Genuchten to describe the relationship between soil water content and suction. Method: Pressure transducers with a range of 0-50 and 0-100 kPa were used, with an analog signal varying between 200 and 4700 mV, whose response was measured by a digital multimeter with the scale adjusted to V or mV. The transducer was adapted by attaching a hypodermic needle Nº 7/30 to the vacuum measurement inlet, leaving the other inlet free for the atmosphere. The transducer was powered by a 9 Vcc battery with rectified voltage to 5 Vcc through a specific integrated circuit for this function. The conversion of the transducer response signal in mV to voltage in kPa was performed using the


INTRODUCTION
The increase in the world population to 8 billion people in 2022 (UNFPA, 2022), has required a growing demand for food that drives the expansion of agricultural areas by increasing the use of water, in particular by irrigation, which has aroused the attention of the world to the sustainable uses of natural resources on the planet, mainly water (Paz et al., 2000).Brazil is a country that holds about 12% of all the fresh water on the planet (Augusto et al., 2012), evidencing the abundance of this resource in the federation, but bitter the reality of poor geophysical distribution in its territory, which has generated discussions and even conflicts (Paz et al., 2000).
In the ranking of water use in Brazil agriculture figures as a villain, since, according to data from the National Agency for Water and Sanitation, irrigation is responsible for 52% of the use of water taken from Brazilian water sources (ANA, 2019).This scenario has fostered discussions and the search for techniques that will lead to an increase in the efficiency of water use, which includes investments in science and technology for the development of more modern water application equipment, the formation of qualified human resources, and techniques for managing irrigation.
The irrigation slide applied in most studies is not based on agronomic criteria (Souza et al., 2023).Therefore, it is necessary to determine the blade according to the needs of the plants.
To carry out the management of irrigation there are several techniques available, which are based on the monitoring of the climate, the plant and the soil, or a set of combined factors (Pires et al., 1999).Soil-based management has a direct relationship with the water requirement of the crop, the amount of water stored in the root zone and its availability.A very useful instrument that has been used since its invention by Gardner and collaborators in 1922 is the tensiometer, which has undergone modifications and adaptations over time, but without altering its working principle (Azevedo & Silva, 1999).New developments in electronics make it possible to technologically update agricultural activities (Rodríguez et al., 2022).Thus, it is possible to optimize the use of tensiometers for the management of irrigation.
In the 1990s, for example, Saad and Libardi (1992), with the intention of facilitating the decision of the farmer in the management of irrigation, proposed a system for reading the tension of water in the soil by bands of colors.The researchers divided the indicator scale of the voltage relative to 80% of the Available Water Capacity (CAD) into 4 color bands (blue, green, yellow and red).This procedure prevented the irrigation system operator from having to take readings and calculate soil moisture.This adaptation seems to have been the first step towards the automation of irrigation using tensiometers.
At the end of last century, irrigated agriculture experienced a good deal of evolution in the area of instrumentation and electronics, with innovative proposals for devices capable of monitoring the irrigated fields and acting in an automatic manner in water application systems.
Initiating the evolutionary sequence was the work of Medici (1997), which developed an automatic irrigation system actuator, based on the introduction of electrical contacts in the mercury path tube on the measurement scale.This made it possible to activate electrical devices from a pre-defined voltage, similar to the proposal of Saad and Libardi (1992), but automatic.Afterwards, Teixeira and Coelho (2003), among others, developed and calibrated an electronic tensiometer with automatic reading based on pressure and temperature transducers coupled to an analog-to-digital converter and to a microcontroller.Queiroz et al. (2005) developed a logic gate circuit, which coupled to a set of three mercury tensiometers, was able to automatically activate the irrigation system based on the indication of at least two of the three tensiometers.The system was still capable of controlling the lamina applied as a function of the tension of water in the soil, by means of automatic timers.Queiroz et al. (2008) presented Software and Hardware capable of performing precision irrigation in a central pivot, using sensors and radio frequency communication devices, which, together, could command the pivot tower by changing the blade applied in each field.
In the evolutionary scale of irrigation management with tensiometers, by the aforementioned reports, it was noted the departure of visual systems (color bands), evolving to mechanical and electromechanical devices and finally the digital ones based on transducers, converters, microcontrollers and visual interface man-machine.The most modern required specific skills and knowledge in microelectronics and instrumentation for its implementation.Perhaps for this reason, there are no reports in the literature that these automation proposals have become commercial products or that they have been adopted by irrigators as an automation option.Even the device proposed by Medici (1997), which had the patent request and granted -PI 9502962-1 B1 - (Medici, 2001) was not popularized in spite of simplicity and efficiency.
However, nowadays, the mechanical irrigation controller is activated by the tension of water in the soil using a tensiometer as active sensor, developed by Almeida et al. (2017), demonstrating that the tensiometer is still an important instrument for the management of irrigation and revealing the interest of researchers in its improvement for irrigation automation, including seeking simplification.
In view of the increase in the use of water by irrigated agriculture, the low efficiency of irrigation systems and the inadequate handling of irrigation, it is necessary to develop techniques and equipment for the management of irrigation that make it possible to apply the irrigation blade in accordance with the plants' water needs and to provide for an increase in the efficiency of the use of water.To do so, it is very important to employ technological updating to optimize the use of tensiometers in the management of irrigation, thus allowing the irrigator to monitor the tension and humidity of the soil for the decision of when and how to irrigate.
Water wastage in agriculture is due to the rural producer applying excess water, as it usually does not adopt a method of managing irrigation (Turco et al., 2009).These producers face difficulties in their cultivation, since they do not have knowledge about the moment and quantity of water to be applied through irrigation (Farias et al., 2004).Therefore, it is necessary and justifiable to seek practical and accessible solution for measuring the water tension in the soil through tensiometers that are capable of facilitating decision-making by the irrigating farmer about the management of irrigation.
In this context, the following question arises: will the use of a new technique and instrumentation of the tensiometer with pressure sensor and a device for reading the response of this sensor make it possible to determine the matrix potential and soil humidity in a satisfactory manner, to handle irrigation according to the water needs of the plants and to make a more efficient use of water?

MATERIAL AND METHODS
A measuring system was mounted by attaching a needle to the vacuum inlet of the sensor (Figure 1), leaving the other inlet open to the atmosphere (Patm), thus allowing the measurement of the monometric pressure within the tensiometer, which is the fruit of the matrix potential and the gravitational potential.Also in Figure 1, there is a 9 Vdc alkaline battery responsible for the electrical supply of the circuit and a voltage regulator integrated circuit LM 7805 responsible for rectifying the supply voltage of the sensor fixed at 5 Vdc.And finally, you see that the output signal from the sensor was connected directly to the potential difference meter.
Two silicone membrane pressure sensors were used, the first with a range from 0 to 50 kPa (sensor MPX 5050 DP) and the second with a range from 0 to 100 kPa (sensor MPX 5100 DP), which operates with 5 Volt DC power (Vcc) and output signal ranging from 0.2 to 4.7 Vcc.The response function provided by the manufacturer was used to convert the sensor response signal into V to pressure in kPa (Figure 2).As shown in Figure 2, the response function has the output signal as a response to the pressure stimulus (Equations 1 and 2), but in practice one wants equipment that does the opposite, that is, that allows the estimation of the pressure (kPa) as a function of the sensor output signal (V or mV).
To do so, an inversion was made in the variables of the response function to allow what is desired (Equations 3 and 4). Where: P -pressure, kPa; and Vout -sensor output signal, V.
To improve the accuracy of Vout measurement one can adjust the meter scale to mV, which allows one to rewrite Equations 3 and 4 in this way, resulting in Equations 5 and 6.  = 0,011111 *   − 2,222 (5)  = 0,022222 *   − 4,444 In this type of tensiometer it is necessary to make a correction of the water tension in the soil (TAS) as a function of the column of water that is stored inside the apparatus, which contributes to the reduction of reading as a function of the gravitational potential, according to Equation 7 (Azevedo & Silva, 1999). Where: TAS -ground water voltage, kPa; P -sensor read pressure, kPa; L -tensiometer length, cm; and 0.098 -conversion factor from cm.c.a to kPa.
Replacing Equations 5 and 6 in Equation 7, we have an equation that allows the estimation of the Water Voltage in the Ground from the reading of the pressure sensor and the tensiometer length (Equations 8 and 9).
Samples of deformed composite soil were collected and sent to the specialized laboratory for determination of the Characteristic Curve of Water Retention in the soil, which was determined at different voltage levels (1, 2, 4, 10, 30, 50, 100, 500 and 1500 kPa).
Based on the laboratory data (Volumetric Humidity x Water Potential), the Van Genuchten Equation (1980) was adjusted using the Mualem parameter (1976) (Equation 10).The adjustment was made by means of spreadsheet tool with minimization of the average square error. Where:  -volumetric humidity as a function of h, m³ m-3; r and s -residual volumetric and soil saturation humidity, m³ m³-3 ; h -ground water potential, kPa; inlet pressure in the largest pore of the soil, kPa-1; n and m -parameters related to the shape of the curve, dimensionless; and the m of Mualem (1976) = 1 -1/n.
The equipment developed was tested during a complete crop cycle of growing tomatoes in soil inside a vegetation house.To do so, tensiometers were installed at depths of 10 cm (layer 0-20 cm) and 30 cm (layer 20-40 cm), which were monitored once a day, with annotation of the sensor response in mV, later converted to kPa (Equations 8 and 9) and to m³ m -3 (Equation 10).
The management of irrigation in the cultivation of the tomato plant was carried out from the data of the tension of the water in the soil, obtained with the pressure sensors, from the characteristic curve of the retention of water in the soil, and from the Van Genuchten equation adjusted, in this way, it was possible to determine the humidity of the soil in the experimental area.
From the data obtained, graphs were prepared containing the values of water tension and soil humidity observed from 37 to 118 days after sowing, i.e. over 82 days of cultivation of the tomato crop in a protected environment.

RESULTS AND DISCUSSIONS
The values of water tension in the soil in the 0 to 20 cm layer and 20 to 40 cm depth observed from 37 to 118 days after sowing, i.e. over 82 days of cultivation of tomato cultivation in a protected environment, are shown in Figure 3.It was found that over 82 days of observation of the water pressure in the soil, the values obtained with the new instrument (pressure sensor tensiometer and sensor response readout device) ranged from 0.00 kPa (minimum voltage) to 53.33 kPa (maximum voltage) in the two soil layers studied, where the average voltage was 23.64 kPa in the most superficial layer of the soil (0-20 cm), while a lower average voltage of 21.38 kPa was found in the deepest soil layer (20-4 0 cm).
The lower tension observed in the deeper soil layer can be justified by the percolation of water to the deeper soil layers, in addition, the surface soil layer was more exposed to evaporation and extraction of water by plant roots.Therefore, the new device adequately measured the water tension in the soil, since the observed values are in line with those described in the literature.
These results corroborate those obtained by Marouelli & Silva (2008), who found that the irrigation of the tomato plant carried out in the vegetative, fruiting and maturing stages, respectively, with the limit stresses of water in the soil of 35, 12 and 15 kPa, boosted the productivity of the crop.Rodrigues et al. ( 2020) also observed higher production performance of the tomato plant when irrigated at the time when the water pressure in the soil reached 20 kPa.In view of this, it can be verified that the new device is suitable for monitoring the matrix potential of water in the soil.
The soil moisture analyzed from 37 to 118 days after sowing, in the layers from 0 to 20 cm and from 20 to 40 cm deep in the experimental area cultivated over 82 days with the tomato crop, is found in Figure 4. Soil moisture obtained from the soil water tension and the soil water retention curve at depths of 0 to 20 cm and 20 to 40 cm in depth ranged from 0.11 (minimum) to 0.50 m 3 m -3 (maximum), respectively, in both soil layers studied.As seen in the water tension in the soil, the average soil moisture at the depth of 20 to 40 cm was also lower (0.20 m 3 m -3 ) when compared to the surface soil layer of 0 to 20 cm (0.21 m -3).
The lower soil humidity found in the deeper layer can occur as a result of several factors, such as water percolation to deeper layers, water absorption by plant roots, or variation in soil texture in the deeper layer.Sgobi et al. (2011) obtained higher tomato productivity when irrigated by observing the soil water stress of 10 kPa, which is equivalent to 0.17 m 3 m -3 according to the soil water retention curve of this study.Whereas, the study by Marouelli et al. (2003) suggests that soil water stresses of less than 15 kPa, i.e. high-frequency irrigation in the fruiting phase, result in higher yields of the tomato plant.This voltage corresponds to 0.15 m 3 m -3 according to the soil water retention curve of the present study.The values obtained with the pressure sensor are in line with the reports in the literature as suitable for providing greater yield in growing the tomato plant.
The new pressure sensor has the capacity to facilitate the management of irrigation in irrigated crops, besides being of low cost, the sensor has the capacity to carry out the reading of various punch devices.The punching device is also relatively low-cost, as the rural producer can purchase parts such as porous capsules and PVC pipes and assemble his own devices.Also, the use of the equipment is very easy to be carried out, since only with a reading of the sensor and a simple calculation is it possible to get the tension of the water in the soil.Therefore, the use of this sensor aims to reduce errors made in the operation of irrigation management and, in this way, increase efficiency in the use of water and productivity of crops.

FINAL CONSIDERATIONS
The pressure sensor instrumented tensiometer and sensor response readout device enables the determination of the water surface potential in a satisfactory manner.
The matrix tension data obtained with the new pressure sensor are in line with the reports in the literature as suitable for providing greater yield in growing the tomato plant.
The technique and instrumentation proposed in this study allows to measure the tension of water in the soil, to know the soil humidity and to carry out the management of irrigation, applying an irrigation blade according to the needs of the plants, making a more efficient use of water and providing savings of water and electricity.

Figure 1 .
Figure 1.Schematic of sensor connection and hypodermic needle coupling.Source: authors.

Figure 3 .
Figure 3. Water tension in the soil observed during the experimental period.Source: authors.

Figure 4 .
Figure 4. Soil humidity verified during the experimental period.Source: authors.