**3. Measurement of moisture in the soil**

The infiltration process in the soil can be modeled by the Richards equation whose solution implies knowing the hydraulic functions of the soil. These functions depend on some parameters they need for their calibration of the hydraulic properties, determined by means of measurements. The measurements of the hydraulic properties of the soil present numerous complications due to two important factors: the nonlinearity of the conductivity/ suction function and the nonlinearity of the moisture/suction function [12].

Given the problem of the heterogeneity of the porous medium, the modeling of the watersolute-soil-plant system is more complicated. Therefore, obtaining the parameters that allow adjusting the model as close to reality is what takes most of time and money.

The adequate measurement of the water content in soil plays a critical role for the estimation of water and energy balances, as well as for understanding the biological and chemical processes in the entire soil-plant system [13].

The evaluation of soil moisture content at different suction conditions in the field requires considerable time and effort, as well as equipment. The effort, time, and equipment needed will depend on the range of interest required for the data.

The surface and depth to study must be carefully defined. The surface to study will depend on the variability existing in the place. In certain occasions the characteristics of the soil-water vary more with the depth that with the area.

The moisture in the soil depends mainly on the texture or the particle size distribution. On the other hand, the content of organic matter and the composition of the solution phase can play a determining role in soil moisture function or retention function. Organic matter has a direct effect on the retention function due to its hydrophilic nature and an indirect effect due to the modification of the soil structure that can be affected by the presence of organic matter [12].

**3.3. Granular matrix sensors**

**3.4. Dielectric sensors**

*3.4.1. Time domain reflectometry (TDR)*

same location or be used in a portable way.

connecting the fixed sensors to a data logger.

*3.4.2. Reflectometry in the frequency domain (FDR)*

related empirically to the volumetric content of water.

In it, the evolution of the potential over time is measured.

They have been developed recently (they were patented in 1985 and manufactured commercially since 1989). It measures the electrical resistance between two electrodes inserted in a small cylinder composed of a porous material. Each device is covered by a membrane consisting of a stainless steel coupling, externally covered by a rubber that makes the sensor more durable than the plaster block. However, the recorder is calibrated to give the value in water tension, by means of an equation that takes into account the temperature of the soil estimated or measured near the sensor. The size of the pores in the matrix is greater than that of the pores in the gypsum blocks, allowing greater sensitivity in the more humid range of water content in the soil [14].

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The TDR and FDR probes measure the dielectric constant of the medium, which is an intrinsic property of the medium. The FDR system calculates the humidity of a soil by responding to changes in the dielectric constant of the medium using a frequency domain reflectometry

The TDR system consists of an oscilloscope connected to two or three metal rods that are inserted parallel to the ground. If a difference of power is applied to one end of the rods, the energy is transmitted along the ends to the end, where they are reflected to the oscilloscope.

Some equipment consists of two main parts: the electronic unit and the waveguides. The electronic unit contains the oscilloscope and the central processor, which controls all measurement, display, and storage functions. The waveguides can be installed horizontally or vertically and remain permanently on the ground to make periodic measurements in the

The TDR uses a series of conversion tables to convert the dielectric constant to a percentage of moisture in the soil. There are different conversion tables to be used with the different types of waveguides. It is not necessary to have a different table for the different soil types since the dielectric constant depends more on the amount of water than on the other soil components. The apparatus calculates the average value of the humidity over the total length of the waveguides. The apparatus allows manual measurements or continuous measurements by

The FDR method is also known as a capacitance probe. The electrodes and the adjacent floor form a capacitor whose capacity is a function of the dielectric constant of the soil. This is

A capacitance sensor requires a probe calibration for each floor and horizon to obtain an optimal measurement of volumetric moisture. The volume of soil measured is not dependent

technique known as capacitance, while the TDR uses time domain reflectometry [16].

Currently there are different equipment and measurement techniques for moisture content such as capacitance, reflectometry, geo-electric (called dielectrics), and neutrons. Descriptions of them are developed in the following points.

#### **3.1. Neutron probe**

A neutron probe is a sophisticated and accurate piece of equipment that measures the moisture content in soil. It requires calibration and operation by a licensed operator.

This technique is based on the theory that fast neutrons are thermalized when they collide with a body of similar mass, such as hydrogen nuclei. The energy of the neutrons is transmitted to the protons which causes the neutron "bounce or shock" to be much lower.

The application of this technique consists of three steps: (1) emission of fast neutrons from a radioactive source, (2) attenuation of the velocity of the neutrons after successive collisions with the atoms at the point of emission, and (3) accounting for neutrons with attenuated velocity by a detector near the source.

By means of electrical impulses the neutrons captured by the detector are translated into a digital reading.

To convert the reading of the neutron probe to volumetric moisture, a calibration model is necessary, where the volumetric moisture of the soil is the main factor. There are some factors that influence measurements such as hydrogen from organic matter, chlorine, iron, and boron present in the soil, capable of attenuating neutrons and absorbing thermally neutron nuclei [14].

### **3.2. Tensiometers**

Tensiometers are widely used to measure the available water content of the soil when the matrix potential is high [15]. These devices are simple, they are not very expensive, and they are very practical in agricultural systems.

They consist of a porous ceramic capsule permeable to water and solutes, connected to a pressure gauge by means of a transparent plastic tube that is filled with water, in such a way that the column of water in its interior forms a continuous with the water of the solution of the floor in the surrounding space, through the porous capsule.

The values obtained reflect the soil tension, are negative values, and their operating range is 0−80 kPa, below this value the water column breaks, penetrating the air and invalidating the following measurements.

The tensiometers are insensitive to the osmotic potential of water in the soil and therefore do not provide an adequate measurement of the water potential in soils with significant salinity.

Tensiometers are often used in combination with the neutron sprayer, resistance blocks, or psychrometers to cover the full range of soil moisture. They require relatively frequent maintenance, which consists of adding water plus a solution for the control of algae [14].

#### **3.3. Granular matrix sensors**

effect on the retention function due to its hydrophilic nature and an indirect effect due to the modification of the soil structure that can be affected by the presence of organic matter [12]. Currently there are different equipment and measurement techniques for moisture content such as capacitance, reflectometry, geo-electric (called dielectrics), and neutrons. Descriptions

A neutron probe is a sophisticated and accurate piece of equipment that measures the mois-

This technique is based on the theory that fast neutrons are thermalized when they collide with a body of similar mass, such as hydrogen nuclei. The energy of the neutrons is transmit-

The application of this technique consists of three steps: (1) emission of fast neutrons from a radioactive source, (2) attenuation of the velocity of the neutrons after successive collisions with the atoms at the point of emission, and (3) accounting for neutrons with attenuated

By means of electrical impulses the neutrons captured by the detector are translated into a

To convert the reading of the neutron probe to volumetric moisture, a calibration model is necessary, where the volumetric moisture of the soil is the main factor. There are some factors that influence measurements such as hydrogen from organic matter, chlorine, iron, and boron present in the soil, capable of attenuating neutrons and absorbing thermally neutron nuclei [14].

Tensiometers are widely used to measure the available water content of the soil when the matrix potential is high [15]. These devices are simple, they are not very expensive, and they

They consist of a porous ceramic capsule permeable to water and solutes, connected to a pressure gauge by means of a transparent plastic tube that is filled with water, in such a way that the column of water in its interior forms a continuous with the water of the solution of the

The values obtained reflect the soil tension, are negative values, and their operating range is 0−80 kPa, below this value the water column breaks, penetrating the air and invalidating the

The tensiometers are insensitive to the osmotic potential of water in the soil and therefore do not provide an adequate measurement of the water potential in soils with significant salinity. Tensiometers are often used in combination with the neutron sprayer, resistance blocks, or psychrometers to cover the full range of soil moisture. They require relatively frequent main-

tenance, which consists of adding water plus a solution for the control of algae [14].

ture content in soil. It requires calibration and operation by a licensed operator.

ted to the protons which causes the neutron "bounce or shock" to be much lower.

of them are developed in the following points.

62 Soil Contamination and Alternatives for Sustainable Development

velocity by a detector near the source.

are very practical in agricultural systems.

floor in the surrounding space, through the porous capsule.

**3.1. Neutron probe**

digital reading.

**3.2. Tensiometers**

following measurements.

They have been developed recently (they were patented in 1985 and manufactured commercially since 1989). It measures the electrical resistance between two electrodes inserted in a small cylinder composed of a porous material. Each device is covered by a membrane consisting of a stainless steel coupling, externally covered by a rubber that makes the sensor more durable than the plaster block. However, the recorder is calibrated to give the value in water tension, by means of an equation that takes into account the temperature of the soil estimated or measured near the sensor. The size of the pores in the matrix is greater than that of the pores in the gypsum blocks, allowing greater sensitivity in the more humid range of water content in the soil [14].

### **3.4. Dielectric sensors**

The TDR and FDR probes measure the dielectric constant of the medium, which is an intrinsic property of the medium. The FDR system calculates the humidity of a soil by responding to changes in the dielectric constant of the medium using a frequency domain reflectometry technique known as capacitance, while the TDR uses time domain reflectometry [16].

### *3.4.1. Time domain reflectometry (TDR)*

The TDR system consists of an oscilloscope connected to two or three metal rods that are inserted parallel to the ground. If a difference of power is applied to one end of the rods, the energy is transmitted along the ends to the end, where they are reflected to the oscilloscope. In it, the evolution of the potential over time is measured.

Some equipment consists of two main parts: the electronic unit and the waveguides. The electronic unit contains the oscilloscope and the central processor, which controls all measurement, display, and storage functions. The waveguides can be installed horizontally or vertically and remain permanently on the ground to make periodic measurements in the same location or be used in a portable way.

The TDR uses a series of conversion tables to convert the dielectric constant to a percentage of moisture in the soil. There are different conversion tables to be used with the different types of waveguides. It is not necessary to have a different table for the different soil types since the dielectric constant depends more on the amount of water than on the other soil components. The apparatus calculates the average value of the humidity over the total length of the waveguides. The apparatus allows manual measurements or continuous measurements by connecting the fixed sensors to a data logger.

#### *3.4.2. Reflectometry in the frequency domain (FDR)*

The FDR method is also known as a capacitance probe. The electrodes and the adjacent floor form a capacitor whose capacity is a function of the dielectric constant of the soil. This is related empirically to the volumetric content of water.

A capacitance sensor requires a probe calibration for each floor and horizon to obtain an optimal measurement of volumetric moisture. The volume of soil measured is not dependent on the type of soil or water content and approaches a cylinder 10 cm high with a diameter of about 25 cm, assuming there are no spaces with air [17].

*4.1.1. Inhibitors of photosynthesis*

this type of herbicide.

absorbed by the roots.

nitriles, benzothiadiazoles, and amides [23].

tracks, industrial zones, and warehouses [24].

(b) Herbicides that uncouple the electron transport chain.

The inhibitors of photosynthesis can be classified into mobile or systemic herbicides and nonmobile or contact herbicides. The inhibitors of photosynthesis include the chemical families of the triazines, triazinones, triazolinones, phenylureas, and uracils and the contact ones to the

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It is subdivided into four groups, the first three being those that act on the luminous phase of photosynthesis. Besides, they are not only used in agriculture but also to clean land, railroad

Ureas, uracils, and triazines correspond to this group of herbicides. This type of herbicides are applied to the soil and absorbed by the roots, transported via xylem to the aerial part, reach the chloroplasts of the leaves, and there inhibit the light phase. Any plant can be affected by

They have the ability to capture the electrons preventing oxidation and forming free radicals "superoxides." Superoxides are very powerful oxidants, which oxidize the unsaturated lipids of the chloroplast membranes, losing structure, and the chloroplast stops working. These molecules are formulated as bromides and chlorides; they are very soluble and very easily

Acylanilides, hydroxybenzonitriles, dinitrophenols, pyridazines, N-phenylcarbamates belong to this group. In photosynthesis, ATP is synthesized from the thylakoid membrane (sites of the photochemical reactions of photosynthesis) of chloroplast cells of plants. The photon particles from the sunlight excite the chloroplast thylakoid membrane, which in turn converts this excitation into ATP's chemical energy. Found in all forms of life, ATP is often referred to as the "molecular unit of currency" of intracellular energy transfer. It is used by the plant for metabolic processes. They are applied differently. They can present an important toxicity for

They act at some point in the synthesis of lycopene. The most important is the amino triazole.

**a.** Herbicides that alter the biosynthesis of aromatic amino acids: These amino acids are then part of proteins. The herbicide capable of inhibiting the synthesis of aromatic amino acids

(a) Herbicides that inhibit the transfer of electrons by inhibiting photosynthesis.

(c) Herbicides that prevent the formation of adenosine triphosphate (ATP).

animals. Hence, some can be used as herbicides and fungicides.

*4.1.2. Those that alter the biosynthesis of metabolites other than carbohydrates*

(d) Herbicides that alter the biosynthesis of carotenoids.

The most used inhibitors of photosynthesis are type (a).

They are subdivided into three groups:

All capacitance sensors installed in floors, even with similar characteristics, must be calibrated with the aim of improving their accuracy given the influence on the measurement of other factors independent of the moisture content such as pH variability or electrical conductivity inside of the porous matrix.

Capacitance sensors are the most economical and easy to install. In addition, it allows a continuous recording of the moisture values in the soil, enabling direct information and in real time. They are very useful for the planning of alert monitoring systems. They can be used as substitutes for neutron probes.
