Determining soil moisture

1. Visual method

2. Gravimetric method

3. Mathematical (water balance calculation) method

4. Methods of using sensors

4.1. Measurement of electrical conductivity (electrometric method)

4.2. Measurement of the soil moisture tension

4.2.1. Watermark sensor

4.2.2. Volumetric sensor

4.3. Measurement of dielectric constant

4.4. Measurement of thermal characteristics

4.5. Measurement / evaluation by spectral reflection

4.6. Measurement of radioactive radiation

Choice of the method is primarily a matter of budget, the required time for moisture determination, the precision of the result and overall practicality.

1. Visual method

As the name suggests, this method is based on visually reviewing the soil in the field. This kind of method is unreliable due to the subjective point of view. Although, this method can be used by producers for determining the starting point of agrotechnical operations that involve soil tillage. It is not recommended for irrigation purposes.

2. Gravimetric method

Gravimetric method or drying method is a direct method of determining the moisture content by drying the soil sample and measuring the difference between the wet and dry sample weight.

This method is precise and reliable, while recommended mostly for experimental work. A certain amount of soil is placed in a container that is heated to 105 °C and dried until all moisture is evaporated. The resulting difference in mass represents the water that has evaporated. The same mass is then incorporated into a formula to calculate the soil moisture in weight percentages (% mass). For the conversion of the weight into the volume percentage, the data on the volume density of the soil that is determined in the laboratory is required.

Because of its complexity and the need for a large number of sampling and moisture determination (through the vegetation season for irrigation purposes) it is not practical in terms of everyday use. The method can be used for calibration of different sensors/devices and for experimental work.

3. Mathematical (water balance calculation) method

The mathematical calculation of water balance consists of daily computation of reference evapotranspiration.

There are dozens of formulas through which reference evapotranspiration can be obtained, while the reliability of the formula depends on the number of parameters included within. Today, the Penman-Monteith formula is the most widely used for this purpose, which has been shown to be the most accurate method for humid or arid climate. The formula is suitable for calculating evapotranspiration on a daily basis. The following data are required for the calculation: air temperature (average, minimum and maximum), air humidity (minimum and maximum), global radiation and wind speed. In addition to evapotranspiration - precipitation data, crop coefficient (for each stage of vegetation), field water capacity, the maximum amount of easy accessible soil water and irrigation records are also required for the correct calculation of water balance.

Daily reference evapotranspiration represents the amount of water that is lost due to evaporation and transpiration from a large surface that has no water deficit, is covered with grass (8-15 cm in height). From this definition, it can be concluded that information about evapotranspiration does not suit all plants and systems of cultivation. Therefore, different coefficients have been developed. Each plant type has its coefficient that varies on the vegetation phase. At the beginning of the vegetation, the leaf surface is smaller – the transpiration is reduced, and as the plant grows, the water demand increases. In the beginning of vegetation, the coefficients are lower, in the middle phase occurs the coefficient reaches the maximum, while at the end of the vegetation the coefficient decreases again.

The reference evapotranspiration (ETo) is multiplied by the crop coefficient (Kc) to obtain the Crop Evapotranspiration (ETc).

The water balance is calculated according to the formula:

Dv = Dvpd + ETc – Oef – Irr

Dv = deficit of water

Dvpd = deficit of water of the previous day

EToc = crop evapotranspiration (mm)

Oef = effective precipitation (mm)

Irr = irrigation (mm)

The water balance formula may also include the capillary water rising from the deep layers and surface water flows. Due to the complexity of determining these values and the fact that they dont influence on the end result significantly, these values are often excluded from the formula.

This kind of water deficit determination was reserved for a long time for institute and faculty field researches, which is not surprising, due to the number of parameters and time necessary for quality results. Fortunately, with the development of new technologies, nowadays, an affordable agrometeorological station can be purchased, that measures all the data needed to calculate evapotranspiration. There are also computer and web applications that calculate the water balance based on the measured data.

An example of irrigation according to evapotranspiration are vineyards in Australia, Chile, California and other countries where precipitation is not sufficient enough. A good example of irrigation based on evapotranspiration is the CIMIS project in California (The California Irrigation Management Information System), where the California Department of Water Resources (DWR) publishes daily weather parameters with calculated reference evapotranspiration. According to this information, California farmers determine the amounts and the time of irrigation.

In viticulture, a frequently used method is RDI (Regulated Deficit Irrigation) which uses a principle of irrigating only an exact percentage of overall evapotranspiration. Which percentage is used, depends on producers instructions - on the development phase of grapevine and on the desired characteristics (reduce vegetation, increase yield, reduce yield, etc.). Fruit growing, farming and other types of production are also turning into such an approach.

4. Methods of using sensors

Soil moisture determination using sensors is the most widely used method. It is characterized by its simplicity, the speed of data collection which reflects on the producers' ability to react quick and timely.

 4.1. Measurement of the soil moisture tension

This method is based on the principle of measurement of force (negative pressure, tension) that holds moisture in the soil. The tensiometer is most reliable at a higher soil moisture level. It is placed in the plant root zone, and it acts as an "artificial root with a dial".

An example of a tensiometer is the Irrometer. The Irrometer consists of a water-filled sealed tube, a specially vacuumed dial and a porous top that is placed in the ground at the desired depth of the root system. In dry conditions, water is extracted from the instrument, reducing the amount of water in the instrument and creating a sub-pressure that is then read on the dial. The less moisture, the higher values. When irrigating, the approach is opposite. Sub-pressure created due to the dry ground now draws water from the ground back into the instrument, and the dial shows lower values.

Irrometer captures the whole range of possible soil moisture necessary for maximum plant growth. The read values indicate the point of starting and ending the irrigation with the best results for specific crops and conditions. The benefits of such approach are relatively low prices and ease of installation and handling. Despite mentioned, it is necessary to handle with care due to fragile construction.

 4.2. Measurement of electrical conductivity (electrometric method)

The electrometric method is based on measuring the electrical conductivity of the medium, which depends on the soil moisture. There are several types of sensors that measure moisture in the soil, such as gypsum sensors (Watermark Sensor) and sensors whose electrodes are in vitro plastic (Volumetric Sensors).

The principle of work is the same. One electrode sends the current to the other electrode, while in between is a medium with water that causes resistance and reduces the current. Although the principle of work is the same, there are some differences.   

4.2.1. Watermark sensor

Watermark sensors have been in use since 1978. One of the main advantages of such sensors is that they can remain in the soil throughout the vegetation period so we can have access to more detailed information. Watermark sensors are compact, easy to install/maintain and are relatively cheap. The data can be collected using a data logger or the sensor can be connected to the agrometeorological station and the data is automatically sent to the servers that users access by computer or mobile applications.

Watermark sensors are the most known representatives of sensors that measure electrical conductivity. The electrodes are not into contact with the surrounding soil, only with the water penetrating gypsum. Electrical conductivity is measured in a medium that is constant. The sensor is not sensitive to pH, ground type, and soil temperature.

Measured values are converted into centibars (cb), that is, the necessary force that the plant needs to apply that use the water. The values range from 0 to 200 cb, the lower value it is, the soil is more saturated with water and the higher the value is, the water deficit in the soil is greater.

The sensor manufacturer states how to interpret the measured values:

0 - 10 cb = well saturated soil

10 - 30 cb = soil contains an adequate amount of water for normal growth and development of the plant (except for sandy soils, where at 30 cb water deficits start to occur)

30 - 60 cb = common values when irrigation is necessary (except for heavy soils)

60 - 100 cb = common values when it is necessary to irrigate the heavy soil

100 - 200 cb = soil is too dry for maximum yields

Another advantage of Watermark is the ability to set multiple sensors at different depths. This allows better monitoring of soil moisture in the root system, water flow and soil moisture at greater depths.

4.2.2. Volumetric sensor

Electrodes from this sensor are located between two layers of vitro plastic and are well isolated from the contact with the measured soil. The measured dielectric conductivity is converted into soil moisture expressed in volume percentages (%). Just like Watermark sensors, they can be applied in the ground through the whole season, and also on various depths. The soil volume in which the sensor measures soil moisture depends on the size of the sensor and it ranges from 0.3 l to 1 l. Smaller sensors are more suitable for greenhouse production and growing in containers while larger sensors are more suitable for outdoor area measurements. The sensors are standardly calibrated for most soil types.

4.3. Measurement of dielectric constant

This method includes the time domain reflectometry (TDR) and the frequency domain reflectometry (FDR). TDR is considered to be a precise method for estimating soil water content. The principle of operation is to monitor the time required for the electromagnetic signal to pass through the steel sensors driven in the soil and time to reflect back to the receptor. Difference in time is the speed of the feedback signal, and the cause of the the difference is the dielectric constant influenced by the water content in the soil.

Advantages of this technology is its precision, minimum destructiveness soil structure, relative insensitivity to pH of the soil, the possibility of simultaneous measurement of soil EC and the fact that calibration is not necessary for certain soil types. Disadvantages are considered the price of equipment, the possibility of limited use in conditions of high salt or clay content and relatively low sensitivity to changes in soil moisture content.

The FDR method works on a similar principle as the TDR method. It is also based on the observation of the electromagnetic signal, but in terms of voltage monitoring. The cause of the difference between the voltage is the dielectric constant which is related to the amount of moisture in the ground.

4.4. Measurement of thermal characteristics

Measurement of soil thermal properties is an indirect method that exploits changes in soil thermal properties due to the difference in soil water content. The two main methods are heat dissipation and heat pulse.

The heat dissipation method uses a heat source (a heated needle) and temperature sensors (thermocouples), immersed into a porous ceramic that equilibrates with the surrounding soil at a given water content. The needle is heated, and the rate of heat dissipation is measured by the temperature sensors. These changes are affected by the thermal conductivity, which depends on the ceramic water content. The thermal conductivity is then obtained by measuring the differential temperature before and after heating.

In the heat flux method, the pulse of heat is applied at the initial location and its arrival at another location is determined by measuring the soil temperature at the other location. The time required for the pulse of heat to travel is a function of soil thermal conductivity, which is related to water content. The heat dissipation sensors are also used to estimate soil water potential, through calibration of the sensors at specific soil water potentials.

4.5. Measurement / evaluation by spectral reflection

This kind of approach is considered a non-contact method in remote sensing. This method is not a direct measurement, but a moisture assessment in the larger area of observation. Nowadays, as a source of information, data collected from unmanned aerial vehicles appears to have a greater importance than data collected with satellites.

The method is based on the knowledge of the spectral curve of water - water absorbs radiation in the near infrared area, and thus soil with more moisture is less reflective. Based on the obtained data, with software-assisted analysis and interpretation, and a subsequent verification of data on the field, it is possible to estimate the amount of moisture on the captured area.

This approach has certain imperfections such as factors affecting reflection itself, i.e. the amount of organic matter in the soil, tillage methods and soil structure, but because of the advantage of processing a larger area, it has significance in today's agriculture.

4.6. Measurement of radioactive radiation

Soil moisture measurement by neutrons and gamma rays gives favorable results, but when using this measurement method, expertise and precautionary measures are required.

To use such equipment, an authorized person is required, the equipment must be kept in an appropriate place and transported by an appropriate type of vehicle to the measuring site due to hazardous radiation. Because of this reason, this type of moisture measurement is not frequent.

Vedran Krevh, mag. ing. agr.

Tomislav Dvorski, mag.ing.agr.

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