Role of Soil Moisture Sensors in Agriculture (Working and Examples)

The sensor consists of at least two electrodes, typically embedded in or near the soil. One electrode acts as a transmitting (or measuring) electrode, while the other acts as a receiving (or ground) electrode. An electrical field is established between these electrodes. 

A  capacitor contains two conductive plates separated by a non-conductive material or dielectric. In the case of soil moisture sensors, the soil between the electrodes acts as the dielectric material.

When the soil is dry, it has a lower dielectric constant (εr) similar to air. But as moisture increases in the soil, the dielectric constant also rises, altering the capacitance between the electrodes.

The increase in capacitance is translated into an electrical signal, which is then processed by the electronics of the sensor.

This processed signal is further conditioned and interpreted.

Depending on the design and programming, the sensor might convert these changes into readable data, such as percentage moisture content, or trigger actions like providing an output voltage or activating a display indicating soil moisture.

Pros

• High sensitivity
• Fast response time
• Highly accurate with proper calibration
• Less expensive compared to TDR
• Low power consumption
• Less sensitive to drift

Cons

• Able to sense small area
• Easily impactable by soil condition
• Specific calibration needed

Time Domain Reflectometry (TDR) sensor works on the principle of radar based on transmitting signals into the medium and collecting reflected signals.

It sends signals into the earth and listens intently for whispers bouncing back.

TDR accurately assesses the medium’s dielectric constant and transmits signals through two metal probes buried in the soil, it awaits the return of these signals, analyzing their every variation.

These probes, typically 10–30 cm long, establish a connection to the TDR through a coaxial cable. Analyzing the returned signals, especially their magnitude, shape, and sign, TDR provides a complete view of the transmitting medium. 

Pros

• It is very accurate
• No need to do soil calibration
• Provides remote access

Cons

• It is able to consider a very small area 
• It is expensive

The neutron method of measuring soil moisture operates based on this principle: as a source emits fast neutrons into the soil, they’re slowed and scattered by the soil itself.

This action creates a cloud of slow neutrons enveloping the probe.

However, this neutron cloud doesn’t endlessly increase in density; it stabilizes at an equilibrium influenced by how quickly neutrons are absorbed into the soil.

In dry soil, the neutron cloud is less dense and reaches farther from the probe. Conversely, in wet soil, the neutron cloud is denser and spans a shorter distance from the probe.

Pros

• It is very accurate
• Able to measure a large area
• Is not affected by salinity and air gaps around the access tube

Cons

• Soil-specific calibration needed
• It is very expensive and heavy
• Need to have the license  
• Not accurate at slight depths
• No remote access

Tensiometers gauge soil moisture using units of negative pressure, which means tension. It measures the force necessary for plant roots to draw water from soil pores.

Water held in larger pores (macropores) requires less force compared to that in smaller pores (micropores).

When plants extract moisture, it starts from the largest pores. As the soil dries, increased force is needed to extract water from smaller pores.

High tension values indicate the soil is dry, and less tension values indicate the soil is moist. 

Pros

• Works without electricity
• Not expensive
• You will get it in different lengths
• Salinity does not affect it

Cons

• The operative range is small
• The response time is slow
• Maintenance needed more often
• No remote access – manual data collections

A Granular Matrix Sensor for electronically gauging soil moisture, known as the Watermark soil moisture sensor by Irrometer Co. in Riverside, CA, has been patented.

This sensor technology addresses issues found in gypsum blocks such as dissolving and inconsistent pore size distribution by utilizing a confined granular matrix within a metal case.

Functioning on the principle of electrical resistance similar to gypsum blocks, granular matrix sensors feature a gypsum wafer embedded in the matrix.

Electrodes within the sensor are positioned in the granular fill material above the gypsum wafer.

The gradual dissolution of the gypsum wafer serves to moderate the impact of soil solution salinity on the electrical resistance between the electrodes.

According to Larson (1985), the particle size and compression of the granular fill material determine the pore size distribution in these sensors, impacting their response characteristics.

Pros

• Accuracy level is good in medium soils
• Less expensive
• Measures moisture content continuously in the same area

Cons

• Response time is more
• Accuracy is less in sandy soils
• It is very sensitive to temperature and salinity