A History of Soil Moisture Sensors
How Farmers Learned to Listen to the Soil
People have been engaging in farming and agriculture since times immemorial. Soil moisture plays a very important part in the growth of the crops since it is critical for plant nutrient uptake and transpiration.
Without technology, people have to manually check the soil moisture levels making it an extremely labour-intensive process, involving soil sampling, weighing, drying, and reweighing to determine moisture content. This approach is impractical, costly and laborious for routine crop management, hence farmers would prefer to just eyeball it, by assessing soil moisture by digging, evaluating soil color, and gauging texture by hand.
The success of this observational method heavily relied on the experience of the grower and attentiveness hence mostly rendering it unsuitable and unreliable
As technology progressed, multiple soil sensors were invented. Let us discuss a few of them.
Porous Ceramic Cups
First developed in the 1920s, these sensors—commonly known as tensiometers—use a simple but clever design: a small ceramic cup is attached to the bottom of a sealed, water-filled tube and placed directly into the soil. When a vacuum is applied inside the tube, any movement of water in or out of the ceramic cup causes a change in internal pressure. This change reflects how tightly the soil is holding onto its moisture, known as soil matric potential. As the soil dries out, the vacuum becomes stronger (more negative), providing a clear signal that water is becoming harder for plants to access.
To get accurate readings, tensiometers must stay in constant contact with the soil, even small air gaps can throw off the results. They’re usually installed permanently and require different probes for each depth you want to monitor. That means if you’re tracking moisture at 6, 12, and 24 inches, you’ll need separate devices for each level. These tools can also face issues over time: microbial growth or salt buildup (especially in saline soils) can block the ceramic pores and disrupt readings. And like any vacuum-based device, tensiometers need occasional maintenance to keep working properly. Despite their simplicity, they require careful setup and attention to detail to stay accurate and reliable.
Buried Gypsum Blocks
These sensors, like the Watermark type, use embedded electrodes to measure changes in electrical conductivity as soil moisture levels shift. In simple terms, as the soil gets wetter or drier, the sensor detects how easily electricity flows through it, giving a good indication of moisture status. To monitor different depths, you’ll need a separate sensor installed at each level.
While the gypsum blocks themselves are fairly sturdy and built to last, the installation process can be tricky. It involves digging into the soil, placing the sensor, and then backfilling the hole, which can disturb the soil’s natural structure and affect how water moves around the sensor. Another challenge is durability in the field: the wires connecting the sensors are often vulnerable to damage from rodents or burrowing animals, which can interfere with data collection or require costly replacements.
Neutron Probes
Neutron probes are often considered the “gold standard” for measuring soil moisture, especially in mineral soils (thanks to their high accuracy and ability to take repeated readings over time). These devices work by using a radioactive americium-beryllium source that emits fast neutrons into the soil. Because hydrogen atoms in water molecules are excellent at slowing down these neutrons, the amount of backscattered slow neutrons detected gives a direct indication of how much moisture is present. The size of the area being measured typically ranges from a baseball to a basketball, offering a good sense of what’s happening around the probe.
However, these tools come with significant trade-offs. Neutron probes are expensive, require strict licensing (such as from the U.S. Nuclear Regulatory Commission), and must be handled carefully due to the presence of radioactive material. They also need pre-installed aluminum access tubes, adding complexity to set up. Safety concerns, especially when used in dry soils or near the surface, further limit how and where they can be used.
Additionally, readings can be affected by organic matter, boron, or chloride in the soil, which may skew results. For these reasons, neutron probes are rarely used by growers directly and are typically operated by specialized consultants trained in both their usage and regulatory compliance.
Time-Domain Reflectometry (TDR) Sensors
TDR sensors, like the Acclima TDR-315, measure soil moisture by analyzing how an electrical signal behaves underground. They work by sending a quick, high-frequency (RF) pulse through metal prongs inserted into the soil. As this signal reflects back, the sensor analyzes how the soil’s dielectric constant, which changes with moisture levels, affects the signal. The wetter the soil, the more it changes the reflection, allowing for accurate moisture readings.
Much like gypsum block sensors, TDR sensors require a separate unit for each soil depth you want to monitor. Installing them involves digging and burying the sensor, which can disturb the soil structure and affect initial readings. Plus, the cables connected to the sensors can be fragile and are vulnerable to damage from rodents or field equipment.
Because of their precision and sensitivity, TDR probes are widely used in research settings or for targeted monitoring, often as single units placed at or just below the soil surface. While they offer reliable data, their cost, installation complexity, and maintenance needs make them less practical for large-scale or everyday farm use.
Capacitance Sensors
Capacitance sensors, originally co-developed by Dartmouth College and the U.S. Army Corps of Engineers in the late 1990s, are another widely used tool for measuring soil moisture. Like TDR sensors, they rely on the principle that soil’s dielectric constant changes with moisture content. These sensors use two parallel metal tines (electrodes) to measure shifts in the frequency of an oscillating electrical current—shifts that directly relate to how much water is in the soil.
When properly calibrated for the soil type, capacitance sensors can provide accurate and consistent moisture readings. They’re relatively compact and easy to install, making them a popular choice in agriculture and turf management.
However, they do have some limitations. Capacitance sensors only measure a small zone of soil right around the electrodes. That makes them particularly sensitive to air gaps or uneven soil contact, which can throw off the readings significantly. Because of this, installation needs to be done carefully, and the sensors may require frequent checking or recalibration—especially in soils that shrink, crack, or shift easily.
Electronic Tensiometers
Some of the newer soil tension sensors aim to solve the problems found in older porous ceramic tensiometers, such as maintaining a vacuum or dealing with clogged ceramic tips. While they do improve on those fronts, they still require a separate sensor at each soil depth you want to monitor, which can be labor-intensive and costly for full-field coverage.
These sensors measure soil tension, which tells us how tightly water is held in the soil, a key indicator of how available it is to plants. Because of this, many users believe that calibration isn’t necessary. However, without calibrating the sensor to the specific soil type, you can’t know how much water is actually in the ground, only that the soil is getting drier or wetter.
As UC Davis Extension puts it: “Tensiometers do not provide information on the amount of water depleted from the soil unless they have been calibrated for the particular soil type. They therefore indicate when to irrigate, but not how much to irrigate.”
In short, these tools are helpful for timing irrigation, but they fall short when it comes to precisely managing water quantities, which is crucial for optimizing yields and conserving resources.
While all of these sensors, to a great extent, reduced the plight of the farmers, they did come with their fair share of drawbacks. Hence, came into play the modern sensors which will be discussed in a future blog.
Reference:
- Neutron Probe image: https://en.wikipedia.org/wiki/Neutron_probe#/media/File:NeutronProbe-0001.svg
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