Making Sense of Soil Moisture Sensors

By Lynette Von Minden

 Ask any irrigation professional about the hottest products on the market today, and they’ll probably tell you that components offering “Smart” technology are creating the most buzz. Controllers featuring Smart technology can automatically modify watering schedules based on weather conditions for a profound impact on the amount of water used. However, only one type of Smart technology can communicate actual real-time soil moisture levels back to the irrigation controller — a soil moisture sensor.

How they work

Soil moisture sensor technology is based on a concept called “closed-loop” irrigation control. In an open-loop control system, there is no direct link between the output of the system and the conditions being controlled. Root zone moisture is controlled without direct measurement or feedback. On the other hand, a closed-loop control system uses a soil moisture sensor that’s buried underground to periodically measure absolute soil moisture levels and relay that information back to a controller interface. When soil moisture levels dip below a pre-determined threshold, the controller interface should interrupt the irrigation schedule that’s been programmed into the system’s timer. If the sensor recognizes dry soil conditions, the system’s next watering cycle will proceed as originally scheduled. This closed-loop feedback process means that actual plant and turf conditions at the roots are being communicated back to the controller for a more accurate snapshot of a landscape’s moisture needs at any given time.

The advantages of a closed-loop system are plain to see. Soil moisture is measured directly at the root zone. There is no need for seasonal clock adjustment. There is very little need for end-user intervention. The system automatically adjusts for varying temperatures, precipitation and other changes in weather that influence soil moisture content. Perhaps most compelling, customers save up to 40 percent more water than they would even with a well-programmed standard controller — and that figure rises to a staggering 70 percent when a soil moisture sensor is combined with a Smart controller.

However, the realization of all these advantages hinges on the accuracy of the soil moisture sensor at the center of a closed-loop irrigation system. Moreover, it is important that this sensor provide absolute, rather than relative, soil moisture measurements. While relative sensor measurements change with other environmental conditions, requiring frequent calibration and becoming unreliable over time; absolute sensor measurements provide a stable moisture reading regardless of soil type, compaction, electrical conductivity or temperature changes.

Past performance and testing issues

Soil moisture sensors have been around for a while, but irrigation specifiers and contractors have rarely recommended their use to clients. Why? According to Diganta Adhikari, research associate for the Center for Irrigation Technology (CIT) at Fresno State University, early soil moisture sensors gained a reputation for inconsistent performance.

“Soil moisture sensing is not something new,” Adhikari said. “The technology dates back to the 1960s and less-sophisticated, pre-electronic tensiometers. Earlier technology was inaccurate when used in different soil types and different water types like grey water or saltwater.”

Soil moisture sensors from years past have also had other issues, including limited sensor life and durability, frequent maintenance requirements and difficult installation and set-up. Burying the sensor and then wiring it back to the controller used to require a great deal of messy trenching as well. Another issue was a lack of industry-wide testing standards for these products. “For a long time, another problem was that there were no common tests,” said Adhikari. “Every manufacturer had its own in-house testing protocol, so there was no way to compare each device’s test results.”

Fortunately, that situation began to change in the early 2000s when the Irrigation Association (IA) approached the CIT for a standardized testing protocol. After years of back-and-forth among CIT, the IA and manufacturers, the first draft of the standardized protocol was available in 2004, and soil moisture sensor tests could begin. For a fee, the voluntary program allows manufacturers to submit products for testing. Once tests are complete, the manufacturer reviews the data and determines whether the information should be released publicly through the IA.

Sensing a brighter future

As a result of these industry changes, modern soil moisture sensors have improved upon past technology. Rick Foster, senior product manager for Rain Bird’s Accessories Group, explained how new soil moisture technologies measure the time required for an electronic pulse to travel along the sensor’s waveguide. Faster times indicate dry conditions, while slower times indicate wet conditions. These patented technologies digitize the waveform at high frequencies, using signal processing to successfully extract time delay information. The result is absolute, rather than relative, soil moisture measurement. Advantages include inherently stable readings of Volumetric Water Content and accurate measurements, even with changing soil types, temperatures and electrical content.

Soil moisture sensors can be integrated into new irrigation systems, or contractors can use them as an easy retrofit for existing systems, greatly increasing their water efficiency. Installing these sensors can be as simple as selecting a full-sun location for the sensor, burying the sensor three to five inches below the surface and wiring the sensor to the nearest valve.

According to Foster, newer sensors can be wired to the valve instead of back to the controller, requiring far less trenching than previous soil moisture sensors. Installing the controller typically involves wiring the interface to the controller and programming it. Programmers simply set the site’s “turn-on” threshold then program the controller to water on certain days of the week. Then, the soil moisture sensor will suspend irrigation whenever soil moisture exceeds the established threshold based on moisture readings.


Smart, efficient and affordable

Recognizing the greater levels of water efficiency possible with a reliable soil moisture sensor, the IA has designated controllers equipped with this technology as “Smart” controllers. In light of drought and water restrictions, more cities are mandating the use of Smart control for new construction. According to Gary Hartwell, director of public works for the City of Frisco, Texas, all newly constructed homes in Frisco are required to have Smart controllers, and, at a cost of about half that of a Smart controller, soil moisture sensors offer builders and home buyers an affordable way to obtain this level of technology. Like many of the cities with these types of mandates, Frisco’s Public Works Department also offers a rebate that makes the installation of an IA-approved soil moisture sensor even more economical.

“Simply provide us with the invoice,” Hartwell said. “And we’ll provide you with an up to $100 rebate.”

A growing number of other water agencies and purveyors throughout the country are also acknowledging the advantages of soil moisture sensors by making these products eligible for Smart control rebates. When rebates and water savings are taken into consideration, the better soil moisture sensors on the market may save enough water to pay for themselves in less than a year. In addition to saving water and money, the closed-loop technology implemented by soil moisture sensors can help homeowners avoid the common tendency to over water their lawns, making it easier to sustain healthy landscapes that are less susceptible to fungus and shallow root growth.

These sensors also offer advantages that will strike a positive note with the contractors who install them. While previous soil moisture sensors were difficult to set up and required frequent recalibration, newer units can be quickly installed during an initial visit and require little maintenance over time. Most current versions of these products tend to work better with turf-dominated landscapes where zones have similar characteristics, but new soil moisture sensors are being developed that will offer the ability to locate a sensor in each zone, making them useful for just about any property. And because their costs are easily recouped, soil moisture sensors are a relatively easy upgrade to sell to customers.

Today’s soil moisture sensors have made great strides during the past 10 years to become a valued component of just about any irrigation system. They offer contractors and end-users another affordable option to upgrade to Smart control, keeping their landscapes healthy while using considerably less water for a better environment.

Lynette Von Minden is public relations counsel with Swanson Russell.

Article provided by Rain Bird, which recently launched its new SMRT-Y (pronounced “smart why”) Soil Moisture Sensor Kit, which utilizes using a Time Domain Transmissometry (TDT) digital sensor and Digital Signal Processing technologies. For more information, visit

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