Soil moisture sensors give turf just the right amount of water

Imagine your home heating or air-conditioning system couldn’t detect the actual temperature in the house. You would have to program it by guessing how much heating or cooling your house would need tomorrow, next week, next month. There’s a good chance you would end up with a house that was too hot or too cold. Instead, there’s a thermometer built into thermostats to detect and monitor the temperature inside the house so the device can then deliver the correct amount of heating or cooling.


Soil moisture sensors, such as this unit from Baseline Systems, Inc., can help reduce water use and improve plant health by measuring moisture available in the rootzone and activating the irrigation system only when needed.
IMAGE COURTESY OF BASELINE SYSTEMS.

Until recently, many irrigation systems were like a thermostat with no thermometer. They could turn the water on and off based on a predetermined program, but they couldn’t tell how much water was already in the soil, or how much the grass needed. Thanks to the introduction of soil moisture sensors, irrigation systems can now base water delivery not simply on a preset frequency and quantity, but also while factoring in how much is needed.

It’s not that soil moisture sensors are a brand-new idea, it’s just that today’s sensors are far superior to those of the past, explains Mike Clark, irrigation designer and consultant (www.clarkirrigationdesign.com) in Lavonia, Ga. “It used to be that soil moisture monitors were basically a gypsum block sensor that basically absorbed moisture. They didn’t last very long, and they weren’t very reliable, so they didn’t make many inroads in the market,” he explains.

There are more models on the market now, he adds, and the technology employed is vastly improved. “New sensor technology has come out that is a lot more robust – it won’t fail; it doesn’t need to be replaced; it’s a lot more accurate in [determining] actual soil moisture levels; and it can be incorporated into different control systems to automatically make adjustments based on soil moisture levels,” says Clark. “These soil moisture sensors are just that: sensors. When used in conjunction with a smart controller, the two will allow for self-adjustment of the irrigation system’s operating schedule.”

It should be noted that these sensors differ from weather-based sensors that use weather data and evapotranspiration (ET) to calculate how much water is needed. Soil moisture sensors are placed in the soil and can detect the moisture that’s actually available to plant roots.

Clark says there are a number of different manufacturers offering soil moisture sensors, but most use a similar approach. Still, he notes, “Different manufacturers use different materials in their sensors, and they get different data back from their sensors.” Clark typically uses sensors and controllers from Baseline Systems (www.baselinesystems.com), but notes that Rain Bird (www.rainbird.com) now offers a small soil moisture sensor for residential applications, and there are similar options from other manufacturers.

The capabilities of different units depend on their complexity. In simple, less-expensive soil moisture sensors, the goal is to hold or prevent the irrigation system from running until the soil moisture level gets down to a set point. Then the sensor allows the irrigation system to do whatever it has been programmed to do.

More complex (and expensive) soil moisture sensors are “self-adjusting” explains Clark. “The systems read soil moisture levels, and then adjust the operational cycle to either replenish the amount of water that’s lost out of the soil profile, or hold the irrigation controller from operating until the soil moisture has been depleted.”

The set points on the sensors are adjustable, allowing the installer – or lawn care company, etc. – to account for variables ranging from soil type to turfgrass species to sun versus shade, etc. Clark says it’s absolutely critical that these factors be considered. While soil moisture sensors offer impressive advantages, they do require some attention, especially right after installation. “If you’re new to the sensor product and the area you’re working in, generally it takes a number of visits to monitor the site. You might see one area getting a little too much water, and you’ll need to make a small adjustment for that. Or another area might not be getting enough water. Whenever you’re using a soil moisture sensor, you need to tune your irrigation controller system so it’s applying just enough water to keep the plants at the level of health that you want.” The sensors are high-tech, but they can’t evaluate turfgrass health. In that regard, says Clark, “they’re just as stupid as a controller without a sensor.”

By taking the time to adjust the controller after installing the sensors, it’s possible to improve turf health while also saving significant amounts of water. The amount saved will depend on how effectively the irrigation system was operating beforehand, the number of sensors installed and many other factors. Manufacturer claims seem to range from 30 to 70 percent (or more) reduction in the amount of water used.

The location of the soil moisture sensors is important to their proper performance. For example, avoid placing them close to an irrigation head or other location that gets more or less water than average for the rest of the site. Instead, look for a place that best represents the area you’re trying to get information from.

The more sensors you install, the more you can tailor the system to the specific site. “Typically, you would put anywhere from one sensor – if you’re just looking for a simple on-off effect – to multiple sensors in different plant zones and/or microclimates on the site,” says Clark. He usually advises placing one sensor in a sunny turf area and another in a sunny mixed landscape area. He also suggests placing sensors in turfgrass in parking lot islands or other zones likely to be hotter and drier than the landscape as a whole. “You look for those little microclimates to really tune your irrigation system,” he explains.

Soil moisture sensors are wired (rather than wireless) and are connected back to the smart controller. In most cases, an existing irrigation system can be retrofitted with at least the simpler on/off type of soil moisture sensors. “If you’re getting into the smarter versions that are self-calibrating, you usually have to install a new controller with the sensors, but you can use the existing wiring path,” says Clark.

When soil moisture sensors are properly installed (seek the assistance of a qualified irrigation system designer and/or installation professional), and then checked and tuned for accuracy, these small devices can pay big dividends in better looking turf and water savings. “They do a great job of keeping plant material healthy and saving water, as long as they are properly adjusted,” states Clark.

Patrick White is a freelance writer and editor who has covered every aspect of the green industry in the past 15 years. He is based in Middlesex, Vt., and is always on the lookout for unusual stories.