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Soil-Moisture Sensors

03/17/2014 @ 2:21pm

Imagine a corn plant whose roots are able to relay real-time soil-moisture conditions. By telling you if the plant’s thirst is quenched or not, the roots can enable you to optimize your irrigation waterings.

That isn’t the case, of course. These days, though, you can come close. 

Soil-moisture sensors can mimic what roots are experiencing under the soil profile. They can tip you off when it’s time to irrigate or to hold off on watering. Perks include saving water, a key attribute in areas like the western Great Plains where water use is being tightened. Even if you aren’t facing water restrictions, avoiding unnecessary waterings saves you money. 

“By overirrigating, you tend to wash nutrients down through the soil,” says Scott Mauseth, Valley Irrigation product manager. “You also develop a shallow root pattern. That does not help the plant in times of heat or drought, compared with a plant with a deep root base.”

There is potential to boost yields, too. One three-year study by Monsanto, conducted across western Kansas and Nebraska, saved 2 acre-inches of water each year, says Nick Emanuel, president of CropMetrics. Meanwhile, average corn yields increased 6 bushels per acre.

Two sensor types
Soil-moisture sensors generally fall into two categories. 

1. Capacitance sensors. “These are good and accurate sensors,” says George Vellidis, a University of Georgia precision farming specialist. “Essentially, you put them in the ground and they produce a measurement.”

Since a soil moisture threshold should be determined based on soil texture, sensors must be calibrated during each installation. That’s a time-consuming drawback. 

2. Tensiometric sensors. These soil-moisture devices measure the energy level at which water is being held by soil.

“Every plant has to expend energy to move water away from the soil to the plant root,” says Vellidis. “These sensors are not not affected by soil type.” Vellidis prefers these sensors, since no calibration for soil type needs to be made. 

These are more user-friendly versions of older tensiometer units that irrigators have used for decades. Tensiometers can be high maintenance. 

“They break tension, and you have to fill them with water,” says Mike Thurow, president and chief executive officer of Spectrum Technologies.

One advantage is that tensiometric soil-moisture sensors are easier to use. Proper installation is crucial. “But once installed, they are pretty reliable,” says Thurow.

Sensor specifics

Here are four factors to consider when shopping for and installing soil-moisture sensors.

  • Number of sensors. A 100-acre field with the same soil could fare well with one site with two sensors. If a field has three main soil types, though, it’s recommended to have two sensors for each soil type (or six in a field), says Vellidis. Besides being a backup in case of lightning, a second sensor at a site helps balance field microvariability. “You can have a 10% to 20% difference in readings from sensors just a few feet apart,” Vellidis points out. “Even in a uniform field, you should have at least  two sensors in the field. Just putting one sensor in highly variable soils at one site is not optimizing water.”
  • Sensor depth. Depth of sensors can range from 8 to 12 inches down to 4 feet or more. The majority of readings will come from shallower depths, but a more complete picture will come when deeper readings are made. “You will get more incremental value by going deeper in the profile,” says Thurow. Some units also provide a software application that identifies the optimal location for the soil-moisture sensor. “Having the probe in the proper location is another piece of the puzzle to the most effective use of data,” says Emanuel.
  • Pricing. “Up to two to three years ago, many of the companies that produced these products would try to sell the equipment and then sell air time to transmit the data,” says Vellidis. “Now, they are on a different paradigm, where they sell the service and give you the data for a certain price.” Some less-expensive units involve manually reading each sensor site and then manually recording them. The other extreme consists of full-capacity sensors with a satellite link to a website and a cost of $2,000 to $3,500 per location for a year’s worth of use, says Vellidis. Some sensor systems make crop templates that correspond to a plant’s growth cycle and soil type. This information can then be relayed directly to smartphones, from which you can base your decisions.
  • Service. Some systems offer recommendations made from readings. “Unless you can break down the data and give it to users so they can put it into a practical application, it really has no value,” says Mauseth. Check for installation and ongoing support for services. “Locally trained data specialists can provide weekly updates to interpret what the data is telling you,” says Emanuel. “If you are buying a capacitance sensor, you need to work with a company that can provide you good agronomic support,” says Vellidis. Understanding where thresholds are is difficult to do during the calibration process, he adds. A service technician who understands the process will be able to help  you get the system going.

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