Soil moisture sensors tell you when, how much to water

01/20/2012 01:13:00 PM
By Tom Burfield

Growers first experimented with drip irrigation in the 1940s, but with the advent in the 1960s of polyethylene plastics to make the drip tubes, the practice quickly became widespread.

Today, drip is an irrigation method of choice throughout the United States, especially for high-value crops.

The process continues to be refined as technology improves and as water conservation becomes an increasingly significant concern.

At the forefront of these refinements is Gilbert Miller, area vegetable specialist who heads the soil moisture monitor project at Clemson University.

Miller has come up with a system that uses moisture sensors to determine when and how much to water crops.

The sensors have copper bands that send radio waves through the soil at depths of 4, 8, 12, 16 and 20 inches. By monitoring the strength of the radio waves that come back to the transmitter, Miller can determine how much moisture is in the soil.

Signals typically are sent out every 15 minutes, but the timing can be adjusted. The signals are sent to a radio that, in turn, sends them to the Internet, allowing Miller to monitor moisture levels from anywhere he has access to a computer or even a smart phone.

Solar powered

The system is powered by solar panels and can be set up to automatically trigger irrigation. Or Miller can activate irrigation manually.

A computer screen has an icon for each electronic valve that indicates when the valve is open and supplying water to the field.

“The system bases the need for irrigation on what is in the soil,” he says.

Growers already have several models that measure inputs—like rain and irrigation— and outputs—like drainage and evaporation.

But then the grower must figure out theoretically what a plant needs.

“The sensors pretty much do all of that for us,” Miller says.

It’s just as important not to over-irrigate as it is to not under-irrigate, he says.

“We’re trying to apply only as much water as what the plant can take up,” he says. The irrigation is often in short bursts, five or six times a day, rather than inundating the plant once a day or so.

Miller’s goal is to keep the water saturation as close to field capacity as possible— to the point where the water level is stable, not moving, and the water is present for the plant to use as needed.

When 15 percent of the water is depleted, Miller triggers the system to refill the “bucket” not quite to field capacity.

Labor, water saver

The system can help save labor, save water, increase yields, improve the quality of the product and reduce environmental risks, he says.

Miller has been conducting his research in small plots at a research station for five years and now is beginning to conduct tests at larger locations in satellite research sites with varying soil types.

In one such test, he compared his results irrigating with sensors in a test plot in grower Billy Jenkins’ field in Jefferson, S.C., with Jenkins’ traditional method of irrigating once a day.

“We were able to get 15 percent to 20 percent over his yield and used 20 percent less water,” Miller says.

During the test, Jenkins noted that the system started irrigating whenever the water level dropped, no matter the time of day, whereas on the control portion, he would irrigate at certain times each day, whether it was needed or not.

The system would be “worth its weight in gold,” especially on farms of about 40 acres or more, he says.

The number of sensors can vary, depending on the number of soil types in a field. Usually one or two sensors are sufficient for up to 75 acres.

They are fairly durable. Miller has used 12 sensors for five years with few problems, but they are electronic components, he emphasizes, and need to be installed properly.

The Enviro Scan sensors he uses are made by Sentek Pty Ltd. of Kent Town, South Australia. They’re available in the United States from Earthtec Solutions LLC of Vineland, N.J.

A simple manual monitoring system with two sensors and a weather station can cost less than $3,000, says David Lankford, general manager for Earthtec.

A more elaborate setup with automatic irrigation and controller valves can range from less than $5,000 to $30,000, depending on the irrigation system the grower already has.

Sensing improvement

Coosaw Farms in Jefferson has been using sensors for about eight years, says Brad O’Neal, operations director.

“They’ve definitely worked,” he says. “We’re very pleased with them.”

Coosaw Farms uses six sensors to monitor 30 acres of blueberries and 70 acres of cabbage on various soil types. During the spring, they’re used on 65 acres of watermelons along with blueberries.

O’Neal hopes to incorporate more sensors into the farm’s 500 acres over time.

He says quality is improved, and yields can increase by 10 percent, depending on conditions.

He found the system to be most effective in drought years, when low humidity and high temperatures “are really sucking the water out.”

O’Neal says he actually saves more on fertilizer than water, since the system prevents overwatering, which can leach nutrients below the plant root zone.

He’s especially pleased that the data is accessible via the Internet. On a recent vacation, he was able to check to see that pumps were running and monitor the moisture content of fields, even though he was outside the country.

Eventually, sensor-based drip irrigation will be the norm, Miller says he believes. “In the future, we as growers are going to have to account for our irrigation a little bit more rigorously and have a little bit more scientific justification for how we apply our water, when we apply it, how long we apply it,” he says. “Down the road, this will be very necessary.”



Comments (0) Leave a comment 

Name
e-Mail (required)
Location

Comment:

characters left

Feedback Form
Leads to Insight