Every crop has an imaginary bucket of water that must be filled close to the rim without going over it in order to achieve maximum yield. Keeping that bucket full of water has been a challenge since farmers first started tilling the soil.

In three years of testing at the Edisto Agricultural Research and Extension Center in Blackville, S.C., Clemson researchers have succeeded in keeping the bucket full for watermelons. Similar work in the Delta has done the same for cotton.

The silver bullet used in both places is a soil moisture sensor that checks the water profile of these crops and provides data needed to turn on irrigation systems when the water bucket gets down to 15 percent and when the bucket reaches 65 percent capacity. The results have been nothing short of phenomenal in dry years and significant in wet ones.

Gilbert Miller, a horticulturist and researcher at Clemson University’s Edisto Agricultural Research and Extension Center in Blackville, S.C., heads the soil moisture monitor project. Miller says there is nothing magic about managing soil moisture, but good management is predicated on knowing what is going on in the soil and these probes offer a good opportunity to give growers the information they need to make good water management decisions.

David Lankford, a Del Marva farmer and general manager of Earthtec Solutions, says, “We are using what we call a multi-level capacity probe with dual frequency. With one frequency we measure moisture and with the other we measure moisture and salinity. By subtracting the moisture from salinity, we can determine salinity,” Langford says. Each probe is set at 4 inch increments down to 20 inches, explains Lankford, who was instrumental in helping Miller acquire the grant money needed to conduct on-farm testing of this water management system.

The soil monitors can be accessed by several telemetry methods, including cell phones. The soil probes send out an electronic signal that will call in every 15 minutes, if desired, though the system can be set to any time interval default.

These measuring devices provide percentage of water in the soil and through field calibration can estimate field moisture capacity. This allows the grower to check the readings and compare to field capacity. Based on this data, the equipment can actually turn on irrigation, or the grower can do that manually.

Coastal plains soils dominate the soil types in which crops are grown in the Southeast. In most of these soils and with most of the crops grown in them, the water holding capacity is less than the daily demand for moisture. The lack of water holding capacity of these soils makes it virtually impossible to water once a day and get it right without leaching, Lankford says.

With these soil sensors, growers can apply water in smaller increments and keep the bucket of water needed for optimum growth near the top. In the Clemson research ‘near the top’ is when 15 percent of the moisture requirement of the target plant is depleted.

In South Carolina, when water was applied to watermelons when the bucket was 65 percent empty versus 15 percent empty, the yield differential in 2008, an extremely dry year — seven inches of rain all season, was a 50 percent increase in melon yield per acre for the 15 percent empty application.

The 133,000 pounds of watermelons really got the attention of the Clemson researchers. Though subsequent results in years with more normal rainfall have not been as dramatic, the increase in yield, plus the more efficient use of water has been consistent, regardless of climatic conditions during the growing season.

“In that dry year (2008) we were able to conserve our water and use it when the plant needed it the most. So, not only did we get a huge yield increase, but we achieved that level of production by applying just what the plant needed, not over-applying water as often happens in a dry year,” Miller says.

The triggering device on the soil probes, in South Carolina’s sandy soils often call for water twice a day — when only 15 percent of the moisture is depleted. By comparison, when the triggering device is set at 65 percent, the irrigation system may come on only twice a week.

Lankford says frequent irrigation may lead some to believe the field is using more water. In reality, he said, it usually uses less water because there is little or no leaching.

In 2009, a wet year — with 20 inches of rain — the differential was less than 20 percent. In a wet year, you might think drip irrigation wouldn’t make much difference. However, there is almost always a critical period — maybe only two to three weeks — in the growing season when moisture isn’t available.

In the third year of testing (2010), Miller says it looks like the yield differential will be something close to a 20-25 increase in yield per acre for the 15 percent versus 65 percent depletion of moisture in the soil.

“Through harvest this year, we got 13 inches of rainfall. This is extremely sandy soil on which the research plots were planted. Some of the check plot plants that didn’t get triggered by the soil sensors died in the field this year,” Miller says.

The next step, he says, “is to take these sensors to a production situation as see if the dramatic data we have gotten here on the research farm hold up on a commercial operation.”

These sensors measure the amount of moisture in the soil, which is determined by how the target plant is using the moisture. We are going to put these on two commercial farms in the Pee Dee area of the state and one in the southern end of the state.

These sensors will control the irrigation on the farm. When the sensors record a 15 percent deficit — 15 percent of our bucket of water is gone — they will trigger a short irrigation because the sensors can sense how much water got into the root zone. Very short irrigation cycles will be used to keep water and nutrients in the root zone, Miller says.

The commercial growers, who will cooperate in the Clemson study, have large watermelon acreages, but Miller’s research team will be working in 15-25 acre fields.

“There is no doubt these sensors work in a research setting. Sometimes when you go from a research to a commercial setting unknowns can crop up and cause problems. We want to be sure these unknown factors are considered before we recommend using these sensors on a wide-scale basis,” Miller says.

“This year we had a prolonged period of drought at the Edisto Station, then we got 3.9 inches of rain. It only took two days for the automated system to kick-in and start watering our crop,” Lankford says.

“We are learning a lot about how fast water moves through a crop, how well the soil holds it and how quickly we need to get water back to the field. We’ve learned a lot and now we are excited about going to real life situations to see if our research plot data holds up,” Lankford says.

It’s amazing what opportunities technology offers us, says veteran Clemson University Horticulturist Gilbert Miller. “I had an opportunity to travel to Syria recently, and I was able to get online and from there I could access my irrigation system here at the Edisto Station. I could see from my computer in Syria that one of the irrigation lines was down, and I could manually click onto the pump house and get the system running,” Miller says.

Though the South Carolina tests have been primarily with watermelon and vegetable crops, similar results have been recorded in field crops, such as cotton. “In cotton in the Delta, we have seen steady watering gives a better yield and cotton comes off early.

The biggest mistake of applying water to cotton is the tendency to let the water cycle go too long and the cotton plant goes into stress, creating a number of production challenges.

Using the same sensors used in South Carolina in cotton in Arkansas has shown a half bale to one bale yield increase by using regular watering cycles as the plant needs it. In some cases growers have harvested their crop up to 20 days earlier, because the even watering kept the plant stress free throughout the growing season, Lankford says.

With national attention focused on such projects as the Chesapeake Bay Initiative and other water management and water utilization issues, knowing when to water and how much to apply are going to become more critical farm management issues in the future.

e-mail: rroberson@farmpress.com