A soils pH map for the sake of the map is just a lot of information for a price. Using it at least two years on a site-specific basis to bring weak spots in the field up to snuff can increase profit for producers.

That's one of the findings Randy Weisz, North Carolina State University Extension small grains specialist, made in a three-year study on C&H Farms, owned by Darryl Corriher and Tom Hall of China Grove, N.C.

The study, “Precision Farming Economics: A North Carolina Case Study” sought to give growers ideas and answers about whether or not precision agriculture is profitable in North Carolina.

The answer: Yes, variable-rate, precision agriculture is profitable, but only if you use the maps that are created, and only for certain nutrients. Variable-rate phosphorous and potash on the acidic soils of the Piedmont doesn't have much impact on bringing soils into balance in the Piedmont, but lime does.

It might not happen in the first year, it might not happen in the second year, but by the third year variable-rate lime pays off on acidic Piedmont soils where soybeans, corn and wheat play a heavy role in the rotation.

At C&H Farms, precision agriculture meant dividing a field into three parts and giving some thought to field history.

Corriher and Hall are religious in liming their fields to an average 6.4 pH. But when Weisz, Ben Knox, North Carolina Department of Agriculture agronomist and Lonnie Reed, Southern States agronomist, started the study, they found that at least 30 percent to 50 percent of the field was below a pH of 6.0 — even with “religious” limings. “Even with excellent liming and management, you can still have a large percentage of the land with too low of a pH,” Weisz says. “In the Piedmont, we're in acidic, highly erodible soils that are eating up more lime than regional recommendations call for.”

After one-acre grid soil maps indicated the low pH areas of the field, the farmers and researchers “mapped out a prescription of how you will be testing and tracking the pH over the next two years,” Weisz says. “And be creative.” In this case, it was easy to be creative.

The low areas were all in one part of the field, so in the second year Weisz just took samples in that area. “You don't have to make another map, just use the first soil map of the entire field as the guide and soil sample the different areas of the field separately.”

For C&H Farms, the field in question had an average pH reading of 6.4, but a range of 5.1 to 7.0, Weisz says.

“I've heard of variable-rate applicators who just go to the areas that were low and adjust the maps after the first year.”

Farmers could also divide problems areas within a field and treat the area as a separate field for variable-rate purposes. “Manage the field as if it were three small fields.”

The wide range of pH levels at C&H Farms was due to the history of the field. The fence line of a long-since-plowed-under pasture and a terrace separated the field into zone of adequate and low pH levels.

“The question we had was, ‘Was it due to the fact that part of the field hadn't received lime during the 1980s or was there something inherent in that part of the soils?” Weisz asks.

The answer started to appear in the second year and became apparent in the third year.

“This test showed that variable-rate liming does indeed bring the soils up,” Weisz says.

There was no response from variable-rate phosphorous or potash. “Don't mess with variable-rate phosphorous and potash.”

In the study, soybeans responded the most, with higher yields in field areas where the pH level was brought up to target. The study also turned up another tidbit that could save farmers money: Mapping soils every year isn't economical in precision agriculture. “But if you look at those pH maps and use them to develop a long-term management prescription for the field, then it becomes very profitable,” Weisz says. It took the length of the three-year study to turn around the target pH levels within the fields.

Over the course of three years, profit increased to $12 per acre, with the attendant increases in soybean yields.

On wheat, the researchers didn't see a yield response.