FAYETTEVILLE, Ark. — Each pass of a tractor over a field compacts the soil, gradually restricting root growth and inhibiting plants’ ability to take up water and vital nutrients.

Farmers can break up the compaction with deep tillage — subsoiling — based on measurements that indicate when compaction may have become severe enough to diminish yields. Those measurements are made by manually pushing a probe, called a cone penetrometer, into the soil at numerous points around the fields, said Sreekala Bajwa, assistant professor of biological and agricultural engineering at the University of Arkansas Division of Agriculture.

“It’s a very time-consuming and labor-intensive process,” Bajwa said. “I’m trying to find a more efficient inspection system using new technologies.”

Bajwa conducts research on agricultural applications for remote sensing instruments for the Arkansas Agricultural Experiment Station. Among those applications, she and graduate student Subodh Kulkarni are studying two instrument methods for quickly detecting and mapping soil compaction in cotton fields.

One experimental method uses soil electrical conductivity to measure compaction. Bajwa said the ease with which electrical current can move through soil is determined by soil type, organic content, moisture and compaction.

Bajwa and Kulkarni use electromagnetic instruments that induce electrical currents at a shallow depth of 13 inches and a deeper depth of about 39 inches. The instrument readings are compared against traditional cone penetrometer readings. Biomass, growth and yields from the cotton test plots are also figured into the equation to determine compaction and its effect on the crop.

“The advantage of using soil electrical conductivity is that the instrument can be towed around the field behind a tractor or all-terrain vehicle, taking measurements in a fraction of the time of traditional methods,” Bajwa said.

Another method uses reflected light from the leaf canopy to measure plant performance relative to soil condition.

Kulkarni uses a hand-held sensor connected to an instrument pack on his back and a laptop computer to measure sunlight reflected from both a reference reflector, called a Spectralon panel, and the leaf canopy. The instrument, called a spectro radiometer, measures the difference in reflected light between the two surfaces, expressing the result as a ratio that is correlated to the health of the cotton plants.

The health of the plants is related to water and nutrient stress, Bajwa said. If all the plots are receiving the same amount of water and nutrients, and some of them exhibit more stress, then something is restricting water flow to those plants. All other factors being equal, the culprit must be roots stunted by compacted soil.

“Reflectance can be measured by aircraft or even by satellites,” Bajwa said.

The data from both instrument methods can be mapped using global positioning system (GPS) technology. It can also be used to guide variable subsoiling equipment under development by other engineers. The equipment will deep till only areas of a field where soil compaction is a problem, Bajwa said.

Bajwa is also trying to determine just how often subsoiling is necessary. She and Kulkarni measure the biomass and yields of plants in the test plots, which are subjected to varying degrees of intentional compaction.

“There is conflicting data on the effects of compaction,” Bajwa said. “We want to provide more accurate information that can prevent unnecessary and costly subsoiling trips over the field.

Fred Miller is science editor for the Arkansas Agricultural Experiment Station. e-mail: fmiller@uark.edu.