Iowa State University Researchers Use New Way to Measure Nitrate in Soil

The TIRS soil analysis in the laboratory starts with the spinning dish (foreground) with a soil sample spread out along its rim. A nozzle (in the middle of the photo) directs a stream of hot air onto the soil as it spins past making the soil glow in the infrared. The small box above where the hot air stream strikes the soil contains a mirror that directs this infrared light into the spectrometer for analysis. The spectrum of the infrared glow is displayed on the computer monitor at the far right.

AMES, Iowa — A team of Iowa State University scientists have measured nitrate in soil with a unique infrared sensor system opening the possibility of determining the level of this vital nutrient in real time as fertilizer is being applied to fields.

“We were actually surprised that we could get in the parts per million range with the soil moving past the analyzer,” said John McClelland, a research scientist with the Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology and Ames Laboratory-U.S. Department of Energy.

The team used a technology, invented at Ames Laboratory, called transient infrared spectroscopy (TIRS) to measure nitrate in soil and then compared those values to ones obtained using traditional soil testing. TIRS works by measuring light emissions in the infrared spectrum after hot air is applied to the surface of the moving soil. The research was published in the journal Applied Spectroscopy.

The research team stressed that this was a preliminary study in the lab and many steps are necessary before a possible commercial tester could be available.

“There are a lot of steps up that ladder before we’d have any hope of commercialization. The breakthrough here is that no one has been able to come up with a plausible tool for effectively measuring nitrate in the field,” said David Laird, a professor of agronomy.

He and McClelland did the work with Roger Jones, a research scientist with the Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology and Ames Laboratory-USDOE and Sam Rathke, an agronomy research associate.

Laird emphasized that nitrate was measured as opposed to total nitrogen. He said nitrate measurement is especially important because it is the dominant available form of nitrogen for growing crops, but is very mobile in the soil.

“Agricultural soils in Iowa are leaky; that’s why we have so much nitrate going down the Mississippi and contributing to the zone of hypoxia in the Gulf of Mexico,” he said. “One of the best ways to improve nitrogen use efficiency in agricultural production is to spoon-feed nitrogen to the crop while it is growing. To do this, you need to be able to diagnose what the situation is at the critical time; does the crop need more nitrogen or not.”

Some technologies propose determining nitrogen needs by analyzing the color of the growing crop’s canopy. Laird said that approach is complicated by alternative reasons the canopy could start to yellow or not have the right colors, such as disease or environmental conditions.

Another advantage to this technology is the ability to quickly determine the “spatial variability” of nitrate, in which one part of the field may have more than enough nitrogen, and another part of the field may be deficient.

“The late spring nitrate test can do that, but it takes an army of people with soil probes to gather samples, which then have to be analyzed in the lab,” said Laird. “If you could short-circuit all of that using a spectroscopic technique, that allows us in real time to assess the current status of the soil’s nitrate level, it would greatly benefit precision nitrogen management.”

McClelland said the next step for the research is to test several soil types to see if they are able to obtain the same quality of data, and would help simulate field conditions because soil types vary continuously. The initial tests in the lab were on one type of soil that was placed on a disc that rotated under the analyzer’s probe.

“If we get good results on additional soil samples that would be impetus to go on and get funding to go on beyond that,” he said.

If the research progresses to field tests, McClelland said it should be feasible to design a cost effective tool that just looks at the necessary wavelengths instead of the more expensive test machine they used. He has been in touch with French researchers who are working on commercializing a smaller, more compact detector.

Laird said a field model also would need an attachment that would remove any plant material from in front of the probe and open the soil so that the probe could be viewing a fresh patch of soil an inch or two beneath the soil’s surface.