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 surprised that we could get in the parts-per-million range with the soil moving past the analyzer," says John McClelland, a research scientist with the Roy J. Carver Dept. of Biochemistry, Biophysics and Molecular Biology and Ames Laboratory-U.S. Department of Energy. The team used a technology, invented at Ames Lab, called transient infrared spectroscopy, or 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 recently published in a research journal.
More answers needed before a commercial tester becomes available
The ISU 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," says 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 emphasizes that nitrate was measured as opposed to total nitrogen. He says 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, they lose nitrate
"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," says Laird. "One of the best ways to improve nitrogen use efficiency in ag 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 says 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 new 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.
How does new testing method compare to late-spring soil nitrate test?
"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," says 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."
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." If the research progresses to field tests, McClelland says 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's been in touch with French researchers who are working on commercializing a smaller, more compact detector.
Laird says 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.