Measuring Soil Carbon Rapidly With

Near-Infrared Spectroscopy

 

An emerging method of soil carbon measurement involves a tractor and lightwaves. The technology is called near-infrared spectroscopy (NIR). Veris Technologies, based in Salina, Kansas, has developed an NIR machine that can be pulled behind a tractor, collecting carbon measurements “on the go.”

 

The process involves a light that is directed at the soil through a sapphire window. The window is pressed directly against the soil and is pulled 5 cm below the soil surface. The light bounces off the soil, and is broken into wavelengths like a prism. Those wavelengths can be calibrated to produce a carbon map of the field. A companion NIR probe is used to measure soil carbon up to 60 cm below the soil surface to get carbon measurements at a wide range of depths.

 

The advantages are that hundreds of carbon measurements can be taken at a time, and no soil preparation is needed, says Eric Lund of Veris Technologies.

Near-infrared light waves are between visible light waves and microwaves on the electromagnetic spectrum. NIR spectroscopy has been used since the 1950s to test grains, feeds, meat, and other biological materials. It has also been used in the pharmaceutical industry because of its nondestructive nature, according to a presentation titled “Mapping Soil Carbon with On-The-Go Near Infrared Spectroscopy” by Colin Christy of Veris Technologies (http://www.oznet.ksu.edu/ctec/Fall%20Forum%20pdf%20files/Papers_Abstracts/Christy_Veris.pdf). Portable pull-behind NIR technology for in-field measurements has been pursued since the 1980s.

 

NIR takes carbon measurements, says Lund, but it doesn’t take samples. The machine can compare how much relative carbon there is from one area to the next, but cannot determine the actual amount of carbon in the soil until it is calibrated against a standard. To get actual carbon readings, a few core samples need to be pulled from the field and sent to the lab. There, carbon levels are analyzed by using the traditional method of dry-combustion. That data is then calibrated to the NIR measurements using advanced chemometric techniques, which then results in a carbon estimate for each area of the field.

 

For maximum accuracy, this should be done on a field-to-field basis -- and in the future, an area-to-area basis, says Lund. The process can take days if the samples have to be newly calibrated. But if previously collected data are used, carbon levels can be determined minutes after NIR measurements are collected, making the total process relatively quick.

 

Lund says field testing in several states hasn’t revealed any major interferences for the machine when used in most agricultural fields. Possible interferences could be very rocky soil, or soil that is predominantly composed of limestone. NIR only responds to organic carbon, and the machine could underestimate the amount of inorganic carbon in limestone soils. Also, if measurements need to be taken in a forest or other area where tractors are hard to maneuver, there might be problems because of the overall design of the machine. In that case, the companion probe would be used.

 

Lund says that like any technology, there is room for improvement, and that more research needs to be done about carbon measurement in general. Lund and other researchers at Veris Technologies are working to make NIR an easy and efficient way to assist in a future carbon credit program.

 

Lund will be making a presentation of NIR technology for measuring soil carbon at the upcoming CASMGS Forum: Agriculture’s Role in the New Carbon Economy, Dec. 17-18, 2007 at K-State. For more information, see: http://soilcarboncenter.k-state.edu/Fall_Forum_CASMGS.html

 

-- Katie Starzec, CASMGS Communications, Kansas State University

kstarzec@ksu.edu

 

Figure 1. The NIR equipment from Veris Technologies making field measurements using a shank.

 

 

Figure 2. To calibrate the NIR equipment readings, soil probes are first used to determine actual soil carbon levels.