Christy, Colin (Veris Technologies, 601 N.,  Broadway, Salina, KS, 67401; Phone: 785-825-1978; Fax: 785-825-6983; Email:


In-Situ NIR Spectroscopy to Measure Carbon and Other Soil Attributes


C.D. Christy *, D.A. Laird, K.A. Sudduth


This presentation discusses the measurement of soil attributes using diffuse optical reflectance in the near infrared (NIR) region.  The functional groups responsible for absorption in this region of the spectrum are C-H, N-H, and O-H, allowing for the measurement of organic carbon, nitrogen, and moisture.  In addition, the method is responsive to soil pH through a hydrogen bonding phenomenon that tends to shift the position of water absorption peaks.  Consequently, the method can be used to measure both carbon and factors that affect the carbon cycle. Recent studies have demonstrated the utility of near infrared spectroscopy (NIRS) in a  laboratory setting.  This work seeks to extend the method into the field using in-situ measurement apparatus.  Measurements of horizontal variations in carbon are being made with a specially designed shank while vertical changes are measured using a probe.        Calibration of NIRS is performed using chemometric methods, which are based on multivariate statistics.  Representative samples are analyzed using standard analytical methods to provide calibration and validation data.  The calibration results of two studies using the optical shank will be presented along with preliminary results of vertical measurements using a reflectance probe.  In the study where reference analysis was performed for total carbon (N=288), calibration resulted in a root mean squared error of prediction (RMSEP) of 0.35% and a correlation of 0.96 between the actual and predicted total carbon values.