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Title: QUANTITATIVE ANALYSIS OF AGRICULTURAL SOILS USING NEAR-INFRARED REFLECTANCESPECTROSCOPY AND A FIBEROPTIC PROBE

Author
item Reeves Iii, James
item McCarty, Gregory

Submitted to: Near Infrared Spectroscopy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Near-infrared spectroscopy (NIRS) is an analytical technique, based on light, which has been used to determine the composition of materials ranging from animal feeds to drugs. It is much faster than chemical methods and generates no wastes. One way of performing NIRS is to use fiber-optics to deliver the light to the sample. This allows samples to be scanned in their containers or even by placing the probe against the object in question, i.e., a piece of fruit. The objective of this work was to investigate the usefulness of fiber-optic based NIRS in determining various forms of carbon and nitrogen in agricultural soils, which are important in determining soil quality. One hundred and eighty samples obtained from experimental plots at 2 locations with 3 replicate plots under plow and no till practices at each location with three rates of ammonium nitrate fertilizer for each plot were studied. Results were compared to those obtained in which the samples were placed in a spinning cup about 3 cm in diameter (more traditional method). For reasons unknown, compared to results achieved using a spinning cup, the use of the probe resulted in more concentration outliers (Samples poorly determined), otherwise, the final results were quite similar to those achieved using a spinning cup for organic carbon, and various measures of nitrogen. In summary, this work showed that NIRS, using a fiber-optic probe, can successfully determine compositional parameters of agricultural soils.

Technical Abstract: The objective of this work was to investigate the usefulness of near-infrared reflectance spectroscopy (NIRS) in conjunction with a fiber-optic probe in determining various constituents (total N, organic C, active N, biomass and mineralizable N, and pH) in agricultural soils. A NIRSystems model 6500 spectrometer equipped with a fiber-optic probe was used to scan soil samples (n = 180) obtained from experimental plots at 2 locations with 3 replicate plots under plow and no till practices at each location with three rates of NH4NO3 for each plot. Optimal results were achieved when spectra were derivatized and only data from 1100 to 2300 nm used, with every 20 data points averaged from 1900 to 2300 nm. For reasons unknown, compared to results achieved using a spinning cup, the use of the probe resulted in more concentration outliers. Otherwise, the final calibration results were quite similar to those achieved using a spinning cup, with calibration (outliers removed) R2 and % RMSD of 0.96, 0.086; 0.95, 0.0075; 0.88, .0024; and 0.80, 0.0017; for organic C, total N, active N and biomass N, respectively. The R2 and RMSD for pH were 0.80 and .29 pH units, respectively. In summary, this work showed that NIRS, using a fiber-optic probe, can successfully determine compositional parameters of agricultural soils (particularly organic C and total N). However, there appeared to be a greater problem with outliers when using a fiber-optic probe than was true when obtaining spectra using a spinning cup.