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United States Department of Agriculture

Agricultural Research Service

Title: Mid-Infrared Spectroscopic Properties of Humic Acid and Fulvic Acid-Soil Mixtures

item Calderon, Francisco
item Reeves Iii, James
item Vigil, Merle

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 6/1/2010
Publication Date: 6/28/2010
Citation: Calderon, F.J., Reeves Iii, J.B., Vigil, M.F. 2010. Mid-Infrared Spectroscopic Properties of Humic Acid and Fulvic Acid-Soil Mixtures. Meeting Abstract. Presented at the 15th International Humic Substances Society Meeting. June 27 to July 2, 2010. Pto. De la Cruz, Tenerife, Spain.

Interpretive Summary:

Technical Abstract: The detection of humic materials in soils is essential in order to determine organic matter (SOM) stability and C sequestration on agricultural land. Mid-Infrared (MidIR) spectroscopy has been used to characterize SOM quality [1], study extracted soil humic acids [2], develop calibrations for quantifying SOM [3], and to study decomposition of organic matter in soil [4]. However, infrared spectra from soils are the result of a multitude of combined absorbances from organic and mineral bands, and some of the spectral signatures of humic acids can be lost or confounded in soil matrices. In this study, we added different amounts of authentic humic and fulvic acid standards to ashed soil in order to identify reliable spectral MidIR bands for marking the presence and amount of stable organic matter in soil. Several regions of the MidIR spectrum decrease in soils upon ashing, and at the same time are highly absorbed by humic and/or fulvic acids. These regions include 3500-2000, 1830-1520, and 1260-990 cm-1 and should thus be considered organic matter bands in soils. Other MidIR regions outside these bands are predominantly of mineral absorption or from a mixture of organic-mineral absorption. The 2870-2950 cm-1 is one of the few regions of the MidIR soil spectrum where absorbance is almost exclusively from organics, with little absorbance due to mineral sources. Our spectral subtraction approach suggests that 3400-2800 and 1730-1630 cm-1 are regions that can be attributed to stable organic compounds in soil spectra.

Last Modified: 06/26/2017
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