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United States Department of Agriculture

Agricultural Research Service

Title: Specular Reflection and Diffuse Reflectance Spectroscopy of Soils

Authors
item Reeves Iii, James
item Francis, Barry
item Hamilton, Stephen - MICHIGAN STATE UNIVERSITY

Submitted to: Journal of Applied Spectroscopy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 18, 2004
Publication Date: January 31, 2005
Citation: Reeves III, J.B., Francis, B.A., Hamilton, S.K. 2005. Specular reflection and diffuse reflectance spectroscopy of soils. Journal of Applied Spectroscopy. 59:39-46.

Interpretive Summary: Mid- and near-infrared spectroscopy utilize light in regions beyond human sight to determine the composition of materials such as soils. With powders, diffuse reflectance can be used where the light diffuses into the sample and is then reflected back to a detector for quantification. If the light simple reflects from the surface without penetrating the sample, this is called specular reflection and can cause various distortions in the spectrum, e.g., the spectrum changes in a non-uniform fashion with concentration. This is an effect often seen with mid-infrared spectra, but not with near-infrared spectra. In this work, it has been found that distortions, in mid-infrared spectra, due to specular reflection occur for both organic (humic acid) and non-organic fractions (carbonates, silica, ashed fraction of soil). The results explain why the spectrum of calcium carbonate in limed soils did not match that of published spectra and offer an explanation as to why the presence of inorganic-C interferes with the development of mid-infrared calibrations for organic-C in soil, but not for near-infrared calibrations. These results may also have far reaching implications for the use of mid-infrared spectra for quantitative and qualitative analysis of soils. For example, commercial libraries of spectra collected by means other than diffuse reflectance would be largely useless for comparing mineral spectra to soil spectra. Also, the results discussed here may indicate that at least for some soil types (e.g., large differences in mineralogy or C contents), separate calibrations or the use of non-linear calibrations methods will be necessary.

Technical Abstract: Studies on the occurrence and effects of specular reflection in mid-infrared spectra of soils have shown that distortions due to specular reflection occur for both organic (humic acid) and non-organic fractions (carbonates, silica, ashed fraction of soil). The results explain why the spectrum of CaCO3 in limed soils did not match that of published spectra and offer an explanation as to why the presence of inorganic-C interferes with the development of mid-infrared calibrations for organic-C in soil, but not for near-infrared calibrations. These results may also have far reaching implications for the use of mid-infrared spectra for quantitative and qualitative analysis of soils. For example, commercial libraries of spectra collected by means other than diffuse reflectance would be largely useless for comparing mineral spectra to soil spectra. Finally, with forages and grains it is necessary to develop separate calibrations for different products in order to obtain the best results. With soils, this has not seemed to be a problem for many diverse sets of samples where the C contents were in a range of 0 to 5%. Mid-infrared calibrations have also appeared to be more robust than the corresponding NIR calibrations in that fewer outliers are found. However, the results discussed here may indicate that at least for some soil types (e.g., large differences in mineralogy or C contents), separate calibrations or the use of non-linear PLS or other non-linear calibrations methods will be necessary.

Last Modified: 11/21/2014
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