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Spectroscopic Method for Estimation of Soil Organic Carbon

USDA-ARS, 1701 Center Ave., Fort Collins, CO 80526

W. D. Guenzi

USDA-ARS, P.O. Box E, Fort Collins, CO 80522

D. J. Savory

Agronomy Dep., Colorado State Univ.

*Corresponding author.

ABSTRACT

The amount of soil organic carbon (OC) is a good indicator of a soil's productivity potential. Thus, OC analyses are routinely used to evaluate the soil's potential to supply N during a cropping season, or to assess the potential adsorption of a herbicide. Two methods are currently used to determine OC in soil. Both have some limitations for use in processing large numbers of samples for routine analyses. The dry combustion procedure requires expensive instrumentation, while the wet digestion procedure is rather lengthy and the disposal of Cr can be a problem. This study describes a new method for a relatively inexpensive, accurate, and rapid determination of OC in soil. Finely ground soil samples are equilibrated with an extracting solution of 0.25 M KOH and 0.05 M Na₂-EDTA (ethylenediaminetetraacetic acid) for 2 h at 85 °C. After cooling, absorbance of the extract was measured at 260 nm. An empirical equation is used to obtain an indirect estimate of soil OC. The empirical equation was developed from comparative analysis between the dry combustion and the proposed spectrophotometric method (r² = 0.89) by using a series of soils of different genesis and development. The method was further evaluated by analysis of samples from two soil profiles not used in the development of the equation, and the results were in close agreement with those obtained from dry combustion.

Received for publication May 14, 1990.