A reevaluation of the chromic acid colorimetric procedure for soil organic carbon

Two methods are currently used to measure soil organic carbon (SOC), one based on measuring CO2 evolution after high temperature combustion of the sample and the other on measuring dichromate reduction after organic carbon (OC) oxidation. The former method is the more accurate, but requires correction for soils with free calcium carbonate; the latter uses a hazardous heavy metal, dichromate (Cr 6+), but is easier when done colorimetrically (Cr 3+ measured at 625 nm) and is less expensive. If we could minimize the hazard and health concerns associated with the hexavalent chromium (Cr 6+), which is required in excess for complete SOC oxidation, and assure adequate accuracy, this method may be more acceptable especially where samples are few, and free calcium carbonate may be present. A methodology is presented where the amount of soil sample (0.1 to 0.5 g) and dichromate used in the analysis (2.5 mt of 0.167M dichromate) is small. The unreacted dichromate after analysis, is further reduced with glucose or other carbon (C) source to produce the less toxic Cr 3+, which in small quantities, can be discarded to landfills once properly precipitated and neutralized. Accuracy of the colorimetric procedure when compared to the CN Analyzer was adequate for samples with less than 1.3% SOC [2% soil organic matter (SOM)], amounts typical for cultivated soils of the Central Great Plains. For samples above 1.3% SOC (>2% SOM), accuracy was improved by reducing sample size so 2.0 to 5.5 mg of OC was contained in the sample.