Quantification of Organic Carbon in Soils: A Comparison of Methodologies and Assessment of the Carbon Content of Organic Matter Steven Sleutel, Stefaan De Neve, Benoit Singier, and Georges Hofman Department of Soil Management and Soil Care, Ghent University, Ghent, Belgium Abstract: Renewed interest in temporal soil organic carbon (SOC) stock changes has stressed the importance of reliable methods for quantitative assessment of organic compound (OC) content. Particularly with the establishment of modern dry-combus- tion analyzers, which are replacing the traditional wet-oxidation methods, the need for correct relationships between both is of crucial importance for comparison of past and current SOC data in long-term SOC stock change studies. Dry combustion with a Variomax CNS-analyzer was the standard to evaluate three other methods for Belgian agricultural soils. Excellent linear relationships were found with the Walkey and Black method and the Springer and Klee method, whereas a Shimadzu TOC- analyzer slightly underestimated the OC content. Precision of the investigated methods was comparable and tended to be dependent on the sample size used for measurement. The OC oxidation efficiency of the most widely applied method of Walkey and Black for the soils in this study was very close to the generally accepted 75%. Mass loss on ignition at 8008C could be very well related to the soil OC content and the clay content. The traditional factor of 1.724 used to convert OC measurements to organic matter percentages is not valid for the investigated soils, which demonstrates that rather regional-specific factors (in this study 1.911) should be determined and adopted. Keywords: Clay content, dry combustion, organic carbon, wet oxidation Received 17 January 2006, Accepted 2 December 2006 Address correspondence to Steven Sleutel, Department of Soil Management and Soil Care, Ghent University, Coupure Links 653, 9000 Ghent, Belgium. E-mail: steven.sleutel@ugent.be Communications in Soil Science and Plant Analysis, 38: 2647–2657, 2007 Copyright # Taylor & Francis Group, LLC ISSN 0010-3624 print/1532-2416 online DOI: 10.1080/00103620701662877 2647