A critical review of the conventional SOC to SOM conversion factor

Use of a single factor for converting soil organic carbon to soil organic matter is challenged. The basis for this challenge arises from four sources: the original papers published in the nineteenth century, empirical studies published throughout the twentieth century, theoretical considerations of organic matter composition, and a consideration of what led to the popularity and general acceptance of the conventional factor. The conventional factor of 1.724, based on the assumption that soil organic matter contains 58% carbon, applies only to some soils or only to particular components of soil organic matter. Studies published since the end of the nineteenth century have consistently shown that the factor of 1.724 is too low for most soils. In a review of previously published data, the median value for the conversion factor was found to be 1.9 from empirical studies and 2 from more theoretical considerations. A factor of 2, based on the assumption that organic matter is 50% carbon, would in almost all cases be more accurate than the conventional factor of 1.724. A consideration of the possible variation in organic matter composition predicts a range of factor values between 1.4 and 2.5, a range that is narrower than empirical results at least in part because of the interaction between the methods used to estimate organic matter and soil composition. Convenience, authority, and tradition rather than strength of evidence are in large part responsible for the widespread acceptance of the conventional factor.