ReviewA critical review of the conventional SOC to SOM conversion factor
Introduction
Historically, the conversion factor of 1.724, based on the assumption that organic matter is 58% carbon, has been used for converting measurements of soil organic carbon (OC) into estimates of soil organic matter (OM). This factor, known as the “van Bemmelen factor,” appears in research papers and textbooks, either without citation or with a citation to a previously published paper that simply used the factor, rather than to the original source. The original source seems to be generally unknown and challenges to its accuracy and precision have been insufficient to remove it from general use.
Recent interest in sequestering carbon in soils underscores the need for a quick, inexpensive, and accurate measurement of soil carbon. Estimating soil carbon from estimates of soil organic matter is a simple and inexpensive procedure but requires the use of a conversion factor. An obvious choice has been the conventional factor of 1.724. Waxman and Stevens (1930), writing in reference to a conversion factor for estimating nitrogenous compounds, state that “[i]t should be emphasized as well that the factor 1.72 for determining the total organic matter, from the carbon content should also be considered as merely tentative” (p. 105) and predict that “[t]his factor will probably be modified when considerable information has accumulated concerning the chemical composition of the soil organic matter” (p. 113). Considerable information has accumulated and we now have sufficient cause to reexamine the factor. The purpose of this study is to show that the conventional carbon-to-organic matter conversion factor is too low for universal application and fails to account for the significant variation in the carbon content of soil organic matter, as does any single factor.
Section snippets
Establishment of the conventional factor
The first line of evidence against the use of 1.724 comes from a close reading of the earliest papers that initially established the factor (Table 1). These papers were published in German and apparently have not been published in English in their entirety.
The assumption that organic matter is 58% carbon seems to be based on the work of Sprengel (1826) who found “Humussäure” (humus acid) to be 58% carbon, 40% oxygen, and 2% hydrogen (Sprengel (1826) actually reported his results to four
Challenges to the conventional factor
The second line of evidence against the conventional factor comes from empirical data published over the last 120 years (Fig. 1 and Table 2). The earliest English-language paper found to date with data that permit the calculation of a conversion factor is by Warington and Peake (1880) working at Rothamsted, England. They observed that organic matter content estimated from loss-on-ignition was considerably higher than that calculated from carbon assuming organic matter is 58% carbon, an
A comparison of theoretical estimates with empirical observations
A third line of evidence against the conventional factor comes from theoretical estimates. Theoretical considerations made in the manner of Schulze (1849) can provide an estimate of the expected range of values that the conversion factor could reasonably take on. As a point of reference, Wiley (1906, p. 360) reported that values of the carbon content of soils range from 40 to 72% (factors of 2.5 and 1.4), presumably based on experimental results but he cites no sources.
An upper limit of the
Sources of variation
Variability in any estimated factor comes from two sources: the methods used to estimate organic matter and organic carbon, and the natural variability in the composition of soil. Variability contributed by the two sources is not independent. For example, the estimate of organic matter based on the loss-on-ignition method (discussed later) depends on the relative amounts of organic matter and clay in the soil. Estimating the conversion factor ratio OM:OC requires measurements of OC and OM or a
Persistence of the conventional factor
Finally, an argument might be made that the factor 1.724 must have some credibility since it has been used for 150 years. Arguments presented against this position, while being more speculative than the previous lines of evidence, help explain the reason for the widespread adoption and persistence of the conventional factor despite the lack of supporting evidence.
Consider four common reasons for belief: evidence, tradition, authority, and revelation (see Dawkins, 1995). Revelation, based on one
Conclusion
A preponderance of studies over the last 120 years show that the fundamental assumption upon which the conventional factor is based, that carbon comprises 58% of organic matter, is on average too high. As a result, the conventional conversion factor of 1.724 is too low for most soils. This study found a median value for the conversion factor of 1.9 from all studies and 2 from theoretical considerations. A factor of 2, based on the assumption that organic matter is 50% carbon, would in almost all
Acknowledgements
The author wishes to thank the Department of Soil, Water, and Climate at the University of Minnesota for support during the research and writing of this paper. Deepest thanks go to Dr. Susanne Woche at the University of Hannover, for providing translations of the nineteenth century German research papers, and to Dr. Mary Kirkham at Kansas State University who conveyed my request for translation help to Dr. Woche. Discussions with and comments from Dr. Paul Bloom were especially helpful.
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