History of the soil organic matter conversion factor of 1.72


Students of soil science are taught that to determine the amount of soil organic matter, soil organic carbon is measured usually by wet oxidation using potassium dichromate (called Walkley-Black method) or in well-equipped laboratories, using CN analyser and then multiplied by a conversion factor of 1.72 or 1.724. Most textbooks and laboratory manuals do not explain how this factor was obtained, so students generally accept the value without any question just like they do with other constants used in natural sciences. 

Origin of the conversion factor

The conversion factor has a very long history and has practically survived the test of time and modern analytical methods. It is about 150 years old. It was based on studies in the 1820s by the famous agricultural chemist, Carl Sprengel of Goettingen University, that organic matter contains 58 percent carbon. But it was another leading agricultural chemistry pioneer, Emil Wolff from Hohenheim, who introduced the value of 1.724 in 1864. Since then this conversion factor has become universal despite the many later studies showing that it is too low for most soils and that a value of 2.0 is more accurate (Pribyl, 2010). When I was doing my masteral thesis at IRRI in the late 1980s, Dr. H.U. Neue, the head of the Soils Department and a leading expert on organic matter of submerged soils, required us to use a factor of 2.0. 

Oldest records of the conversion factor (Source: Pribyl, 2010) 
In an excellent review about this conversion factor, Pribyl (2010) concluded that convenience, authority and tradition rather than strength of evidence are in large part the reason for the widepsread acceptance of the conversion factor until now. However, this may be a controversial conclusion for other soil scientists in some countries. In France for instance, analytical laboratories use the factor of 1.72 or 2.0. The former (i.e. 1.72) is better suited for cultivated horizons while the latter (i.e. 2.0) is more appropriate for forest topsoils (Baize, 1988).

Who was Emil Wolff?

Dr. Emil von Wolff (30 Aug 1818-26 Nov 1896) was a professor of chemistry and agricultural chemistry at the Hohenheim Academy of Agriculture and Forestry in Stuttgart, Germany (since 1967 named University of Hohenheim) from 1853 to 1894. Wolff was one of the agricultural chemistry pioneers who made major contributions to its development and to that of soil science, plant science and animal science. 

Prof. Emil Wolff (Source: Hohenheim Univ)
Emil von Wolff started his studies in medicine at Kiel University in northern Germany but later shifted to natural science which he finished in Berlin. He obtained his PhD in 1843 in Berlin a year after Justus von Liebig published his most important book on agricultural chemistry. This probably infuenced him to focus his teaching and research on soil and plant chemistry as well as on the composition of organic substances including foods. He wrote several books among which were the “Textbook of Agricultural Chemistry (1847)” and “Ash Analysis of Agricultural Products (1877). 

Wolff belonged to the most influential and highly regarded agricultural scientists of the 19th century and had no doubt contributed to the fame of the Hohenheim school. It is thus a fitting tribute that an important street at the heart of the Hohenheim University campus bears his name: Emil-Wolff-Strasse (Emil Wolff street).

References
Baize D. 1988. Soil Science Analyses. John Wiley & Sons, Chichester.
Leisewitz, C. 1910.Wolff, Emil von. In: Allgemeine Deutsche Biographie 55 (1910), S. 115-117 [Onlinefassung]; URL: http://www.deutsche-biographie.de/pnd115599533.html?anchor=adb
Pribyl D.W. 2010. A critical review of the conventional SOC to SOM conversion factor. Geoderma 156: 75-83

The geoecology of the limestone and shale areas in Samar, Philippines


Contributed by

Dr. Ian A. Navarrete
Humboldt Fellow
Soil Science of Tropical and Subtropical Ecosystems
Buesgen Institute
University of Göttingen, Germany

Geoecology, a term coined some 41 years ago by the geomorphologist Carl Troll who was at the time professor at the University of Bonn, Germany, is a broad integrative term to the study of forms and functions of terrestrial geoecosystem (Huggett, 1995). It emphasizes the interdependency and/or inter-relationships of the ecological biosphere with landscape and hence sometimes equated with landscape ecology. For example, the movement and distribution of solutes across soil landscapes are influenced by the geomorphic position in the soil within the landscape thus influencing soil genesis (Sommer and Schlichting, 1997) and vegetation development (Huggett, 1975). 

Fig 1. Relation of primary forest and grasslands of Samar
During our fieldwork at the Samar Island Natural Park (along the Paranas-Taft road at about 300 m above sea level) in Feb 2012, we observed two typical grassland ecosystems occurring near or far from the primary forests (Fig 1A).

The first type is the grassland that occurs in the lower residual limestone soil or at the margin of the primary forest. The soils in such grassland are younger as indicated by poor soil profile development. The dominant grass is Paspalum conjugatum which in many areas occur in association with Chromolaena odorata. The second type is the grassland in the degraded rolling and hilly areas usually away from primary forests. The soils in these areas are different from the soils in the primary forest on the upper slopes in that they are mature, reddish, and deep (Fig 1B). They appear to have formed from the limestone residue or from the shale (underlying the limestone) that is widely exposed in the rolling areas. The dominant grass is Imperata cylindrica

Fig 2. Primary forest soil in Samar
The soils of the primary forest (limestone forest) on the upper and usually steep slopes are generally very thin and are underlain by consolidated limestone rocks (Fig 2). The presence of nutrient-enriched weathering pockets (where deposition of nutrient and decomposition of organic matter take place) of the limestone parent material, and the high annual rainfall explain the lush growth of the forest vegetation. It also partly explains the high tree species diversity of the forest.

(Members of the team: V.B. Asio, Ariel Bolledo, Mark Moreno, Pearl Carnice, Richel Lupos, Forester Elpidio Cabahit Jr. from the Samar Island Natural Park, and myself)

References

Huggett RJ (1975). Soil landscape systems: a model of soil genesis. Geoderma 13: 1-22.
Huggett RJ (1995). Geoecology: An Evolutionary Approach. Routledge, London.
Sommer M, Schlichting E (1997). Archetypes of catenas in respect to matter-a concept for
structuring and grouping catenas. Geoderma 76:1-33.