Comparison in the determination of soil organic matter by parcial and complete oxidation in different soils of the VII Region

Francisco J. Matus B.1, Victor Hermosilla A., Christian R. Maire G. y Samuel Ortega F.

In this study, two method of soil organic matter (SOM) determination were compared: Walkley and Black (1934) (WB) and Nelson and Sommers (1982) with two heating periods; 30 or 120 minutes (NS30 y NS120). Furthermore, it was investigated if the percentage of clay and silt fraction of soils (which determine the degree of soil aggregation), affect SOM determination. Walkley and Black method recovered 56% of SOM obtained by NS120 . Therefore, a correction factor of 1.79 (100/56) can be used to compare the results between WB and NS120. Nelson and Sommers with heating for 30 minutes recovered 21% less SOM than the extensive heating with NS120. Thus, if a quantitation is needed, NS120 is recommended. In general, the statistical error and its variation, decreased as the amount of organic matter increased. This was also true for the coefficients of variation of each determination. These results suggest that the number of replicates should be taken in account for soils with SOM < 2%. On the other hand, clay and silt content of the soils did not affect the recovery of SOM, after extensive heating for 120 minutes was applied, since no differences were established between soils dispersed with ultrasound and non-treated soils.

Keywords: Soil organic matter, dichromate organic carbon oxidation, c1ay and silt content of the soils.
1 Departamento de Producción Agrícola, Facultad de Ciencias Agrarias, Universidad de Talca, Casilla 747, Talca, Chile.