Comparison between colorimetric and titration methods for organic carbon determination in acidic soils

Dry combustion CO2 determination is an accurate method to measure organic carbon in soil, but is not always used because it requires expensive equipment. Wet oxidation methods are then widely used. The oxidation conditions in these chemical procedures are still searching and in some cases can be difficult to decide the appropriate method for organic carbon (OC) determination. The objective of this paper was to compare the efficiency of organic carbon oxidation of the Walkley and Black titration (OCWB) and the Anderson and Ingram colorimetric (OCC) methods in acid soils from two humid premontane regions of Venezuela. Both methods were applied to 184 samples of sandstone- and schist-derived soils covering different soil depth and different vegetation (forest to savanna). The standard procedure to evaluate the efficiency of the wet oxidation methods was the dry combustion (OCDC) using a C analyzer. The OCWB method recovered 100% of OC in sandstone-derived soils and 94% in schist-derived soils. The OCC method, however, underestimated the OC, recovering only 53% of OC in the sandstone-derived, and 41% of OC in the schist-derived soils. Statistical analysis indicates that the relation-ship between OCWB and OCC methods is independent of parent material which allows us to use a conversion factor (0.49) between these methods.