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The efficiency of oxidation of organic C by the Walkley‐Black method was studied by comparing with determinations using dry combustion equipment and gravimetric determination for the evolved CO2. Fifty surface soils (0–30 cm) were studied, in groups of 10, each one belonging to one of the following taxonomic suborders: Fluvents, Andepts, Tropepts, Udults, and Ustults. Samples from 17 Andept profiles were also analysed to study changes of oxidation efficiency with depth.The results for the surface samples indicated that the commonly used conversion factor was somewhat too small. It is recommended that 75% oxidation be used as the base for conversion to total C, as 86% of the studied samples deviated in < 2% from this value.It was observed that the organic matter in the deeper horizons is more easily oxidized through the Walkley‐Black procedure, perhaps because plant residues are less abundant and there is an accumulation of fulvic acids. In some cases the oxidation is almost complete. This oxidation is rather variable and it was not correlated with the texture or pH of these soils. It is believed that the extent of oxidizability can allow conclusions on the relative age of horizons in Andept profiles and indicates the presence of buried horizons.

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