Comparison of different methods for the determination of total organic carbon and humic substances in Brazilian soils
Palavras-chave:
Walkley-Black method, dichromate-oxidation, loss on ignitionResumo
Aiming to compare three different methods for the determination of organic carbon (OC) in the soil and fractionsof humic substances, seventeen Brazilian soil samples of different classes and textures were evaluated. Amounts of
OC in the soil samples and the humic fractions were measured by the dichromate-oxidation method, with and without
external heating in a digestion block at 130 °C for 30 min; by the loss-on-ignition method at 450 °C during 5 h and
at 600 °C during 6 h; and by the dry combustion method. Dry combustion was used as reference in order to measure
the efficiency of the other methods. Soil OC measured by the dichromate-oxidation method with external heating
had the highest efficiency and the best results comparing to the reference method. When external heating was not
used, the mean recovery efficiency dropped to 71%. The amount of OC was overestimated by the loss-on-ignition
methods. Regression equations obtained between total OC contents of the reference method and those of the other
methods showed relatively good adjustment, but all intercepts were different from zero (p < 0.01), which suggests
that more accuracy can be obtained using not one single correction factor, but considering also the intercept. The
Walkley-Black method underestimated the OC contents of the humic fractions, which was associated with the
partial oxidation of the humin fraction. Better results were obtained when external heating was used. For the organic
matter fractions, the OC in the humic and fulvic acid fractions can be determined without external heating if the
reference method is not available, but the humin fraction requires the external heating.
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