Soil carbon determination by high temperature combustion a comparison withdichromate oxidation procedures and the influence of charcoal and carbonate carbon on the measured value

The measurement of organic carbon in soils has traditionally used dichromat e oxidation procedures including the Wakley and Black and the Heanes method s. The measurement of carbon in soils by high temperature combustion is now widely used providing a rapid automated procedure without the use of toxic chemicals. This procedure however measures total carbon thus requiring som e means of correction for soil samples containing carbonate and charcoal fo rms of carbon. This paper examines the effects of known additions of charco al to a range of soil types on the results obtained by the Walkley and Blac k, Heanes and combustion methods. The results show, that while the charcoal carbon does not react under Walkley and Black conditions, some proportion does so with the Heanes method. A comparison of six Australian Soil and Pla nt Analysis Council reference soil samples by the three methods showed good agreement between the Heanes method, the combustion method and only slight ly lower recoveries by the Walkley and Black procedure. Carbonate carbon wi ll cause an overestimation of soil organic carbon by the combustion method thus requiring a separate determination of carbonate carbon to be applied a s a correction. This work shows that a suitable acid pre-treatment of alkal ine soils in the sample boats followed by a drying step eliminates the carb onate carbon prior to combustion and the need for an additional measurement . The measurement of carbon in soils by high temperature combustion in an o xygen atmosphere has been shown to be a rapid and reliable method capable o f producing results in good agreement with one of the established dichromat e oxidation procedures.