Total Carbon, Organic Carbon, and Organic Matter

Total carbon (C) in soils is the sum of both organic and inorganic C. Most organic C is present in the soil organic matter fraction, whereas inorganic C is largely found in carbonate minerals. This chapter describes two dry combustion and one wet combustion procedures for total C analysis. Not all soils contain inorganic C because of dissolution during soil formation of carbonate minerals originally present in parent material. However, organic C is present in all agricultural soils. Total C determinations for soil involve conversion of all forms of C in soils to CO2 by wet or dry combustion and subsequent quantitation of evolved CO2 by gravimetric, titrimetric, volumetric, spectrophotometric, or gas chromatographic techniques. Because of the problem associated with determining the organic matter content of a soil, it is strongly suggested that investigators determine and report the organic C concentration as a measure of the organic matter in a soil.