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 CO 2 by wet or dry combustion and subsequent quantitation of evolved CO 2 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.