This study examined soil carbon levels across a wide range of cracking clay soils used for growing cotton in Australia by using a simple carbon fractionation procedure. The soils studied included reference and cropped sites. The procedure employed determines soil carbon fractions based on their ease of oxidation to obtain the labile (CL) and non-labile (CNL) carbon components. Based on the total carbon (CT), labile carbon (CL), and non-labile carbon (CNL) of a cropped soil relative to a reference soil, various monitoring indices were developed. It was found that cultivation has generally led to a decrease in the organic carbon status of the soils. The effect of cultivation was found to be more pronounced in the CL and the carbon management index (CMI) than in the CT and CNL. The changes in the ratio of CL to CNL as a result of cultivation have been variable. The CMI has generally declined during cultivation, and since the CMI has incorporated the changes taking place in CT, CL, and CNL, the use of this index can provide very useful results in monitoring of organic matter status of soils.