A KMnO4 oxidation method for estimating labile soil C (CL) was evaluated for use in a soil testing mode to identify soils where soil quality and productivity is likely or unlikely to respond positively to increased levels of CL. Four sets of paired fields of the same soil series (within each set) but contrasting soil management history (continuous cropping vs. long-term sod) were studied. Fields with sod history initially tested higher in total soil C and CL than fields with cropped management history. Within each field two treatments (winter rye cover crop or no cover crop) were applied in each of four blocks. Crop and soil functional responses to rye, when significant, were higher in fields that initially tested lower in CL indicating that the KMnO4 method used has some predictive value as a soil test. The method could be used in field testing kits for evaluation of soil C.