Permanganate (KMnO4) oxidizable carbon (POXC), an estimate of labile soil C, was evaluated for use as a soil test to identify soils where soil functions will improve with improved soil organic matter (SOM) management.  Paired fields having similar soils but contrasting management histories (continuous cropping vs. long-term sod) were studied at three sites.  Fields historically under sod tested higher in total organic C (TOC) and POXC than historically cropped fields.  Across all sites, POXC was strongly related to TOC.  Both soil C measures were positively correlated with water stable aggregates (WSA), mineralizable C (C_min), and mineralizable N (N_min).  Within each field, two cover crop treatment levels were applied: rye and no cover crop.  We examined relative responses of WSA, C_min, and N_min to rye.  After at least two years of treatment, significant negative correlations were observed between the relative response of WSA to rye with POXC and TOC at sites with finer textured soils.  Similar correlations were seen across all sites between the relative response of N_min to rye with POXC and TOC.  No significant correlations were seen between relative responses of C_min to rye and soil C parameters. Permanganate oxidizable C and TOC were comparable at predicting significant relative soil functional responses to rye.  These results suggest that POXC may be useful for identifying soils where soil functions will benefit from improved SOM management. The POXC methodology is rapid and simple, enabling on-site or laboratory soil testing.