A suitable soil quality index should be sensitive to changes in soil functions, but not too sensitive to seasonal and among-site variations in soil conditions. Recent studies have suggested that the total nitrogen and organic matter contents of undisturbed soils (under climax vegetation) are in equilibrium with biological and biochemical properties. Accordingly, such equilibrium is disrupted when the soil is subjected to disturbance or stress events. The measurement of this disruption, which can be expressed mathematically, can then be used as a soil quality index that is spatially and temporally robust. In this study, we aimed to evaluate these hypotheses in soils from the H.J. Andrews Experimental Forest in Oregon. Both O and A horizons were sampled from eight local sites, including 4 old growth and 4 second growth, in Spring/2005 and Fall/2006. Soil samples were analyzed for enzyme activities (phosphomonoesterase, B-glucosidase, laccase, N-acetyl glucosaminidase, protease and urease), N-mineralization, respiration, microbial biomass, pH, mineral nitrogen (NO3-, NH4+) and soil organic matter (SOM) contents. The results showed variation in biological and biochemical soil properties that were closely correlated with the SOM content. Multiple regression analysis of SOM content against chemical and biochemical variables showed that microbial biomass, respiration and phosphomonoesterase activity were able to explain 95% of the variation in SOM. The model fit did not change statistically when site age, sampling date, or soil horizon were included as covariates in the regression analysis. These results support the existence of a balance between the organic matter and biological and biochemical properties in these soils, and that this balance is spatially and temporally constant. Further experiments are in progress to test if the application of stresses to these native soils will disrupt the predicted equilibrium between SOM and the three biochemical variables.