The effects of cultivation in hillside (> 30% slope) on soil C and N stocks after forest clearance and subsequent cultivation in new re-growth forest sites not always induce rapid soil organic matter (SOM) loss in subtropical areas. In the present study we evaluated the sensitivity to SOM changes of material floatable in water (Fw), coarse sand (> 250 mm), fine sand (50-250 mm), silt (2-50 mm) and clay (0-2 mm) fractions by the impact of cultivation of Mexican Ferrasols. This research also determined the relative degree of C and N saturation in the silt and clay size particles. The soil physical fractions were obtained after ultrasonic vibration and sedimentation. Soil organic matter stock (0-20 cm depth) in cultivated sites ranged from 54 to 146 Mg C ha-1 and from 3.9 to 7.1 Mg N ha-1 and in the forest soil 46-94 Mg C ha-1 and 3.7-5.1 Mg N ha-1. The relative distribution of C and N was lower in the coarse sand and highest in the silt fraction; the latter paralleled the changes of whole soil C and N. Coarse sand was the most sensitive fraction responding to cultivation, thus representing a diagnostic pool to assess the SOM shifts under cultivation. Half of our studied soil showed a low degree of C and N saturation in the silt and clay fraction indicating potential for accumulation of C and N, while the other half were well above the saturation limit.