Fertility trials for wheat (Triticum aestivum) were conducted in high (2,800 meters above sea level (masl)) and lower (2,300 masl) altitudes of the central high valley of Mexico. Check yields of wheat (i.e., without fertilizer) in the lower altitude zone were found to be correlated with soil organic matter (SOM) levels (r2= 0.79). This relationship broke down, however, when check yields from higher sites with higher levels of SOM were included. A greenhouse study suggested that the field results were due largely to a temperature effect on N mineralization. Sixty-nine sites were sampled and analyzed for a range of soil characteristics in order to identify the basis for different SOM levels and to establish whether the contribution of SOM to soil properties other than N varied with altitude or SOM level. The levels of SOM were related primarily to altitude (assumed to be a temperature effect) and less so to silt content. Despite the relatively large range in SOM levels (8-fold differences), the contribution of SOM from different altitudes and at different levels to cation exchange capacity (CEC), water availability, and equilibrium bulk density were similar. The effect of pH on CEC is stressed and the beneficial effects of SOM on a range of soil properties are quantified. Unknowns in respect to the ‘active’ fractions for various SOM effects are discussed.