This research introduces the concept of a CO2 fertilization factor for soil carbon (σCF). The σCF is a measure of an ecosystem's capacity to increase soil carbon storage in response to elevated carbon dioxide levels. This paper describes the mathematical derivation of σCF and illustrates how σCF can be determined experimentally, using data from a white oak study. I have developed this concept to compare the results of carbon dioxide enrichment experiments having different soil carbon turnover times, different levels of CO2 enrichment, and different lengths of exposure to elevated carbon dioxide levels. The σCF can also be used to estimate increases in soil carbon uptake due to observed contemporary increases in atmospheric carbon dioxide levels. Although the approach used here may seem oversimplified, I present it as a simple way of estimating the extent to which elevated levels of CO2 could increase soil carbon storage. I have determined a σCF of 1.18 for a white oak ecosystem using soil carbon and radiocarbon measurements. If major terrestrial ecosystems have similar σCF values, CO2 fertilization may be transferring enough carbon from the atmosphere to soil to balance the global carbon budget.