Mechanistically modeling the biogeochemical processes in land models requires one to resolve the vertical diffusion of different dissolvable volatile gases, such as CH₄, O₂, and CO₂. However, many existing models use ad hoc techniques to represent the dual-phase diffusion of these gases, representing one important structural uncertainty for earth system models. We proposed three equivalent formulations of this dual-diffusion problem and contended that the gas-phase primary form is most straightforward to implement for land biogeochemical modeling. We then solved the gas-phase primary form using a finite volume based algorithm and evaluated its accuracy against three analytical solutions: steady-state CO₂ diffusion in soil, steady-state CH₄ diffusion in soil, and transient tracer diffusion in two different sequentially aligned medias. The results indicated that our method achieved good numerical accuracy with maximum relative errors less than a few percent. We expect that our method will help land biogeochemical modelers to develop simple and robust algorithms for modeling volatile gas diffusion in variably saturated soil.