Knowledge of gas exchange dynamics is important in determining the suitability of growing media used in nursery and greenhouse production. However, gas diffusion measurement methods are difficult to use directly in potted growing media and a rapid, simpler, and reliable approach appears desirable for routine assessment of gas diffusivity. This study compares gas diffusivity and pore efficiency estimates from gas diffusion chamber measurement with indirect estimates obtained from water storage and flow measurements and point of air entry values for various substrates. Four peat substrates with bark in variable particle sizes, a mineral soil, and silica sand were packed into aluminum cores in four replicates and then saturated for 72 h. After equilibration at −0.8 kPa of water potential on a tension table, the concentrations of N2 diffusing through these cores were measured in a gas diffusion chamber and gas diffusivity calculated from the gas concentration change in time. Diffusivity was also calculated with the water desorption curve and the saturated hydraulic conductivity, also measured on the same core (indirect approach). The gas diffusivity estimates obtained by gas diffusion chamber measurement were significantly correlated (R2 = 0.49, slope not different from 1) with those obtained from the indirect approach. Estimates obtained for pore efficiency were much closer and less variable than those for gas diffusivity (R2 = 0.80, slope not different from 1). The indirect approach may be a useful tool for the rapid assessment of gas exchange dynamics in growing media under undisturbed conditions.