Douglas fir [Pseudotsuga menziesii Mirb. (Franco)] bark (DFB), sphagnum peatmoss, and pumice are the most common substrate components used in the Oregon nursery industry. The objective of this study was to document the effect of peat and pumice addition on the physical and hydrological properties of DFB soilless substrates. A secondary objective was to determine if measured properties of mixed soilless substrates can be accurately predicted from the known properties of the individual components. Treatment design was a 3 × 3 factorial with three rates each of sphagnum peatmoss and pumice (0%, 15%, and 30% by vol.) added to DFB. The resulting nine substrates were measured for total porosity, air space, container capacity, and bulk density using porometers. Moisture characteristic curves were generated by measuring water content along a continuous column. Adding pumice to DFB decreased total porosity, container capacity, available water, and water-buffering capacity but increased bulk density. Adding peatmoss to DFB increased total porosity, container capacity, and available water but decreased air space and bulk density. Comparison of predicted values against measured values indicated that bulk density could be predicted reliably; however, all other physical properties could not be accurately predicted.