Soilless substrates, such as peat, pine bark, and coir, are widely used as growing media in containerized crops for their favorable characteristics, including low bulk density, balanced air exchange and water retention, disease resistance, and low pH and salinity. However, improper irrigation of these media can have negative outcomes such as root asphyxia, pathogen development, and reduced plant growth. Understanding pore size distributions, water dynamics, and gas diffusivity of these substrates is essential to promote plant growth. The effects of different particle sizes of soilless media on processes such as infiltration, hydraulic conductivity, and gas diffusivity are also not well understood. The characterization of these effects is important for the overall improvement of container crop production. This thesis presents three studies that aimed to characterize the physical and hydraulic properties of pine bark substrates, both unamended and amended with peat or coir. The first study looked at three substrate types: unamended, unscreened pine bark, peat-amended pine bark, and coir amended pine bark. Three methods were employed to quantify pore distributions: non-equilibrium infiltration measurements, equilibrium water retention characterization, and scanning electron microscopy. We characterized pore distributions during wetting and drainage for the three substrates. Coir-amended bark had the largest water-conducting porosity, highest hydraulic conductivity, and most water retention. Unamended pine bark had the highest microporosity, and the addition of peat and coir lowered macroporosity, with peat having the greater effect. The total porosity inferred from the infiltration method was significantly smaller than that inferred from drainage experiments due to assumptions related to pore shape. The second study focused on defining hydraulic conductivity and water retention for pine bark substrates of five different particle sizes, <1 mm, 1-2 mm, 2-4 mm, 4-6 mm, and an unscreened fraction. We utilized the same methods from the first study. The resulting data showed that the smallest particle sizes (i.e., <1 mm and 1-2 mm) had the highest hydraulic conductivity and greatest water retention. The three larger sizes had lower hydraulic conductivity and poor water retention, including the unscreened fraction, which more closely followed the results of the 2-4 mm size. The final study examined gas diffusivity of the five pine bark particle sizes at different moisture levels: 60% moisture content (initial conditions), saturated at the bottom of the sample, near-saturated at the sample bottom, and drained from saturation to container capacity. We used a one-chamber gas diffusion setup to find gas diffusion coefficients (Ds). The results displayed an inverse relationship between Ds values and substrate water content. In addition, the larger particle sizes were less sensitive to changes in water content due to their well-draining large pores. Proper balance of aeration and water retention is necessary for the success of soilless growing media. Overall, the smaller particle size fractions had the best water retention and hydraulic conductivity rates while the larger fractions had the largest Ds coefficients. This work contributes valuable knowledge on the physical and hydraulic properties of different size fractions of pine bark substrates, which can assist nursery growers in optimizing water usage for sustainable container crop production. / Master of Science / Since the 1950's soilless substrates have been an important resource for growing a variety of fruits, vegetables, flowers, and ornamental plants. Soilless growing media have become more popular choices for containerized plant production compared to natural soils due to improved air exchange, increased disease resistance, and more plants per acre. They are also favored because they help conserve resources, reduce agricultural waste, and minimize transportation requirements as compared to traditional cropping methods. The most popular types of soilless media include peat, coir, compost, and pine bark. In the U.S., pine bark is the main substrate used, as it is renewable and widely available. Growers still face many issues when using containerized crop production. For example, pine bark is susceptible to water runoff which can cause environmental problems and increase costs from this loss of water and fertilizer. Further characterizing of water and gas dynamics in of pine bark growing media is important for conserving water and fertilizer resources while optimizing plant growth in this container cropping industry. Pore characteristics, aeration, and water movement are key factors of substrates to be described to solve these challenges. This project aimed to apply soil physics strategies to soilless media, focusing on describing pore sizes, water movement, water holding capacity, and air movement in pine bark substrates. We utilized three methods throughout this study. For the first method, we took infiltration measurements to examine how water moved into the media, while the second utilized controlled drainage experiments to observe how water moved out of the media. The final method was characterizing gas movement through the substrates at different water contents and particle sizes. The results found showed that the smaller particle sizes and pine bark mixed with peat and coir had increased ability to retain water and allow water movement as compared to the larger particle sizes and unamended pine bark. In contrast, the larger particles had less water retention but improved gas movement. These results could be applied by stacking different particle sizes or mixes over one another could optimize water retention in the top of the container and drainage and gas movement in the bottom of the container. Overall, the application of this work is to create best management practices for growers to be able to balance water retention and gas movement in order to optimize plant growth.