Quantifying functional pore sizes in pine bark growing media

Water limitations and concerns about nutrient runoff make it important for nursery producers to select substrates with proper water-holding and infiltration properties. Pore size distribution is a crucial property influencing water dynamics within soilless growing media. Pore sizes can be determined using equilibrium-based measurements, such as drainage water retention experiments. Another approach involves non-equilibrium infiltration measurements, typically with water supplied under tension, which can better identify pores that become dynamically activated during wetting events. In this study, we compared two procedures to quantify pore sizes in a pine-bark substrate that was unamended versus amended with peat or coir. The first procedure used drainage measurements with a hanging water column to create moisture retention curves. The second procedure involved interpreting measurements collected from a tension infiltrometer at three source tensions and saturation. The two approaches provided different yet complementary estimates for pore size distributions. The tension infiltrometer-based measurements implied that only a small proportion of the total pore space was involved during infiltration, with water-conducting porosities of <10^‑4 cm³ cm^‑3. Both methods indicated that coir may be a particularly useful amendment since it retains some macro-porosity (which is useful for allowing gas exchange in wet media) while increasing the water retention of the pine bark substrate. These results can guide future efforts to characterize and optimize growing media such as pine bark substrates, with the end goal of maximizing nursery production while minimizing pollution and water losses.