Quantifying functional pore sizes in pine bark growing media

Authors: WOLCOTT, CAROLINE - Virginia Polytechnic Institution & State University; Owen Jr, James - Jim; STEWART, RYAN - Virginia Polytechnic Institution & State University

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/19/2023
Publication Date: 2/26/2024
Citation: Wolcott, C.C., Owen Jr, J.S., Stewart, R.D. 2024. Quantifying functional pore sizes in pine bark growing media. Acta Horticulturae. 1389:215-226. https://doi.org/10.17660/ActaHortic.2024.1389.24.
DOI: https://doi.org/10.17660/ActaHortic.2024.1389.24

Interpretive Summary: Pine bark-based substrates are the favored growing media in many parts of the United States, France, Spain, New Zealand, and Australia for specialty crops grown in containers. Advantages of pine bark include that it is relatively low cost, has a wide range of particle sizes that provide desired water retention properties, and suppression of pathogens. However, pine bark is also susceptible to becoming water repellent and undergoes other factors that reduces its ability to store and supply water. The ability of pine bark to store and transfer water is related to pore characteristics, making it critical to have suitable methods to quantify and understand how pore structure and size distributions influence water retention, water availability to plants, and water movement through soilless substrates. This project seeks to apply soil physics strategies to soilless media to gain a better understanding of the hydraulic and physical properties of the substrates. We compared two different methods of quantifying macropore (i.e., large pore) distributions in pine bark that was unamended versus amended with peat or coir during controlled drainage versus when being watered (i.e., infiltration). Both methods indicated that pine bark had large pores, equal to or greater than 0.3 mm, and the addition of peat reduced large pores substantially. Results also suggest that coir may be a particularly useful amendment since it retains some large pores, which is useful for allowing gas exchange in wet media, while increasing the water retention of the pine bark substrate. When comparing methodology, the total amount of porosity that was inferred using the infiltration method was 3 to 4 orders of magnitude smaller than that inferred from the drainage experiments. The biggest inconsistencies were related to the assumption that the pores were cylindrical tubes. Despite such discrepancies, the two methods explored here offer insight into important hydraulic properties of the growing media. The infiltration experiment also quantified portions of the pore space that may be filled with water but does not actively influence water flow. This ‘non-active’ pore space may act as a water supply for plants growing in the media, depending on its ability to become filled and retain water during irrigation or other water application. Therefore, future work may specifically consider how these different portions of the pore space influence plant water uptake and growth.

Technical Abstract: Water limitations and concerns about nutrient runoff make it increasingly 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 porous media. One way to determine pore sizes is 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. 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 calculated to be <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