ARS | Publication request: Change in physical properties of pine bark and switchgrass substrates over time

Submitted to: Journal of Environmental Horticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 25, 2012
Publication Date: September 1, 2012
Citation: Altland, J.E., Krause, C.R. 2012. Change in physical properties of pine bark and switchgrass substrates over time. Journal of Environmental Horticulture. 30(3):113-117.

Interpretive Summary: Regionally sourced alternatives to pine bark have been explored, including use of straw materials such as switchgrass. Switchgrass substrates can be engineered to have optimum physical and chemical properties at the time of potting. Changes in chemical properties of switchgrass substrates over time have been studied; however, little is known about how physical properties change over time. Pine bark is considered to be ideally stable over the production period of most containerized plants. The objective of this research was to document the change in physical properties of switchgrass compared to pine bark substrates. Air space decreased in all substrates, but decreases were greater with switchgrass. Shrinkage was also greater in switchgrass substrates. Vigorous root growth may act as a biological scaffolding to support substrates and reduce shrinkage, especially in substrates that are otherwise prone to shrinkage. Our data show that while switchgrass substrates can initially have ideal physical properties, there will be greater shrinkage in the absence of vigorous root growth and thus may not be suitable for production of slow-rooting crops.

Technical Abstract: Alternatives to pine bark for nursery crop substrates have been proposed, including the use of straw materials such as switchgrass. While straw substrates can be developed with suitable physical properties measured immediately after mixing, little is known about how the physical properties of straw-based substrates change over time. The objective of this research was to measure the change in air space (AS), container capacity (CC), total porosity (TP), and bulk density (Db) over time of a switchgrass-based substrate compared to a pine bark substrate. Switchgrass and pine bark substrates were packed into 15 cm (6 in) tall aluminum cores and placed in a production greenhouse with or without a single hibiscus plant. Physical properties of the substrates were measured at the beginning of the experiment and 9 to 10 weeks later when the plants were nearly too large for their containers. Air space decreased over time, primarily as a function of root growth and shrinkage. Container capacity increased slightly across all treatments over time. Bulk density changed very little over time. The switchgrass substrate was more prone to shrinkage than the pine bark substrate, although vigorous hibiscus root growth reduced shrinkage in switchgrass substrates.