Change in Physical Properties of Pine Bark and Switchgrass Substrates Over Time

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.