Examining a new laboratory method for packing soilless substrate to ensure consistent, appropriate bulk density when measuring static physical properties

Authors: ALRED, BRIANNA - University Of Tennessee; Owen Jr, James - Jim; ESPINOZA, ALEXANDRIA - The Ohio State University; SUN, XIAOCUN - University Of Tennessee; FULCHER, AMY - University Of Tennessee

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2024
Publication Date: 9/11/2024
Citation: Alred, B., Owen Jr, J.S., Espinoza, A., Sun, X., Fulcher, A. 2024. Examining a new laboratory method for packing soilless substrate to ensure consistent, appropriate bulk density when measuring static physical properties. HortScience. 59(10):1477–1481. https://doi.org/10.21273/HORTSCI17953-24.
DOI: https://doi.org/10.21273/HORTSCI17953-24

Interpretive Summary: The amount of substrate packed in each container or volume may not be the same when packed in the laboratory by scientists versus in the field by growers. Resulting small differences in how loose or compacted a substrate effect water and air availability to the crop. The amount of substrate also varies by practitioner regardless if scientist or grower. Our goal was to 1) identify how field packing differs from laboratory and 2) modify current laboratory methods to reproducibly better imitate how growers pack containers in the field. Growing media comprised of 100% pine bark, coir, or peat, and blends of each were compared. Using discs and shims consistently packed experimental units similar to growers for each substrate blend when compared to conventional tapping technique. Small, but notable differences were observed in water retention and air space when packed using the different methods.

Technical Abstract: Soilless substrate cores are typically prepared in a laboratory by filling a sample ring with substrate then tapping the ring on a benchtop. Substrate packing method will largely determine the bulk density (Db), which subsequently impacts substrate physical property results. Bulk density produced in the laboratory can be inconsistent across samples, and may not align with the Db at a production nursery or “expected” published ranges. Additionally, it is unknown if ranges identified as “typical” using a sample ring in a laboratory reflect Db occurring in production size-containers packed using commercial potting practices. Packing to an insufficient Db can cause substrate shrinkage following saturation and drying, and along with packing to a non-representative Db can compromise the accuracy, precision, and applicability of subsequent static and dynamic physical properties. Therefore, our objectives were to 1) emulate nursery practices and document the Db associated with a potted 2.8-L (#1) container, 2) develop and test the “shim and compression” method to determine if it consistently packs sample rings to a Db commensurate with that of a 2.8-L nursery container, and 3) demonstrate how static physical properties and parameters derived from moisture characteristic curves (MCC) are affected by the new “shim and compression” sample ring packing method. In this experiment, we use an espresso tamp exerting 5 newtons of pressure per cm2 in combination with 7.5 cm diameter, 0.2 mm thick, stainless-steel shims for 100% pine bark, coir, and peat, and blends of each. We found that the Db achieved using a range in number of presses and discs is largely dependent on the substrate, but that the desired Db (i.e., Db of 2.8-L potted containers) can be consistently achieved for each substrate using this technique. There was no effect of disc number on Db (P-value = 1.000) for any substrate. There was no effect of tamp number (P-value = 0.0602) for all substrates except peat-amended. For peat-amended, 5 tamps yielded a greater Db than 1 tamp (P-value = 0.0324). MCCs produced by packing the sample ring using the “shim and compression” method were very consistent with standard deviations =0.006 regardless of substrate. Packing method nominally impacted easily available water (EAW; -1 to -5 kPa) and water buffering capacity (WBC; -5 to -10 kPa). To our knowledge this is the only report of Db observed in commercial container production facilities (i.e., “native”) using nursery production-relevant container sizes. Yet, additional research is needed to establish the Db at nursery potting lines using a range of container sizes and more comprehensively test alternative packing methods.