Identifying native bulk density for static physical properties analysis

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

Submitted to: Acta horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2024
Publication Date: 12/6/2024
Citation: Alred, B., Owen Jr, J.S., Espinoza, A., Fulcher, A. 2024. Identifying native bulk density for static physical properties analysis. Acta horticulturae. 1409:121-124. https://doi.org/10.17660/ActaHortic.2024.1409.17.
DOI: https://doi.org/10.17660/ActaHortic.2024.1409.17

Interpretive Summary: Container production of nursery crops relies on soilless substrates as an anchor for root growth and development and a reservoir of both water and mineral nutrients. Laboratory instruments can provide insights into single factors such as maximum water holding capacity and minimum air space; whereas, moisture retention and storage determined by moisture characteristic curves provide insight to the dynamic or ever-changing properties that occur during crop production. Each of these parameters are interrelated and directly affected by bulk density (Db), the dry mass of substrate per given volume. Our objective was to document the Db associated with a potted 2.8 L (#1) container, when imitating common nursery worker potting techniques in support of future research developing a new packing method for soilless substrate cores. Soilless substrate sample cores are typically packed in the laboratory, since in situ procedures are not well defined nor proven. Thus, packing methods utilized are assumed to replicate conditions used to pack containers in production, bearing in mind the considerable impact that sample Db would have on physical properties and ultimately plant growth and development. We found that the in situ Db for each of the substrates, regardless of how they were packed, i.e., dropped and manually compressed or just manually compressed, irrigated or not, were close in value. Regardless, our hypothesis that a substrate sample in a 5 cm × 8 cm ring may not represent the same Db as when in a nursery container size. The Db achieved in this experiment varied by substrate and did not always match Db listed by historical references focused on laboratory methods. In the future, additional tests using a greater number of replications, a wider range of container sizes, and a commercial potting line should be conducted to establish in situ Db norms and evaluated for applicability in lab analysis.

Technical Abstract: Soilless substrate rings are commonly prepared in the laboratory rather than sampling in situ. Rings are packed by filling with loose substrate before being tapped on the benchtop; however, this method can yield inconsistent bulk density (Db) across samples and result in values outside the “typical” ranges. For example, a 100% pine bark substrate packed with the tap method yielded 0.14 g cm-3 instead of the reported “typical” 0.20 g cm-3, yet 100% Sphagnum peat packed with the tap method was 0.08 g cm-3, in the center of the “typical” range, 0.06 to 0.10 g cm-3. Additionally, ranges identified as “typical” in a laboratory may not reflect Db occurring through normal potting practices. Packing to an insufficient Db can cause substrate shrinkage following saturation and drying, compromising the accuracy, precision, and applicability of subsequent static and dynamic physical properties. Our objective was to document the Db associated with a potted 2.8-L (#1) container as an initial step toward developing a new laboratory packing method for substrates cores that will consistently achieve substrate Db commensurate with in situ nursery Db. We demonstrate the influence of container potting practices and post-potting irrigation on Db.