Physical and Hydraulic Properties of Commercial Pine-bark Substrate Products Used in Production of Containerized Crops

in HortScience

Pine bark is the primary constituent of nursery container media (i.e., soilless substrate) in the eastern United States. Pine bark physical and hydraulic properties vary depending on the supplier due to source (e.g., lumber mill type) or methods of additional processing or aging. Pine bark can be processed via hammer milling or grinding before or after being aged from ≤1 month (fresh) to ≥6 month (aged). Additionally, bark is commonly amended with sand to alter physical properties and increase bulk density (Db). Information is limited on physical or hydraulic differences of bark between varying sources or the effect of sand amendments. Pine bark physical and hydraulic properties from six commercial sources were compared as a function of age and amendment with sand. Aging bark, alone, had little effect on total porosity (TP), which remained at ≈80.5% (by volume). However, aging pine bark from ≤1 to ≥6 months shifted particle size from the coarse (>2 mm) to fine fraction (<0.5 mm), which increased container capacity (CC) 21.4% and decreased air space (AS) by 17.2% (by volume) regardless of source. The addition of sand to the substrate had a similar effect on particle size distribution to that of aging, increasing CC and Db while decreasing AS. Total porosity decreased with the addition of sand. The magnitude of change in TP, AS, CC, and Db from a nonamended pine bark substrate was greater with fine vs. coarse sand and varied by bark source. When comparing hydrological properties across three pine bark sources, readily available water content was unaffected; however, moisture characteristic curves (MCC) differed due to particle size distribution affecting the residual water content and subsequent shift from gravitational to either capillary or hygroscopic water. Similarly, hydraulic conductivity (i.e., ability to transfer water within the container) decreased with increasing particle size.

Contributor Notes

Funding for this work was provided, in part, by the Virginia Agricultural Experiment Stations, Agricultural Research Service, and the Hatch Programs of the National Institute of Food and Agriculture, U.S. Department of Agriculture.

Trade or brand names used in this publication do not constitute a guarantee or warranty of the products by Virginia Polytechnic Institute and State University, U.S. Department of Agriculture, North Carolina State University, or Louisiana State University and do not imply their approval to the exclusion of other products or vendors that may also be suitable.

Research Horticulturist.

Associate Professor.

Assistant Professor.

Corresponding author. E-mail: james.altland@ars.usda.gov.

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    Fresh (A) and aged (B) loblolly pine bark from supplier 1; fresh (C) and aged (D) loblolly pine bark from supplier 2; and fresh (E) and aged (F) loblolly pine bark from supplier 3. Bark of both ages from each supplier was amended with 10% (by volume) fine or coarse sand for determination of effects on physical properties.

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    Observed relationship of volumetric water content and water potential generated via the evaporative method to fit moisture characteristic parameters of aged pine bark from three sources [X (=), Y (), Z (Õ)] that provide soilless substrate for containerized crop production in the eastern United States.

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    Modeled unsaturated hydraulic conductivity (K) of aged pine bark (Pinus taeda) from three North Carolina bark sources [X (—), Y (···), Z (- - -)] that provide soilless substrate for containerized crop production in the eastern United States.

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