Influence of Sand on Physical Properties of Pine Bark Substrates of Different Ages and from Different Sources

Bark has been an important component in horticultural substrates for years (and is rapidly increasing in greenhouse professional mixes and in retail potting soil mixes), and in the nursery industry bark is the most common substrate component used in the United States. Bark is obtained as a byproduct of the timber industry (pulp mills, saw mills, pole peeling operations, etc.) when it is striped off logs after harvest. Sand is still often added to bark nursery substrates. Whether it be for the added weight to prevent container blow overs or just because it is “tradition” and they always have done it so they continue to do it. The objective of this work was to evaluate the influence of two grades of sand on the physical properties of pine barks of different ages and from different sources/suppliers. To determine the influence of sand on physical properties of pine bark substrates, four sources and two different ages (fresh and aged) were obtained and tested. Samples of each bark material were separated into three subsamples (3 L): one without sand, one amended with 10% (by vol) small grade fine sand, and one amended with 10% (by vol) large grade coarse sand. Once blended, substrates were wet up to 60% moisture content to prevent shrinkage/swellage during porometer testing. A total of 12 samples were tested for physical properties using the NCSU porometer method. The effect of sand on the air and water properties of each pine bark was the same regardless of source or age. Bark amended with small grade sand showed an increase in water and a decrease in air space (compared to bark without sand). Those amended with large grade sand did not show a significant change in water (only varied 1% or 2%), however, did exhibit a decrease in air space. For both small and large grade sand amended barks, total porosity decreased by 5% or more as compared to bark without either sand. Whether sand can increase, decrease, or have absolutely no effect on air and water properties is completely dependent on the bark it is being added to if the particle size variations are very different.