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Pine Bark Particle Separation Improves as Moisture Content Decreases at Time of Screening

Authors:
Jeb S. Fields Hammond Research Station, Louisiana State University Agricultural Center, 21549 Old Covington Highway, Hammond, LA 70403, USA

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Kristopher S. Criscione Hammond Research Station, Louisiana State University Agricultural Center, 21549 Old Covington Highway, Hammond, LA 70403, USA

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Abstract

Bark particle screening is a critical secondary processing stage when engineering bark-based horticultural substrates. There are several factors that can influence bark screening efficiency; however, the bark moisture content immediately before screening may have the largest impact. The objectives of this study were to determine the effect bark moisture content has on bark particle separation across two commonly used screen apertures and the subsequent static physical properties of the screened bark. The moisture contents examined herein ranged from 50%, 55%, 60%, 65%, and 70% and were gravimetrically determined. The screen apertures used were 6.3 mm and 9.5 mm. The results showed that moisture content has a considerable effect on both screening yield and the physical properties. Generally, as moisture content increased, bark yield (i.e., bark processed through the aperture) decreased. Moreover, as moisture content increased, the proportions of fine bark particles adhered to coarse bark increased, shifting the air-filled porosity: water-holding capacity of the substrate. In summation, the drier moisture content had the greatest (i.e., most equal) separation, regardless of screen aperture. Future research should identify the interaction between feed rate and moisture content.

In soilless culture, bark-based substrates continue to be a leading soilless substrate in container production (Gruda 2021). Bark was previously (< 1970s) considered a by-product of the timber industry and was deemed a useless, costly waste disposal issue, as most would be discarded by being buried or burned (Naasz et al. 2009). This was especially true since lumber (wood or xylem) was prioritized to function as a material for fuel and energy usage and for the construction or fabrication of buildings or other objects (Raviv and Leith 2008). However, political pressure at the time aimed to reduce environmental and agricultural pollution by repurposing discarded raw material into a reusable item. Considering that bark typically comprises ∼10% of a tree’s volume (Bunt 1988), sustainability initiatives in the timber industry have discovered several agricultural sectors to divert bark waste, such as in paper products (Harkin and Rowe 1971), biofuels (Nosek et al. 2016), cork (wine bottles), and in the horticulture industry as both a decorative mulch (Raviv and Leith 2008) and growing medium (Baker 1957; Bunt 1988; Gruda 2021; Pokorny 1979). Typically, softwood genera such as Pinus, Pseudotsuga, or Picea are used in horticultural operations because of their greater lignin contents.

Bark includes the inner (phloem) and outer (rhytidome) fractions of the tree bark and there are several environmental (i.e., tree age and size, location, time of year harvested, weather conditions, and specific species) and cultural (type and order of processing; aging, hammer-milling, and screening) factors that can influence bark quality and yield (Raviv and Leith 2008). However, post-debarking, bark often requires several types of secondary processing for suitability in container production. Fresh bark (i.e., bark recently harvested from the tree) can commonly contain phytotoxins such as terpenes and phenols that can inhibit or restrict plant growth, have high nitrogen-immobilization rates due to an undesirable C:N ratio (Solbraa 1979), and low pH values (∼4; Gruda 2021). A commonly used solution to prepare bark for horticultural use is to age or compost the bark for >6 months (Kaderabek et al. 2016), or to leach the soluble compounds with water (Raviv and Leith 2008). This emphasizes the importance of secondary processing for the integrity and biological stability of bark-based substrates.

An important consideration of freshly harvested bark that warrants further refining and processing is that unprocessed bark is large in particle size, requiring hammer-milling or screening (Pokorny 1983). A bark particle size suitable for horticultural substrates has been identified to be >0.6 mm (Pokorny and Delaney 1975), whereas most composted/aged bark substrates range between 0.1 to 20 mm (Gruda 2021). In view of this, decreasing the initial particle size (either pre- or post-aging) is often necessary. Today, most screen apertures during the hammer-milling process vary from 4.0 to 9.5 mm (Fain et al. 2008; Jackson et al. 2009). Bark particle size has a profound impact on substrate hydraulic characteristics, which can influence containerized plant growth and development (Fields et al. 2017). Bark substrates are well-known for their large particle sizes, which create high air-filled porosity and low water-holding capacity, when compared with other horticultural substrates such as peat or coir (Raviv and Leith 2008). A careful balance between a particle size distribution (PSD) that is too coarse or too fine can lead to either insufficient water availability or poor aeration, respectively (Mathers et al. 2007).

During the bark particle fractionation (i.e., hammer-milling and screening process), a critical factor that influences its efficiency is the moisture content of the bark immediately before screening (Stewart et al. 2019). Jackson et al. (2010) found that increasing bark moisture content decreased the quantity of bark particles screened. Because of the high surface tension of water and its adhesive and cohesive properties, greater moisture content enables the bark particles to adhere to each other or form a restrictive mat on the screen aperture, blocking further separation. However, limited research has been conducted to quantify the effect moisture content has on bark screening efficiency. Thus, the objective of this study was to determine the relationship between increasing moisture content and bark fractionation efficiency.

Materials and Methods

Bark moisture content preparation.

Locally sourced, aged loblolly pine (Pinus taeda) bark (Phillips Bark Processing Co, Brookhaven, MS, USA) was filled in 15 plastic bags (0.03 m3 each) and sealed to prevent evaporative moisture loss. The moisture content (MC) of each bag was gravimetrically determined (55% ± 0.01% SD) by weighing three wet samples, drying them at 105 °C for 48 h, and reweighing the samples using Eq. [1]. Thus, five MC treatments were selected to gauge an accurate representation of common bark pile MC before screening: 50%, 55%, 60%, 65%, and 70% MC.
wet weightdry weightwet weight 

Considering the bark MC ranged within the MC treatment values, samples were adjusted to either decrease or increase the MC. A porometer analysis, as described by Fonteno and Harden (2010), was used with unprocessed (unscreened) bark to estimate the dry weight of the 0.03 m3 bark samples (n = 3). This was accomplished by assuming bulk density (Db) values 0.17 g⋅cm−3 ± 0.00 SD with a corresponding mass of 4919 g ± 59 SD. Consequently, the volume of water needed to be decreased (via evaporation; 50% MC) or increased (55%, 60%, 65%, and 70% MC) for each MC treatment was determined gravimetrically. Treatments contained target weights of 9838 (50% MC), 10,931 (55% MC), 12,298 (60% MC), 14,055 (65% MC), and 16,397 g (70% MC).

Each MC treatment contained three replicates (5MC treatments × 3replicates = 15total bags). The bark with 50% MC treatment remained in the plastic bag with the bag open and bark surface exposed to the atmosphere to encourage evaporative loss to reduce the MC. The bags were continuously weighed until the target weight was reached. For the remaining treatments, 72 h before screening to allow for ample equilibration, tap water was measured in a graduated cylinder and slowly added to each bag in its respective treatment until the desired weight was achieved. Once all MC treatments achieved the targeted weights, the treatments were prepared for screening.

Bark processing.

Immediately before bark fractionation, three ∼50-g bark samples were collected from each bag to measure the MC. The actual MC of the 50%, 55%, 60%, 65%, and 70% MC treatments were 52% ± 2% SD, 58% ± 2% SD, 61% ± 5% SD, 65% ± 1% SD, and 68% ± 1% SD, respectively. In addition to the five MC treatments, there were two screen aperture treatments, a 6.3 and a 9.5 mm. Considering each bag was a replicate, 0.014 m3 (half of each bag replicate was screened in either the 6.3 or 9.5 mm screened) of bark was removed and placed in a 0.03-m3 container and immediately processed. The bark was passed through a continuous flow screen (CF-1; Gilson Company Inc. Model, Lewis Center, OH, USA) fitted with either the 6.3- or 9.5-mm aperture screen, set to 569 rpm, and screen level was maintained at 5° inclined slope. The bark particles were passed through the screen at a rate of 8000 cm3⋅min−1.

Measurements.

The data presented within the article regarding the 6.3-mm aperture screen treatments was collected from a project published by Criscione et al. (2022). For the remainder of the article, bark that was processed through the screen will be denoted as “fines,” and bark that did not pass through the screen will be referred to as “coarse” bark. Multiple measurements were evaluated during and immediately after each replicate was screened:

  • - The duration of time needed for all the bark to be passed through the screen.

  • - The mass (g) of the fine and coarse barks.

  • - The volume (m3) of the fine and coarse barks.

After the bark replicate within each MC treatment was processed, the bark was placed in a plastic bag and sealed to prevent further moisture loss.

Physical properties.

Substrate physical properties [container capacity (CC), air space (AS), total porosity (TP), and Db] were measured via porometers for each coarse and fine replicates within all MC treatments post screening. Thereafter, each replicate within the MC treatments was measured for its PSD by sieving three, 100 g dry substrate replicates through a Ro-Tap shaker (Rx-29; W.S. Tyler, Mentor, OH, USA) for 5 min using a column of stacked sieves with aperture sizes of 6.3, 2.0, 0.7, 0.5, 0.3, and 0.1 mm, with a catch pan at the bottom.

Data analysis.

All data presented in tables and figures with corresponding statistical analysis were analyzed in JMP Pro (16.2.0; SAS Institute, Inc., Cary, NC, USA) using analysis of variance and Tukey’s honestly significant difference test at the α = 0.05 significance level. Pearson correlation coefficient values were also calculated in JMP Pro (16.2.0) to correlate screening parameters across different types of measured yield.

Results

Physical properties.

Among the coarse particles screened through a 6.3-mm aperture, there were no differences detected within CC (P = 0.4820), AS (P = 0.1211), and TP (P = 0.2375) across MC treatments (Table 1). Coarse particles screened at 60% MC contained lower Db values than 65% and 70% MC treatments (P = 0.0162; Table 1). Differences in static physical properties were more pronounced in bark fines, whereas more significant differences were observed across MC treatments in CC (P = 0.0239) and AS values (P < 0.0001; Table 1). Bark screened at 65% MC had significantly lower Db values than all other MC treatments (P = 0.0003; Table 1).

Table 1.

Static physical properties of pine bark substrates screened under five different initial moisture contents. After screening, coarse (>6.3 or 9.5 mm) and fine (<6.3 or 9.5 mm) bark particles are produced.

Table 1.

Bark screened with a 9.5-mm aperture had more obvious effects on static physical properties than bark screened with a 6.3-mm aperture (Table 1). There were interactions between MC and partitioned bark particles with all static properties. As MC increased in coarse bark, CC (P < 0.0001) and TP (P = 0.0168) increased, whereas AS (P = 0.0470) and Db (P = 0.0156) decreased (Table 1). These results were contrasted in bark fines, whereas MC increased; AS (P < 0.0001), TP (P < 0.0001), and Db (P < 0.0001) generally increased; and CC (P = 0.0149) decreased (Table 1).

MC had similar influences on PSD values across screen aperture (Table 2). For both screen apertures, generally as MC increased, there were fewer particles with larger diameters and greater proportions of finer particles for coarse bark (Table 2; Fig. 1). Contrasting these results, finer bark particle proportions were generally greater in the lower MC treatments when screened with a 6.3-mm aperture, whereas there were no differences observed in fine bark across treatments screened with a 9.5-mm aperture (Table 2). There were strong relationships between the particle size proportions and their subsequent static physical properties (Table 3). In both screen apertures, there were strong relationships regarding extra-large particle abundances and AS (positive), CC (negative), and Db (negative) (Table 3). This relationship was contrary to finer particle proportions, where AS, CC, and Db related negatively, positively, and positively, respectively (Table 3).

Table 2.

Particle size distribution of pine bark substrates screened under five different initial moisture contents. After screening, coarse (>6.3 or 9.5 mm) and fine (< 6.3 or 9.5 mm) bark particles are produced.

Table 2.
Fig. 1.
Fig. 1.

Particle size distribution curve of screened bark with a (A) 6.3-mm screen aperture and (B) 9.5-mm screen aperture under different initial moisture contents. Each error bar is constructed using a 95% confidence interval of the mean. Data regarding the bark screened with a 6.3-mm aperture was collected from Criscione et al. (2022) and adapted.

Citation: HortScience 58, 9; 10.21273/HORTSCI17205-23

Table 3.

Pairwise Pearson correlations across static physical properties, particle size distribution, and screening measured parameters. All data regarding bark treatments screened with a 6.3-mm aperture were collected from Criscione et al. (2022).

Table 3.

Screening measurements.

MC had significant effects on bark screening yields for both screen apertures (Table 4). Using a 6.3-mm aperture, as MC increased, the quantity of time that bark remained on the screen after the final feed increased (P < 0.0001), and coarse bark volume (P = 0.0049), mass (P < 0.0001), and particle separation ratio on a volume (P < 0.0001) and mass (P < 0.0001) basis increased (Table 4). These effects were opposite for the fine bark, whereas MC increased, the volume (P < 0.0001) and mass (P < 0.0001), as well as the particle separation ratio based off volume (P < 0.0001) and mass (P < 0.0001) decreased (Table 4).

Table 4.

Bark screened at five initial moisture contents and yield was measured in various methods during and immediately after screening. Bark particles were screened with a 6.3-mm or 9.5-mm aperture.

Table 4.

Using a 9.5-mm aperture, MC had comparable effects as in the 6.3-mm screened bark treatments (Table 4). The quantity of time that bark remained on the screen after the final feed increased (P < 0.0001) as MC increased (Table 4). Coarse bark volume (P = 0.0002), mass (P < 0.0001), and particle separation ratio on a volume (P = 0.0005) and mass (P < 0.0001) basis increased as MC increased (Table 4). Moisture contents influence was also opposite in the screened fine bark; as MC increased, the volume (P = 0.0067) and mass (P = 0.0041), as well as the particle separation ratio based off volume (P = 0.0005) and mass (P < 0.0001) decreased (Table 4).

Bark particle size screened with a 6.3-mm aperture had stronger relationships with screened bark volume and mass than bark screened with a 9.5-mm aperture (Table 3). Using a 6.3-mm aperture, extra-large particles had a stronger relationship than other particle diameters for volume, mass, and particle separation based off volume and mass (Table 3). However, in the 9.5-mm treatments, the particle diameter had little relationships on screening yield data (Table 3).

Discussion

Bark particle screening partitions bark fragments into two or more diameter classifications. The process can influence the static physical properties of the bark, subsequently determining its behavior and performance as a horticultural substrate (Wallach 2008). This is primarily due to the strong relationships observed between pore and particle diameter, and their interaction with substrate water retention capabilities (Hillel 2004). The tension at which water can be held is directly related to the pore diameter opening (Nimmo 2004) or rather its effective pore space (Wallach 2008) in bark substrates, considering bark is “plate-like” in structure and results in irregularities in bark pore sizes and shapes. Smaller-diameter particles generally contribute to enhanced micropore volume and tightly held water, whereas larger-diameter particles contribute to macropore volume, which drain more freely (Zazirska et al. 2021). Thus, differentials between bark particle diameter homogeneity generally shifts substrate AS:CC ratio because of changes in pore size (Altland et al. 2018).

As shown herein, the water-holding capacity of the screened bark increased with increasing MC, irrespective of screen aperture (Table 1). This was likely attributed to the increase in smaller particle proportions, because the fine particles likely adhered to the coarse bark during screening (Table 2; Jackson et al. 2010). Considering the relative relationship between screened bark and its separated particles (i.e., unscreened bark that is partitioned to yield fine and coarse bark), generally fewer fine particles remained in the fine bark substrate, regardless of screen aperture, with increasing MC (Table 2; Fig. 1). This highlights that the initial MC of bark before screening has similar effects as the static physical properties and particle diameter distribution of the harvested bark, regardless of screen size; however, it is apparent that there is a stronger influence when a larger screen aperture is used. To expand, coarse bark processed with a 9.5-mm aperture overall contained significantly greater extra-large particle proportions than bark processed through a 6.3-mm aperture (P = 0.0001; Table 2). Thus, differences will be more pronounced when fine particles are added or removed (Tables 1 and 2). Nevertheless, harvested fine bark generally contained adequate statistic physical properties, although coarse particle treatments did not result with recommended nursery substrate static physical property standards for either CC (0.45–0.65 cm3⋅cm−3) or AS (Table 1; 0.10–0.30 cm3⋅cm−3; Bilderback et al. 2013).

The fine particles screened by the 6.3-mm aperture increased in AS, then significantly decreased. The authors hypothesize the relationship between AS and CC is a parabolic association. AS generally increases as more fine particles are adhered to the coarser bark, as a result of the cohesion and adhesion characteristics of water. As the moisture content increases in the 70% MC, and there was more capillary water, increased amounts of water were sufficient enough to add mass to the particles, separating more fine particles from the coarse bark, resulting in additional fine particles when screened at 70%. This is similar to the wet sieving process described in mineral soils (Ma et al. 2023). Shifting the AS:CC ratio can have a negligible influence on substrate TP, as observed with bark screened using a 6.3-mm aperture (P = 0.3947; Table 1). This follows the fundamental geometric principle that a group of uniform spherical objects will always occupy 66.7% (vol.) of a cylindrical container (Jury and Horton 2004), regardless of sphere volume. Although bark particles are relatively plate-like, this principle more-or-less follows the results herein (Fields et al. 2021). Yet, this was not observed in the 9.5-mm screened treatments of this study (P = 0.0095; Table 1).

Screening bark at greater MC yielded the slowest times for both screen apertures to be cleared of bark (P < 0.0001; Table 4). This is likely because of the increased proportions of medium and fine particles blocking screen apertures (Table 2; Jackson et al. 2010). Again, MC had a similar effect on bark output for both aperture treatments (Tables 3 and 4). Jackson et al. (2010) stated that greater MCs result in decreased screened bark proportions. Using bark with lower MC before screening resulted in greater partitioning (i.e., more fine particles are processed) than bark with greater MC because of fewer aperture obstructions and particle adherence (Table 4). Although screening bark with lower MC improves bark separation and yield can be increased (Table 4), caution should be exercised. For example, Fields et al. (2014) demonstrated that pine bark with a low (25%) MC was unable to achieve suitable CC values, even after 10 hydration events, because of the hydrophobic nature of the bark. Thus, screening bark at drier MC may require further processing, such as surfactant incorporation (Fields et al. 2014). Other considerations should include bark quality discrepancies. Fields et al. (2017) screened bark with a 4-mm aperture at 66.4% MC and received practically an equal (∼50%) partition by volume, further validating that there can be considerable variations in differently sourced bark (Kaderabek et al. 2016; Stewart et al. 2019).

Conclusion

Bark particle screening post-debarking is a critical secondary processing stage when engineering bark-based horticultural substrates. There are multiple factors that can influence the yield of bark separation; however, bark MC immediately before screening may have the largest impact. This study examined the effect of MC on bark screening yield and the resulting static physical properties of the screened bark under various screen apertures.

The results herein demonstrated that MC has similar effects on bark separation, regardless of screen aperture. Greater bark MC resulted in a more unequal separation, while the drier MCs had the greatest separation. As MC increased, the proportion of fine bark particles among the coarser bark increased because of fine particle adherence. Overall, screening bark at drier moisture contents will generate the greatest yield, although caution is advised for moisture contents below 50% due to potential hydrophobicity. Thus, the authors recommend that before screening to decrease the moisture content as low as possible above or near 50% MC. Future research should delve deeper into if feed rate can combat the effects of greater moisture on bark separation.

References Cited

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  • Fig. 1.

    Particle size distribution curve of screened bark with a (A) 6.3-mm screen aperture and (B) 9.5-mm screen aperture under different initial moisture contents. Each error bar is constructed using a 95% confidence interval of the mean. Data regarding the bark screened with a 6.3-mm aperture was collected from Criscione et al. (2022) and adapted.

  • Altland JE, Owen JS, Jackson BE, Fields JS. 2018. Physical and hydraulic properties of commercial pine-bark substrate products used in production of containerized crops. HortScience. 53:18831890. https://doi.org/10.21273/HORTSCI13497-18.

    • Search Google Scholar
    • Export Citation
  • Baker KF. 1957. The U.C. System for producing healthy container-grown plants. California Agric. Expt. Sta. Extension Service Manual 23, Univ. California, College of Agriculture Publication, Berkeley, CA, USA.

  • Bilderback TE, Owen JS, Altland JE, Fain GB, Jackson BE, Riley ED, Kraus HT, Fonteno WC. 2013. Strategies for developing sustainable substrates in nursery crop production. Acta Hort. 1013:43–56.

  • Bunt AC. 1988. Media and mixes for container-grown plants. 2nd ed. Unwin Hyman Ltd, London, England.

  • Criscione KS, Fields JS, Mizell A. 2022. Quantifying the influence of moisture content on bark screening for improved particle separation. Combined Proceedings IPPS 72:102–110.

  • Fain GB, Gilliam CH, Sibley JL, Boyer CR. 2008. Wholetree substrates derived from three species of pine in production of annual vinca. HortTechnology. 18:1317. https://doi.org/10.21273/HORTTECH. 18.1.13.

    • Search Google Scholar
    • Export Citation
  • Fields JS, Owen JS Jr, Scoggins HL. 2017. The influence of substrate hydraulic conductivity on plant water status of an ornamental container crop grown in suboptimal substrate water potentials. HortScience. 52:14191428. https://doi.org/10.21273/HORTSCI11987-17.

    • Search Google Scholar
    • Export Citation
  • Fields JS, Fonteno WC, Jackson BE. 2014. Hydration efficiency of traditional and alternative greenhouse substrate components. HortScience. 49:336342. https://doi.org/10.21273/HORTSCI.49.3.336.

    • Search Google Scholar
    • Export Citation
  • Fields JS, Owen JS, Altland JE. 2021. Substrate stratification: layering unique substrates within a container increases resource efficiency without impacting growth of shrub rose. Agronomy. 11:1454. https://doi.org/10.3390/agronomy11081454.

    • Search Google Scholar
    • Export Citation
  • Fonteno WC, Harden CT. 2010. North Carolina State University horticultural substrates lab manual. North Carolina State University, Raleigh, NC.

  • Gruda N. 2021. Soilless culture systems and growing media in horticulture: An overview, p 1–20. In: Gruda N (ed). Advances in horticultural soilless culture. Burleigh Dodds Science, Cambridge, UK.

  • Harkin JM, Rowe JW. 1971. Bark and its possible uses. United States Department of Agriculture Forest Service (Research note FPL; 091). https://www.fs.usda.gov/research/treesearch/5760.

  • Hillel D. 2004. Introduction to environmental soil physics. Elsevier Academic Press, San Deigo, CA.

  • Jackson BE, Wright RD, Seiler JR. 2009. Changes in chemical and physical properties of pine tree substrate and pine bark during long-term nursery crop production. HortScience. 44:791799. https://doi.org/10.21273/HORTSCI.44.3.791.

    • Search Google Scholar
    • Export Citation
  • Jackson BE, Wright RD, Barnes MC. 2010. Methods of constructing a pine tree substrate from various wood particle sizes, organic amendments, and sand for desired physical properties and plant growth. HortScience. 45:103112. https://doi.org/10.21273/HORTSCI.45.1.103.

    • Search Google Scholar
    • Export Citation
  • Jury WA, Horton R. 2004. Soil physics. Wiley, Hoboken, NJ, USA.

  • Kaderabek L, Jackson BE, Fonteno W. 2016. Changes in the physical, chemical, and hydrologic properties of pine bark over twelve months of aging. Intl Plant Prop Soc Proc. 41:313317.

    • Search Google Scholar
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Jeb S. Fields Hammond Research Station, Louisiana State University Agricultural Center, 21549 Old Covington Highway, Hammond, LA 70403, USA

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Kristopher S. Criscione Hammond Research Station, Louisiana State University Agricultural Center, 21549 Old Covington Highway, Hammond, LA 70403, USA

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Contributor Notes

J.S.F. is the corresponding author. E-mail: JFields@agcenter.lsu.edu.

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