Abstract
Soilless substrate stratification is increasing in popularity in the greenhouse and nursery industry globally. The concept of stratifying substrates entails stacking two substrates with different physiochemical properties to augment vertical moisture balances and redistribution for quicker establishment, greater root growth, and quicker time to market. Stratified substrate research to date has estimated or assumed that the static physical properties of a stratified system as the mean of the individual strata components. No research to date has verified or rejected this assumption using a stratified column. The research herein measured the static physical properties of 1) peatlite (85% peat: 15% perlite), 2) unprocessed <12.7 mm aged bark, 3) fine bark particles (≤6.3 mm), and 4) coarse bark particles (≥6.3 mm). Moreover, these physical properties were measured via 1) using the standard promoter analysis (7.6 cm core), 2) extending the core height by stacking two standard porometer cores (15.2 cm height) of the same strata component atop each other (to identify how water storage and air-filled porosity changes), and 3) stratifying either peatlite over unprocessed bark or fine bark over coarse bark. The results showed that extending the height of the porometer increased drainage and decreased water storage across conventional and stratified systems alike, illustrating the benefit of using cores equivalent to container height when making cultural decisions to manage water efficiently. When stratifying substrates, the system as a whole stores less water and has more air-filled porosity than nonstratified composite profiles (100% peatlite; 100% unprocessed bark) due to gravitational forces draining the higher portion of the container. Assumptions regarding the static physical properties of a stratified system can be made with the standard or extended porometer core for coarse-textured bark substrates used generally in the nursery industry with reasonable accuracy (<5% difference), meaning that nursery growers interested in stratifying their substrates can assess their stratified static physical properties using standard measurements. However, assumptions cannot accurately be assessed for finer peat-based stratified profiles used in greenhouse production and may require further refinement for estimations. The broader implications of this research highlight the storage capacities of a stratified substrate system, which may influence growers’ decisions in application and irrigation management.
Soilless substrate stratification is an increasingly popular substrate management technique that involves stacking unique substrates within a single container (Fields et al. 2021). Typically, a fine or fibrous substrate is layered above a coarse material to redistribute the water and air throughout the vertical container profile. In the greenhouse, a peatlite is placed atop <9.5 mm bark, whereas in nursery a fine bark or fiber-amended bark is placed atop a coarse bark (>6.3 mm). In container substrates at equilibrium, a moisture content gradient is present from the top of the container (i.e., drier) to the container base as a result of gravitational and matric forces acting on the moisture within (Bilderback and Fonteno 1987; Milks et al. 1989; Owen and Altland 2008). This phenomenon occurs regardless of the substrate material, provided that the substrate is homogeneous throughout the container. In a stratified container, the finer particles toward the surface increase microporosity (Drzal et al. 1999), which ultimately results in lower water potential and greater water storage; whereas the coarser material in the sub-strata increases drainage and reduces perched water table effects (Criscione et al. 2024). The resulting moisture gradient is reduced in stratified substrates. In recent years, this stratification technique has been increasingly implemented by growers globally due to the potential improvement of irrigation and fertilizer efficiency (Criscione et al. 2022a; Fields et al. 2021) and reduction of peat and other costly substrate materials (Fields and Criscione 2023). Research has also indicated slower water infiltration rates in a nursery bark–based stratified substrate systems compared with conventional containers filled homogenously with one bark substrate (Criscione et al. 2022b). This results in more uniform wetting fronts and ultimately enhanced irrigation efficiency, resulting in greater water storage per irrigation event (Criscione et al. 2024).
With substrate stratification becoming more prevalent in both the United States and global horticultural markets, it is important for scientists to evaluate physical properties of stratified system in a consistent, objective, and standardized manner. Traditionally, the North Carolina State University (NCSU) porometer has been used to analyze soilless substrate physical properties (Fonteno and Bilderback 1993), becoming a staple experiment in soilless substrate research laboratories. The method measures four fundamental substrate static physical properties using a 7.62-cm core, including 1) the minimum proportion of air-filled porosity (AS) and 2) the maximum poportion of water stored within the substrate pores (container capacity; CC) after all gravitational drainage has occurred. These two static metrics calculate 3) the substrate total porosity (TP) and 4) dry bulk density (Db). The porometer method provides a means to standardize soilless substrate static physical properties and allow universal comparisons of past, present, and future substrates. The measured porometer core is engineered to be representative of the center of a greenhouse container profile (3.81 cm), providing an informative understanding regarding the in situ container substrate properties. Using the porometer method allows any soilless media composition to be analyzed, ranging from stand-alone components (i.e., Sphagnum peat) to varying blended composites (i.e., a peat:perlite mix). However, as soilless substrate research continues to evolve, new and emerging engineered substrates have been developed to extend beyond blending traditionally used components.
Individual substrate strata have been evaluated via porometer analysis to gather information regarding the CC and AS (Criscione et al. 2022a; Fields et al. 2021), where assumptions can be made on the water and air distribution within a stratified profile. However, these assumptions have limitations, as height and gravitational adjustments are not taken into consideration, nor will the presence of any perched or transient water tables be reflected. Yet, no reports to date have been able to confirm accurate static physical properties of the entirety of a stratified substrate profile. Accurate assessment of substrate physical properties can benefit the horticultural industry as growers continue to embrace stratification practices, allowing more informed irrigation and fertilization decisions.
The overall objective of this research was to augment the NCSU porometer procedure to ascertain accurate assessment of substrate physical properties of the most basic forms of conventional and stratified substrates (peatlite; pine bark; stratified peatlite over pine bark; stratified fine bark over coarse bark). To do this, individual 7.6- and 15.2-cm-tall cores (7.6 cm i.d.) were packed to standard density and vertically stacked to more accurately reflect stratified substrates in containers (i.e., 50:50 top:bottom ratio). In addition, the 7.6 cm and 15.2 cm would represent young plants (i.e., transplants) and small floriculture or foliage and nursery crops. This manuscript aims to identify 1) the variation in core height, 2) if porometer cores can be overlayed upon each other to determine the physical properties, with a focus on redistribution of water retention in a stratified system, and 3) how we can compute the physical properties of a stratified system using individual strata cores.
Materials and Methods
Substrate preparation.
Approximately 0.17 m3 of aged pine bark less than 12.7 mm (Pinus taeda; Phillips Bark Processing Co., Brookhaven, MS, USA) was collected. A 0.06-m3 subset of the bark was set aside. The remainder was screened with a 6.3-mm continuous flow screen (Gilson; CF-1; Lewis Center, OH, USA) at a rate of 0.012 m3·min−1 (5° slope; 569 r·min−1). The resulting substrates were bark particles ≥6.3 mm (coarse bark) and ≤6.3 mm (fine bark). In addition to the processed bark substrates, 0.03 m3 of a commercial peat:perlite mix (85:15; Pro-Mix LP 15; Quakertown, PA, USA) was hydrated for use.
Substrate physical properties—standard porometer analysis.
The static physical properties of all substrates were analyzed via NCSU porometer analysis (n = 3) as described by Fonteno and Bilderback (1993). Briefly, the primary 7.6 cm h × 7.6 cm i.d. (347.5 cm3) cylindrical core was attached atop an identical base core (347.5 cm3) and below an extended top core (15.2 cm h × 7.6 cm i.d.; 695 cm3), creating a column (30.4 cm h × 7.6 cm i.d.; 1737.5 cm3). The substrate was slowly filled in the column gently tapped five times from a 10-cm height. The top core was removed, after which particles were gently discarded until flush with the primary (7.6 cm; 347.5 cm3) core. The lid was attached, and cores inverted so the base core was reoriented on the top. Thereafter, the base core was detached, and particles were gently removed until flush. The core containing the substrate and lid was weighed to measure the pack weight. All core pack weights were within 5% within a respective substrate treatment.
A base plate with perforated holes that rotate between an open and closed configuration was attached to the primary core. With the lid removed, the core + baseplate was then placed in a modified Buchner funnel. The Buchner funnel was sealed on the bottom, base plates were opened to allow water entry, and deionized water was slowly added inside the funnel. Thus, the substrate was saturated bottom-up via capillary action and the water table was equilibrated to evacuate all air-filled pores and completely saturate the substrate. Water was added to the funnel until reaching one-third of the core height. After 5 min, more water was added to raise the water level to two-thirds the core height before waiting an additional 5 min before raising the water to create a meniscus between the funnel and core top. After ≈30 min of saturation, base plates were rotated and closed for 15 more min, and Buchner funnels were drained. Lids were loosely placed on top to minimize evaporation. After 45 min of saturation, base plates were slowly opened, and leachate was collected. Cores were allowed to drain for >1 h.
Air-filled porosity was measured by dividing the gravitational water (drain volume; cm3) by the total core volume (347.5 cm3). Container capacity was measured by taking the substrate wet weight, subtracting the dry weight to receive the total proportion of water within the sample, and dividing that by the total core volume. TP was the summation of AS and CC. Bulk density was calculated by taking the dry weight of the substrate and dividing it by the volume of the core. After all gravitational drainage occurred, base plates were removed, lids attached, and the substrate wet weight was measured. Substrate was dried for 48 h at 105 °C.
Substrate physical properties—modified porometer.
In a stratified porometer, regardless of the substrate, each 15.2-cm core was packed using the same process described by Fonteno and Bilderback (1993) (Fig. 1A). However, the procedure for the ensuing steps was modified. For example, a standard packed peatlite core is stacked atop a normally packed unprocessed bark core. Standard horticulture paper is placed atop the sub-strata core (in this case unprocessed bark), the core is slowly rotated and placed atop the peatlite core to minimize rotation and particle migration (Fig. 1B). The paper is carefully removed between the two cores (Fig. 1C). Following removal, standard adhesive tape is weighed and tightly wrapped around the core interface to securely attach the two cores and create a seal to prevent leaking (Fig. 1C and D). Both lids are attached, and the entire stacked core is weighed to record a new stacked pack weight. All pack weights are maintained within 5% of their respective treatment. Base plates were attached to the sub-strata core (for example, unscreened bark) and cores were inverted.
Representation of an extended/stratified porometer core. (A) Two separate standard cores (7.6 cm; 347.5 cm3) packed normally with respective substrates. (B) A standard white horticulture paper was placed atop the sub-strata, carefully inverted on top the top strata core. (C) Paper was carefully removed, and the cores were tightly secured using strong adhesive tape. (D) A base plate was attached to the bottom core and the cores were carefully inverted. (E) An 18.9-L container with bolts attached to the bottom was used to raise the water table to the top of the extended porometer core. (F) Saturated cores were placed back in the modified Buchner funnel to allow drainage to occur and collection of leachate.
Citation: HortScience 59, 12; 10.21273/HORTSCI18188-24
Because the height of the modified Buchner funnel was insufficient to adequately bring the bottom-up water table to the top of the core, an 18.9-L container was used. Bolts were attached at the bottom of the 18.9-L container to rotate the base plate (Fig. 1E). All base plates were opened, and water was again slowly added at the same time and proportions as suggested by Fonteno and Bilderback (1993). After 45 min of saturation, base plates were closed, and the stacked cores were removed from the bucket before placing on the modified Buchner funnel to drain the core (Fig. 1F). All leachate was collected, and the cores were drained >1 h. Once the stacked cores finished draining, base plates were removed and reweighed to record substrate wet weight. The tape was then removed, and the substrate was placed in an oven for 48 h at 105 °C.
The stratified porometer air-filled porosity was measured by dividing the leachate (cm3) by the total core volume (695 cm3). Container capacity was measured by taking the substrate wet weight, subtracting the dry weight to receive the total proportion of water within the sample, and dividing that by the total core volume (695 cm3). TP was the summation of AS and CC. Bulk density was calculated by taking the dry weight of the substrate and dividing it by the volume of the core (695 cm3).
Four substrates assessed: 1) unscreened pine bark, 2) a peatlite mix, 3) fine bark particles, and 4) coarse bark particles. These four substrates were all packed normally and evaluated by the standard 7.6-cm porometer (Fonteno and Bilderback 1993). Then, substrates were evaluated by the extended 15.2 porometer by stacking 1) two unscreened 7.6-cm pine bark cores atop each other (SS-U), 2) two peatlite substrate 7.6-cm cores atop each other (SS-PL), 3) two fine bark 7.6-cm cores atop each other (SS-FB), 4) two coarse bark 7.6-cm cores atop each other (SS-CB), then two 15.2 stratified porometers, including 5) a 7.6-cm peatlite core stacked atop an 7.6-cm unscreened bark core (SS-P), and a 6) 7.6-cm fine bark core stacked atop a 7.6-cm coarse bark core (SS-B).
Results and Discussion
Standard porometers.
The static physical properties were different across substrates measured by the standard porometer (Table 1). Screening bark particles ≥6.3 and ≤6.3 mm significantly decreased and increased substrate CC, respectively, when compared with the unprocessed bark, whereas peatlite contained the greatest CC (P < 0.0001; Table 1). This was inverted for AS, where peatlite had the lowest AS, and coarse bark had ≈0.10 cm3·cm−3 more AS than unprocessed bark (Table 1). This is the result of increasing or decreasing the proportions of micro/macroporosity across substrates. Smaller-diameter pores store more water, where larger diameter pores generally release more water (Handreck and Black 2002). TP was unaffected by screening bark particles (P = 0.1686; Table 1).
The static physical properties of a substrates measured by the standard porometer analysisi (7.6 cm core; 347.5 cm3), including a 1) peatlite (85% peat: 15% perlite), 2) unprocessed <12.7-mm aged bark, 3) fine bark particles (≤6.3 mm), and 4) coarse bark particles (≥6.3 mm). In addition, these same four substrates were also measured for their static physical properties using an extended porometer (15.2 cm; 695 cm3).
Extended porometer.
Moisture retention and distribution within container substrates is dictated by two primary forces: gravity (in the form of gravitational potential) and matric potential. The height of a container or column of substrate will adjust the gravitational potential, with taller columns draining more than shorter columns of equal volume. In other words, the reference point where the gravitational forces are imposed on the substrate column, often represented by Z0, changes with height (Fig. 2). This additional drainage is most observable in the upper portion of the container as this will have the greatest gravitational force. By comparison, container width has little to no influence on drainage capacity of a substrate material, where the water table at the container base would remain at a constant height given the same substrate. Matric potential, or how tightly water is held in a pore, can be a function of the pore size distribution, with smaller pores exerting a greater tension on water than large pores. A substrate matrix with a uniform particle size distribution has consistent matric tensions throughout the substrate profile. The magnitude of gravitational forces becomes greater as container height increases. Milks et al. (1989) showed that gravitational tensions increase 1 hPa with every centimeter of height; therefore, taller containers will experience more gravitational potential, which changes moisture distribution. In other words, there are greater gravitational forces at the top of a container than bottom, which results in a moisture gradient that extends throughout the vertical container column (Bilderback and Fonteno 1987). Thus, it was hypothesized that extending the height of the porometer would influence moisture characteristics and ultimately shift water storage and air-filled porosity ratio (Owen and Altland 2008).
Illustrating the influence changing the container height has on average pressure head via 1) extending the porometer height (15.4 cm; left), 2) stacking two 7.2-cm porometers atop each other (15.4; middle), or 3) using a standard porometer core.
Citation: HortScience 59, 12; 10.21273/HORTSCI18188-24
Extending the height of the porometer core affected the CC and AS of finer particle substrates, namely peatlite (less CC: P < 0.0001; more AS: P < 0.0010) and fine bark (less CC: P < 0.05) when compared with the standard porometer (Table 1). However, extending the height of the core did not statistically affect CC in unprocessed or coarse pine bark. This was hypothesized because previous reports have demonstrated that the magnitude of the moisture gradient can be greater in finer particles than larger textured substrates (Owen and Altland 2008). Substrates that contain a finer particle distribution in general contain smaller-diameter pores (and more pores) that ultimately have an influence on moisture retention characteristics (Fields et al. 2018; Handreck and Black 2002). Smaller pore diameters retain more water than large pores because of greater capillary forces (Hillel 2004); thus, in finer particles there is more variation for moisture distribution.
The extended porometer cores ultimately store less water and contain more AS than the standard porometer core. This was expected because the height of the core is increased by 100%, and it has been shown that taller containers drain more (Owen and Altland 2008); thus, resulting in more gravitational pore water draining (Fig. 2). In the extended porometer cores, there were similar linear decreases in CC as in the standard porometers, where peatlite contained the greatest CC and coarse bark contained the lowest (P < 0.0001; Table 1). Again, analogous to the standard porometers, there were similar increases in AS across substrates (P < 0.0001; Table 1). Peatlite in the extended core had the greatest TP, where all the other extended cores contained similar TP (Table 1).
Stratified porometer.
The concept of stratification, layering finer textured substrates atop of coarser particles, was originally designed to spatially redistribute the AS:CC within the container (Fields et al. 2021). The TP of the stratified system remains the same as nonstratified profiles, where only AS and CC are being redistributed in more strategic locations across the vertical container profile (Table 2). Essentially, stratified substrates modify where matric forces are more strongly or weakly imposed in the container. Criscione et al. (2024) showed that more available water is held in the top strata layer, while overall the system is storing less water.
The static physical propertiesi of a substrate measured by a modified porometer analysis. Stratified porometer cores were also assessed, in which a 1) peatlite (85% peat: 15% perlite) porometer core was stacked atop a unprocessed bark porometer core, and 2) a fine bark (<6.3 mm) porometer core was stacked atop a coarse bark (>6.3 mm) core resulting in an extended porometer (15.2 cm; 695 cm3).
Stratifying fine and coarse bark particles decreased CC (P < 0.0001) and increased AS (P < 0.0001; Tables 1 and 2) when compared with the extended unprocessed pine bark. The results are also consistent for peatlite stratified atop unprocessed bark, where the stratified peatlite profile stored less water overall than the nonstratified extended peatlite column (Tables 1 and 2). The authors hypothesize that the system as a whole stores less water than nonstratified profiles, which is due to the hydrophysical properties of the coarser sub-strata. Most of the water stored in a substrate, irrespective of the substrate type, is in the lower layers of a container profile (Bilderback and Fonteno 1987; Milks et al. 1989; Owen and Altland 2008). Thus, placing a coarser substrate in the bottom half reduces the perched water table effect, which significantly contributes to the greater AS/reduced CC (Table 1). Fields et al. (2024) operated on an assumption that the static physical properties of the stratified system would be approximated by the mean of the two individual substrates and reported that a 50:50 layered stratified profile stored less water and contained greater AS.
It was previously hypothesized that averaging static physical properties of the top and bottom strata can accurately estimate the physical properties of the stratified profile (Fields et al. 2021, 2024). Because the height of the porometer core will modify the AS and CC, it is difficult to assess the physical properties of a stratified system using the standard porometer core measurements. In other words, estimations of the physical properties of a stratified substrate system are difficult to attain with a standard porometer core because the estimated values are generally overestimated for CC and underestimated for AS (Table 3). However, if the porometer core is extended to account for the increase in height, this hypothesis can be confirmed in coarser substrates (i.e., bark; Table 3). The results herein suggest that reasonably accurate estimates can be attained by averaging the static physical properties of the two separate strata (<5% difference) in bark-based stratified systems using the standard 7.6-cm core. Estimations to date are less accurate (>5% difference) for peat-based stratified systems (Table 3). This is attributed to the vastly different physical properties between strata (e.g., peatlite and pine bark), as water resists gravitational forces in finer materials.
The static physical propertiesi of a substrates measured by a modified porometer analysis. Stratified porometer cores were also assessed, where a 1) peatlite (85% peat: 15% perlite) porometer core was stacked atop a unprocessed bark porometer core, and 2) a fine bark (<6.3 mm) porometer core was stacked atop a coarse bark (>6.3 mm) core resulting in an extended porometer (15.2 cm; 695 cm3). Calculated stratified averages were attained from individual porometer core measurements.
Conclusion
For testing stratified substrates, especially those involving coarse materials like pine bark, both standard and extended porometer methods can provide accurate measurements of water and air storage. However, for fine-textured, peat-based stratified systems, the extended porometer method is recommended for more accurate representations of hydrophysical properties, as the standard method may lead to overestimations of water storage and underestimations of air-filled porosity.
Previous assumptions of linear relationship between container height and physical properties of a stratified system (i.e., taking the average of the two individual strata components) were not accurate for fine-textured peat-based stratified systems; however, with reasonable accuracy (<5% differences), assumptions can be made using either the extended or standard porometer core values for coarse-texture (pine bark) substrates.
As growers continue to adopt stratified substrates to improve resource efficiency and reduce peat reliance, further exploration into the physics of stratification becomes necessary. A more robust understanding of how individual substrate materials complement each other in stratified systems can further prepare growers with better information. A long-term decision-making tool for assessing substrate physical properties would benefit integrated root zone management decisions.
References Cited
Bilderback TE, Fonteno WC. 1987. Effects of container geometry and media physical properties on air and water volumes in containers. J Environ Hort. 5(4):180–182. https://doi.org/10.24266/0738-2898-5.4.180.
Criscione KS, Fields JS, Owen JS Jr. 2022. Root exploration, initial moisture conditions, and irrigation scheduling influence hydration of stratified and non-stratified substrates. Horticulturae. 8(9):826. https://doi.org/10.3390/horticulturae8090826.
Criscione KS, Fields JS, Owen JS Jr, Fultz L, Bush E. 2022. Evaluating stratified substrates effect on containerized crop growth under varied irrigation strategies. HortScience. 57(3):400–413. https://doi.org/10.21273/HORTSCI16288-21.
Criscione KS, Fields JS, Owen JS, Gentimis A. 2024. Stratified substrates enhance water storage and distribution between irrigation events. Soil Sci Soc Am J. 88(2):387–402. https://doi.org/10.1002/saj2.20636.
Drzal MS, Fonteno WC, Cassel DK. 1999. Pore fraction analysis: A new tool for substrate testing. Acta Hortic. 481:43–54. https://doi.org/10.17660/ActaHortic.1999.481.1.
Fields JS, Owen JS Jr, Altland JE, van Iersel MW, Jackson BE. 2018. Soilless substrate hydrology can be engineered to influence plant water status for an ornamental containerized crop grown within optimal water potentials. J Amer Soc Hort Sci. 143(4):268–281. https://doi.org/10.21273/JASHS04251-17.
Fields JS, Criscione KS. 2023. Stratified substrates can reduce peat use and improve root productivity in container crop production. HortScience. 58(3):364–372. https://doi.org/10.21273/HORTSCI17019-22.
Fields JS, Criscione KS, Owen JS. 2024. Conventional sphagnum peatlite substrate can be reduced by 50% in stratified substrate systems while growing equivalent size and quality Pentas lanceolata. Sci Hortic. 331:113121. https://doi.org/10.1016/j.scienta.2024.113121.
Fields JS, Owen JS Jr, Altland JE. 2021. Substrate stratification: Layering unique substrates within a container increases resource efficiency without impacting growth of shrub rose. Agron J. 11(8):1454. https://doi.org/10.3390/agronomy11081454.
Fonteno WC, Bilderback TE. 1993. Impact of hydrogel on physical properties of coarse-structured horticultural substrates. J Am Soc Hortic Sci. 118(2):217–222. https://doi.org/10.21273/JASHS.118.2.217.
Handreck K, Black N. 2002. Growing media for ornamental plants and turf (3rd ed). University of New South Wales Press, Sydney, Australia.
Hillel D. 2004. Introduction to environmental soil physics. Elsevier Academic Press, San Deigo, CA, USA.
Milks RR, Fonteno WC, Larson RA. 1989. Hydrology of horticultural substrates: II. Predicting physical properties of substrate in containers. J Am Soc Hortic Sci. 114(1):53–56. https://doi.org/10.21273/JASHS.114.1.53.
Owen JS, Altland JE. 2008. Container height and douglas fir bark texture affect substrate physical properties. HortScience. 43(2):505–508. https://doi.org/10.21273/HORTSCI.43.2.505.