Abstract
Substrate stratification is an emerging substrate management strategy involving layering multiple substrate materials within a single container to modify physiochemical characteristics of the substrate system. Specifically, stratifying allows growers and researchers to rearrange the air–water balance within a container to modify hydraulic characteristics. Moreover, fertilizer can be incorporated into just the upper strata to reduce leaching. Research to date has shown benefits associated with resource efficiency, production timing, and weed control. With the associated benefits for substrate stratification, interested growers will need pragmatic solutions for onsite trials. Therefore, the objective of this study was to identify a cost-effective solution for growers interested in exploring stratification options. As such, this research was designed to identify a single-screen bark separation to generate fine and coarse bark textures suitable for use as the top and bottom substrate strata. Loblolly pine bark (Pinus taeda) was screened with either a 4.0-mm, 1/4-inch, or 3/8-inch screen, with the particles passing through the screen (unders) separated from retained particles (overs). Stratified substrate systems were engineered with an individual screen wherein the fines were layered atop the coarse particles from the same screen. ‘Natchez’ crepe myrtle (Lagerstroemia indica) liners were planted in either of the three stratified substrate treatments or a nonstratified control. Substrate physical characteristics were assessed for each strata by pre- and postproduction properties to identify changes of substrate. The final growth index of the crop was unaffected by the substrate treatment (P = 0.90); however, stratified substrates did increase dry root weight (P = 0.02), with the smallest screen (4.0 mm) resulting in the greatest root weight. Separation of roots between the two strata indicated the presence of more roots in the upper strata in all substrates. However, the stratified substrates resulted in a greater shift in root location, encouraging increased rooting in the upper strata with fine particles, with the largest screen (3/8 inch) resulting in the greatest differentiation between upper and lower rooting. Each stratified treatment had increase in water-holding capacity in the lower (coarser) strata without changes in the upper strata. Thus, we conclude that single screens can be used to build stratified substrate systems. Moreover, screen aperture size may be used to achieve different outcomes with regard to root growth and development as well as water–air balance. Further research may indicate that screen selection may be used to target specific crop needs.
The nursery industry is an expanding sector in agriculture that requires intensive use of water and fertilizer to produce salable crops. Many nursery growers rely on bark-based soilless substrates as the primary component of their growing media. Bark-based substrates often have increased proportions of large-diameter pores, which result in relatively low water-holding capacity (Pokorny, 1979) and allow for ample aeration (Jackson et al., 2006). This is advantageous because it reduces risks associated with excessive irrigation/precipitation (Mathers et al., 2005) and subsequent root disease (Baker, 1957). However, the downside to the low water-holding capacity is growers must irrigate frequently to replenish the water reservoir that is rapidly lost (Fields et al., 2021). Thus, nurseries tend to use large quantities of water, with estimates of 19,000 gal/acre/d spent on crop irrigation during the peak season (Fulcher and Fernandez, 2013). Leaching from high-irrigation application volume and frequency has been shown to result in runoff of agrichemicals such as mineral nutrients, pesticides, and large quantities of applied irrigation (Dumroese and Haase, 2018). As the industry seeks to become more sustainable, better water management strategies must be developed.
Soilless substrate stratification is an emerging substrate management strategy. It involves stacking two unique substrate layers atop one another to reorganize water retention characteristics in the container profile (Fields et al., 2020). This practice has been identified as a possible technique to increase irrigation water efficiency while improving plant growth simultaneously (Criscione et al., 2022). The placement of coarse particles in the lower half of the container promotes drainage where a perched water table is observed (Owen and Altland, 2008), whereas using substrates with high water-holding capacity and increased hydraulic and chemical retention in the upper half may provide more available resources for plant roots and may limit drying from evapotranspiration and gravitational forces. Together, this process supports a balanced rooting environment in the early stages of root growth and exploration in the upper half of the container, while allowing ample drainage in the lower half of the container. In practice, layering fine bark or bark amended with fibers such as peatmoss or coir on top of coarse bark will promote water holding in the upper (typically drier) portion of the container and will promote drainage in the lower (typically wetter) portion of the container. Thus, we are encouraging water retention and distribution to be more uniform throughout the container profile (i.e., promoting water retention in dryer regions and promoting drainage in wetter regions), while maintaining a similar overall porosity. Substrate stratification has also been shown to reduce weed germination (Khamare et al., 2022); however, this strategy involves layering coarse particles on top of finer particles. Thus, the upper portion of the container would dry quickly, inhibiting germination.
To date, Fields et al. (2021) and Criscione et al. (2022) have identified benefits associated with water and fertilizer management in nursery production using stratified bark substrates. Their work showed that layering fiber-amended bark atop coarse bark can increase resource (i.e., fertilizer and water) efficiency while producing similar crops. Criscione et al. (2022) also showed that stratified substrates can support quality plant production under irrigation levels that were too low to produce healthy plants grown in traditional pine (Pinus sp.) bark substrates. Both Fields et al. (2021) and Criscione et al. (2022) reduced fertilizer inputs by 20% and produced equal or larger plants in stratified substrates when compared with crops grown in traditional bark substrates.
With prior research effectively demonstrating the benefits of substrate stratification, it is important to identify pragmatic options to integrate this management strategy easily into production. It is hypothesized that a single screen could be used to divide a pine bark substrate into fractions to achieve stratification. Therefore, the objective of this study was to evaluate the growth effects of a nursery crop grown in single-screen, stratified pine bark substrates.
Materials and methods
Substrate preparation
Stabilized loblolly pine (Pinus taeda) bark (Phillips Bark Processing Co., Inc., Brookhaven, MS), which was aged ≈6 months, was amended with 4.6 lb/yard3 dolomitic lime (Lime-Rite Pelletized Dolomitic Lime; Imerys Carbonates, Roswell, GA) and 1.6 lb/yard3 micronutrients (Micromax G90505; ICL Specialty Fertilizers, Dublin, OH), and blended in a ribbon soil mixer for 10 min. Bark particles were then separated by diameter using a continuous flow screener (model CF-1; Gilson Company Inc., Lewis Center, OH) set to the standard 569 repetitions/min. The bark was separated using screens with aperture sizes of 4.0 mm, 1/4 inch, or 3/8 inch. A receptacle was placed under the sieve to collect the fractionated bark that was processed through the screen (unders); the bark that did not pass through (overs) was discharged into a separate receptacle. The bark particles were processed through the screen at a rate of 0.42 ft3/min, and the process was stopped every 10 min to remove debris from the screen. This process was continued until at least 3.5 ft3 of bark fines and 3.5 ft3 of coarse bark were collected. An additional 7.0 ft3 of unscreened pine bark was also retained.
Physical properties
The substrate static physical properties, including air space (AS), container capacity (CC), total porosity, and bulk density, were measured via the North Carolina State University porometer according to the process described by Fonteno and Harden (2010). The particle size distribution of each substrate was then evaluated by passing three 100-g oven-dry samples of each substrate through six sieves (6.3, 2.0, 0.7, 0.5, 0.3, and 0.1 mm), with a collection pan at the bottom. Each sample was agitated with a sieve shaker (Ro-Tap shaker Rx-29; W.S. Tyler, Mentor, OH) for 5 min. When finished, particles retained in each sieve were weighed and compiled into four size classifications: extralarge (>6.3 mm), large (2.0–6.3 mm), medium (2.0–0.7 mm), and fines (<0.7 mm).
Crop growth experiment
Ten 3.0-gal containers (1200 Blow Molded-Classic Line; Nursery Supplies, Kissimmee, FL) were filled with one of four substrate treatments: three stratified treatments and a control. The stratified treatments consisted of filling the bottom half of the container with the overs of a specific screen and filling the upper half with the unders from the same screen. Thus, treatments consisted of particles that passed through a 4.0-mm screen layered on top of particles that did not pass through the 4.0-mm screen (SS4), particles that passed through a 1/4-inch screen layered on top of particles that did not pass through the 1/4-inch screen (SS6), and particles that passed through a 3/8-inch screen layered on top of particles that did not pass through the 3/8-inch screen (SS9). The final treatment was a control and consisted of nonstratified pine bark throughout. The stratification was done using a template in which a line was drawn at exactly half the height of the container. When filled, each container was dropped once from ≈4 inches to settle the substrate.
On 23 June 2020, 5-cm ‘Natchez’ crepe myrtle (Lagerstroemia indica) liners were transplanted into each of the containers. Each container was then top-dressed with 80% of the medium-high recommended rate of 18N–2.6P–10.0K controlled-release fertilizer [Osmocote 18–6–12 (8–9 months), ICL Specialty Fertilizers]. Containers were then placed on an open-air nursery gravel pad under overhead irrigation. The containers were irrigated daily at 0700 hr for 30 min until 21 d after initiation (DAI) of the study. After 21 DAI, irrigation was adjusted to 20 min at 0700 hr and 10 min at 1400 hr.
Plant growth index [GI = (Plant height + Widest width + Perpendicular width)/3] was measured every 30 ± 1 d on three randomly selected replicates of each treatment, with the exception of 1 and 128 DAI (initiation and culmination of study, respectively), when every plant was measured. Substrate leachate fertility was measured twice (at 58 and 100 DAI) via pour-through analysis on three random replicates of each substrate treatment. Plants were irrigated to effective CC (maximum water-holding capacity in practice), allowed to equilibrate, and 500 mL deionized water was applied to displace pore water (LeBude and Bilderback, 2009). The displaced pore-water samples were analyzed for electrical conductivity (EC) and pH with a portable probe (GroLine HI 9814; Hanna Instruments, Woonsocket, RI).
Harvest
Photographs depicting roots and shoots were taken of representative replicates of each treatment on 144 DAI. Afterward, roots were separated from the shoots. The roots of half (five replicates) of each treatment were separated further by slicing the root system at the stratification interface. All roots were then washed of substrate particles. The root and shoot fresh weights were normalized by drying at 73 °F for 7 d and then weighed. Plant compactness was calculated as the ratio of shoot dry weight to shoot height; thus, larger values represent more compact plants (van Iersel and Nemali, 2004). Foliar nutrition from the 10 youngest, fully mature leaves from three plants per treatment was then analyzed via plant digestion (Deena II Automated Digestion System; Thomas Cain Inc., Omaha, NE) by the Louisiana State University Agricultural Center Soil Testing and Plant Analysis Laboratory (Baton Rouge, LA). Digested plant samples were then analyzed for agricultural metals through inductively coupled plasma spectroscopy (Spectro Arcos; SPECTRO Analytical Instruments GmbH, Kleve, Germany).
Data analysis
The data presented in the table and figures with associated statistics were analyzed using statistical software (JMP Pro version 16.0; SAS Institute, Inc., Cary, NC), with Tukey’s honestly significant difference test (α = 0.05), to separate substrate static physical properties, particle size fractions based on dry weight, crop responses, and foliar nutrition means across eight substrates (Table 1). In addition, analysis of variance was used to determine any statistically significant differences among the means of the substrate static physical properties, particle size fractions based on dry weight, crop responses, and foliar nutrition. In addition, the se was calculated (JMP Pro version 16.0) to determine differences from the mean of the samples.
Substrate static physical properties (container capacity, air space, total porosity, and bulk density) and particle size distribution of conventional bark fractions used in stratified substrate research.
Results and discussion
Substrate fractionation
The screen aperture size influenced directly total volume (P = 0.02; Fig. 1) and total dry weight (P = 0.01) of the fractions. Moreover, linear regression analysis of the separation by aperture diameter resulted in a linear relationship on both a volume (R2 = 0.81, F = 0.0010, P < 0.01) and a weight (R2 = 0.80, F = 0.0005, P < 0.10) basis. As the screen aperture increased in size, the substrate fractions became more uniform and even. The 4.0-mm, 1/4-inch, and 3/8-inch screens had a 1:4, 4:5, and 2:1 unders-to-overs dry weight ratio, respectively.
Fractionated bark particles through a screen that resulted in coarse bark particles [overs (>4.0 mm, 1/4 inch, and 3/8 inch)] and fine bark particles [unders (4.0 mm, 1/4 inch, and 3/8 inch)]. Letters represent differences among means of three substrates separately in unders and overs. (A) Total volume of each screen after separation. (B) A nonproportional photograph of the individual substrate fractions. With coarse bark particles (overs) atop fine bark particles (unders); 1 mm = 0.0394 inch, 1 inch = 25.4 mm.
Citation: HortTechnology 32, 5; 10.21273/HORTTECH05018-22
Substrate physical characteristics
Decreasing bark particle diameter increased substrate CC and decreased AS (Table 1). Increasing bark diameter had inverse effects, where substrate CC was decreased (Table 1). The coarse particles were similar in AS to the unscreened bark (Table 1). Total porosity was relatively similar across the substrates (Table 1). The 4.0-mm and 1/4-inch unders had the greatest bulk density whereas the 4.0-mm and 1/4-inch coarse bark had the least (Table 1). The 1/4- and 3/8-inch overs bark contained the greatest proportions of extralarge particles (>6.3 mm; Table 1).
Root growth has been shown to alter substrate physical characteristics (Altland et al., 2011; Judd et al., 2015), and more importantly these changes to the physical environment influence water movement (Cannavo et al., 2011). In general, through the process of crop production, substrates’ physical properties are modified over time, where CC is increased and AS is decreased (Altland et al., 2018). After 128 d of production, the unscreened bark increased in CC and decreased AS by nearly 0.1 cm3⋅cm–3, changing equally in both the top and bottom strata (Table 1). Although there were very small roots present in the samples, the postexperiment assessment was done on substrate particles that had been cleared from roots. The 4.0-mm, 1/4-inch, and 3/8-inch coarse particles all increased CC by ≈0.12 cm3⋅cm–3 over the course of the experiment, whereas all the fine particles remained the same (Table 1). Furthermore, the small particles increased in all coarse strata postexperiment by ≈0.05 g⋅g–1, which corresponds to 5% of the total sample shifting to fines through the production cycle. This shift to smaller particles is likely a contributing factor for the increased CC in the lower strata, likely a result of both particle breakdown and migration of fine particles from the upper portion. In addition, there is some expectation of particle settling through the production cycle that may also contribute minimally to the change in CC. The only AS value to change was the 4.0-mm coarse, which decreased by ≈0.13 g⋅g–1, again likely a result of both particle migration downward through the substrate profile and breakdown. Overall, there were few differences in total porosity between pre- and postexperimentation, aside from the increase in 1/4-inch coarse particles, which was an equal increase to CC.
There were no changes in extralarge particle proportions throughout the duration of the study; however, 1/4-inch coarse bark increased in large particle percentages, possibly because of marginal shrinkage of extralarge particles (Table 1). The 4.0-mm fine bark treatments were the only substrates to increase significantly in bark fines (Table 1).
Crop physical growth and development
Overall, there were few differences in GI among plants grown in the four substrate treatments (Fig. 2). The only observable differences being at 60 DAI, when plants grown in SS4 were smaller than plants grown in the other substrate treatments, and 110 DAI, when plants grown in SS4 and SS6 were larger than plants grown in the control treatment. By the end of the study, there were no detectable differences in overall plant GI (Fig. 2).
Growth index of containerized ‘Natchez’ crepe myrtle (Lagerstroemia indica) grown in four experimental substrate treatments. Experimental substrate treatments include a control of conventional pine bark (nonstratified) and three stratified systems. The stratified systems all contained 1) coarse bark particles (>4.0-mm screen) paired complementarily in the upper strata with fine bark [<4.0 mm (SS4)], 2) coarse bark particles (>1/4 inch) paired complementarily in the upper strata with fine bark [<1/4 inch (SS6)], and 3) a coarse bark particle (>3/8 inch) paired complementarily in the upper strata with fine bark [<3/8 inch (SS9)]; 1 mm = 0.0394 inch, 1 inch = 25.4 mm, 1 cm = 0.3937 inch.
Citation: HortTechnology 32, 5; 10.21273/HORTTECH05018-22
There were no differences observed in mean foliar concentrations (±sd) of nitrogen (3.6% ± 0.6%, P = 0.22), phosphorus (0.6% ± 0.1%, P = 0.18), potassium (1.9% ± 0.2%, P = 0.47), and sulfur (0.4% ± 0.04%, P = 0.31) among substrate treatments at the culmination of the study. However, substrate treatment did affect foliar calcium (1.3% ± 0.3%, P = 0.01), magnesium (0.4% ± 0.04%, P = 0.00), and iron (126.7 ± 22.0 ppm, P = 0.04), where SS4 generally had the greatest proportions among the other treatments. This could be a result of the increase in smaller particles supporting added nutrient retention (Altland et al., 2014).
Nearing the end of the study (100 DAI), pour-through analysis identified that there were no differences in pour-through pH values among the four substrate treatments (P = 0.33). However, there were differences observed in pour-through EC values (P = 0.02), with SS4 having the greatest EC value. Altland et al. (2014) observed that cation exchange capacity increases as particle size decreases as a result of increased surface area, resulting in greater access to exchange sites. Therefore, the greater proportions of medium and fine particles in SS4 in the upper layer may have resulted in increased nutrient retention and slower ion transport (nutrient leaching) through the upper layer. However, the stratification process ultimately did not result in any major changes in the overall growth of the crops.
At harvest, there were no detectable differences in population means (±sd) in the overall root-to-shoot ratio (0.74 ± 0.2, P = 0.20), change in GI (difference from initial to final GI) (37.8 ± 6.8 cm, P = 0.53), and compactness (0.43 ± 0.2 g⋅cm–1, P = 0.43) among the crops grown in our study. This indicates that no substrate treatment influenced water or nutrient retention that resulted in shoot growth differences. However, overall root dry weight was influenced by substrate treatment (P = 0.02), with the plants growing in SS4 having the greatest dry root weight (P = 0.02). Increased root growth in SS4 may explain the reduced GI observed 60 DAI (Fig. 2), in which the plants in the SS4 may have allocated greater proportions of resources to root growth. This possibility may warrant additional investigation in the future.
Further investigation into root location and architecture indicates that there was a greater root weight in the upper strata than the lower strata (P = 0.03) in all treatments (Fig. 3). The difference in upper and lower strata root dry weight differed greatly between the individual treatments, with all stratified treatments having more differentiation than the plants grown with the control treatment (Fig. 3). However, it should be noted that SS6 had even fractions of unders and overs, with a similar root growth as the control (Fig. 3). Plants grown in SS4 and SS6 both had a 1.8 upper-to-lower rooting ratio, with plants grown in SS9 having a 2.7 upper-to-lower rooting ratio. Last, plants grown in the control treatment had a 1.5 upper-to-lower root ratio. Thus, we can conclude that the extreme coarseness of the lower strata in SS9 (i.e., bark > 3/8 inch) was less conducive for root growth when compared with the upper strata. This may have further limited the rooting volume of the container; however, in this instance, there were no shoot differences. The exploration of containerized plant roots is limited by many factors, including a finite container volume, environmental factors (temperature and wind fluctuations), and quick changes in resource availability (Polak and Wallach, 2001). The differences in rooting may indicate further limitations to root exploration in some stratified systems. With many nursery crops destined for transplanting into landscapes, superior root growth and exploration is critical to reducing transplant shock (Grossnickle, 2005). Thus, further investigation into root growth and expansion within different strata must be conducted.
Root dry weight partitioning within the upper 50% (red bars) and lower 50% (blue bars) of the substrate profile. ‘Natchez’ crepe myrtle (Lagerstroemia indica) was grown in one of four experimental substrate treatments. The control was a nonstratified (unscreened) substrate. Stratified substrate treatments were engineered by separating conventional pine bark with a single screen that contained apertures of 4.0 mm (SS4), 1/4 inch (SS6), and 3/8 inch (SS9); 1 mm = 0.0394 inch, 1 inch = 25.4 mm, 1 g = 0.0353 oz.
Citation: HortTechnology 32, 5; 10.21273/HORTTECH05018-22
Conclusion
Substrate stratification has been described as a potential solution for improving resource management in container crop production. However, growers will need pragmatic solutions or options to incorporate substrate stratification trials into their operation. The purpose of our study was to identify whether growers could use a single screen to separate bark particles effectively into two fractions suitable for substrate stratification. All three of the screen sizes use in this research (4.0 mm, 1/4 inch, and 3/8 inch), as well as the nonstratified control, resulted in an equal overall crop GI. However, stratified substrates resulted in greater root weight than in plants grown in the control treatment. Moreover, root growth was enhanced in the upper half of the stratified substrates, whereas in nonstratified substrates, there was more of a balanced growth among the upper and lower halves.
Overall, stratified substrates had little influence on crepe myrtle shoot growth. The differences in root architecture and root exploration may provide further tools to support growers. This research presents the opportunity to identify whether individual strata can be engineered specifically for added production benefits.
Units
Literature cited
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