2005 study.

On 17 Aug. 2005, japanese holly liners (10-cm tall in 64-cm³ containers) were potted in 2.8-L plastic containers [17 cm (h) × 17 cm (d)] containing either PB or PTS. Pine tree substrate was produced from loblolly pine trees (≈30 cm basal diameter) that were harvested at ground level and delimbed on 25 July 2005 in Warsaw, VA. Trees were then chipped (including bark) with a Morbark Chipper (Winn, MI) operated by Wood Preservers Inc. (Warsaw, VA) on 26 July 2005. Wood chips (2.5 cm × 2.5 cm × 0.5 cm) were further ground in a hammer mill (Meadows Mills, Inc., North Wilkesboro, NC) on 27 July 2005 to pass through a 6.35-mm screen. Pine tree substrate was used fresh (uncomposted) and amended with 0.6 kg·m⁻³ calcium sulfate (CaSO₄) based on previous work by Saunders et al. (2005) that showed improved growth in herbaceous species when CaSO₄ was incorporated in PTS. No preplant amendments are required with PB for japanese holly production. Osmocote Plus (15N–3.9P–10K; O.M. Scott Horticulture Products) was preplant-incorporated in PB and PTS at rates of 3.5, 5.9, 8.3, or 10.6 kg·m⁻³ (5.3, 8.9, 12.5, or 16 g·m⁻³ of N), respectively. Plants were grown on benches in a greenhouse in Blacksburg, VA, and irrigated as needed with beaker-applied water to achieve an approximate 30% leaching fraction. Average day and night temperatures in the greenhouse were 26 and 22 ^oC, respectively. Substrate solution was extracted using the pourthrough method (Wright, 1986) every 4 weeks and analyzed for pH and EC using a Hanna HI 9811 instrument (Hanna Instruments, Woonsocket, RI). Nutrient analysis was conducted on substrate solution taken on 7 Oct. 2005 and analyzed for nitrate (NO₃-N) with an Orion ion selective electrode (Thermo Electron, Beverly, MA) and P, K, Ca, and Mg concentrations were determined with a Spectro Ciros Vision ICP (Spectro Analytical Instrument, Mahwah, NJ). On 22 Nov., shoots were severed at the substrate surface and roots were washed, dried at 65 °C for 4 d, and weighed. Samples of the most recently matured leaves from four plants per treatment were harvested and analyzed for N, P, and K (Quality Analytical Laboratories, Panama City, FL).

As a measure of microbial activity in PB and PTS, substrate CO₂ efflux levels were determined as an indicator of the potential for N immobilization to occur (Wang et al., 2003). Substrate CO₂ efflux (μmol CO₂ m⁻²·s⁻¹) was determined at the end of the experiment (23 Nov. 2005) on three container replications of both substrates at each fertilizer rate using a LI-6400 (LI-COR, Lincoln, NE) fitted with a soil CO₂ flux chamber designed to take nondestructive CO₂ measurements from the substrate-filled containers. A 10.2-cm diameter × 5.1-cm high plastic collar was inserted 0.5 cm into the container substrate surface allowing the soil CO₂ flux chamber to be inserted and positioned securely above the substrate surface for CO₂ efflux measurements.

Physical properties including air space, container capacity, total porosity, and bulk density were determined on three replicate samples of each substrate at the beginning of the experiment using the North Carolina State University Porometer method as described by Fonteno et al. (1995). Cation exchange capacity (CEC) was determined by A & L Eastern Laboratories (Richmond, VA) using the AOAC International Official Method (Thrope, 1973).

2007 study.

An experiment similar to the previous experiment was conducted from 1 Mar. to 25 May 2007. Pine tree substrate was produced from 12-year-old loblolly pine trees (≈25 cm in basal diameter) that were harvested at ground level and delimbed on 19 Feb. 2007 in Blackstone, VA, and chipped with bark intact on 20 Feb. 2007 with a Bandit Chipper (Model 200; Bandit Industries, Inc., Remus, MI). Wood chips were then further ground in a hammer mill on 21 Feb. 2007 to pass through a 6.35-mm screen. Physical properties of both substrates were determined using the method described in the 2005 study. Osmocote Plus (15N–3.9P–10K) was incorporated in PB and PTS at rates of 3.5, 5.9, 8.3, or 10.6 kg·m⁻³ (5.3, 8.9, 12.5, or 16 g·m⁻³ of N), respectively. In addition to japanese holly, this experiment also included azalea. Because azaleas require lower fertilizer rates (compared with holly) for optimal growth (Yeager et al., 2007), this species was included in this study to provide growth data on a woody species with a relatively low nutrient requirement when grown in PB and PTS. Fertilizer rates for azalea were 1.2, 3.5, 5.9, and 8.3 kg·m⁻³ (1.8, 5.3, 8.9, or 12 g·m⁻³ of N), respectively. Both substrates were preplant amended similar to the 2005 study. Plants were grown on a gravel ground bed in a greenhouse in Blacksburg, VA, and overhead-irrigated at a rate of 4.5 cm·h with a Basic Grower boom sprayer (McConkey Co., Sumer, WA) as needed to achieve an approximate 30% leaching fraction. Species were grouped separately on the ground bed and irrigated independently during the entire experiment. Substrate solution was extracted using the PT method every 2 weeks and analyzed for pH and EC for both species. Nutrient analysis was conducted on substrate solution taken from each species on 30 Apr. 2007 in the same method as previously described for the 2005 study. On 25 May 2007, shoots of both species were severed at the substrate surface and the roots of japanese holly were washed, dried at 65 °C for 4 d, and weighed. Samples of the most recently matured leaves from five plants, per treatment, per species were harvested and analyzed for N, P, and K by the Penn State Analytical Testing Laboratory, University Park, PA. Substrate CO₂ efflux was measured on three container replications of each treatment for japanese holly.

The experimental design was completely randomized with six single container replications per treatment in 2005 and five replications per treatment per species in 2007. All data in both studies were analyzed with the analysis of variance procedure using SAS (version 9.1; SAS Institute, Cary, NC) and subjected to regression analysis using SigmaPlot (version 9.01; SPSS, Chicago, IL) with the exception that substrate physical properties were subjected to analysis of variance within the GLM procedure with treatment means separated by least significant difference analysis. Standard error bars are presented in Figures 1 through 3 with the linear and quadratic regression responses presented.

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2005 study.

There was a significant substrate × fertilizer rate interaction (P ≤ 0.001) for shoot dry weight: at fertilizer rates of 3.5 and 5.9 kg·m⁻³, shoot dry weight was higher for PB than PTS; at 8.3 kg·m⁻³, dry weight was approximately equal for the two substrates; at 10.6 kg·m⁻³, dry weight was higher for PTS than PB (Fig. 1A). Root dry weights were not influenced by fertilizer rate (P ≤ 0.145) but were higher in PB than PTS at the 3.5 and 5.9 kg·m⁻³ rates and equal in both substrates at the 8.3 and 10.6 kg·m⁻³ rates (Fig. 1B). There was a significant substrate × fertilizer rate interaction (P ≤ 0.002) for substrate CO₂ efflux. Efflux levels were higher in PTS than in PB at each fertilizer rate with the magnitude of difference decreasing as fertilizer rate increased (Fig. 1C). At the lowest fertilizer rate (3.5 kg·m⁻³), substrate efflux levels in PTS were four times higher than in PB but were nearly the same at the highest fertilizer rate (10.6 kg·m⁻³). In contrast to PTS, substrate efflux levels slightly increased in the PB substrate as fertilizer rates increased, resulting in similar efflux levels at the highest fertilizer rate (Fig. 1C).

Substrate solution EC values increased with increasing fertilizer rate and at any particular fertilizer rate levels were higher in PB than in PTS (Table 1), showing that higher fertilizer rates were required for PTS compared with PB to achieve comparable substrate EC levels. Substrate solution pH was higher in PTS at each fertilizer rate compared with PB and decreased as fertilizer rate increased (Table 1). The pH of PB did not change with increasing fertilizer rates. There was a significant substrate and fertilizer rate response for all nutrient concentrations (except for K) in the substrate solution sampled on 7 Oct. 2005 (Table 2). Similar to the EC levels previously discussed, nitrate (NO₃-N) concentrations were higher in PB at each fertilizer rate compared with PTS. Solution concentrations of P, K, Ca, and Mg were also higher in PB compared with PTS (Table 2).

Table 1. Average electrical conductivity (EC) levels and pH values of substrate solution for japanese holly grown in pine bark or pine tree substrates for 3 months when fertilized with four rates of Osmocote Plus 15N–3.9P–10K^z.

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Table 2. Substrate solution nutrient concentrations sampled on 7 Oct. 2005 and 20 Apr. 2007 for japanese holly grown in pine bark or pine tree substrate fertilized with four rates of Osmocote Plus 15N–3.9P–10K^z.

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Leaf tissue nutrient concentrations were generally higher in PB-grown plants compared with PTS-grown plants at each fertilizer rate (Table 3). All nutrient levels generally increased as fertilizer rate increased for PB up to the 10.6 kg·m⁻³ rate, and nutrient levels increased in PTS plants up to the 8.3 kg·m⁻³ rate (Table 3). Tissue N levels in PB-grown plants were above the recommended sufficiency range of 1.80% to 2.80% (Mills and Jones, 1996) at all fertilizer rates, and tissue N levels in PTS-grown plants were within the recommended range at the 5.9, 8.3, and 10.6 kg·m⁻³ fertilizer rates (Table 3).

Table 3. Leaf tissue analysis taken on 22 Nov. 2005 and 25 May 2007 for japanese holly grown in pine bark or pine tree substrate and fertilized with four rates of Osmocote Plus 15N–3.9P–10K.

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Total porosity was similar for both PB and PTS and was within the recommended range outlined in The Best Management Practices Guide for Producing Container-Grown Plants (BMP) for physical properties of container substrates (Yeager et al., 2007) (Table 4). Air space was significantly higher in PTS (38.2%) than in PB and was above the recommended BMP range. Container capacity was higher in PB (56.1%) than in PTS (45.2%), but both were within the BMP range (45% to 65%) for both substrates. Bulk density of PTS was similar to PB but was below minimum recommended value of 0.19 g·cm⁻³ (Table 4).

Table 4. Physical and chemical properties of unamended pine bark and pine tree substrates used in the production of japanese holly in 2005^z.

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2007 study.

Like with the 2005 experiment, there was a significant substrate × fertilizer rate interaction (P ≤ 0.025) for japanese holly shoot dry weight: at the fertilizer rate of 3.5 kg·m⁻³, shoot dry weight was higher for PB than PTS; at rates of 5.9, 8.3, and 10.6 kg·m⁻³, dry weight was similar for PB and PTS plants (Fig. 2A). The data also indicate that maximum shoot dry weight for japanese holly in PB is at 3.5 kg·m⁻³ or less, and for PTS is between the 3.5 and 5.9 kg·m⁻³ rates (Fig. 2A).

There was a substrate × rate interaction for japanese holly root dry weights (P ≤ 0.017). Root growth in both PTS and PB decreased as fertilizer rate increased (Fig. 2B), but in contrast to 2005, root growth was generally higher in PTS-grown plants than PB-grown plants. There was a significant substrate × fertilizer rate interaction (P ≤ 0.001) for substrate CO₂ efflux. Similar to 2005 results, substrate CO₂ efflux levels were higher in PTS than in PB at each fertilizer rate with the magnitude of difference decreasing slightly as fertilizer rate increased (Fig. 2C). At all fertilizer rates (3.5, 5.9, 8.3, and 10.6 kg·m⁻³), substrate CO₂ efflux levels were twice as high in PTS as in PB (Fig. 2C). Like with the 2005 experiment, substrate solution EC levels increased with increasing fertilizer rates in both substrates (Table 1). Also similar to 2005, substrate solution pH was higher in PTS than PB regardless of fertilizer rate, and pH decreased with increasing fertilizer rate in PB and PTS (Table 1). Similar to the 2005 data, nitrate concentrations in the substrate solution were generally higher in PB compared with PTS and substrate solution concentrations of P, K, Ca, and Mg were in most cases higher in PB compared with PTS (Table 2). Similar to 2005, japanese holly leaf tissue nutrient levels generally increased in response to increasing fertilizer rate for PB and PTS-grown plants (Table 3). In contrast to 2005 data, leaf nutrient levels at the 8.3 and 10.6 kg·m⁻³ fertilizer rates for N and at the 5.9, 8.3, and 10.6 kg·m⁻³ rates for P (Table 3) were higher in PTS-grown plants compared with PB-grown plants. Tissue N levels were above the recommended sufficiency range (1.8% to 2.8%) at all fertilizer rates for plants grown in both substrates except for the 3.5 kg·m⁻³ rate in PTS. Potassium levels were higher in PTS-grown plants at all fertilizer rates (Table 3). Physical property data of PTS and PB in 2007 were similar to 2005 data and therefore are not reported.

There was a significant substrate × fertilizer rate interaction (P ≤ 0.013) for azalea shoot dry weight: at the fertilizer rates of 1.2 and 3.5 kg·m⁻³, shoot dry weight was higher for PB than PTS; at rates 5.9 and 8.3 kg·m⁻³, dry weights were similar for PB and PTS plants (Fig. 3) with maximum shoot dry weight occurring at 3.5 kg·m⁻³ for PB and at 5.9 kg·m⁻³ for PTS. Substrate solution EC levels for azalea increased with increasing fertilizer rate in both substrates and were higher in PB than in PTS at all rates (Table 5). This once again shows that higher fertilizer rates are required in PTS compared with PB to achieve similar substrate EC levels. Similar to japanese holly, substrate solution pH was higher in PTS than PB regardless of fertilizer rate (Table 5). pH decreased in PB as fertilizer rate increased in contrast to pH values of PTS that did not change with fertilizer rate (Table 5). Substrate solution nitrate concentrations increased with increasing fertilizer rate in both substrates and were lower in PTS compared with PB at each fertilizer rate (Table 5). Substrate solution P and K concentrations were generally higher in PB than PTS at each fertilizer rate (Table 5). Azalea N and K leaf tissue nutrient concentrations increased in response to increasing fertilizer rate in both PB-grown and PTS-grown plants (Table 5). Phosphorus concentrations were generally the same at all fertilizer rates in PB-grown plants, whereas in PTS-grown plants, P concentrations increased with fertilizer up to the 5.9 kg·m⁻³ rate. Tissue N concentrations were higher in PB than in PTS at all fertilizer rates. Tissue N concentrations were within the sufficiency range (2.0% to 3.0%) (Mills and Jones, 1996) for PB plants at the 3.5, 5.9, and 8.3 kg·m⁻³ fertilizer rates, but only at the 5.9 and 8.3 kg·m⁻³ rates for PTS plants.

Table 5. Leaf tissue analysis taken on 25 May 2007, average electrical conductivity (EC) levels, and pH values over the experiment and substrate solution nutrients sampled on 20 Apr. 2007 for azalea grown in pine bark or pine tree substrate and fertilized with four rates of Osmocote Plus 15N–3.9P–10K.

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This study demonstrates that a higher rate of fertilizer is required for japanese holly and azalea to achieve shoot growth in PTS comparable to shoot growth in PB. Both japanese holly and azalea achieved shoot growth in PTS comparable to shoot growth in PB with ≈2.4 kg·m⁻³ additional fertilizer for PTS. For example, shoot growth of japanese holly was similar at 5.9 kg·m⁻³ in PB and 8.3 kg·m⁻³ in PTS in 2005 (Fig. 1A) and shoot growth was similar at 3.5 kg·m⁻³ in PB and 5.9 kg·m⁻³ in PTS in 2007 (Fig. 2A). The different growth response in japanese holly to fertilizer rate in 2005 versus 2007 may be attributable in some way to the seasonal effects (light and temperature differences effecting irrigation need and frequency) between the fall (2005) and spring (2007) experiments. In addition, the placement of the 2007 study on graveled ground beds of the greenhouse instead of on benchtops (2005 study) may have resulted in less air circulation and decreased drying of the containers resulting in a lesser need for water. Lower irrigation rates and frequency in 2007 could have reduced the amount of nutrient leaching from the substrates and resulted in higher substrate EC averages, which occurred in 2007 compared with 2005 (Table 1). Niemiera and Leda (1993) showed N (NO₃-N and NH₄-N) leaching in a PB substrate increases with increasing leaching fraction (irrigation volume) resulting in lower substrate solution EC levels and lower concentrations of substrate solution N. In contrast, a lower leaching fraction (less irrigation volume) causes less nutrient leaching and results in higher substrate solution EC levels and N concentrations. Their results help to explain the higher EC levels reported in the 2007 study, which received generally lower and less frequent irrigation applications. The higher EC levels in 2007 than in 2005 are likely the reason for higher tissue N concentrations at all fertilizer rates for plants grown in both PB and PTS in 2007 (Table 3). Higher tissue N concentrations at lower fertilizer rates in 2007 could explain the increased growth response in japanese holly at lower fertilizer rates compared with plants grown in 2005.

Maximum shoot growth of azalea occurred at the same fertilizer rates (3.5 kg·m⁻³ for PB and 5.9 kg·m⁻³ for PTS; Fig. 3) as japanese holly in 2007, which was unexpected because azalea is considered a woody plant with lower nutrient requirements compared with japanese holly. The substrate solution EC levels and nutrient concentrations for azalea plants grown in PB and PTS were generally lower (Table 5) at the same fertilizer rates (3.5, 5.9, and 8.3 kg·m⁻³) compared with levels seen in the substrate solution of japanese holly plants (Table 2) also in the experiment. The lower EC and substrate solution nutrient levels in azalea may be the result of different growth rates between azalea and japanese holly with azalea having more shoot growth than japanese holly (≈18 g for azalea and 11 g for japanese holly; Fig. 3) suggesting that azalea plants were using more nutrients from the substrate solution for growth (lowering the EC and nutrient levels) and requiring more frequent irrigations than japanese holly, leading to potentially more nutrient leaching. In contrast, the smaller holly plants required less irrigation and therefore conceivably had less nutrient leaching during this experiment causing the higher (and statistically similar) substrate solution EC levels between substrates (Table 1) and generally higher nutrient levels (Table 2) in both PB and PTS in 2007. Tissue N concentrations in azalea were not within the sufficiency range (2.0% to 3.0%) until the 3.5 kg·m⁻³ fertilizer rate for PB plants (2.4% tissue N) and at the 5.9 kg·m⁻³ fertilizer rate for PTS-grown plants (2.3% tissue N) (Table 5). At these fertilizer rates, maximum shoot growth occurred in each substrate.

Lower substrate solution EC for PTS compared with PB has been reported previously by Wright and Browder (2005) and Wright et al. (2006). The reason for this difference may be twofold. First, PTS is more porous and has a significantly lower CEC compared with PB (Table 4), which could result in more nutrient leaching from PTS. The second may relate to higher rates of microbial N immobilization demonstrated with PTS. Substrate CO₂ efflux for PTS (Figs. 1C and 2C) was higher than in PB and likely related to the higher carbon: nitrogen ratio of PTS (550:1) compared with PB (50:1) (Bollen and Lu, 1957; Tisdale et al., 1993). Similar to our results (Figs. 1A and 2C), previous work has shown a reduction in substrate CO₂ efflux as fertilizer rate increased (Maas and Adamson, 1972) possibly as a result of the increased salt concentration, which apparently either modified the microflora in the substrate or reduced their activity (Allison, 1965). This study provides preliminary evidence that N immobilization could be a viable reason for the higher fertilizer requirement in PTS. Substrate solution pH was lower for PB than for PTS reflective of inherently higher pH of PTS, which may be the result of the chemical nature of freshly ground wood. The pH levels for both substrates in these studies are acceptable for both japanese holly and azalea. There was no visual substrate shrinkage or decomposition in PTS during the 3-month experiment, which is consistent with observations from other unpublished studies by these authors. Future studies to investigate the rate and impact of decomposition of PTS in containers during long-term production will be reported. In addition to evaluating PTS decomposition in containers, different methods of PTS storage and the effect of storage on PTS over time will be investigated.

A higher fertilizer requirement for PTS compared with PB is of concern and must be considered when using PTS as a substrate for woody nursery crops. Fertilizer recommendations for growing woody crops in PTS will help growers maximize plant growth and minimize fertilizer use, which can reduce both fertilizer costs and the potential for excess nutrient losses in to the nursery runoff system. Further work is needed to determine the extent of nutrient leaching in PTS as well as establish irrigation requirements during crop production with PTS. The addition of various proportions of PB, as well as other organic materials and composts, to PTS may increase the CEC and reduce nutrient leaching as well as improving (decreasing) the carbon:nitrogen ratio of PTS (currently at 550:1) potentially reducing N immobilization and reducing the need for additional fertilizer for PTS. Manufacturing PTS to have more small particles (fines) could result in higher container capacity and less porosity to reduce irrigation (and nutrient) leaching. The reason for lower substrate solution nutrient levels in PTS needs further investigation to more fully understand and manage fertility requirements when growing plants in PTS during short- and long-term production. Future studies plan to investigate the influence of a relatively high microbial activity in PTS (evident by increased substrate CO₂ levels) on substrate nutrient levels, the rate and significance of nitrogen immobilization, and on substrate decay over longer plant production cycles and in larger containers.