Growth and Quality Response of Four Container-grown Nursery Crop Species to Low-phosphorus Controlled-release Fertilizer

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  • 1 US Department of Agriculture, Agricultural Research Service, US National Arboretum, Floral and Nursery Plants Research Unit, Otis L. Floyd Nursery Research Center, 472 Cadillac Lane, McMinnville, TN 37110
  • | 2 US Department of Agriculture, Agricultural Research Service, Application Technology Research Unit, 1680 Madison Avenue, Wooster, OH 44691
  • | 3 School of Plant and Environmental Sciences, Virginia Tech, 409 West Campus Drive, 401-A Saunders Hall, Blacksburg, VA 24061

The amount of phosphorus (P) conventionally recommended and applied to container nursery crops commonly exceeds plant requirements, resulting in unused P leaching from containers and potentially contributing to surface water impairment. An experiment was replicated in the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley ecoregions of Virginia to compare the effect of a low-P controlled-release fertilizer (CRF, 0.9% or 1.4% P depending on species) vs. a conventional CRF formulation (control, 1.7% P) on plant shoot growth, crop quality, and substrate nutrient concentrations of four species: ‘Natchez’ crape myrtle (Lagerstroemia indica × Lagerstroemia fauriei), ‘Roblec’ Encore azalea (Rhododendron hybrid), ‘Radrazz’ Knock Out rose (Rosa hybrid), and ‘Green Giant’ arborvitae (Thuja plicata × Thuja standishii). In both ecoregions, the low-P CRF resulted in 9% to 26% lower shoot dry weight in all four species compared with those given the conventional formulation, but quality ratings for two economically important species, ‘Radrazz’ Knock Out rose and ‘Green Giant’ arborvitae, were similar between treatments. When fertilized with the low-P CRF, ‘Roblec’ Encore azalea and ‘Natchez’ crape myrtle in both ecoregions, and ‘Green Giant’ arborvitae in the MACP ecoregion had ∼56% to 75% lower substrate pore-water P concentrations than those that received the control CRF. Nitrate-nitrogen (N) concentrations in substrate pore water at week 5 were more than six times greater in control-fertilized plants than in those that received a low-P CRF, which may have been a result of the greater urea-N content or the heterogeneous nature of the low-P CRFs. Lower water-extractable pore-water P and N indicate less environmental risk and potentially increased crop efficiency. Our results suggest low-P CRFs can be used to produce certain economically important ornamental nursery crops successfully without sacrificing quality; however, early adopters will need to evaluate the effect of low-P CRFs on crop quality of specific species before implementing on a large scale.

Abstract

The amount of phosphorus (P) conventionally recommended and applied to container nursery crops commonly exceeds plant requirements, resulting in unused P leaching from containers and potentially contributing to surface water impairment. An experiment was replicated in the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley ecoregions of Virginia to compare the effect of a low-P controlled-release fertilizer (CRF, 0.9% or 1.4% P depending on species) vs. a conventional CRF formulation (control, 1.7% P) on plant shoot growth, crop quality, and substrate nutrient concentrations of four species: ‘Natchez’ crape myrtle (Lagerstroemia indica × Lagerstroemia fauriei), ‘Roblec’ Encore azalea (Rhododendron hybrid), ‘Radrazz’ Knock Out rose (Rosa hybrid), and ‘Green Giant’ arborvitae (Thuja plicata × Thuja standishii). In both ecoregions, the low-P CRF resulted in 9% to 26% lower shoot dry weight in all four species compared with those given the conventional formulation, but quality ratings for two economically important species, ‘Radrazz’ Knock Out rose and ‘Green Giant’ arborvitae, were similar between treatments. When fertilized with the low-P CRF, ‘Roblec’ Encore azalea and ‘Natchez’ crape myrtle in both ecoregions, and ‘Green Giant’ arborvitae in the MACP ecoregion had ∼56% to 75% lower substrate pore-water P concentrations than those that received the control CRF. Nitrate-nitrogen (N) concentrations in substrate pore water at week 5 were more than six times greater in control-fertilized plants than in those that received a low-P CRF, which may have been a result of the greater urea-N content or the heterogeneous nature of the low-P CRFs. Lower water-extractable pore-water P and N indicate less environmental risk and potentially increased crop efficiency. Our results suggest low-P CRFs can be used to produce certain economically important ornamental nursery crops successfully without sacrificing quality; however, early adopters will need to evaluate the effect of low-P CRFs on crop quality of specific species before implementing on a large scale.

Phosphorus (P) is an essential plant mineral nutrient, and soilless substrates used for container-based nursery production contain insufficient P levels for crop requirements (Yeager and Wright 1982a). Thus, P fertilization is necessary to produce a salable container-grown crop. In outdoor container nursery production, P and other plant-essential nutrients are commonly supplied by controlled-release fertilizers (CRFs) incorporated into the substrate (preplant) or applied to the substrate surface after transplanting. Orthophosphate [i.e., the plant-available forms of P: dihydrogen phosphate (H2PO4) and hydrogen phosphate (HPO42−)] is susceptible to leaching from the substrate and container drainage holes during irrigation (Yeager and Barrett 1984; Yeager and Wright 1982a). Studies that have quantified effluent P during production of container-grown plants in pine bark-based substrates (Pinus sp.) reported 7% to 47% of P applied via CRF leached from containers (Broschat 1995; Million et al. 2007a, 2007b; Tyler et al. 1996a, 1996b). When P fertilizer leaves the container, it may run off to adjacent surface waters. Phosphorus contamination of nitrogen (N)-enriched surface waters can trigger eutrophication, resulting in harmful algae blooms and hypoxic conditions that threaten aquatic ecosystems. The impact of P runoff on surface water quality has resulted in increased environmental regulation (Ruark et al. 2014; Westermann 2005), a trend that will likely continue in an effort to remediate and preserve impaired waterways (Thornton et al. 2014).

Conventional fertilizers applied to container-grown nursery crops, including both water-soluble and controlled-release forms, contain unnecessarily high amounts of P or are applied at rates that commonly exceed crop requirements for optimal growth and quality (Majsztrik et al. 2011). Reducing P fertilization to levels minimally sufficient for maximal crop growth can improve P uptake efficiency (percentage of applied P taken up by the plant) while mitigating P leaching from containers and subsequent runoff (Owen et al. 2008; Ristvey et al. 2007). Numerous studies have been conducted to determine critical P fertilizer levels needed to sustain unrestricted shoot growth when producing woody landscape plants in containers (Havis and Baker 1985; Ristvey et al. 2007; Shreckhise et al. 2018, 2019; Wright and Niemiera 1985; Yeager and Wright 1982b). However, maximal shoot growth may not necessarily coincide with perceived quality, and the latter is ultimately more important to growers and consumers in ornamental horticulture. Baas et al. (1995) reported that intentionally restricting root zone P to growth-limiting levels improved the appearance of ‘Impulse’ busy lizzie (Impatiens walleriana) despite reduced shoot weight, height, and width. Henry et al. (2018) found that fertigating New Guinea impatiens (Impatiens hawkeri) and summer snapdragon (Angelonia angustifolia) with between 3 and 5 mg⋅L–1 P limited shoot growth without affecting visual quality, and finished plants were similar in form to those treated with a commercial plant growth regulator and fertilized with nonlimiting P concentrations. Few studies have investigated beneficial P restriction on woody nursery crops, although Hansen and Nielsen (2001) showed that P limitation reduced plant height, had no effect on flower count, and improved postproduction quality of ‘Poulbian’ Bianca Parade® rose (Rosa sp.) by delaying flower senescence and reducing root death.

Shreckhise et al. (2019) found that fertilizing container-grown ‘PIIHM-II’ Bloomstruck bigleaf hydrangea (Hydrangea macrophylla) and ‘Helleri’ Japanese holly (Ilex crenata) with experimental low-P CRFs containing 1.3% or 1.7% P, respectively, resulted in similar shoot and root dry weights compared with those that received the control (CRF containing 2.6% P). However, plant quality was not assessed, and additional research is needed to ensure other economically important nursery crop species can be produced with low-P CRFs. Expanding on this research, our objective was to compare the effect of a commercially available CRF (1.7% P, control) vs. an experimental low-P CRF (0.9% or 1.4% P, depending on plant species) on shoot growth, plant quality, foliar nutrient levels, and pore-water nutrient concentrations of four economically important woody plant species grown in two Virginia ecoregions that vary in climate, topography, and ecology.

Materials and methods

The experiment was conducted during Summer 2016 and replicated simultaneously in two different Virginia ecoregions: the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). The MACP ecoregion was at the Virginia Tech Hampton Roads Agriculture Research and Extension Center in Virginia Beach, VA, USA [lat. 36°53'31"N, long. 76°10'44'W; elevation, 8 m; U.S. Department of Agriculture (USDA) Plant Hardiness Zone 8a] and the RV ecoregion was at the Virginia Tech Urban Horticulture Center in Blacksburg, VA, USA (lat. 37°12'59"N, long. 80°27'50"W; elevation, 629 m; USDA Plant Hardiness Zone 6b). Daily maximum, minimum, and average temperatures as well as cumulative daily rainfall at each ecoregion are shown in Fig. 1. Averaged over the course of the study, maximum, minimum, and average daily temperatures (± SD) were 29.3 ± 4.25 °C, 21.3 ± 3.44 °C, and 25.2 ± 3.45 °C, respectively, at the MACP ecoregion and 28.1 ± 2.94 °C, 16.8 ± 2.91 °C, and 22.1 ± 2.16 °C, respectively, at the RV ecoregion. Cumulative rainfall at the MACP ecoregion (708 mm) was more than twice that received at the RV ecoregion (315 mm) by the end of the study.

Fig. 1.
Fig. 1.

Cumulative daily rainfall (blue shaded area) and maximum (upper dotted line), minimum (lower dotted line), and average (solid line) daily temperatures over the course of the 17-week experiment located in the Middle Atlantic Coastal Plain (MACP) or Ridge and Valley (RV) ecoregions of Virginia; (1.8 × °C) + 32 = °F; 1 mm = 0.0394 inch.

Citation: HortTechnology 32, 5; 10.21273/HORTTECH05058-22

Middle Atlantic Coastal Plain

‘Natchez’ crape myrtle (Lagerstroemia indica × Lagerstroemia fauriei; Bennett’s Creek Nursery, Smithfield, VA), ‘Roblec’ Encore azalea (Rhododendron hybrid; Lancaster Farms, Suffolk, VA), ‘Radrazz’ Knock Out rose (Rosa hybrid; Star Roses and Plants, West Grove, PA), and ‘Green Giant’ arborvitae (Thuja plicata × Thuja standishii, Bennett’s Creek Nursery) were obtained in 18-cell (‘Green Giant’ arborvitae and ‘Natchez’ crape myrtle), 21-cell (‘Roblec’ Encore azalea), or 32-cell (‘Radrazz’ Knock Out rose) flats. ‘Roblec’ Encore azalea liners were comprised of two rooted cuttings per cell, whereas all other species had a single rooted cutting per cell. To avoid competition, one of the two ‘Roblec’ Encore azalea cuttings within each cell was severed at substrate level. Visible existing fertilizer was shaken from the liner root ball as 20 liners per species were planted individually into 1-gal black plastic containers (C400; Nursery Supplies, Chambersburg, PA). Substrate used for potting ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae consisted of loblolly pine (Pinus taeda) bark (5/8-inch screen; age, 4–6 months; Pacific Organics, Henderson, NC) amended with 1.5 lb/yard3 of a granular micronutrient fertilizer (Micromax; ICL Specialty Fertilizers, Summerville, SC) and 3.5 lb/yard3 each of ground dolomitic limestone (97% calcium carbonate equivalent; Rockydale Quarries Corp., Roanoke, VA) and pulverized dolomitic limestone (94% calcium carbonate equivalent; Old Castle Lawn and Garden, Thomasville, PA). The micronutrient fertilizer was comprised of 6% calcium (Ca), 3% magnesium (Mg), 12% sulfur (S), 0.1% boron, 1% copper, 17% iron (Fe), 2.5% manganese, 0.05% molybdenum, and 1% zinc derived from dolomite, ferrous sulfate monohydrate, manganese sulfate, zinc sulfate, copper(II) sulfate pentahydrate, sodium tetraborate, and sodium molybdate dihydrate. ‘Roblec’ Encore azalea were potted using pine bark amended with 1.5 lb/yard3 micronutrient fertilizer and 2.0 lb/yard3 ground dolomitic limestone to supply Ca and Mg while maintaining a low substrate pH. All amendments were incorporated into the pine bark using a 1-yard3 ribbon mixer for a duration of 5 min. Total porosity, container capacity, and air space (± SD) of the substrate were 84.1% ± 1.0%, 53.2% ± 0.4%, and 30.9% ± 1.5% (by volume), respectively, and bulk density was 0.21 ± 0.01 g⋅cm–3 [North Carolina State University porometer method (Fonteno et al. 1995)].

On 18 May 2016, ‘Natchez’ crape myrtle, ‘Green Giant’ arborvitae, and ‘Radrazz’ Knock Out rose were top-dressed with ∼3 g N, between the medium and high label-recommended rates, from one of two 8- to 9-month (80 °F) polymer-coated CRFs (Harrell’s LLC, Lakeland, FL): 17 g of 18.4N–1.4P–10.2K (low-P CRF) or 17 g of 18N–1.7P–6.6K (control; commercially available and commonly used per personal communication with growers). ‘Roblec’ Encore azalea, which reportedly have a relatively low nutrient requirement (Bilderback et al. 2013), were top-dressed with 17 g of 18N–0.9P–10K (low-P CRF) or the control. Nitrogen in the low-P CRFs was derived predominantly from polymer-coated urea, whereas N in the control CRF was about equal parts polymer-coated urea-N, ammonium (NH4)-N, and nitrate (NO3)-N. Additional details on the contents of each CRF are reported in Table 1. All surface-applied fertilizer was subsequently hand-incorporated into the top 1 inch of substrate. ‘Roblec’ Encore azalea and ‘Natchez’ crape myrtle were pruned to an approximate height of 6.5 and 8 inches, respectively; ‘Radrazz’ Knock Out rose and ‘Green Giant’ arborvitae were left unpruned because size variance between replicates was negligible.

Table 1.

Ammonium-nitrogen (NH4-N), nitrate-N (NO3-N), urea-N, total N, phosphorus (P), potassium (K), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn) content in the commercially available, homogeneous controlled-release fertilizer [CRF (control)] and two experimental heterogeneous low-P CRFs.

Table 1.

Ten replicates of each treatment (20 plants per species) were placed 0.2 m apart on a gravel pad in a completely randomized design separated by species. Plants were respaced throughout the study as needed to accommodate growth. For the first 13 weeks after planting (WAP), all plants received one daily 15-min cycle (1.5 ± 0.16 cm) of overhead irrigation at 0900 HR delivered via impact sprinklers (model 2045-PJ SBN-1, 5/32-inch orifice; Rain Bird, Azusa, CA) on 48-inch risers. To decrease the leaching fraction, ‘Green Giant’ arborvitae and ‘Roblec’ Encore azalea were moved to an adjacent overhead irrigation at 13 WAP in which plants received 0.7 ± 0.08 cm water in a daily, 15-min cycle for the remainder of the study. Leaching fraction (proportion of applied water volume leached from the container) was measured on three plants per species at 5, 9, 13, and 17 WAP. Mean leaching fraction values for ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae were 0.39 ± 0.26, 0.52 ± 0.43, 0.28 ± 0.18, and 0.65 ± 0.43, respectively. Relatively high leaching fraction values for ‘Roblec’ Encore azalea and ‘Green Giant’ arborvitae were a consequence of scheduling irrigation to avoid water stress of the plant species with the highest water demand as a result of the unavailability of a second irrigation system before 13 WAP. ‘Radrazz’ Knock Out rose and ‘Natchez’ crape myrtle were sprayed with 0.03 fl oz/gal bifenthrin (Onyx; FMC Corp., Philadelphia, PA) 5 WAP using a hand-pump backpack sprayer with a cone nozzle to control Japanese beetle (Popillia japonica).

By 9 WAP, ‘Natchez’ crape myrtle had become rootbound and were transplanted into 3-gal, black plastic containers (C1200; Nursery Supplies, Inc.). The day of transplanting, pine bark was amended with 1.5 lb/yard3 micronutrient fertilizer (Micromax) and 3.5 lb/yard3 each of ground and pulverized dolomitic limestone. Substrate constituents and blending method were the same as that used at experiment initiation. During repotting, all existing fertilizer and substrate were retained, and an additional 48 g of respective fertilizer was surface-applied and incorporated into the top 1 inch of substrate to achieve the label-recommended medium rate of 65 g for a 3-gal container.

At 5, 9, 13, and 16 WAP, substrate pore water (i.e., water retained in the container between and within substrate particles) was extracted from three replicates per treatment for each species via the pour-through method (Wright 1986). Pore-water extractions were performed 1 h after a normal irrigation event by hand-pouring 120 and 350 mL deionized water over the surface of the substrate of the 1- or 3-gal containers, respectively, and collecting at least 50 mL of the subsequent leachate. Within 8 h of pore-water extraction, an 8-mL aliquot of each sample was filtered (pore size, 0.2 μm; 30-mm-diameter polyvinylidene fluoride syringe filter; Thermo Fisher Scientific, Waltham, MA) and stored at –18 °C for later ion analysis. The remaining portion of each sample was analyzed for pH and electrical conductivity (EC) using a benchtop meter (Orion 4-Star Plus pH/Conductivity Meter, Thermo Fisher Scientific). The 8-mL samples frozen for ion detection were thawed and analyzed at ∼25 °C for NH4, NO3, and phosphate (PO4) concentration using an ion chromatography system (ICS-2100, Thermo Fisher Scientific). Anion- and cation-exchange columns as well as the autosampler used by the ion chromatography system were similar to those described in Hoskins et al. (2014), except both columns were 4-mm i.d. × 250-mm length and a different anion-exchange column (AS19, Thermo Fisher Scientific) was used. Element concentrations and alkalinity of irrigation water were determined by Brookside Laboratories (New Bremen, OH). Irrigation water contained < 0.19 mg⋅L–1 NH4-N, 3.08 mg⋅L–1 NO3-N, < 0.08 mg⋅L–1 PO4-P, 12.7 mg⋅L–1 Ca, 4.1 mg⋅L–1 Mg, 5.5 mg⋅L–1 potassium (K), 37.4 mg⋅L–1 S, < 0.1 mg⋅L–1 Fe, < 0.2 mg⋅L–1 aluminum, and 28.3 mg⋅L–1 total alkalinity.

At 16 WAP, nine volunteers completed plant quality evaluations. Participants, unaware of fertilizer treatments, rated each plant from 1 point (poor) to 5 points (excellent) based on their perception of quality. At 16 WAP, foliar tissue samples were collected from each of five randomly selected replicates from each treatment, triple-rinsed with deionized water, dried at 55 °C for at least 72 h, weighed, and sent to Brookside Laboratories to determine N, P, K, Ca, Mg, and S concentrations. Foliar tissue sample weights were later added to shoot dry weight (SDW). A foliar tissue sample from ‘Natchez’ crape myrtle and ‘Radrazz’ Knock Out rose was comprised of 15 to 30 recently matured leaf blades collected from a single plant. ‘Roblec’ Encore azalea and ‘Green Giant’ arborvitae tissue samples consisted of 8 and 15 1.5-inch terminal stem cuttings, respectively, as recommended by Bryson et al. (2014). At 17 WAP, shoots (i.e., aboveground stems and leaves) were severed level with the substrate, dried at 55 °C until weight remained stable, and weighed for SDW.

Ridge and Valley

Materials and methods for the RV ecoregion were similar to those used at the MACP ecoregion with the following exceptions. Overhead irrigation was delivered via #7 upright mini-Wobblers (7/64-inch orifice; Senninger, Clermont, FL) on 54-inch risers. Plants were irrigated for 20 min to apply 0.9 ± 0.07 cm daily at 0640 HR for the first 8 weeks. At 8 WAP, ‘Natchez’ crape myrtle were moved to an adjacent irrigation system where plants were irrigated daily for 28 min, applying 1.3 ± 0.12 cm daily starting at 0620 HR. ‘Radrazz’ Knock Out rose, ‘Green Giant’ arborvitae, and ‘Roblec’ Encore azalea irrigation was increased to 28 min at 12 WAP. Irrigation water contained < 0.46 mg⋅L–1 NH4, 1.29 mg⋅L–1 NO3, 0.39 mg⋅L–1 PO4, 11.6 mg⋅L–1 Ca, 4.4 mg⋅L–1 Mg, 3.9 mg⋅L–1 K, 5.2 mg⋅L–1 S, < 0.1 mg⋅L–1 Fe, < 0.2 mg⋅L–1 aluminum, and 50.2 mg⋅L–1 total alkalinity. At 5 and 7 WAP, ‘Radrazz’ Knock Out rose was sprayed with 0.11 fl oz/gal copper octanoate (Liquid Copper Fungicide; Bonide Products Inc., Oriskany, NY) using a 2-gal hand-pump sprayer with a cone nozzle to control powdery mildew (Podosphaera pannosa). ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, and ‘Radrazz’ Knock Out rose were sprayed with 0.03 fl oz/gal bifenthrin (Onyx) at 10 WAP using a similar sprayer to control Japanese beetle. Leaching fraction was measured on three plants per species at 5, 9, and 16 WAP. Mean leaching fraction values for ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae were 0.49 ± 0.27, 0.63 ± 0.23, 0.24 ± 0.18, and 0.56 ± 0.35, respectively. At 17 WAP, plant quality evaluations were completed by six volunteers, foliar tissue samples were collected for nutrient analysis, and shoots of all plants were harvested to determine dry weight.

Statistical analysis

Shoot dry weight, quality ratings, and foliar nutrient concentrations were analyzed separately for each species for main effects of CRF treatment and ecoregion as well as their interaction via two-way analysis of variance. t-Tests were conducted to determine significance of the CRF treatment effect on pore-water pH, EC, P, and NH4-N at each of the four sampling dates (n = 3) and when pooled over time (n = 12). Pore-water P data were log-transformed to achieve normality and homoskedasticity before analyses; however, nontransformed values are reported. Correlations between SDW and quality rating were determined by calculating the Pearson correlation coefficient. All data were processed using statistical software (JMP Pro version 15; SAS Institute Inc., Cary, NC).

Results and discussion

Shoot dry weight and quality

Shoot dry weight of all four species was influenced by the CRF treatment and ecoregion main effects but not the CRF treatment × ecoregion interaction; accordingly, only main effects are reported (Table 2). ‘Green Giant’ arborvitae, ‘Roblec’ Encore azalea, ‘Natchez’ crape myrtle, and ‘Radrazz’ Knock Out rose had, respectively, 18%, 16%, 26%, and 9% less SDW when fertilized with the low-P vs. control CRF. In contrast, Shreckhise et al. (2019) reported that ‘PIIHM-II’ Bloomstruck bigleaf hydrangea fertilized with CRFs containing 1.3% or 1.7% P had similar SDWs regardless of the ecoregion in which they were produced. Plants of all four species grown at the RV ecoregion had a 34% to 45% greater SDW than those grown in the MACP ecoregion. Greater SDW of plants produced at the RV ecoregion may have been a consequence of lower temperatures in this ecoregion. Supraoptimal root-zone temperatures, which are common during the summer for plants produced in black containers in the middle Atlantic United States, have been shown to reduce SDW and root dry weight of woody nursery crops by more than 50% (Walden and Wright 1995; Witcher et al. 2020). Although we did not record root-zone temperature in our study, air and substrate temperature are correlated positively (Gheysari et al. 2010); thus, the 3.1 °C higher average air temperature in the MACP ecoregion relative to the RV ecoregion suggests the root-zone temperature was also higher.

Table 2.

Shoot dry weight (SDW) and quality ratings (n = 10) of container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae when fertilized with an experimental, low-phosphorus (P) controlled-release fertilizer (CRF) or a conventional (18N−1.7P−6.6K) CRF (control) after being produced for 17 weeks in two Virginia ecoregions: Middle Atlantic Coastal Plain (MACP) or Ridge and Valley (RV).

Table 2.

Similar to SDW, quality ratings for all four species were unaffected by the CRF treatment × ecoregion interaction. Controlled-release fertilizer treatment had no effect on quality of ‘Green Giant’ arborvitae or ‘Radrazz’ Knock Out rose, whereas ‘Roblec’ Encore azalea and ‘Natchez’ crape myrtle had higher quality ratings when fertilized with the control vs. the low-P CRF. With the exception of ‘Roblec’ Encore azalea, which had a higher mean quality rating in the MACP ecoregion, quality ratings were similar across ecoregions.

Shoot dry weight and quality rating had a strong, positive correlation in ‘Green Giant’ arborvitae (r = 0.71, P = 0.001), ‘Roblec’ Encore azalea (r = 0.72, P < 0.001), and ‘Natchez’ crape myrtle (r = 0.82, P < 0.001), and a moderate positive correlation in ‘Radrazz’ Knock Out rose (r = 0.52, P = 0.019) in the MACP ecoregion. In the RV ecoregion, this positive correlation was strong in ‘Roblec’ Encore azalea (r = 0.81, P < 0.001), moderate in ‘Green Giant’ arborvitae (r = 0.55, P = 0.012) and ‘Natchez’ crape myrtle (r = 0.68, P = 0.001), and nonsignificant in ‘Radrazz’ Knock Out rose (P = 0.229). Quality evaluation results for ‘Green Giant’ arborvitae and ‘Radrazz’ Knock Out rose, and the relationship between quality and SDW in ‘Radrazz’ Knock Out rose indicates plants with greater SDWs are not necessarily more desirable. Hansen and Nielsen (2001) showed that a reduction in height in ‘Poulbian’ Bianca Parade® rose in response to mild P limitation corresponded with improved postproduction plant quality. As rose is among the top-valued nursery crops in the United States, with > $168 million in annual sales (U.S. Department of Agriculture, National Agricultural Statistics Service 2020), fertilizing roses with a species-specific low-P CRF could have major implications for improving P use efficiency and reducing P runoff from container nurseries.

Pore-water PO4-P, NO3-N, NH4-N, EC, and pH

At both sites, pore-water P concentrations at week 5 were generally similar between treatments, with the exception of ‘Green Giant’ arborvitae and ‘Radrazz’ Knock Out rose in the RV ecoregion in which the low-P CRF generated greater concentrations compared with the control (Fig. 2). At weeks 9, 13, and 17, pore-water P concentrations of plants fertilized with the control were greater than or similar to those of plants fertilized with the low-P CRF; however, the sampling weeks during which pore-water P concentrations differed between treatments were generally inconsistent across species and sites. When averaged across time, ‘Roblec’ Encore azalea and ‘Natchez’ crape myrtle fertilized with the low-P CRF had 56% to 75% lower pore-water P concentrations than those that received the control CRF at both sites. ‘Green Giant’ arborvitae pore-water P concentrations (pooled across time) responded to CRF treatments in the MACP ecoregion, with 58% less P in those given the low-P CRF, but not in the RV ecoregion. ‘Radrazz’ Knock Out rose pore-water P concentrations were unaffected by CRF treatments at both sites. For ‘Natchez’ crape myrtle and ‘Roblec’ Encore azalea, consistently (i.e., at both ecoregions) less time-averaged pore-water P concentrations when fertilized with the low-P CRF compared with the control suggests substrate-P availability may have affected SDW in these two species. Furthermore, pore-water P concentrations from ‘Roblec’ Encore azalea and ‘Natchez’ crape myrtle were consistently less than 2.5 mg⋅L–1, and increasing pore-water P concentrations within the range of 0.5 to 5 mg⋅L–1 has been shown to shift P fertility status from limiting to sufficient for shoot growth of various container-grown woody plant species (Havis and Baker 1985; Shreckhise et al. 2018, 2019). The pore-water P differences between CRF treatments for ‘Roblec’ Encore azalea, specifically, can be attributed to the lower P content in the low-P CRF used for ‘Roblec’ Encore azalea (0.9% P, ∼50% of the control CRF P content) compared with that used for the other three plant species (1.4% P, ∼75% of the control CRF P content).

Fig. 2.
Fig. 2.

Mean pore-water phosphorus (P) concentrations (± SD) determined in pour-through extracts at 5, 9, 13, and 17 weeks after planting collected from container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae fertilized with either a conventional (18N–1.7P–6.6K) controlled-release fertilizer (CRF) formulation [control ()] or an experimental low-P CRF (- - -) that contained either 18.4N–1.4P–10.2K (for ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae) or 18.2N–0.9P–10.2K (for ‘Roblec’ Encore azalea). The experiment was replicated in two Virginia ecoregions: the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). Asterisks indicate a significant difference between treatments within the same species, sampling time, and ecoregion (n = 3, α = 0.05). Embedded tables contain time-averaged means and P values for the CRF treatment main effect (n = 12); 1 mg⋅L–1 = 1 ppm. Ctrl., control.

Citation: HortTechnology 32, 5; 10.21273/HORTTECH05058-22

Pore-water NO3-N concentrations over time, reported in Fig. 3, suggest the control and low-P CRFs had different NO3-N release timing, despite having the same labeled release longevity (8–9 months) and coating material. In ‘Green Giant’ arborvitae, ‘Roblec’ Encore azalea, and ‘Radrazz’ Knock Out rose, pore-water NO3-N was greatest at week 5 or 9 if fertilized with the control, and at week 9 or 13 if fertilized with the low-P CRF. At week 5 in all species and at both sites (except ‘Natchez’ crape myrtle in the MACP ecoregion), NO3-N concentrations from plants fertilized with the control CRF were within the Best Management Practices recommended range of 15 to 25 mg⋅L–1 (Bilderback et al. 2013) and at least six times greater than in those given the low-P CRF, in which concentrations were between 0.4 and 5.2 mg⋅L–1 NO3-N. After 5 WAP, with few exceptions, NO3-N concentrations were similar between treatments. Averaged over time, pore-water NO3-N concentrations were similar between treatments in all species at both sites except ‘Natchez’ crape myrtle in the RV ecoregion, in which concentrations were more than nine times greater in the control-fertilized plants compared with those fertilized with the low-P CRF. The earlier NO3-N availability in substrate pore water of plants fertilized with the control vs. the low-P CRF may have contributed to differences in final SDW between CRF treatments. The reason for the differing release curves between the control and low-P CRFs is unclear, but may be related to the heterogeneous nature of the low-P CRF compared with the homogeneous control CRF. Shreckhise et al. (2019), who measured pore-water EC from container plants grown with the same experimental low-P CRF used in our study, also reported that the heterogeneous low-P CRF had a different nutrient release pattern compared with the homogeneous control CRF. Another possible explanation for the observed difference in NO3-N release among the CRFs used in our study is the difference in N source. Urea was the predominant N form in the low-P CRF, whereas N in the control CRF was derived from about equal parts urea-N, NO3-N, and NH4-N (Table 1).

Fig. 3.
Fig. 3.

Mean pore-water nitrate-nitrogen (NO3-N) concentrations (± SD) determined in pour-through extracts at 5, 9, 13, and 17 weeks after planting collected from container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae fertilized with either a conventional (18N–1.7P–6.6K) controlled-release fertilizer (CRF) formulation [control ()] or an experimental, low-phosphorus (P) CRF (- - -) that contained either 18.4N–1.4P–10.2K (for ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae) or 18.2N–0.9P–10.2K (for ‘Roblec’ Encore azalea). The experiment was replicated in two Virginia ecoregions: the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). NO3-N values > 30.7 mg⋅L–1 could not be measured accurately because of instrument limitations and thus are reported as equal to 30.7 mg⋅L–1. Therefore, means could not be compared statistically. 1 mg⋅L–1 = 1 ppm. Ctrl., control.

Citation: HortTechnology 32, 5; 10.21273/HORTTECH05058-22

At both sites on weeks 5 or 9, pore-water NH4-N concentrations from ‘Green Giant’ arborvitae, ‘Roblec’ Encore azalea, and ‘Radrazz’ Knock Out rose fertilized with the control CRF tended to be greater than those that received the low-P CRF, whereas at weeks 13 and 17, NH4-N concentrations were similar between the two CRF treatments (Fig. 4). Ammonium-N concentrations in substrate pore-water of ‘Natchez’ crape myrtle at both sites were similar between CRF treatments at weeks 5, 9, and 17, but greater in control-fertilized vs. low-P-fertilized plants at week 13. In all except ‘Roblec’ Encore azalea in the MACP ecoregion, pore-water NH4-N concentrations averaged over the course of the experiment were similar between treatments. ‘Roblec’ Encore azalea fertilized with the control in the MACP ecoregion had ∼3-fold greater pore-water NH4-N concentrations than those fertilized with the low-P CRF. Temporarily greater pore-water NH4-N concentrations in control-fertilized ‘Roblec’ Encore azalea may have affected final SDW and quality, as azalea have been shown to have greater vegetative growth and improved visual appearance with an increasing NH4-N-to-NO3-N ratio in applied fertilizer (Clark et al. 2003).

Fig. 4.
Fig. 4.

Mean pore-water ammonium-nitrogen (NH4-N) concentrations (± SD) determined in pour-through extracts at 5, 9, 13, and 17 weeks after planting collected from container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae fertilized with either a conventional (18N–1.7P–6.6K) controlled-release fertilizer (CRF) formulation [control ()] or an experimental, low-phosphorus (P) CRF (- - -) that contained either 18.4N–1.4P– 10.2K (for ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae) or 18.2N–0.9P–10.2K (for ‘Roblec’ Encore azalea). The experiment was replicated in two Virginia ecoregions: the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). Asterisks indicate a significant difference between treatments within the same species, sampling time, and ecoregion (n = 3, α = 0.05). Embedded tables contain time-averaged means and P values for the CRF treatment main effect (n = 12); 1 mg⋅L–1 = 1 ppm. Ctrl., control.

Citation: HortTechnology 32, 5; 10.21273/HORTTECH05058-22

The CRF treatment had no effect on the mean EC (pooled over time) within each species and ecoregion (Fig. 5). EC values from plants produced in the MACP and RV ecoregions ranged from 0.27 to 0.86 mS⋅cm–1 and 0.08 to 0.91 mS⋅cm–1, respectively. Although differences in EC among CRF treatments were detected at some sampling events, CRF treatment effects were generally inconsistent across ecoregions and species. One exception was the pattern of greater EC from control- vs. low-P-fertilized plants at 5 WAP in ‘Green Giant’ arborvitae (MACP only), ‘Roblec’ Encore azalea (MACP and RV), and ‘Natchez’ crape myrtle (RV only). This trend of greater EC in control- vs. low-P-fertilized plants at 5 WAP is in line with differences in pore-water NO3-N and NH4-N between CRF treatments.

Fig. 5.
Fig. 5.

Mean pore-water electrical conductivity (EC) (± SD) determined in pour-through extracts at 5, 9, 13, and 17 weeks after planting collected from container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae fertilized with either a conventional (18N–1.7P–6.6K) controlled-release fertilizer (CRF) formulation [control ()] or an experimental low-phosphorus (P) CRF (- - -) that contained either 18.4N–1.4P–10.2K (for ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae) or 18.2N–0.9P–10.2K (for ‘Roblec’ Encore azalea). The experiment was replicated in two Virginia ecoregions: the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). Asterisks indicate a significant difference between treatments within the same species, sampling time, and ecoregion (n = 3, α = 0.05). Embedded tables contain time-averaged means and P values for the CRF treatment main effect (n = 12); 1 mS⋅cm–1 = 1 mmho/cm. Ctrl., control.

Citation: HortTechnology 32, 5; 10.21273/HORTTECH05058-22

Pore-water pH within each sampling period was generally similar among CRF treatments, with a few exceptions (Fig. 6). Averaged over time, pore-water pH of ‘Roblec’ Encore azalea in the MACP ecoregion was 0.3 U greater than those fertilized with the low-P CRF compared with those that received the control CRF. ‘Roblec’ Encore azalea in the RV ecoregion, as well as ‘Green Giant’ arborvitae, ‘Natchez’ crape myrtle, and ‘Radrazz’ Knock Out rose at both sites, had similar time-averaged pH values among CRF treatments. The ecoregion main effect on pH was significant in all four species (P < 0.05) with pH values 0.6 U greater in the RV ecoregion in ‘Roblec’ Encore azalea and 0.2 to 0.3 U greater in the MACP ecoregion in ‘Green Giant’ arborvitae, ‘Natchez’ crape myrtle, and ‘Radrazz’ Knock Out rose.

Fig. 6.
Fig. 6.

Mean pore-water pH (± SD) determined in pour-through extracts at 5, 9, 13, and 17 weeks after planting collected from container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae fertilized with either a conventional (18N–1.7P–6.6K) controlled-release fertilizer (CRF) formulation [control ()] or an experimental, low-phosphorus (P) CRF (- - -) which contained either 18.4N–1.4P–10.2K (for ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae) or 18.2N–0.9P–10.2K (for ‘Roblec’ Encore azalea). The experiment was replicated in two Virginia, USA ecoregions: Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). Asterisks (*) indicate a significant difference between treatments within the same species, sampling time, and ecoregion (n = 3, α = 0.05). Embedded tables contain time-averaged means and P values for the CRF treatment main effect (n = 12). Ctrl., control.

Citation: HortTechnology 32, 5; 10.21273/HORTTECH05058-22

Foliar nutrients

The CRF treatment × ecoregion interaction term was not significant for foliar nutrient concentrations in all species with a few exceptions (i.e., N and K in ‘Roblec’ Encore azalea, and Ca and Mg in ‘Radrazz’ Knock Out rose); thus, pooled means are reported to demonstrate main effects (Table 3). The CRF treatment effect on foliar nutrient concentrations varied across species.

Table 3.

Foliar nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) concentrations (n = 5) of container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae when fertilized with an experimental, low-Pi controlled-release fertilizer (CRF) or a conventional (18N−1.7P−6.6K) CRF (control) after being produced for 17 weeks in two Virginia ecoregions: Middle Atlantic Coastal Plain (MACP) or Ridge and Valley (RV).

Table 3.

‘Green Giant’ arborvitae fertilized with the low-P CRF had lower foliar P, but greater foliar N, K, and Mg concentrations compared with those fertilized with the control. Despite these differences, all measured nutrients were within the survey range reported by Bryson et al. (2014) for western redcedar (T. plicata), which is a parent of ‘Green Giant’ arborvitae.

‘Roblec’ Encore azalea grown with the low-P vs. control CRF were lower in foliar P, Ca, Mg, and S concentrations. Foliar N in ‘Roblec’ Encore azalea, which was influenced by a significant CRF treatment × ecoregion interaction, was greater in plants fertilized with the control (1.59%) vs. the low-P CRF (1.46%) in the MACP ecoregion (P = 0.046), but in the RV ecoregion, foliar N was similar between treatments (P = 0.253). Although the interaction term was also significant for foliar K in ‘Roblec’ Encore azalea, simple effects of CRF treatment on foliar K at the MACP and RV sites were not significant (P = 0.072 and 0.486, respectively). Greater foliar P concentrations in ‘Roblec’ Encore azalea fertilized with the control vs. the low-P CRF is in line with greater mean pore-water P concentrations from control-fertilized ‘Roblec’ Encore azalea (Fig. 2). Differing foliar N concentrations in the MACP ecoregion but not in the RV ecoregion is consistent with pore-water NH4-N results, where the mean pore-water NH4-N concentration was greater from control- vs. low-P-fertilized plants in the MACP ecoregion, but concentrations were similar between treatments in the RV ecoregion (Fig. 4).

In ‘Natchez’ crape myrtle, the low-P CRF resulted in greater foliar Mg concentrations relative to those fertilized with the control, whereas all other measured nutrients were unaffected by CRF treatment. Although foliar Mg concentrations were different between treatments, Mg concentrations in both treatments were within the survey range of 0.33% to 0.45% reported for ‘Natchez’ crape myrtle (Bryson et al. 2014). The overall absence of a CRF treatment effect on foliar nutrient concentrations measured at the end of the experiment is evidence that lower SDW and quality ratings of ‘Natchez’ crape myrtle fertilized with low-P CRF was a result of growth-limiting concentrations of one or several mineral nutrients early in the experiment that later became sufficiently available (e.g., pore-water NO3-N) (Fig. 3).

‘Radrazz’ Knock Out rose foliar nutrient concentrations were mostly unaffected by CRF treatment, with the exception of N, which was greater in those fertilized with the low-P CRF. Foliar Ca and Mg in ‘Radrazz’ Knock Out rose, which were influenced by a significant CRF treatment × ecoregion interaction, were greater in control- vs. low-P-fertilized plants in the MACP ecoregion (P < 0.05) and greater in the low-P- vs. control-fertilized plants (Ca, P < 0.05), or were similar across CRF treatments (Mg, P = 0.102) in the RV ecoregion. ‘Radrazz’ Knock Out rose foliar Ca and Mg concentrations in the MACP ecoregion, regardless of CRF treatment, were not likely growth limiting. Jeong et al. (2011) showed that withholding Ca or Mg when fertilizing ‘Karina Parade’ rose with an otherwise complete nutrient formula resulted in obvious visual deficiency symptoms (e.g., interveinal chlorosis or necrosis, collapsed flower stalks, white patches along leaf margins) before shoot growth was affected. As concentrations of all measured nutrients were either similar among CRF treatments or greater in plants that received the low-P CRF, foliar nutrient analysis for ‘Radrazz’ Knock Out rose provided little insight into possible reasons for differences in SDW between CRF treatments. As was previously suggested for ‘Natchez’ crape myrtle, delayed NO3 release from the low-P CRF may have limited shoot growth initially, but when NO3 became available, plants had enough time to absorb it such that foliar N concentrations were no longer indicative of N deficiency when foliar samples were harvested.

Ecoregion affected foliar concentrations of all six measured macronutrients in at least one of the four species, and in instances in which the ecoregion effect was significant, greater concentrations were generally found in MACP plants, with few exceptions (Table 3).

Lower foliar N and S concentrations in all species grown in the RV ecoregion, all of which also had greater SDWs at the time of foliar tissue harvest than those produced in the MACP ecoregion, may have been a result of nutrient dilution—a phenomenon commonly ascribed to when plant biomass increases without a parallel increase in nutrient uptake, resulting in a decrease in concentration of that nutrient in the plant tissue (Jarrell and Beverly 1981). Similarly, Martin and Ruter (1996) reported reduced plant size and a concomitant increase in foliar N, P, K, and Ca in container-grown ‘Muskogee’ crape myrtle produced in Tempe, AZ, which were exposed to higher maximum and minimum temperatures, greater solar radiation, higher root-zone temperatures, and less rainfall compared with those grown in Tifton, GA, during the same growing season. However, in our study, foliar Ca and Mg concentrations in ‘Roblec’ Encore azalea, and P and Ca concentrations in ‘Green Giant’ arborvitae did not follow this trend, because concentrations were greater in those grown at in RV ecoregion than in those in the MACP ecoregion. In ‘Roblec’ Encore azalea, greater foliar Mg and Ca in RV vs. MACP plants may be pH related because ‘Roblec’ Encore azalea was also the only species with greater time-averaged pore-water pH values in the RV (pH = 6.4) vs. the MACP (pH = 5.8) ecoregion, and Ca and Mg in organic substrates have been shown to increase in availability as pH increases in the range of 5.8 to 6.4 (Peterson 1982). The reason for greater foliar P and Ca in ‘Green Giant’ arborvitae grown in the RV vs. the MACP ecoregion is unclear. Ecoregion effects on foliar nutrient concentrations observed in our research support the notion proposed by others that foliar nutrient analysis of container-grown nursery crops has limited practical application when diagnosing plant nutritional disorders (Clark and Zheng 2015a, 2015b; Wright and Niemiera 1987).

Conclusion

This research demonstrated that growing ‘Radrazz’ Knock Out rose or ‘Green Giant’ arborvitae in containers with CRF containing 1.4% P can be accomplished without a reduction in plant quality. Using this CRF formulation would equate to a 25% reduction in P when compared with commercially available “low-P” CRFs marketed to container nurseries. ‘Radrazz’ Knock Out rose and ‘Green Giant’ arborvitae are major nursery crops produced throughout the United States; thus, commercial adoption of a reduced-P CRF for these two species alone could have major implications for improving P use efficiency and reducing P loads in nursery runoff. Further research is needed to explore possible value-added benefits of using low-P CRFs, such as compact plants that require less pruning or fewer applications of plant growth regulators. A CRF with < 1.7% P is not yet advisable for use on all nursery crops. ‘Natchez’ crape myrtle SDW and quality were both affected negatively when grown with the low-P CRF under experimental conditions. In addition, a CRF with 0.9% P reduced growth and quality of a crop with purported low nutrient requirements (i.e., ‘Roblec’ Encore azalea), suggesting CRFs with <1.4% P may have limited use in container nursery production. Caution should be taken when trialing or adopting any custom-blended, low-P CRF because the experimental low-P CRF used in this study had a different NO3-N release pattern compared with the homogeneous, commercially available CRF of the same labeled release longevity and polymer coating material. We suspect the method of combining nutrients to generate a complete fertilizer (i.e., homogeneous, multinutrient prills vs. a blend of individually coated fertilizer salts) and the N source (predominantly urea-N vs. about equal parts urea-N, NH4-N, and NO3-N) may influence N release rates, although additional research is needed to support this conjecture.

Units

TU1

References

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  • Wright, R.D 1986 The pour-through nutrient extraction procedure HortScience 21 227 229

  • Wright, R.D. & Niemiera, A.X. 1985 Influence of N, P and K fertilizer interactions on growth of Ilex crenata Thunb. ‘Helleri’ J. Environ. Hortic. 3 8 10 https://doi.org/10.24266/0738-2898-3.1.8

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  • Wright, R.D. & Niemiera, A.X. 1987 Nutrition of container-grown woody nursery crops Hortic. Rev. 9 75 150 https://doi.org/10.1002/978111806 0827.ch3

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  • Yeager, T. & Barrett, J. 1984 Phosphorus leaching from 32P-superphosphate-amended soilless container media Hort-Science 19 216 217

  • Yeager, T.H. & Wright, R.D. 1982a Phosphorus requirement of Ilex crenata Thunb. cv. Helleri grown in a pine bark medium J. Am. Soc. Hortic. Sci. 107 558 562

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  • Yeager, T.H. & Wright, R.D. 1982b Pine bark-phosphorus relationships Commun. Soil Sci. Plant Anal. 13 57 66 https://doi.org/10.1080/00103628209 367244

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Contributor Notes

We thank Julie Brindley, Velva Groover, and Anna Birnbaum for technical assistance, as well as Pacific Organics Inc. for donating pine bark and Harrell’s LLC for formulating a novel analysis and donating fertilizer.

Funding was provided by the Virginia Agricultural Experiment Station and the Hatch Program of the National Institute of Food and Agriculture (SCRI 2014-51181-22372), US Department of Agriculture (USDA), the Horticultural Research Institute, and the Virginia Nursery and Landscape Association.

The use of trade or brand names in this publication does not constitute a guarantee or warranty of the product by Virginia Tech or USDA, Agricultural Research Service and does not imply its approval to the exclusion of other products or vendors that also may be suitable.

J.H.S. is the corresponding author. E-mail: jacob.shreckhise@usda.gov

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    Cumulative daily rainfall (blue shaded area) and maximum (upper dotted line), minimum (lower dotted line), and average (solid line) daily temperatures over the course of the 17-week experiment located in the Middle Atlantic Coastal Plain (MACP) or Ridge and Valley (RV) ecoregions of Virginia; (1.8 × °C) + 32 = °F; 1 mm = 0.0394 inch.

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    Mean pore-water phosphorus (P) concentrations (± SD) determined in pour-through extracts at 5, 9, 13, and 17 weeks after planting collected from container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae fertilized with either a conventional (18N–1.7P–6.6K) controlled-release fertilizer (CRF) formulation [control ()] or an experimental low-P CRF (- - -) that contained either 18.4N–1.4P–10.2K (for ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae) or 18.2N–0.9P–10.2K (for ‘Roblec’ Encore azalea). The experiment was replicated in two Virginia ecoregions: the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). Asterisks indicate a significant difference between treatments within the same species, sampling time, and ecoregion (n = 3, α = 0.05). Embedded tables contain time-averaged means and P values for the CRF treatment main effect (n = 12); 1 mg⋅L–1 = 1 ppm. Ctrl., control.

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    Mean pore-water nitrate-nitrogen (NO3-N) concentrations (± SD) determined in pour-through extracts at 5, 9, 13, and 17 weeks after planting collected from container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae fertilized with either a conventional (18N–1.7P–6.6K) controlled-release fertilizer (CRF) formulation [control ()] or an experimental, low-phosphorus (P) CRF (- - -) that contained either 18.4N–1.4P–10.2K (for ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae) or 18.2N–0.9P–10.2K (for ‘Roblec’ Encore azalea). The experiment was replicated in two Virginia ecoregions: the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). NO3-N values > 30.7 mg⋅L–1 could not be measured accurately because of instrument limitations and thus are reported as equal to 30.7 mg⋅L–1. Therefore, means could not be compared statistically. 1 mg⋅L–1 = 1 ppm. Ctrl., control.

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    Mean pore-water ammonium-nitrogen (NH4-N) concentrations (± SD) determined in pour-through extracts at 5, 9, 13, and 17 weeks after planting collected from container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae fertilized with either a conventional (18N–1.7P–6.6K) controlled-release fertilizer (CRF) formulation [control ()] or an experimental, low-phosphorus (P) CRF (- - -) that contained either 18.4N–1.4P– 10.2K (for ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae) or 18.2N–0.9P–10.2K (for ‘Roblec’ Encore azalea). The experiment was replicated in two Virginia ecoregions: the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). Asterisks indicate a significant difference between treatments within the same species, sampling time, and ecoregion (n = 3, α = 0.05). Embedded tables contain time-averaged means and P values for the CRF treatment main effect (n = 12); 1 mg⋅L–1 = 1 ppm. Ctrl., control.

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    Mean pore-water electrical conductivity (EC) (± SD) determined in pour-through extracts at 5, 9, 13, and 17 weeks after planting collected from container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae fertilized with either a conventional (18N–1.7P–6.6K) controlled-release fertilizer (CRF) formulation [control ()] or an experimental low-phosphorus (P) CRF (- - -) that contained either 18.4N–1.4P–10.2K (for ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae) or 18.2N–0.9P–10.2K (for ‘Roblec’ Encore azalea). The experiment was replicated in two Virginia ecoregions: the Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). Asterisks indicate a significant difference between treatments within the same species, sampling time, and ecoregion (n = 3, α = 0.05). Embedded tables contain time-averaged means and P values for the CRF treatment main effect (n = 12); 1 mS⋅cm–1 = 1 mmho/cm. Ctrl., control.

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    Mean pore-water pH (± SD) determined in pour-through extracts at 5, 9, 13, and 17 weeks after planting collected from container-grown ‘Natchez’ crape myrtle, ‘Roblec’ Encore azalea, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae fertilized with either a conventional (18N–1.7P–6.6K) controlled-release fertilizer (CRF) formulation [control ()] or an experimental, low-phosphorus (P) CRF (- - -) which contained either 18.4N–1.4P–10.2K (for ‘Natchez’ crape myrtle, ‘Radrazz’ Knock Out rose, and ‘Green Giant’ arborvitae) or 18.2N–0.9P–10.2K (for ‘Roblec’ Encore azalea). The experiment was replicated in two Virginia, USA ecoregions: Middle Atlantic Coastal Plain (MACP) and Ridge and Valley (RV). Asterisks (*) indicate a significant difference between treatments within the same species, sampling time, and ecoregion (n = 3, α = 0.05). Embedded tables contain time-averaged means and P values for the CRF treatment main effect (n = 12). Ctrl., control.

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  • Wright, R.D. & Niemiera, A.X. 1985 Influence of N, P and K fertilizer interactions on growth of Ilex crenata Thunb. ‘Helleri’ J. Environ. Hortic. 3 8 10 https://doi.org/10.24266/0738-2898-3.1.8

    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
  • Yeager, T. & Barrett, J. 1984 Phosphorus leaching from 32P-superphosphate-amended soilless container media Hort-Science 19 216 217

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    • Search Google Scholar
    • Export Citation
  • Yeager, T.H. & Wright, R.D. 1982b Pine bark-phosphorus relationships Commun. Soil Sci. Plant Anal. 13 57 66 https://doi.org/10.1080/00103628209 367244

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    • Export Citation
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