Materials and methods

Shrubs grown in 10-L polypropylene containers were transplanted on 10–13 July 2013 into a Margate fine sand soil (siliceous, hyperthermic Mollic Psammaquent, pH 6.5) and another nearby plot having 30–40 cm of calcareous fill soil (mostly crushed limestone, pH 8.4) added to the surface in Davie, FL. The latter soil is typical of soils used for planting in street medians, highway rights-of-way, and many new commercial and residential landscapes in Florida. Physical and chemical properties of these soils are provided in Broschat (2015).

The design was split plot (two soil types) with three shrub species (bush allamanda, ixora, and surinam cherry) and seven fertilizer treatments, with eight replicate plants per species and treatment completely randomized within each soil type. Plants were spaced 8 ft apart in all directions.

Fertilizers were applied at the time of transplanting and every 3 months thereafter. Treatments included 1) no fertilizer, 2) a 24N–0P–9.2K turf fertilizer (24–0–11; Lesco, Cleveland, OH) applied at a rate of 41.7 g per plant, 3) the same turf fertilizer applied at 83.4 g per plant, 4) a 8N–0P–10K–6Mg palm fertilizer (8–0–12; Nurserymen’s Sure Gro, Vero Beach, FL) applied at 125 g per plant, and 5) the same palm fertilizer applied at 250 g per plant. The turf and palm fertilizer application rates provided equivalent amounts of N equal to 10 and 20 g per shrub (2 and 4 lb/1000 ft²) of N per application for the low and high rates, respectively. These high rates were used because Gilman (1988) reported that chinese hibiscus growth and quality response to N fertilization was still linear at 15 lb/1000 ft² of N per year, and it was not known how these other species would respond. Two additional treatments (6 and 7) provided equivalent K levels to those of treatments 4 and 5 (12.5 and 25 g/shrub of K, respectively) from a 0N–0P–13.3K–6Mg palm fertilizer (0–0–16, Nurserymen’s Sure Gro). Fifty percent of the N in the turf fertilizer was in controlled release form (polymer-coated urea) with the remainder being water soluble (urea). Water soluble K was provided by potassium chloride, but 2% iron (Fe) and 1% manganese (Mn) were in rather insoluble sucrate forms. All of the N, K, and Mg in the palm fertilizers were controlled release (polymer-coated urea, polymer and sulfur-coated potassium sulfate, and kieserite) and 1.25% Mn, 1.17% Fe, 0.15% zinc (Zn), 0.06% copper (Cu), and 0.6% boron (B) were provided by manganese sulfate, iron sulfate, iron ethylenediaminetetraacetate (EDTA), iron diaminetriaminepentaacetate (DTPA), zinc sulfate, copper sulfate, and sodium borate. All fertilizers were applied by broadcasting uniformly over a 1 m²-area surrounding each plant.

All shrubs received ≈2 cm of water from overhead irrigation three times per week during the first 6 months and twice per week thereafter. An area of ≈1 m² around all shrubs was kept weed-free with glyphosate. All shrubs were measured at the time of transplanting and every year thereafter for total height and width in two opposite directions (within and across rows). The two width measurements and total height were multiplied together and the total divided by 10,000 to obtain a single size value. Growth was calculated as the size at the end of 6 months (establishment period) minus initial size. Growth during the maintenance phase was calculated as the final size after 2.5 years minus the size at the end of the establishment phase. At the time these measurements were taken, all shrubs were subjectively rated for severity of N, P, K, Mg, Fe, and Mn deficiencies on a scale of 1 to 5 with a rating of 1 being severe and 5 being completely free of deficiency symptoms for that element. The deficiency severity ratings for each of these elements were averaged for each plant to obtain a single nutrient deficiency rating score. Shrubs were similarly rated on a 1 to 5 scale for relative density. Inflorescences were also counted for each plant.

Because plant quality variables such as height, widths, shrub density, and nutrient deficiency severity ratings are typically highly intercorrelated, principal component analysis was performed on the data to reduce the four original variables to a single index of overall quality, namely, the scores on the first principal component (Broschat, 1979). All quality data were standardized to a mean of 0 and a sd of 1 to eliminate the effects of differences in scale among the original variables using PROC STANDARD (SAS version 9.4; SAS Institute, Cary, NC). Principal component analysis was done using PROC PRINCOMP with scoring by PROC SCORE. In this analysis, the first principal component typically contains high positive correlations for most or all of the original variables with the first principal component, making it a useful index of overall quality. Scores for each shrub on the first principal component were further subjected to analysis of variance (ANOVA) (PROC GLM) with mean separation by the Waller–Duncan k-ratio method (P = 0.05) to determine treatment effects. Because soil type was found to have a significant effect on plant quality scores for all three species, data from each soil type were analyzed independently.

At the end of the experiment (10 Oct. 2015), leaf samples consisting of the youngest fully expanded leaves on each shoot were collected from each shrub for nutrient analysis. Unwashed leaf samples were dried, ground, digested using a modified Kjeldahl procedure (Hach et al., 1987), and analyzed for N using an autoanalyzer (Seal Analytical, Mequon, WI), P by the ascorbic acid method (Kuo, 1996), and K, Mg, Fe, and Mn by atomic absorption spectroscopy (PerkinElmer, Waltham, MA). Leaf nutrient concentration data for each element were analyzed using ANOVA with mean separations by the Waller–Duncan k-ratio method.

Results and discussion

A summary of the principal component analyses showed that all plant quality variables had high positive correlations with the first principal component, which typically accounted for about half or more of the variation included in the original four variables (Table 1). For surinam cherry and ixora, shrub density was uniform among plants, and there were no flowers at the time of evaluation on the surinam cherries so these variables were excluded from the principal component analysis. For all three species, the first principal component accounted for more than half of the total variance of the original variables.

Table 1.

Eigenvectors or correlations of original plant quality variables with the first principal component for three species of shrubs grown in two soil types and fertilized with two rates of one turf and two palm fertilizers (n = 112).

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At the end of the 6-month establishment period, there were differences in overall quality among treatments only for ixora on the sand soil and surinam cherry on the fill soil (Table 2). In both cases it was one of the 8–0–12 treatments that resulted in the highest quality although the two 24–0–11 treatments produced statistically equivalent plants. Some N seems to be essential for newly planted ixoras on the sand soil as the unfertilized controls, and the 0–0–16 palm fertilizer treatments had some of the lowest plant quality scores. Nitrogen fertilization did not affect allamanda quality during the first 6 months. Shober et al. (2013) found very little difference in quality of indian hawthorne, rose, or sweet viburnum after 6 months when fertilized with varying levels of N. Differences among surinam cherry treatments on the fill soil were minor as only the two 0–0–16 treatments were statistically poorer than the low rate of 8–0–12, the best treatment. In general, the lack of response after 6 months suggests that insufficient growth had occurred during that short time for statistically significant differences in plant quality to have developed. Alternatively, there may have been sufficient residual controlled release fertilizer in the containers at the time of transplanting to sustain plant growth and quality until the root system had extended beyond the original container volume. Broschat and Moore (2010) showed inconsistent or no responses to fertilizer type and N rate in chinese hibiscus during the first 6 months after transplanting on similar soils.

Table 2.

Overall plant quality (scores on the first principal component) of shrubs after 6-month establishment period on two soil types with two rates of turf and palm fertilizers (n = 8).

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Overall quality of ixora at the end of the study did not differ among fertilizer treatments (Table 3). Although individual plants typically showed moderate to severe stunting and deficiency symptoms of N, P, K, Mg, Fe, and Mn on both soils, these effects were not consistently associated with any particular treatment. When the measured plant quality variables were analyzed individually (without principal component analysis), treatment differences were noted only for N, Mg, Mn, and Fe deficiency ratings on the sand soil (data not shown). In that case, the two rates of 8–0–12 palm fertilizer resulted in the fewest nutrient deficiency symptoms, whereas the untreated controls and low rate of 24–0–11 turf fertilizer resulted in the most severe nutrient deficiencies.

Table 3.

Overall plant quality (plant scores on the first principal component) of shrubs grown on two soil types and fertilized with two rates of turf and palm fertilizers at the end of 3 years (n = 8).

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Fertilizer treatments significantly affected the plant quality of allamanda on both soil types (Table 3). In both cases, the low rate of 8–0–12 resulted in the highest plant quality scores and the unfertilized controls the lowest. On the fill soil, the low rate of 24–0–11 and both rates of 0–0–16 had equally low quality scores. These data suggest that some N is necessary for allamanda growing on these two soil types in south Florida, but when N is provided some additional K may also be needed. However, K, Mg, and micronutrients, in the absence of N, were not effective in improving allamanda quality, particularly on the fill soil. Some N, from either a high N turf fertilizer or a palm type fertilizer, was also found to improve N deficiency rating, but not plant size, in dwarf allamanda, a related species on the same sand soil (Broschat et al., 2008). On the other hand, some N, from either a turf or palm fertilizer, increased both plant size and Fe deficiency ratings in nandina (Nandina domestica) on a Bonneau fine sand soil in north Florida (Broschat et al., 2008).

Surinam cherry seemed to respond strongly to N application rate, with the high rate of 24–0–11 resulting in the highest quality on both soil types (Table 3). On the sand soil, the equivalent high N rate from 8 to 0–12 produced similarly high quality plants. The no N treatments consistently produced the poorest quality plants in this species. Gilman (1988) also found a positive relationship between N application rate and plant size and color in chinese hibiscus.

Fertilizer treatment affected foliar nutrient concentrations, but not in the same manner as plant quality. In allamanda, the highest quality treatments (the low rate of 8–0–12 and high rate of 24–0–11) also had lower nutrient concentrations for several elements (Table 4). Similarly, the poorest quality plants (controls) tended to have the highest nutrient concentrations for elements such as N, P, and Mg. The negative correlations between overall plant quality and Mg concentrations in this species were statistically significant (r = −0.363, P = 0.017 and r = −0.300, P = 0.05 for the sand and fill soils, respectively). These trends can be attributed to dilution effects in the plant tissue. Foliar Fe concentrations were unaffected by fertilizer treatment and are often poorly correlated with plant quality (Mills and Jones, 1996).

Table 4.

Leaf nutrient concentrations of nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), manganese (Mn), and iron (Fe) of allamanda grown in two soil types with two rates of turf and palm fertilizers at the end of 3 years (n = 6).

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In allamanda grown on fill soil, leaf P concentrations were lowest for those treatments that contained N (both rates of 24–0–11 and 8–0–12) (Table 4). Foliar Mg concentrations were highest on treatments producing the smallest plants on both soil types (unfertilized controls) because of dilution. Manganese concentrations in allamanda were extremely low for all treatments except for the two 8–0–12 treatments. Concentrations below 50 µg·g⁻¹ are considered to be low (Jones et al., 1991) and Mn deficiency likely caused stunting in plants receiving other treatments. This stunting from Mn deficiency may have been responsible for the elevated concentrations of other elements on the smallest plants. Manganese deficiency rating was the only plant quality variable that was negatively correlated with plant size and all other plant quality variables (data not shown). Foliar Mn concentrations in dwarf allamanda were similarly found to be highest in plants fertilized with palm fertilizers that contained 1% to 2% Mn (Broschat et al., 2008).

Although ixora quality was unaffected by fertilizer treatment, leaf nutrient concentrations of all elements except N and Fe differed significantly among treatments on both soil types (Table 5). Phosphorus concentrations tended to be lowest in those treatments that contained N and produced slightly larger plants, whereas K concentrations were highest in the treatments that contained large amounts of K (both rates of 8–0–12 and 0–0–16). Foliar Mg concentrations were lowest for those treatments having high rates of N or high N:Mg ratios (high rate of 8–0–12 and both rates of 24–0–11). As with allamanda, ixora Mn concentrations were extremely low for all treatments in both soils with only the two 8–0–12 treatments on the sand soil producing foliage with Mn concentrations above10 µg·g⁻¹. Jones et al. (1991) consider Mn concentrations below 40 µg·g⁻¹ to be deficient in this species.

Table 5.

Leaf nutrient concentrations of nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), manganese (Mn), and iron (Fe) of ixora grown in two soil types with two rates of turf and palm fertilizers at the end of 3 years (n = 6).

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Surinam cherry showed significant differences in leaf nutrient concentrations for all elements except Mg and Fe on the sand soil and all elements on the fill soil (Table 6). Phosphorus concentrations tended to be highest in those treatments that resulted in the least growth (controls and both rates of 0–0–16) because of dilution effects. Foliar P concentrations for most other treatments were below the 0.21% threshold for deficiency in this species (Jones et al., 1991). Potassium concentrations were lowest in the treatments having the highest N:K ratio (the 24–0–11 treatments). On the calcareous fill soil, leaf Mg concentrations were higher in the unfertilized controls and the high rate of 0–0–16 than in the high rate of 24–0–11. Again, this was likely because of dilution as the high rate of 24–0–11 produced by far the largest plants and the unfertilized controls and both rates of 0–0–16 resulted in the smallest plants. Unfertilized control plants and those receiving the low rate of 24–0–11 had the lowest leaf Mn concentrations on both soil types, but both 0–0–16 treatments had similarly low Mn concentrations on the sand soil. Although visual Mn deficiency symptoms were not apparent on this species, all treatments in this study resulted in foliar Mn concentrations below the minimum 25 µg·g⁻¹ recommended for this species by Jones et al. (1991).

Table 6.

Leaf nutrient concentrations of nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), manganese (Mn), and iron (Fe) of surinam cherry grown in two soil types with two rates of turf and palm fertilizers at the end of 3 years (n = 6).

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In conclusion, with the exception of ixora grown in sand soil, where N-containing fertilizers of either type improved quality, fertilizer type and rate had little or no effect on overall plant quality of allamanda, ixora, or surinam cherry during the first 6 months following transplanting. Once established, plant quality of allamanda and surinam cherry was influenced by fertilizer type and rate on both soil types. The high rate of 24–0–11 consistently produced high quality plants of both species on both soil types, but equivalent quality was obtained with the low rate of 24–0–11 or either rate of 8–0–12 in all cases except for surinam cherry grown on fill soil. Thus, although high K, Mg, and micronutrient fertilizers are essential for palms growing in these same soils (Broschat, 2015; Broschat et al., 2008), these eudicot shrubs appear to grow just as well with less expensive turf fertilizers. Although plants grown on the calcareous fill soil were always smaller and more nutrient-deficient than those growing on the native sand soil, the types of nutrient deficiencies observed and plant responses to these fertilizer types were similar on both soil types. Thus, ornamental shrubs do not require different fertilization programs when grown on these two soil types in south Florida.

Units

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