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Harvey J. Lang

Euphorbia pulcherrima Willd. `V-14 Glory Red', `V-17 Marble', and `Jingle Bells 3' were grown in various peat-based potting media amended with ground, non-composted woody stem core of kenaf (Hibiscus cannabinus L.) and fertilized at every irrigation with a 17N–2.2P–13.8K fertilizer at 300 ppm N. Kenaf is an annual tropical species grown in several Gulf Coast states as a fiber crop. Growth and overall quality of all cultivars was best in media containing 50% to 70% coarse grind of kenaf (by volume) and was similar to a commercial soilless control medium (Sunshine #1, SunGro Horticulture, Inc.). Media containing greater than 70% fine grind kenaf resulted in significantly smaller plants with chlorosis of both lower and upper leaves, and also resulted in undesirable shrinkage of media in the containers. Addition of 19N–2.6P–10K Osmocote to kenaf-amended media at a rate of 3.5 kg·m–3 resulted in significantly better plants with less chlorosis than similar media without Osmocote. Media containing kenaf also resulted in significantly less water-holding capacity than Sunshine #1 and required morefrequent irrigation to sustain the growth of plants.

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Harvey J. Lang

Analysts of potting media for pH and electrical conductivity (EC) can be a useful tool for monitoring the nutritional status of greenhouse grown plants. This research examined the variability associated with procedures involved in the determination of pH and EC in greenhouse potting media. Three commonly used methods, the 1:5 dilution, the 1:2 dilution and the saturated media extract, were examined on several different commercial potting media. Because of the different dilution volumes used, there were significant differences in pH and EC between the three methods for all media tested. Within each method, results varied baaed on whether readings were taken in the slurry, solution phase, or extract, with extracts resulting in consistently higher pH, but lower EC values. There was a significant effect of medium-solution equilibration time on both pH and EC, with variability decreasing after 30 minutes of equilibration. Samples taken from the upper half of pots had higher EC readings than those collected from the bottom half of pots only on plants fertilized with N concentrations greater than 200 ppm. There was also slight variability between the different calibrated instruments used in determining pH and EC. Details of each study along with grower recommendations will be discussed.

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Dario Ramirez and Harvey J. Lang

Two studies were conducted to determine the effect of applied Fe concentration on the occurrence of phylloclade marginal chlorosis in holiday cactus. Schlumbergera sp. `White Christmas', `Twilight Tangerine', `Christmas Charm', and `Lavender Doll' were grown in either solution culture or a peat-based medium and supplied with a nutrient solution containing Fe-EDTA at either 0, 0.18, 0.36, 0.54, or 0.72 mm. Growth decreased while marginal chlorosis increased on all cultivars as Fe-EDTA treatment levels increased. Phylloclade marginal chlorosis developed more severely and consistently in plants grown in peat-based media versus solution culture. Marginal chlorosis and accumulation of Fe in the potting medium occurred very slowly, and first symptoms were not observed until after about 2 months of treatment. Symptomatic phylloclade tissue had a greater Fe concentration than corresponding asymptomatic phylloclade tissue. Cultivars varied in tissue Fe concentration and symptom severity across Fe-EDTA treatments. There were no significant correlations in other tissue micronutrient concentrations with either Fe treatment or symptom development. The pH of either the culture solution or peat-based medium leachate did not change significantly throughout either study, and there were no significant amounts of Fe+2 detected in solution. Chemical name used: ferric ethylenediaminetetraacetic acid, monosodium salt (Fe-EDTA).

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Dario Ramirez and Harvey J. Lang

Production of holiday cactus has been limited by the common occurrence of marginal chlorosis of the phylloclades, which can lead to losses in crop quality. This work was conducted to determine if poor growth and phylloclade yellowing could be correlated to applied Fe concentration. Rooted cuttings of Schlumbergera truncata `White Christmas', `Twilight Tangerine', 'Christmas Charm', and `Lavender Doll' were transplanted into a modified Hoagland's solution, adjusted to a pH of 6.3, containing Fe-EDTA at either 0,10,20,30, or 40 mg·liter–1 Fe. Plants were grown in a controlled environmental chamber under 16 h daylength for 16 weeks at 22/18C day/night temperature. Plants grown under 0 and 10 mg·liter–1 Fe had significantly greater fresh weight, height, and root length than plants grown under higher Fe concentrations for all cultivars. Comparison of tissue analysis results revealed a direct correlation between poor growth and levels of Fe within the tissue. There was no correlation, however, between Fe concentration and phylloclade edge yellowing, as yellowing occurred sporadically in all treatments. Comparison studies in the greenhouse of plants grown in peat: perlite medium showed similar trends.

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Timothy R. Pannkuk and Harvey J. Lang

`Barbados' New Guinea impatiens were grown in 500 cm3 pots under drip irrigation and fertilized with solutions containing either 6, 12 or 18 mM N at two leaching fractions (0 and 30-50%). Soil moisture within the pots was characterized and controlled using electronic microtensiometers. The 0% leachate plants were automatically irrigated to container capacity when the soil tension in the pot reached 4.5 kPa, while the 30-50% leachate plants were irrigated for five minutes at a time whenever the soil tension was at or slightly greater than 4.5 kPa. After 6 weeks, there was only a slight effect of fertilizer concentration on height, width, or leaf number of the plants. Plants grown under 0% leachate did not differ significantly in overall growth and performance than those that were leached at every irrigation. Electrical conductivity of the potting media was highest in the 0% leachate treatments (highest readings in upper one-third of the pot) and increased as fertilizer concentration increased. After 6 weeks of production, total volume of irrigation solution leached per pot range6 from 2.2 to 3.6 liters for plants grown under the 30-50% leachate treatment.

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Harvey J. Lang and George C. Elliott

Autotrophic nitrifying organisms were enumerated in soilless potting media using the most probable number (MPN) technique. Populations of NH4 + and NO2 - oxidizing organisms varied widely between two soilless media—Metro-Mix 220 and 350. Estimates for NH4 + oxidizing organisms ranged from 0.7 to 7.8 × 105 organisms/cm3, while NO2 - oxidizers ranged from 1.3 to 9.5 × 105 organisms/cm3. Population numbers were similar to those typically reported in soils. There was a significant effect of medium type, NH4 + N : NO3 - N fertilizer ratio, and planting on MPN counts of both groups of organisms, with significant interaction between several of the factors. Estimates of NH4 + oxidizers were not linearly correlated with NH4 + oxidizing activity, implying low counting efficiency, heterotrophic nitrification, or rate-limiting substrate NH4 + level. In a separate study, a soilless potting medium was inoculated with pure cultures of either Nitrosomonas europaea or Nitrobacter agilis. Rates of NH4 + and NO2 - oxidation increased, respectively, as inoculum volume increased. Inoculation with nitrifying bacteria may help in the overall management of N in the rhizosphere and be feasible alternatives for the prevention of either NH4 + or NO2 - phytotoxicity with fertilizers containing urea or NH4 +.

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Harvey J. Lang and Don C. Wilkerson

Experiments were conducted to determine the effectiveness of paclobutrazol in solid spike form as compared to foliar spray or medium drench applications for height control of several foliage and flowering plants grown in 8-inch or 10-inch hanging baskets. Paclobutrazol was applied as either a 20 or 40 mg·liter–1 foliar spray, 1 or 2 mg·liter–1 medium drench, or 200 or 400 mg·liter–1 spike insertion per basket. Begonia × tuberhybrida `Nonstop Apricot' and Begonia × hiemalis `Barbara' treated with paclobutrazol were significantly shorter than nontreated controls. Drench applications were more effective than either spray or spike treatments for both species, with Hiemalis begonia showing severe dwarfing at both the 1 and 2 mg·liter–1 drench. Paclobutrazol treatments did not significantly affect flower number for either species. Syngonium podophyllum `White Butterfly' and Epipremnum aureum showed similar trends as the begonias; however, relative reductions in height were not as great. Plants appeared to be slightly less stretched than nontreated plants.

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Harvey J. Lang and George C. Elliott

Effects of medium, NH4: N03, ratio, and N fertilizer rate on the development of NH4 oxidation in soilless potting media were evaluated. In two separate experiments, NH4 oxidizing activity increased to a maximum (4 to 6 weeks of cropping) and then dropped off sharply. Ammonium oxidation activity varied significantly among types of soilless potting media. Media fertilized with 1 NH4-N: 3 N03-N had higher rates of NH4 oxidation than media fertilized with ratios of either 1:1 or 3:1. Nitrogen fertilization at 15 mm gave consistently higher oxidation rates than fertilization at 30 mm. In general, media samples that had been cropped with plants had higher nitrifying activity than unplanted samples. Ammonium oxidation rate over all observations was significantly correlated with medium pH (r = 0.50). pH values above 6.8 were necessary but not completely sufficient for relatively high rates of NH4 oxidation. Rates of oxidation were insignificant with pH values <5.6.

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Harvey J. Lang and Timothy R. Pannkuk

New Guinea impatiens `Barbados' (Impatiens ×hawkeri) were fertilized with solutions containing N at 6, 12, or 18 mmol·L-1 delivered from a drip irrigation system with either minimum leaching or standard leaching (0.3 to 0.4 leaching fraction). Irrigation was monitored and controlled by computers using microtensiometers placed in representative pots of each treatment. In two separate experiments, growth index, fresh mass, and dry mass were dependent upon both fertilizer concentration and irrigation treatment. Maximum growth overall was achieved at 12 mmol·L-1 N regardless of irrigation treatment; however, standard-leached plants receiving N at both 6 and 18 mmol·L-1 produced larger plants than did similarly fertilized minimum-leached plants. Leaf scorch, spotting, or marginal necrosis did not occur in any of the treatments. Leaf N, P, and K concentrations were highest in plants treated with N at 18 mmol·L-1, but Ca, Mg, and several micronutrients were highest in plants at 6 mmol·L-1 N. At the end of the cropping period for both experiments, growing medium electrical conductivity (EC) in the uppermost one-third layer of the pot was two to four times as high as that in the bottom two-thirds (root zone) layer. Root-zone EC ranged from 0.6 to 4.0 dS·m-1 and increased as fertilizer concentration increased. Standard leaching had little effect in reducing root-zone EC except in plants fertilized with N at 18 mmol·L-1. All plants continued to perform well and flower after 4 weeks in a simulated interior environment. Minimum-leach drip irrigation used ≈35% less solution than did standard irrigation with leaching, and eliminated N runoff.

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George C. Elliott and Harvey J. Lang

Fungicides were applied at label rates to two commercial soilless potting media in which Iris siberica L. crowns had been potted and were subsequently grown under greenhouse conditions. Effects of fungicides on urea hydrolysis were inconsistent and generally insignificant. Ammonium oxidation was inhibited to varying degrees by Truban, Benlate, Banol, and Subdue. In a subsequent experiment, the same fungicides were added to cropped samples of the same media in vitro, followed 12 hours later by a solution containing urea and ammonium. Urea hydrolysis was essentially unaffected by fungicide treatments. Subsequent oxidation of ammonium was inhibited by Truban and Banol only in one medium. Transient accumulation of nitrite was inhibited by Truban but stimulated by Benlate in both media. When added to pure cultures of Nitrosomonas europea and Nitrobacter agilis, Truban completely inhibited oxidation of ammonium and nitrite. Benlate partially inhibited oxidation of ammonium and nitrite, while Subdue and Banal partially inhibited oxidation of ammonium but not nitrite. Chemical names used: [Methyl 1-(butylcarbamoyl)-2-benzimidazolecarbamate] (benomyl); N- (2,6-dimethylphenyl) -N- (methoxyacetyl)alanine methyl ester (metalaxyl); [2-chloro-6-(trichloromethyl)pyridine (nitrapyrin); 5-Ethoxy-3-(trichloromethyl)-1,2,4-thiadiazole (ethazol); Propyl[3-(dimethylamino)propyl]carbamate monohydrochloride (propamocarb).