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Ricardo Campos and David Wm. Reed

Salinity is a limiting factor in plant growth. The combination of water high in soluble salts and water-soluble fertilizers can induce salt damage in plants. The objective of this work was to investigate the effects of salinity in irrigation water on optimal fertilization rates in Spathiphyllum `Petite'. The combination of 5 levels of fertilizers and 5 salinity levels were tested. Maximum growth was observed at 250 mg l-1 N and no salts, and with 2000 mg l-1 salts at 125 mg l-1 N. As salt levels increased, height and leaf area decreased. Tissue calcium, sodium, and chloride increased with increasing levels of salinity. Tissue nitrogen, phosphorus, and potassium generally increased with increasing levels of fertilizers, and were not affected by salinity level. It is possible that high sodium and chloride concentrations in leaves, petioles, and roots produced an ion toxicity.

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Trisha Blessington Haley and David Wm. Reed

Two experiments were conducted to investigate the effect of K fertilizer rates on growth of New Guinea impatiens (Impatiens Hawkeri Bull.), vinca (Catharanthus roseus (L.) G. Don) and petunia (Petunia ×hybrida Hort. Vilm.-Andr.) in a recirculating subirrigation system. Based on a variety of growth parameters, a broad range of K concentrations allowed maximum growth, notably 1 to 6 mM for New Guinea impatiens `Ovation Salmon Pink Swirl', 2 mm for New Guinea impatiens `Cameo' and `Illusion', 2 to 8 mm for vinca `Pacifica Apricot', and 2 to 16 mm for petunia `Trailing Wave Misty Lilac'. Thus, the lowest concentration that allowed maximum growth was 1 to 2 mm K. A third experiment compared the optimum K concentration and K balance of vinca grown with recirculating subirrigation versus top-watering. Based on a variety of growth parameters of vinca `Pacifica Red', the lowest concentration that allowed maximum growth was 2 mm K with recirculating subirrigation and 4 mm K with top-watering. The K balance demonstrated that subirrigated plants were twice as efficient in K use compared to the top-watered plants. Leachate loss was the major contributor to inefficiency in top-watered plants. Electrical conductivity (EC) of the growing medium remained below the recommended level of 1.2 dS·m-1 in both irrigation methods at K concentrations of 16 mm and below in the bottom layer and 8 mm and below in the middle layer. In the top layer of the growing medium, EC was above the recommended level at all K concentrations tested in subirrigation at all concentrations, and in top-watering at 16 mm and above.

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Luis A. Valdez-Aguilar* and David Wm. Reed

Mixture experiments were used to study the effect of Rb, K, and Na in combination with a number of bicarbonate concentrations on bean plants grown in hydroponics in a controlled environmental chamber. The objective was to separate the cation effect from the bicarbonate effect. The first experiment was a 3-component mixture-amount experiment using various ratios of Rb, K, and Na at 0 and 7.5 mm of bicarbonate. In the 0 mm bicarbonate control, the pure blends were ranked: Rb > Na > K for their effect on reducing shoot dry mass. The high toxicity to the Rb ion was probably due to direct Rb toxicity in addition to any general salinity effect. At 7.5 mm bicarbonate, shoot dry mass was decreased with all the counter-ions compared to the 0 mm bicarbonate control, and their toxicity was ranked: Rb > Na ≈ K. The next series of experiments were 2-component mixture-amount experiments at various ratios of K and Na at 2.5, 5 and 7.5 mm bicarbonate. In the 0 mm bicarbonate control, shoot dry mass decreased with increasing proportions of Na, indicating a specific Na toxicity. The same trend was observed at 2.5 mm bicarbonate. In the 7.5 mm bicarbonate treatment, both Na and K were equally toxic. At low concentration of bicarbonate, the Na is more toxic than the bicarbonate. At higher concentrations of bicarbonate, both Na and bicarbonate exhibit similar levels of toxicity.

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Luis A. Valdez-Aguilar* and David Wm. Reed

Tolerance to alkalinity was evaluated in Rose `Pink Cupido', Vinca `Apricot Delight', Chrysanthemum `Miramar', and Hibiscus `Bimini Breeze' and `Mango Breeze'. Plants were potted in a sphagnum peat moss-based growing medium and irrigated with water containing 0, 2.5, 5, 7.5 and 10 mm of Na bicarbonate. In rose, shoot mass was significantly decreased and chlorosis increased at the 5 mm treatment, indicating that the alkalinity toxicity is between 2.5 and 5 mm. In chrysanthemum, the concentration of Na bicarbonate did not significantly affect shoot mass, but caused a significant increase in leaf chlorosis at 5 mm or higher Na bicarbonate. This indicates an alkalinity toxicity level between 2.5 and 5 mm. In Vinca, shoot dry mass was not affected significantly, but leaf chlorosis was significantly increased with 5 mm of Na bicarbonate. This indicates an alkalinity toxicity level between 2.5 and 5 mm. In hibiscus `Mango Breeze', shoot mass was significantly increased at 2.5 and 5 mm, but was significantly decreased at 7.5 mm and above. Leaf chlorosis was significantly increased with a concentration of 5 mm and above, indicating that in hibiscus `Mango Breeze' the alkalinity toxicity level is between 5 to 7.5 mm. In hibiscus `Bimini Breeze', shoot mass was not significantly reduced, but leaf chlorosis exhibited a significant decrease at 7.5 mm. this indicates that in hibiscus `Bimini Breeze' the alkalinity toxicity level is between 7.5 and 10 mm. Growing medium pH increased with increasing levels of Na bicarbonate. The species showed varying capacity for acidification of the growing medium. All species, except rose and vinca, neutralized the alkalinity effect of 2.5 mm, but none of the species neutralized the effect of 5 mm and higher Na bicarbonate.

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Daphne L. Richards and David Wm. Reed

New Guinea impatiens (Impatiens hawkeri Bull.) `Illusion' were grown in a recirculating subirrigation system under various rates and placements of 14N-6.1P-11.6K (Osmocote; Scotts-Sierra, Marysville, Ohio) resin-coated, controlled-release fertilizer (CRF). Four CRF placements (incorporated, top-dressed, bottom, and dibble) were tested. Incorporated placement yielded slightly greater dry weights than the other placements. A rate experiment tested incorporating from 0.5 to 2 times the fertilizer manufacturer's recommended rate of 7.11 kg·m-3. All shoot growth parameters (height, leaf number, shoot, and root fresh and dry weight) exhibited a significant quadratic response, as exemplified by shoot dry weight, where shoot dry weight increased up to the 1.5× rate, after which shoot dry weight decreased. A quadratic response surface model revealed that the optimum rate response ranged from 1.16× rate for height to 1.47× rate for shoot dry weight. The lower bound of the 95% confidence interval (CI) would be the lowest rate at which one could expect maximum growth response. The lower bound of the 95% CI varied from 0.56× rate for height to 1.30× rate for shoot dry weight. Thus, the lowest rate that would be within the 95% CI for all growth parameters, and thus yield maximum growth response, would be the 1.30× rate. Electrical conductivity (EC) of the growing media increased significantly with increasing CRF rate. At all rates, EC was significantly greater in the top layer than in the middle and bottom layers. Only in the 1.75× and 2× rates did EC exceed the recommended EC levels in the middle and bottom layer. All rates >0.75× exceeded recommended EC in the top layer. Release characteristics and total nutrient balance of the CRF was compared in subirrigated and top-watered systems. There was no significant difference between top-watered and subirrigated treatments for the amount of K recovered in plant tops and released from prills. By day 84, in subirrigation, 46% of the K was still in the prills, 41% was recovered in the plant tops, and 22% was recovered in the medium. Similar results were obtained in the top-watering treatment, except that a lesser amount was recovered in the medium (9%) and a small amount (4%) was recovered in the leachate. The uptake of K by plants and release of K by the CRF were inversely proportional and linear with respect to time. Of the K released from the prills, 77% and 83% were recovered in the plant tops for subirrigation and top-watering, respectively, indicating very high fertilizer use efficiency.

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Matthew W. Kent and David Wm. Reed

Greenhouse cultural methods must change rapidly to minimize runoff and to keep pace with environmental regulation aimed at protecting water resources. Two experiments were designed to investigate the effect of N fertilization rate on New Guinea impatiens (Impatiens ×hawkeri) and peace lily (Spathiphyllum Schott) in an ebb-and-flow subirrigation system. Maximum growth response for impatiens was centered around 8-mM N levels as measured by root and shoot fresh and dry weight, height, leaf number, leaf area, and chlorophyll concentration. For peace lily, growth peaked around 10 mM N. Growing medium was divided into three equal layers: top, middle, and bottom. Root distribution favored the middle and bottom layers, and the relative distribution of roots was consistent as N level increased. Soluble salts remained low in middle and bottom layers at N concentrations below 10 mM, but increased significantly for all soil layers at levels above 10 mM. The top layer contained two to five times higher soluble salt levels than in the middle or bottom layers at all N levels. Increased nitrate concentration mimicked increases in soluble salts, while pH decreased as N concentration increased for both impatiens and peace lily.

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Matthew W. Kent and David Wm. Reed

Greenhouse cultural methods must minimize runoff to keep pace with environmental regulation aimed at protecting water resources. Two experiments were designed to investigate the effect of N fertilization rate on New Guinea impatiens (Impatiens ×hawkeri) and peace lily (Spathiphyllum Schott) in an ebb-and-flow subirrigation system. Maximum growth response for impatiens was centered around 8 mm N levels as measured by root and shoot fresh and dry weight, height, leaf number, leaf area, and chlorophyll concentration. For peace lily, growth peaked at about 10 mm N. Growing medium was divided into three equal layers: top, middle, and bottom. Root distribution favored the middle and bottom layers, and the relative distribution of roots was consistent as N level increased. EC remained low in middle and bottom layers at N concentrations below 10 mm, but increased significantly for all layers at levels above 10 mm. The EC for the top layer was 2 to 5 times higher than in the middle or bottom layers at all N levels. Increased nitrate concentration paralleled increased EC, while pH decreased as N concentration increased for impatiens and peace lily.

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Nancy Morgan Todd and David Wm. Reed

New Guinea impatiens (Impatiens hawkeri Bull.) were grown in a recirculating ebb-and-flow subirrigation system under increasing levels of salinity stress from a mixture of NaCl and CaCl2 (1:1 equivalent ratio, 2:1 molar ratio) and recommended production levels of other nutrients. Growth and quality decreased as salinity level increased, with a 75% to 80% growth reduction at 18 mol·m-3 NaCl-CaCl2 compared to controls. Among controls, root mass distribution was 10%, 50%, and 40% in the top, middle, and bottom layers of the root zone, respectively. In the highest salinity treatment (18 mol·m-3 NaCl-CaCl2), most of the root mass was in the middle layer (80%), while the root mass in the top and bottom layers was reduced to 5% and 15%, respectively. The electrical conductivity (EC) of the growing medium was high in the top layer in all treatments, but only exceeded maximum recommended levels in the middle and bottom layers in the 4·mol·m-3 or higher treatments. Initial postproduction leaching caused the salts in the top layer to migrate to the middle and bottom layers, which in some experiments induced a rapid and transient wilting. Up to six leaching and drying cycles of a 0.20 leaching fraction were required to reduce EC in all layers to recommended levels. Overall, salable plants of good quality and size were produced with up to 2 mol·m-3 (total 152 mg·L-1) NaCl-CaCl2 in the recirculated nutrient solution.

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Nancy K. Todd and David Wm. Reed

Concerns over groundwater contamination due to greenhouse runoff have caused many growers to turn to subirrigation as an alternative watering method. One reported problem is the movement of salts to the top layer of the rootzone due to zero leaching. Many growers are faced with the added challenge of subirrigating plants with poor-quality water than contains a high salt content before the addition of fertilizer. An experiment was conducted to investigate the movement of salts in the root zone and the effects on root development and overall plant growth. Plants were grown using water treated with NaCl + CaCl2 (1:1 equivalent basis) at the following total concentrations: 0, 2, 4, 6, 8, 10, 14, and 18 mM. Treatment time was 10 weeks (marketable stage). At harvest, height was measured and plants were cut off at the soil line and divided into shoots (stems and leaves) and roots for fresh and dry weight. Leaf area was measured. The root zone was divided into three layers—top, middle, and bottom (≈3 cm each). Roots were separated from each soil layer and soil samples collected for measuring EC and pH using 1:2 dilution. Soil samples showed EC in the top layer of the root zone was much higher than the middle and bottom layers. Root weight also decreased substantially in the top layer of the root zone. Height, FW, DW, and leaf area of plants did decrease with increasing salt concentration, indicating that the detrimental effects of poor-quality water on subsequent plant growth, especially in a subirrigation system.

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Harvey J. Lang, Claire-Lise Rosenfield and David Wm. Reed

Ficus benjamina L. and Dracaena marginata Lam. were grown in a modified Hoagland's nutrient solution containing either 0, 0.22 or 5.52 mg Fe3+/liter (HEEDTA or EDTA). F. benjamina grew well at all Fe levels and showed mild chlorosis only at 0 mg Fe/liter. For D. marginata, growth decreased and chlorosis increased as solution Fe level decreased. F. benjamina exhibited a high capacity for Fe3+ reduction, which increased as Fe level decreased, reaching a maximum below 0.06 mg·liter-1 D. marginata exhibited a low capacity for Fe3+ reduction, which was slightly enhanced at 0.1 to 0.15 mg·liter-1. In both species, reduction occurred in the presence of roots, with minimal reduction in their absence. This result indicates that Fe3+ is reduced at the root surface and not by reductants released into the solution. F. benjamina increasingly lowered pH as solution Fe decreased, and always lowered pH more than D. marginata at all Fe levels. Total and extractable Fe concentration of leaves did not correlate well with chlorosis, whereas total Fe content per plant correlated highly with chlorosis. Chemical names used: N-hydroxyethyl-ethylenediamine-triacetic acid (HEEDTA), ethylenediamine tetraacetic acid (EDTA).