Azaleas are generally known as light feeders, requiring low levels of N fertilization in production (Larson, 1993). They do not respond well to high N rates, which can increase plant susceptibility to fungal disease and reduce winter hardiness (Witt, 1994). According to Michałojć and Koter (2012), the optimal nutrient levels in azalea leaves are 1.88% to 2.20% N, 1.0% to 1.7% potassium (K), and 0.60% to 1.20% calcium (Ca). Under overhead irrigation, increasing N rate from 1.0 to 1.5 or 2.0 lb N/yard3 resulted in lower shoot dry weight and shoot height of Rhododendron sp. ‘George Tabor’, and 1.5 lb N/yard3 was recommended for resin-coated controlled-release fertilizer (Nutricote 17N–3.1P–6.7K, 180 d at 77 °F) (Million et al., 2007). In another study, increasing N fertilization rate was found to promote shoot growth of Rhododendron ‘Karen’, with decreasing N and phosphorus (P) fertilization rates promoting root growth (Ristvey et al., 2007). Optimal plant growth was maintained using an intermediate fertilization rate (100 mg N and 5 mg P per week) compared with lower (25 mg N) or higher (250 mg N) fertilization rates. Increasing fertilization rates (N and P) increased the amount of nutrient leached to the environment and decreased uptake efficiency (Ristvey et al., 2007). Bi et al. (2007) reported similar results where N uptake efficiency of 1-year-old rhododendron (Rhododendron ‘P.J.M.’) and azalea (Rhododendron ‘Cannon’s Double’) declined linearly with an increasing rate of N fertilization from 5 to 20 mm. The investigation of the nutrient requirements of a specific species is important for optimizing nutrient management and reducing environmental contamination. Optimum nutrient applications for a species are often only valid under a certain set of growing conditions, resulting in certain nutrient availability in the substrate to plant root systems (Bi et al., 2007; Million et al., 2007; Ristvey et al., 2007). Given sufficient N supply, N uptake was considered to be subject to a plant’s internal regulation depending on growth rate (Gastal and Lemaire, 2002). Therefore, determination of optimum N rate should be based on both substrate availability of the nutrient (N supply) and crop growth (N demand).
Fertilizer application rates are commonly determined in nursery production based on the assumption that water availability does not limit nutrient uptake and that container capacity should be maintained to promote plant growth and nutrient uptake (Beeson, 1992; Scagel et al., 2011). However, it may be impractical to maintain container capacity in production. Nutrient availability has been shown to decline with low soil water content, which becomes a limiting factor for nutrients to become soluble and be delivered to root surface (Marschner, 2012). Nutrient uptake can be further decreased when a dry substrate has impaired root growth, especially in a dry climate (Marschner, 2012). Because soil moisture conditions can be altered by altering irrigation frequency (Levin et al., 1980), the influence of irrigation frequency on plant growth and nutrient uptake has been investigated. Neilsen et al. (1995) reported that high irrigation frequency improved tree growth of ‘Gala’ apple (Malus domestica) than low irrigation frequency, with less effect on leaf nutrient concentration. Canopy volume, trunk cross-sectional area, dry weight, shoot length, leaf area, and total root dry weight of young ‘Hamlin’ orange (Citrus sinensis) trees were significantly reduced by low irrigation frequency (Marler and Davies, 1990). Increased irrigation frequency was found to reduce N leachate more than continuous irrigation using the same total irrigation quantity (Fare et al., 1994).
Traditionally, when plants are grown in plastic containers, evaporative loss of water is mainly through the substrate surface rather than the container sidewall because plastic containers are impervious to water. Use of biodegradable containers as a sustainable alternative to plastic containers can alter water consumption characteristics of container-grown plants (Koeser et al., 2013a; Wang et al., 2015). Biocontainers constructed with materials such as peat, wood fiber, straw, or paper are highly porous and tend to require more frequent irrigation and a higher total amount of water than traditional plastic containers (Evans et al., 2010; Koeser et al., 2013a; Wang et al., 2015). With increasing water loss between irrigation events, some biocontainers produced smaller plants of ‘Yellow Madness’ petunia (Petunia ×hybrida) (Koeser et al., 2013a). However, sidewall water loss from biocontainers was reported to reduce substrate temperature, which helped alleviate heat stress and enhanced plant survival at locations with hot summer conditions (Nambuthiri et al., 2015).
Growth response of Encore® azaleas under different N fertilization levels and irrigation frequencies when grown in biocontainers compared with plastic containers has not been determined. Considering the container used to grow nursery plants may have considerable influence on plant water consumption characteristics, container choice may alter the irrigation requirement and nutrient uptake of a given species. Therefore, the objectives of this study were to 1) investigate the influence of increased irrigation frequency on growth and N uptake of Encore® azalea ‘Chiffon’, 2) compare growth response of plants grown in a black plastic container with a biocontainer made from recycled paper, and 3) determine the optimum N fertilization rate based on irrigation frequency and container type. Results of this study will provide valuable reference for the fertilization and irrigation requirements of Encore® azalea ‘Chiffon’, a dwarf, slow-growing cultivar.
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