Increased demand for container nursery stock for commercial forestry, restoration/wildlife plantings, and landscaping has generated a need to optimize watering methods, media types, and nutrient management (Oliet and Jacobs, 2012). Subirrigation has recently been demonstrated as an effective alternative to overhead irrigation, especially for container seedling propagation of broadleaves (Davis et al., 2011a; Schmal et al., 2011). In subirrigation systems, irrigation water is delivered from beneath container trays and (optionally) recycled, which reduces wastewater, nutrient-leaching losses, and uneven water distribution associated with overhead systems (Morvant et al., 1997; Schmal et al., 2011). Subirrigation has been shown to substantially conserve irrigation water (Dumroese et al., 2006; Landis and Wilkinson, 2004) and promote plant growth at reduced nutrient applications compared with overhead systems (Beeson and Knox, 1991; Pinto et al., 2008). Subirrigated forest tree seedlings are usually of equal or better quality to those produced under comparable culture using overhead irrigation systems (Davis et al., 2008, 2011a) with these differences being maintained after field establishment (Bumgarner et al., 2008; Davis et al., 2011b).
Supplemental nutrition through fertilization is essential in these soilless media nursery systems (Landis et al., 1989). When not optimized for a given species and cultural conditions, however, fertilization can result in nutritional disorders such as induced deficiency of other nutrients and/or ion toxicity (Salifu and Jacobs, 2006; Salifu and Timmer, 2003). Despite the demonstrated potential of subirrigation, detailed analyses of fertilization responses specific to these systems is lacking (Schmal et al., 2011). This may be particularly important given observations of persistent residual fertilizer salts in the medium and holding tanks under subirrigation systems that recycle runoff water for multiple irrigation cycles (Dumroese et al., 2006, 2011).
Fertilization in any form can alter rhizosphere chemical and physical properties including pH, EC, ion availability, and ψS. For example, when nitrate (NO3–) is taken up preferentially over ammonium (NH4+), OH– will be released from the root to maintain charge balance and the media pH will increase. The opposite is true if NH4+ is taken up (Jacobs and Timmer, 2005). Changes in pH result in altered ion availability, which can create deficiency of important nutrients and toxicity of other ions, such as aluminum, reducing root growth and development (Jacobs and Timmer, 2005).
EC of media solution is an indication of fertilizer salts in the medium (Landis et al., 1989) and higher EC levels reflect greater salt buildup. Greater fertilizer inputs and decreased media moisture exponentially increase EC of the media solution, which can be detrimental to plant growth (Jacobs and Timmer, 2005; Landis et al., 1989). Furthermore, the addition of fertilizer can lower media solution ψS, resulting in a disrupted water potential gradient. A disturbed gradient can cause physiological drought in plants, and plants may close stomata in response. Although stomatal closure conserves tissue water and could prevent lethal desiccation, it is done at the cost of CO2 assimilation (Burdett, 1990). Consequently, optimal plant growth is not achieved because of limitations in net photosynthesis.
Problems associated with reduced ψS may become exaggerated for plants grown in subirrigation systems as a result of the upward flow of ions with water by capillarity, resulting in high soluble salt levels in the upper layers of growing media (Davis et al., 2008; Dumroese et al., 2006, 2011; Pinto et al., 2008). Tolerance to high EC is species-specific, although tolerance levels among broadleaves are not well understood (Jacobs and Timmer, 2005). Thornton et al. (1988) reported that EC levels greater than 1.0 dS·m−1 caused damage to northern red oak (Quercus rubra L.) seedlings including foliar discolorations and reduced leaf dry mass production, suggesting the potential sensitivity of this species.
In a previous study (Bumgarner et al., 2008), we examined the effects of media composition and fertility on growth of northern red oak seedlings under subirrigation vs. overhead irrigation systems. Subirrigated seedlings had greater field diameter growth, although nursery fertilization (1.2 g N/plant) resulted in reduced seedling field survival and height growth compared with the control (unfertilized treatment). Thus, the current study was designed to more closely examine the effects of a wide range of fertility rates applied during subirrigation on media pH, EC, and on the physiology and morphology of container-grown northern red oak seedlings.
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