Large irrigation volumes leach nutrients from containers because the growing substrate used in nursery production generally has low nutrient and water-holding capacity (Arreola et al., 2006; Beeson, 2006; Fare et al., 1994; Huett, 1997). Leaching from containers can be decreased by increasing irrigation frequency but not total volume. Less frequent irrigation may decrease plant growth as a result of nutrient shortage rather than water shortage, and more frequent irrigation may compensate for certain nutrient deficiencies (Buljovcic and Engels, 2001; Scheiber et al., 2008; Silber et al., 2003; Xu et al., 2004).
Many commercially important qualities of container-grown plants are a function of nutrients and water availability during production (Cameron et al., 2008; Sharp et al., 2008). Nitrogen is the primary nutrient that drives growth and therefore demand for other nutrients (Marschner, 1995). One of the largest challenges in container nursery nutrient management is that water availability may have unforeseen impacts on nutrient availability and vice versa. Maintaining a balance of water and nutrient availability to optimize growth and quality of container plants requires knowledge of how these factors interact. The influence of water and N management on uptake of other nutrients during production for many crops has not been evaluated fully. With some woody perennial plants, growth is enhanced more by minimizing water stress than by increasing fertility (Rose et al., 1999; Tan and Hogan, 1997).
Nitrogen availability influences growth and uptake and storage of N and other nutrients by evergreen and deciduous cultivars of Rhododendron spp. (Bi et al., 2007a; Harris et al., 2006; Scagel et al., 2007, 2008a, 2008b). Negative growth responses to excess N can occur from increased salinity, disruption of the balance between N and other nutrients, or increased water stress (Bi et al., 2007b; Cabrera, 2004). Certain N application rates can increase uptake of other nutrients; however, growth of plants with a high N status can be limited by availability of other nutrients if nutrient management strategies are not altered for high N rates. Irrigation scheduling can alter growth of Rhododendron (Keever and Cobb, 1985; Million et al., 2007). Improved knowledge of the combined influence of irrigation and nutrient management during nursery production is needed to develop integrated nursery production practices targeted at improving plant quality and decreasing production inputs.
Recently, we described how both N deficiency and high plant N status increase water stress in container-grown Rhododendron (Scagel et al., 2011). Watering plants more frequently decreased water stress of plants fertilized with the highest N rate and had little impact on alleviating water stress of N-deficient plants. Altering irrigation frequency changed N availability in the growing substrate or the ability of roots to absorb N. In addition, we found that transitory increases in plant water stress from different irrigation frequencies alters N uptake and use and plant form without detectable changes in total plant biomass. The influence of N rate on the uptake of other nutrients reported in our previous research (Scagel et al., 2008a, 2008b) may have been partially a function of differences in water stress.
The effects of cultural practices on nutrient content can be complicated to interpret in terms of drawing conclusions related to nutrient uptake when plant size is also influenced by the cultural practices and cultivars evaluated. Differences in nutrient uptake between cultivars and treatments can be partially attributable to scaling effects of plant growth on nutrient content (Righetti et al., 2007). For example, the increasing rate of N application can increase total biomass and N content of Rhododendron (Bi et al., 2007b; Scagel et al., 2007, 2008a). This indicates that plants absorbed more N at greater N rates (e.g., greater N uptake) but it is not clear whether greater N uptake is solely a function of plant size or a combination of plant size and increased capability to absorb N.
Previously, we used covariate analyses to account for differences attributable to plant size in comparisons of nutrient uptake between cultivars and treatments (Scagel et al., 2008b). Our research on the effects of water stress on N uptake indicated plant size was unaffected by our irrigation treatments (Scagel et al., 2011). Lack of significant biomass response to the low level of water stress achieved by our irrigation treatments allows us to more readily interpret changes in nutrient content as changes in nutrient availability, the efficiency of nutrient uptake, and, in perennial crops, differences in nutrient storage. We report the influence of irrigation frequency and rate of N application on uptake of other nutrients in one deciduous and two evergreen cultivars of Rhododendron grown in containers for 1 year. Additionally, we also evaluated whether nutrient concentrations in different structures and ratios of N to other nutrients in leaves and whole plants could be used to detect the influence of irrigation frequency on the relationship between N and other nutrients.
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