Micronutrient supplements were applied to container rhododendron (Rhododendron L. × `Girards Scarlet' [Girard Evergreen Hybrid Group]) in three forms: uncoated micronutrient fertilizer; slow-release, NPK-plus-minors fertilizer; and biosolids compost (15% v/v). Control plants received no supplement. While all micronutrient treatments had significantly higher foliar Mn or Cu concentrations than controls 1 year after potting, they did not increase growth (dry weight) or plant quality. At 1, 3, and 12 months after potting, the compost treatment had significantly higher diethylenetriaminepentaacetic acid (DTPA)-extractable levels of Mn, Fe, and Zn in the medium. Only one micronutrient fertilizer treatment increased extractable micronutrient concentrations (Cu) on all testing dates. Correlations between medium-extractable and foliar micronutrient concentrations were low (r2 < 0.30). Vigorous growth in the control treatment suggested that adequate levels of micronutrients were supplied by the pine bark-hardwood bark-peat-sand medium. September concentrations [ppm (mg·L-1)] as low as 2.0 Mn, 17.8 Fe, 0.3 Cu, 4.2 Zn, and 0.9 B in DTPA extracts produced acceptable growth in rhododendron through the following June.
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