Rate of Nitrogen Application During the Growing Season and Spraying Plants with Urea in the Autumn Alters Uptake of Other Nutrients by Deciduous and Evergreen Container-grown Rhododendron Cultivars

Abstract

The influence of fall sprays with urea on the uptake of nutrients other than nitrogen (N) was assessed using 1-year-old container-grown Rhododendron L. (Rhododendron ‘H-1 P.J.M’) and azalea (Rhododendron ‘Cannon's Double’) grown with different rates of N. Plants were grown with a complete fertilizer containing different N rates from May to Sept. 2004 sprayed or not with urea in the fall of 2004 and grown with a complete fertilizer containing different N rates in the spring of 2005. Urea sprays altered uptake of nutrients other than just N although fertilizer application with other nutrients ceased before plants were sprayed with urea. Across a wide range of plant sizes and N status, urea sprays increased net phosphorus (P), copper (Cu), and manganese (Mn) uptake and decreased net potassium (K) and magnesium (Mg) uptake during the year of urea application. Spraying plants with urea altered nutrient demand and storage in different plant structures during the winter. For azalea, urea sprays increased P demand by roots, Mn demand by 2004 stems, and Cu demand by stems. Urea also decreased storage of K in roots and 2004 stems of azalea and Mg in roots. For rhododendron, urea sprays increased P demand by 2003 stems and 2004 leaves and Mn demand by 2004 leaves. Urea sprays also decreased storage of K and Mg in 2004 leaves of rhododendron. For both cultivars, urea sprays increased mobilization of iron (Fe) from storage and demand for Fe in stems. Spraying Rhododendron with urea in the fall altered uptake and demand for certain nutrients during the following spring. Urea sprays in the fall of 2004 increased uptake and possibly demand for P, K, and sulfur during the spring of 2005 for both cultivars, the uptake of calcium by rhododendron, and the uptake of Mg and Mn by azalea. Our results indicate that when growers spray plants with urea in the fall, spring fertilizer practices may need to be modified to account for increased uptake or demand of certain nutrients.