Nitrogen balance for two container-grown woody ornamental plants

Abstract

The woody ornamentals Ilex opaca ‘Hedgeholly’ and Lagerstroemiaב Tonto’ plants were grown in containers for 9 months, and fertilized with complete nutrient solutions differing in applied nitrogen ([N] A) concentration: 15, 30, 60, 120, 210, and 300 mg l −1. Shoot dry biomass and leaf area increased significantly with applied N concentrations up to 60 mg l −1, but higher levels caused significant reductions in these parameters. Root dry weights decreased with increases in applied N. Tissue N concentrations and leaf chlorophyll levels increased in a typical saturation response with N supply, and maximum dry weight yields for Ilex and Lagerstroemia plants were observed at leaf N concentrations of 2.53 and 2.65%, respectively. Plants of both species fertilized with 15–60 mg l −1 produced leachate N concentrations that were on average below the actual [N] A, whereas plants fertilized with 120–300 mg l −1 resulted in leachate N concentrations that were higher than [N] A. Across the two species, N leaching losses (as a fraction of total N applied) increased from an average of 17.4% in plants receiving 15 mg l −1 of N to an average of 52.0% for those fertigated with 210 and 300 mg l −1. The fraction of applied N that was taken up by plants in both species decreased significantly with increases in [N] A, averaging 46.8 and 7.2% for the 15 and 300 mg l −1 treatments, respectively. The N fraction retained by the growing medium at the end of the experiment was not significantly affected by [N] A, averaging 6.4 and 3.7% of the applied N in Ilex and Lagerstroemia, respectively. Interestingly, and regardless of [N] A, an average of 31% (range 22.8–40.6%) of the total applied N could not be accounted for at the end of the experiment in both species, and this fraction was attributed to gaseous N losses by denitrification.