Nitrogen Nutrition of Containerized Thuja x ‘Green Giant’

Rooted stem cuttings of ‘Green Giant’ arborvitae (Thuja L. x ‘Green Giant’) were grown in 3.8 liter (#1) plastic containers containing a pine bark: sand (8:1 by vol) substrate. Plants were fertilized three times weekly for 15 weeks with a complete nutrient solution at N application rates (NARs) of 0, 10, 20, 40, 80, 160, or 320 mg/liter (ppm) supplied as ammonium nitrate (NH₄NO₃). Both shoot and root dry weights were significantly affected by NAR. Maximum shoot weight, as predicted by a quadratic plateau model, was reached at a NAR of approximately 100 mg/liter (ppm), representing a 200% increase over controls [0 mg/liter (ppm) N]. Maximum and minimum root dry weights occurred at 0 mg/liter (ppm) N and approximately 50 mg/liter (ppm) N, respectively, representing a 37% decrease. Root length decreased 36% at a NAR of approximately 50 mg/liter (ppm). Root diameter (root area ÷ root length) increased before reaching a plateau at approximately 130 mg/liter (ppm) N indicating that root length decreased faster than root area at low N concentrations. Shoot concentrations of N, P, Mg, and S were maximized at approximately 71,41,48, and 52 mg/liter (ppm) N, respectively. Uptake of K and Ca were unaffected by N concentrations.