184 Gilman and Marshall: Fertilizer Rate and Number of Applications Impact Growth of Trees comparison among various studies because tree size or the area of soil receiving fertilizer varies among studies, or one of these values was not reported. Tested amounts of N/cm caliper (23 g N/cm cali- per, Fini et al. 2007; 29 g N/cm caliper, Watson 2010; up to 58 g N/cm caliper, Struve 2002) and amounts above which there was no response (13 to > 26 g N/cm caliper depending on N source, Neely et al. 1970; 40 to 80 g N/cm caliper, Perry and Hick- man 1998; 4.5 to 36 g N/cm caliper depending on tree size, Gilman et al. 2000) are within an order of magnitude of the historical rate from the cur- rent study [mean = 32 g N/cm caliper (range = 21 to 41 g N/cm caliper) across both species and all three years]. However, it would be hard to make a case for applying more than 0.33 of the historical rate based on current caliper and height data, which would amount to about 11 (32 g × 0.33 historical rate) g N/cm caliper/year. This falls near the bot- tom of the range of cited studies. Perhaps the natu- rally fertile soils in many nurseries combined with N in rainfall (Pribble and Janicki 1999) or irriga- tion in eastern North America, where most studies have been conducted, precluded growth responses to more than a small amount of fertilizer. Total NOx-N applied/tree (2.4 mg/L in well water × 3224 L irrigation/tree annually × four-year study period = 31.2 g N) through irrigation during the current study was 15.3% of the total N applied in fertilizer at the 0.33 historical rate (51 + 163 + 196 + 196 g N = 607 g N fertilizer/tree for the historical rate ÷ 0.33 = 202.3 g N). Although small, this background N apparently supplied enough N to support growth that was more than 90% of that of fertilized trees. Results may have been different if the N-rich shoots pruned from trees were removed from the plots. CONCLUSIONS Reducing fertilizer amount to one-third of what was historically used in a shade tree field nursery, and reducing number of applications from three to either one or two, had a negligible impact on growth of one species of Ilex and Quercus. Nurseries can use the outlined approach along with N recovery rates from wood in harvested trees to find the most efficient strategy for applying fertilizer to their nurs- ery crops. Managers could conduct efficiency tests similar to the one outlined here in an attempt to find the smallest amount of fertilizer required to pro- ©2014 International Society of Arboriculture duce crops. A few selected nurseries have done this. Although the current study tested a 0.33 historical rate, the data and conversations with growers sug- gest that even lower rates should be included in future tests. Aſter finding the lowest rate and smallest number of applications in a three- to four-year test, researchers recommend incor- porating the most efficient rate into a couple of fields, adjusting as needed, and eventually applying the new rate to the entire operation. LITERATURE CITED Broschat, T.K., D.R. Sandrock, M.L. Ellliott, and E.F. Gilman. 2008. Effects of fertilizer type on quality and nutrient content of estab- lished landscape plants in Florida. HortTechnology 18:278–285. Cabrera, R.I., and D.R. Devereaux. 1999. Crape Myrtle post- transplant growth as affected by nitrogen nutrition during nursery production. Journal of the American Society for Horti- cultural Science 124:4–98. Chadwick, L.C., P.E. Tilford, and C.F. Irish. 1957. A study of some methods of fertilizing shade trees. Proceedings American Soci- ety Horticulture Science 55:519–526. Conover, C., and J. Joiner. 1974. Influence of fertilizer source and rate on growth of woody ornamentals transplanted to stress conditions. Florida Agricultural Experiment Station Journal Series No. 5469, 7 pp. University of Florida, Gainesville. Fini, A., F. Ferrini, P. Frangi, G. Amoroso, and F. Minoprio. 2007. Growth and physiology of field-grown Acer pseudoplatanus L. trees as influenced by irrigation and fertilization. Proceedings Southern Nurseryman Association 52:51–58. Gilman, E.F. 1987. Response of hibiscus to soil applied nitrogen. Proceedings Florida State Horticulture Society 100:356–357. Gilman, E.F., and T.H. Yeager. 1990. Fertilizer type and nitrogen rate affects field-grown laurel oak and Japanese ligustrum. Proceedings Florida State Horticulture Society 103:370–372. Gilman, E.F., T.H. Yeager, and D. Kent. 2000. Fertilizer rate and type impacts magnolia and oak growth in sandy landscape soil. Journal of Arboriculture 26:177–182. Hensley, D.L., R.E. McNeil, and R. Sundheim. 1988. Management influences growth of transplanted Magnolia grandiflora. Journal of Arboriculture 14:204–207. Ingram, D.L., B. Roach, and M. Klahr. 1998. Effects of controlled- release fertilizers on growth and nutrient content of field-grown nursery crops. Proceedings Southern Nurserymen Association Annual Conference 43:122–127. Jacobs, H.L. 1930. Fertilization of shade trees. Bulletin 5, 16 pgs. The Davey Tree Expert Company, Kent, Ohio. Khatamian, H., J.C. Pair, and R. Carrow. 1984. Effects of turf competition and fertilizer application on trunk diameter and nutrient composition of honeylocust. Journal of Arboriculture 10:156–159. Klein, I., I. Levin, B. Bar-Yosef, R. Assaf, and A. Berkovitz. 1988. Drip nitrogen fertigation of ‘Starking Delicious’ apple trees. Plant and Soil 119:305–314. Mathers, H., L. Case, and K. Daniel. 2012. Tree response to fertilizer formulation in the field. Proceedings Southern Nurserymen’s Association Annual Conference 57:128–131.
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