Arboriculture & Urban Forestry 36(4): July 2010 RESULTS & DISCUSSION Significant tree responses to the fertilization treatments were lim- ited (Table 1). In the third year only, caliper increase of the inner area-full rate treatment was greater than all other treatments. Twig growth and leaf area were not increased by any treatment combina- tion in any year. Even the more thorough sampling in the third year did not reveal differences among treatments, though it did produce lower and likely more representative, average twig growth mea- surements for all treatments, compared to previous years. In the third year, SPAD measurements of the inner area-full rate and the inner area-half rate treatments were marginally significantly (P = 0.086) greater than the control. The trees were also visibly greener. Nutrient uptake nearer the tree may have been enhanced by greater natural fine root development (Watson 1996). Concen- tration of nutrients can increase root proliferation and uptake (Robinson 1994; Hodge 2004). It may be that concentrating the nitrogen fertilizer applications closer to the base of the tree can take advantage of the naturally higher fine root density, in addi- tion to any further fine root stimulation resulting directly from the nitrogen application. Future research should include moni- toring changes in fine root development accompanying above- ground responses to placement and rate of fertilizer application. The lack of any response to the treatments until the third year was not expected, but could possibly be explained by the annual growth cycle of this temperate deciduous species and the nature of the fertilizer product. Since 71% of the nitrogen was in a slow- release formulation, and application was in May, annual twig growth may have been completed by the time sufficient nitrogen was released, absorbed, and transported to the shoots, preventing a growth response in the first year. If the slow-release formulation was also ineffective in increasing bud size in the first year, twig growth increase in the second year may have still been limited, though an increase in SPAD readings could have been expected. The third year could have been the first growing season when fertilizer was present to affect both bud formation in the previ- ous season and current season growth. If increased uptake was dependent on an increase of fine root density in response to the more concentrated fertilizer, it may have taken time to develop. Another possible factor in the limited growth response from the fertilizer treatments could be related to moderate level of natural fertility on the site. Similar Ozaukee soils in the re- gion average 4.3% carbon, 0.34% nitrogen, 12.65 C/N (Bry- ant Scharenbroch, The Morton Arboretum, pers. comm.). The limited overall response to fertilization application raises ques- tions as to how much fertilization is appropriate for trees in well-maintained urban landscapes where fertilizers are applied to lawns and other plantings. Further studies on different spe- cies, application rates, and patterns are needed on more urban, less fertile sites. A broader approach to providing a healthy soil environment (physical and biological, as well as chemical) may also be needed in comprehensive plant health care programs. Fertilizer recommendations are currently based on soil surface area. Basic geometry calculations show the inner half of the total soil area within the dripline extends 70% of the radius to the drip- line. This was the inner area used in this study, but may actually be similar to the way fertilizer is often applied in practice, depending on the soil surface accessible, or how “from the trunk to near the dripline” (Smiley et al 2002), is interpreted. If anything, the results of this study may reinforce the recommendation to not apply fer- 181 tilizer beyond the dripline (Smiley et al 2002). The results of this study should not be interpreted as a mandate for basal application of fertilizer, as has become standard for some insecticides and growth regulators. Further research on less fertile sites could pro- vide more information on how to apply fertilizers more effectively. Acknowledgments. This study was funded, in part, by a grant from The Care of Trees, Wheeling, Illinois. LITERATURE CITED American National Standard Institute. 2004. American National Stan- dard for Tree Care Operations – Tree, Shrub, and Other Woody Plant Maintenance – Standard Practices (Fertilization) (A300, Part 2). Tree Care Industry Association, Manchester, New Hampshire. Barton, L., and T.D. Colmer. 2005. Irrigation and fertilizer strategies for minimizing nitrogen leaching from turfgrass. Agricultural Water Management 80:160–175. Chamuah, G.S. 1988. The effect of nitrogen on root growth and nutrient uptake of young tea plants (Camellia sinensis L.) grown in sand cul- ture. Fertilizer Research 16:59–65. Cregg, B., C. Rios, J. Hart, and D. Briggs. 2004. Fate of nitrates in field nursery production systems. USDA Forest Service Proceedings RMRS-P-33. 50–54. Cripps, R.W. 1992. N fertilization of field grown woody Ornamentals. Proceeds of the SNA Research Conference 37:128–132. Funk, R.C. 2000. The effect of fertilizer source and placement on tree growth. pp. 83–86. In Siewert, A, A. Siewert, B. Rao, and D. Marion (eds). Tree and Shrub Fertilization: Proceedings from an International Conference on Tree and Shrub Fertilization. May 17–18, 2000, Fair- lawn, Ohio. Gold, A.J., W.R. DeRagoon, W.M. Sullivan, and J.L. Lemunyon. 1990. Nitrate-nitrogen losses to groundwater from rural and suburban land uses. Journal of Soil & Water Conservation 45:305–310. Green, T., and G.W. Watson. 1989. The Effect of Organic Surface Mulch on the Establishment of Bare Root Trees. Journal of Arboriculture 15:268–272. Guillard, K., and K.L. Kopp. 2004. Nitrogen fertilizer form and associ- ated nitrate leaching from cool-season lawn turf. Journal of Environ- mental Quality 33:1822–1827. Hodge, A. 2004. The plastic plant: root responses to heterogeneous sup- plies of nutrients. New Phytologist 162:9–24. Ingram, D.L., B. Roach, and M. Klahr. 1998. Effects of controlled-re- lease fertilizers on growth and nutrient content of field-grown nursery crops. Proceeds of the SNA Research Conference 43:124–127. Juntunen, M-L, and R. Rikala. 2001. Fertilization practice in Finnish forest nurseries from the standpoint of environmental impact. New Forests 21:141–158. Lanphear, L. 2000. Tree Fertilization: An Arborist’s Perspective. pp. 143–150. In: A Siewert, A. Siewert, B. Rao, and D. Marion (eds). Tree and Shrub Fertilization: Proceedings from an International Conference on Tree and Shrub Fertilization. May 17–18, 2000, Fair- lawn, Ohio. Larcher, W. 1975. Physiological Plant Ecology. Springer-Verlag, Berlin and New York. Mangiafico, S.S., and K. Guillard. 2007. Nitrate leaching from Kentucky bluegrass soil columns predicted with anion exchange membranes. Soil Science Society of America Journal 71:219–224. National Resource Conservation Service Web Soil Survey ©2010 International Society of Arboriculture
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