180 Gary Watson: Broadcast Nitrogen Fertilization Rates and Placement on the Growth of Green Ash Trees unit of root is also increased (Chamuah 1988; Watson 1994; re- view papers by Robinson 1994 and Hodge 2004). Applying fertil- izer in a smaller area nearer the trunk, thereby increasing the rate per unit area of soil, could take advantage of these root stimulat- ing and absorption efficiency effects, thereby increasing uptake. Reports on the effectiveness of different application meth- ods vary. In the field, there was no reduction in pecan [Carya illinoensis (Wangenh) C. Koch] yield when nitrogen rates were cut in half and applied to a limited area by drip irriga- tion rather than by broadcasting (Whorley 1995). Several stud- ies have reported no difference between broadcast application, drill holes filled with granules, and liquid injection applica- tions (Neely et al. 1970; Funk 2000), or only a slightly greater growth response from holes filled with granules (Smith 2000). This study utilized young trees, growing on a soil where a quick release nitrogen formulation had proven effective in stimulating growth of similar trees in the past (Neely et al. 1970). The objective of this study was to measure tree growth responses in relation to placement and rate of a slow re- lease nitrogen fertilizer application to determine if fertilizers can be applied more effectively, or if rates could be reduced. METHODS A site with ‘Summit’ green ash (Fraxinus pennsylvanica ‘Summit’), 15 cm average caliper, growing in established turf at 4.8 m spacing on Ozaukee silt loam soil was used for this study (National Resource Conservation Service). Trees exhibited typical growth and color for their species and age. The turf was mowed only two or three times each growing season. No supplemental irrigation was provided. The degree of overlap of root systems of adjacent trees was investigated prior to initiation of fertilization treatments. To avoid disturbance to the experimental trees, trenches were dug at the mid- point between similar trees on the site with the same spacing (2.4 m from each tree). This midpoint was typically less than 30 cm beyond the dripline. The trenches exposed very few tree roots over 6 mm diameter. Overlap of root systems was not considered a problem. The maximum fertilization area for each tree was a 4.8 m diameter square (23 m2 ). This area was slightly wider than the dripline, which is commonly designated as the soil surface area where fertilizer should be applied (Smiley et al. 2002). Fertilizer was applied to all or a portion of this area: the en- tire square, the inner half of the total soil surface area (3.4 m diameter square area), or the outer half of the total soil sur- face area (the outer 0.7 m of the 4.8 m diameter square area). The trees were fertilized with Nu-Gro Nitroform® granular 38- 0-0 fertilizer, a urea formaldehyde nitrogen source with 71% of the nitrogen from a water-insoluble fraction. Annual application rate variations were based on 1.92 kg N/100 m2 over the maximum 23 m2 application area, or 0.44 kg N/tree. When only half of the area was fertilized, either the entire amount was applied to maintain a consistent amount per tree, or the amount was cut in half to main- tain a consistent rate per unit of soil surface area. See Table 1 for a complete listing of treatment combinations. The 1.92 kg N/100 m2 rate was chosen because it is the mid-point of the standard tree fertilization rates used by arborists (ANSI 2004). There were five replications of each fertilizer treatment, applied annually for three years on May 16, 2002; May 29, 2003; and May 11, 2004. Late in each growing season, when annual growth was complete and terminal buds were set, terminal twig growth was measured on two vigorous, nonapical branches just above the center of the crown, on opposite sides of the tree. The final year, when trees were destruc- tively harvested, there was an opportunity to sample more extensively throughout the crown. Ten branches from each tree were measured. Two undamaged fully-expanded leaves from the same branches were used to measure leaf area on a Delta-T (video) Area Meter. Rela- tive chlorophyll content was measured on each of the leaves with a Minolta SPAD-502 meter. Caliper was measured annually at 30 cm above grade. Since caliper was not measured, the season prior to initi- ation of treatments, increase for the first year could not be calculated. One-way ANOVA (P ≤ 0.05, Normality, P > 0.05) with separation of means by the Holm-Sidak Method (SigmaStat 3.0, SPSS Science) was used to compare caliper increase, twig growth, leaf area, and SPAD measurements from each year. Table 1. Growth responses of green ash trees to three different placements of fertilizer and two rates. Caliper increase (cm) Entire root zone-full ratez Inner area-full rate Inner area-half rate Outer area-full rate Outer area-half rate Control P value Entire root zone, full rate Inner area-full rate Inner area-half rate Outer area-full rate Outer area-half rate Control P value 2002 48.36 48.68 49.40 49.96 46.60 47.42 0.68 2003 1.9 1.3 1.0 1.2 0.9 1.1 0.18 SPAD 2003 47.82 50.20 44.50 47.28 43.60 47.46 0.60 z Entire root zone = 4.8 m diameter square, full rate = 1.92 kg N/100 m2 five replications of each treatment. y Values followed by the same letter are not significantly different at P ≤ 0.05 x Values followed by the same letter are not significantly different at P ≤ 0.10 2004y 1.2b 2.9a 1.4b 1.0b 0.9b 1.4b 0.03 2004x 37.99de 42.12ae 40.72ace 40.16de 37.69cd 36.26bd 0.08 2002 41.9 39.8 34.8 38.7 46.2 50.3 0.84 2002 72.5 61.6 82.5 58.6 85.1 77.7 0.32 73.9 61.0 74.3 60.9 64.8 0.39 Leaf Area (cm2 2003 100.9 110.7 101.8 102.8 94.4 98.7 0.72 ) 2004 102.5 122.2 160.3 126.2 146.7 105.0 0.27 , or 0.44 kg N/tree. Square footage or fertilizer amount was halved for other treatments. There were Twig growth (cm) 2003 63.7 2004 15.5 14.0 14.0 17.0 14.6 13.3 0.88 ©2010 International Society of Arboriculture
July 2010
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